Summary of MASW Publications

 

IVA-01-01

Modal separation before dispersion curve extraction by MASW method

Abstract

Accurate extraction of dispersion curves is the most critical part with any surface-wave method. Although a multichannel method like Multichannel Analysis of Surface Waves (MASW) method proves most effective for this purpose, there are several acquisition parameters that need to be set properly for the optimal extraction. One of them requires a long range of receiver spread to separate phase velocity of one mode from those of other modes. Sometime, however, this requirement can not be met due to an unacceptable lateral inhomogeneity in the near-surface materials being surveyed and therefore multichannel records are often collected with a relatively short spread length. In this case the interference between the different modes of surface waves can be so severe that neither fundamental nor highermode dispersion curve can be extracted with a reliable confidence. In addition to this multimodal effect by surface waves, other types of seismic event such as channel (guided) waves can cause similar harmful effect during the analysis. When this occurs, a simple multichannel processing technique that mutes the interfering wavefields in the offset-time (x-t) domain can significantly enhance the resolution of multimodal dispersion curves. This is demonstrated by using both synthetic and real shot gathers.

                              Full Paper     IVA-01-01.PDF 103KB

IVA-00-02

Mapping Poisson’s ratio of unconsolidated materials from a joint analysis of surface-wave and refraction events

Introduction

Poisson’s ratio ( σ ) of the near-surface materials is one of the key parameters in various types of geotechnical projects. It is usually associated with the integrity of the materials from the engineering perspectives. A two-dimensional (2-D) distribution map of σ , therefore, would have an invaluable value.  Seismically, σ can be determined if P- (Vp) and S-wave (Vs) velocities are known. This would indicate that two separate (P- and S-wave) surveys should be performed in order to obtain the separate maps for Vp and Vs. Running both types of survey for one project will be expensive in terms of equipment, data processing, and overall time. In addition, S-wave survey is generally known as being much more difficult to acquire good quality data than the P-wave survey.  We can obtain Vp and Vs fields by using several methods. However, each of these methods requires individual data acquisition and processing techniques. For example, the reflection method is not suitable for studying the very near surface because the shallower we aim our target of investigation the more expensive it becomes and the more our data will be contaminated by waves considered as noise. On the other hand, the refraction method is incapable of detecting “hidden layers” (Burger, 1992) such as a high velocity thin layer or a low velocity layer “sandwiched” between two high velocity layers. Such “hidden layers” cause erroneous interpretation of the data. An improved way of interpretation of refraction data is by using refraction tomography. Still, we need an initial model that is close to the true Vp distribution as well as smoothing constraints (Stork and Clayton, 1991.) in order to achieve reliable results during inversion.  Reflection and refraction Vs methods share the same problems as with the Vp field. In addition there are equipment and acquisition difficulties and there is the possibility for misinterpretation due to S-P-S wave conversion (Xia et al., 1999).  Recently, an economic seismic method has been used to produce a Vs profile (plot of Vs vs.  depth) by analyzing surface waves (ground roll) on a multichannel record. This multichannel analysis of surface waves (MASW) method (Park et al., 1999a) can produce a 2-D near surface Vs map when the multichannel records (shot gathers) are acquired in a consecutive manner similar to conventional reflection survey (Miller et al., 1999a). Since the MASW method SAGEEP 2000 employs the conventional seismic approach in which vertical source and receivers are used the near-surface Vp information can be associated with the first arrivals from the shot gathers.  Refraction tomography, for example, can be applied to the shot gathers to obtain the near surface Vp map.  The goal of this paper is to propose a method for obtaining accurate and reliable 2-D σ distribution map in the near surface in an economic manner. The economy is achieved by performing a single near surface seismic survey using a vertical source for acquiring data for two wave fields. The accuracy and reliability is achieved using the analysis results of one wave field (surface wave) as a priori information for the inversion of another (body wave).

                              Full Paper    IVA-00-02.PDF 1.23MB

IVA-00-01

Joint analysis of surface-wave and refraction events from river-bottom sediments

Summary

An underwater seismic survey was performed in the Fraser River Delta area near Vancouver, B.C., Canada, using hydrophones.  Data were analyzed using multichannel analysis of surface waves (MASW), and refraction tomography inversion provided an accurate P-wave velocity (Vp) profile for riverbottom sediments in the upper 30 m.  To optimize the compressional wave velocity function in the 30 to 70 m depth interval, Vp information from a nearby well was used as a priori information.Using shear wave velocity profiles from surface-wave inversion as a priori information allows a fast and reliable inversion of the first arrival events interpreted on an underwater shot gather. The inverted Vp profile, strongly suggesting the presence of gas within water-bottom sediments, corroborates a nearby land well. This example demonstrates the usefulness of joint inversion and its potential for detecting near-surface anomalies. between measured and calculated first arrivals. The match is improved by iteratively changing the Vp model until a satisfactory solution has been reached.  As described previously, through joint analysis of this shallow marine data set a reliable Vp solution will be obtained for the shallow sediments independent of the measurements made in the nearby control well.

                              Full Paper    IVA-00-01.PDF 827KB

MIL-01-03

Love waves: A menace to shallow shear wave reflection surveying

Summary

Shallow shear wave reflection profiling, in comparison to compressional wave surveys, requires a greater awareness of and ability to discriminate the many sources and associated forms coherent noise can take on shot gathers and the manifestation of that noise on CMP stacked sections. Shallow shear wave reflection surveying is an attractive alternative to compressional wave reflection surveys in some situations due in large part to the increased resolution potential at a given dominant reflection frequency, the polarized nature of the energy, insensitivity to pore materials, and the relationship of shear wave velocity to rigidity/stiffness of materials. Unlike compressional wave arrivals where reflections are the only coherent hyperbolic events, reflections and surface waves can appear hyperbolic on shear wave shot gathers. This unique difference should eliminate the complete confidence placed in unique interpretations of all hyperbolic events with a zero offset apex as reflections. Without careful attention to true measured velocities (ground truth) and complete removal of all energy arriving within the noise cone, Love waves will likely stack coherently on CMP sections. Interpretations made of stacked shear wave data which do not include sample shot gathers where reflection events are clearly identifiable outside the noise cone should not be considered legitimate and need to be assumed coherent noise.

                              Full Paper    MIL-01-03.PDF 1.43MB 

MIL-01-02

SHEAR WAVE VELOCITY FIELD TO DETECT ANOMALIES UNDER ASPHALT

Abstract

Non-invasive mapping of anomalies beneath asphalt at depths from 2 m to as deep as 50 m has been successful using MASW in a variety of near-surface settings. Anomalies that include fracture zones within bedrock, dissolution/potential subsidence features, voids associated with old mine works, and erosional channels eched into the bedrock surface have been effectively identified in the shear wave velocity field calculated by the Multichannel Analysis of Surface Waves (MASW) method. By acquiring many individual multichannel surface wave data gathers on even spacings along a continuous transect, a series of 1-D shear wave profiles obtained by inverting surface wave dispersion curves can be generated that form a 2-D shear wave velocity field beneath the transect. The cell size of the shear wave velocity field depends on the frequency range of the data and source spacing. By contouring the shear wave velocity field, variations representative of anomalous subsurface can easily be interpreted. The method itself focuses on surface wave data  with frequencies ranging from 2 to over 60 Hz, which can be directly correlated to depth of investigation and is completely insensitive to cultural noise and surface conditions (e.g., asphalt, gravel, cement, etc.). By incorporating CMP style roll-along acquisition with multichannel acquisition, sufficient redundancy and smoothing exists to confidently interpret anomalies that are evident across several 1-D profiles. Case histories from several uniquely different sites with uniquely different problems provide empirical evidence supporting the utility of this method. Mapping bedrock beneath an asphalt parking lot at depth from 2 to 7 m was successfully accomplished at a site in Olathe, Kansas. Preliminary analysis of this site’s hydrologic characteristics, based primarily on borehole data, suggested that fractures and/or an unmapped buried stream channel was influencing fluid movement along the drill-defined bedrock surface. High velocity gradients within the shear wave velocity field were used as diagnostic of the bedrock surface, while localized lateral decreases in the shear wave velocity below the bedrock surface were considered characteristic of fracture zones or erosional channels. Delineating voids resulting from extensive underground mining of lead and zinc in southeastern Kansas was critical to pavement evaluations and determinations of road stability. Void features interpreted on shear wave velocity profiles were consistent with extensive borehole data collected at this site. Subsidence features obscured by development and masked from other geophysical methods by power line noise, mechanical noise, reinforced concrete, and requirements for non-invasive methods were distinguishable on 2-D shear wave velocity field data. Subsidence features interpreted on data acquired through occupied houses in western Florida were correlated with existing drill data and verified by drilling based on interpretations of those data.  Pits and trenches were located beneath asphalt surfacing at an old refinery site in eastern Illinois through coincident analysis of phase and amplitude distortions on surface wave data with the 2-D shear wave velocity field. Pipes placed 3 to 5 ft deep in trenches, infilled with native soils, and then covered with asphalt produced a distinctive signature on surface wave data. Advantages of using the shear wave velocity field, calculated from surface waves to detect, delineate, and/or map anomalous subsurface materials include the nsensitivity of MASW to velocity inversions and cultural noise, ease of generating and propagating surface wave energy in comparison to body wave energy, and its sensitivity to changes in velocity.

                              Full Paper    MIL-01-02.PDF 1.50MB

MIL-01-01

COMPARING SHEAR-WAVE VELOCITY PROFILES INVERTED FROM MULTICHANNEL SURFACE WAVE WITH BOREHOLE MEASUREMENTS

Abstract

Recent field tests illustrate the accuracy and consistency of estimating near-surface shear (S) wave velocities calculated using multichannel analysis of surface waves (MASW) (Park et al., 1999; Xia et al., 1999; Miller et al., 1999). To evaluate the technique in a variety of near-surface conditions and through a wide range of velocities, MASW-derived S-wave velocity profiles (S-wave velocity vs. depth) were compared to direct borehole measurements at four North American sites. A detailed study of the effects of the total number of recording channels, sampling interval, source offset, and receiver spacing on the inverted S-wave velocity was conducted at a test site in Lawrence, Kansas. Optimization of the method provided generally applicable rules of thumb that have resulted in differences between inverted S-wave velocities between the MASW method and borehole measurements to be as low as 18 percent, with potential improvement as low as 9 percent (Figure 1).  A surface wave survey was performed in Wyoming to determine shear-wave velocities in near-surface materials (upper 7 m) as a direct result of mode converted shear wave refraction data. In the 0 to 6 m range, the average difference between S-wave velocities estimated from the MASW method and those measured from suspension logging is less than 15 percent (Figure 2).  Validation of the MASW technique requires comparison between several borehole-derived velocity profiles as well as blind testing. MASW-derived S-wave velocity profiles were statistically compared to S-wave velocity profiles measured in seven boreholes in the unconsolidated sediments of the Fraser River Delta, near Vancouver, B.C., Canada. An overall difference of approximately 15 percent was observed between the direct borehole measurements and inverted S-wave velocities from the seven well locations. A blind test of the stand-alone accuracy of MASW was conducted at an eighth well. For this blind test, S-wave velocity measurements made in and interpreted from the borehole were not available during MASW data processing.  Differences between S-wave velocities using MASW and those measured in the blind test borehole was 9 percent (Figure 3). Inverted S-wave velocities calculated using the MASW technique at a landfill site in Johnson County, Kansas, are within 15 percent of borehole measurements, which were treated as ground truth. Mode conversions along sloping subsurface refracting horizons can result in misidentification of shear wave velocity.  No systematic difference between these results were observed in data from any of these test sites. The MASW method provided reliable S-wave velocity profiles within the upper 30 meters below the ground surface.

                              Full Paper    MIL-01-01.PDF 119KB   

MIL-00-01

SHEAR WAVE VELOCITY FIELD FROM SURFACE WAVES TO DETECT ANOMALIES IN THE SUBSURFACE

Abstract

Mapping the bedrock surface at depths ranging from 1 m to as deep as 30 m, identifying potential fracture zones within bedrock, and delineating dissolution/potential subsidence features can be effectively done in a variety of near-surface settings using the shear wave velocity field calculated by the multichannel analysis of surface waves (MASW) method. A 2-D shear wave velocity field accurate to within 15% can be produced by acquiring many individual multichannel surface wave data gathers and inverting the surface wave dispersion curves to form a series of 1-D shear wave profiles. Individual discrete cells that make up the shear wave velocity field provide a measure of resolution potential and are dependent on the frequency range of the data and source spacing. Variations in the shear wave velocity field representative of an anomalous subsurface can easily be interpreted on shear wave velocity field contours.  As generally applied, the method focuses on surface wave energy with frequencies ranging from 2 to over 60 Hz (frequency and associated wavelengths are indicative of depth of investigation) and is relatively insensitive to cultural noise and surface conditions (e.g., asphalt, grass, gravel, mud, etc.). By incorporating common mid-point (CMP)-style roll-along acquisition (seismic reflection method) with multichannel acquisition, sufficient redundancy and resolution exists to interpret anomalies evident across several 1-D profiles.  Case histories from four different sites with uniquely different problems provide empirical evidence supporting the utility of this method.  1) A map of the shear wave velocity field imaging depths from 2 to 7 m allowed correlation of the bedrock surface to high velocity gradients within the shear wave velocity field with localized lateral decreases in the shear wave velocity below the bedrock surface characteristic of fracture zones or erosional channels at a site in Olathe, Kansas.  2) Delineating drill confirmed dissolution features beneath undisturbed alluvial overburden provided information integral to the design and construction of a power plant in Alabama.  3) Subsidence features interpreted on data acquired through occupied houses in western Florida were correlated to drill data and verified by drilling based on interpretations of those data.  4) Pits and trenches were located at an old refinery site in eastern Illinois through coincident analysis of phase and amplitude distortions on surface wave data with the 2-D shear wave velocity field.  Advantages of using the shear wave velocity field, calculated from surface waves to detect, delineate, and/or map anomalous subsurface materials include the insensitivity of MASW to velocity inversions and cultural noise, ease of generating and propagating surface wave energy in comparison to body wave energy, and its sensitivity to changes in velocity.

                              Full Paper    MIL-00-01.PDF 1.39MB  

MIL-99-02

Using MASW to map bedrock in Olathe, Kansas

Abstract

The shear wave velocity field, calculated using the Multichannel Analysis of Surface Waves (MASW) method (Park et al., 1999; Xia et al., in press) was used to map the bedrock surface at depths of 2 to 7 m and identify potential fracture zones within bedrock at a site in Olathe, Kansas. Preliminary analysis of this site’s hydrologic characteristics, based primarily on borehole data, suggested that fractures and/or an unmapped buried stream channel was influencing fluid movement along the drill-defined bedrock surface. Since topographic variations on the surface of bedrock can influence the transport and eventual fate of contaminants released at or near the ground surface, determining the nature and location of anomalous bedrock was critical to establishing the environmental risk at this site. High velocity gradients within the shear wave velocity field were used as diagnostic of the bedrock surface, while localized lateral decreases in the shear wave velocity below the bedrock surface were considered characteristic of fracture zones or erosional channels. Calculating the shear wave velocity field from surface wave arrivals can generally be accomplished with a high degree of accuracy regardless of cultural noise. The insensitivity of MASW to cultural obstacles and noise was demonstrated at this site (e.g., a 185,000 m2 asphalt parking lot, electrical and mechanical noise from nearby industrial facilities, traffic noise from the adjacent highway, exploratory drilling on the asphalt parking lot, and aircraft noise). The depth-to-bedrock map produced using shear wave velocity data only possesses significantly higher resolution with less than 0.3 m in difference observed between the interpreted bedrock depth from surface wave data and from drill confirmed bedrock. Advantages of mapping the bedrock surface with the shear wave velocity field calculated from surface waves include the insensitivity of MASW to velocity inversions, ease of generating and propagating surface wave energy in comparison to body wave energy, and its sensitivity to lateral changes in velocity.

 

                              Full Paper    MIL-99-02.PDF 250KB   

MIL-99-01

Seismic techniques to delineate dissolution features in the upper 1000 ft at a power plant site

Summary

Shallow seismic techniques (Steeples and Miller, 1990; Park et al., 1999; Xia et al., in press) were used to enhance the effectiveness of a drilling program designed to locate dissolution features large enough to put the integrity of equipment or environment at Alabama Electric Cooperative’s proposed Damascus site at risk (“A” on Figure 1). Dissolution features will directly impact engineering design specifications and future plant safety. This applied research program identified acoustic characteristic unique to voids, subsurface subsidence, and/or karst features; evaluated the potential of acoustic methods to enhance drill-assisted mapping of major stratigraphic units and structural features; identified the maximum and minimum depths of seismic investigation; estimated resolution potential (vertical and horizontal features detectable and resolvable); defined optimum geometries and equipment; and incorporated production 2½-D reflection and shear wave profiles with exploratory drilling. The shear wave velocity field provided valuable information about areas that might be at risk of subsiding.  From that feasibility survey, areas studied with “young” sinkholes, directly tied to karst features, produced pronounced velocity inversions in close proximity to large velocity gradients (generally forming a closure on contoured cross-sections). These anomalous areas were interpreted to indicate increased stress associated with roof rock loading over rubble zones or void areas. Data from both the reflection and shear wave profiles from the feasibility and 2½-D surveys possessed several unique features that are probably related to dissolution and subsidence.

 

                              Full Paper    MIL-99-01.PDF 1.89MB   

PAR-02-04
Lamb waves observed during MASW surveys

Abstract

Two fundamental modes of asymmetric and symmetric types of Lamb waves are observed from the near-surface seismic surveys by multichannel analysis of surface waves (MASW) method at two sites. One site consisted of loose soil charged with gas at shallow depth (at one and half meter). The other site was on top of hard, massive limestone possibly having a less stiff layer of shale or horizontal fracture at depth of approximately five meters. Lamb waves, originally misinterpreted as Rayleigh waves, were first identified from the prominent trend of both inverse (fundamental-mode asymmetric) and normal (fundamentalmode symmetric type) dispersion on the phase velocity image constructed from a multichannel analysis technique.  Theoretical Lamb wave dispersion curves are then calculated and matched with the observed trend of dispersion on the image to calculate parameters such as thickness, and Sand P-wave velocities. These field examples indicate that Lamb waves can find useful applications in near-surface seismic surveying. Possible application may include detection of coal beds and plate voids as well as the cases illustrated here.

                              Full Paper    PAR-02-04.PDF 144KB         

PAR-02-03

Optimum Field Parameters of an MASW Survey

Abstract

The importance of key acquisition parameters of the multichannel analysis of surface wave (MASW) method such as offset, receiver, and source are briefly discussed by using field examples collected over two types of soil sites: the most common type being moderately wet and compact, and the other type being dry and fairly hard. For most common soil sites, the offset range of 10-100 m is optimal for record-ing the fundamental mode surface waves in the frequency range of 5-50 Hz, and in the phase velocity range of 50-1000 m/sec. In general, MASW is a seismic method most tolerant to field parameters among all other methods.

                              Full Paper    PAR-02-03.PDF 334KB  

PAR-02-02

FILTERING SURFACE WAVES

Summary

As surface waves become more commonly utilized in near-surface seismic investigations, the necessity of filtering them in the level of different modes often arises. This is because the surface wave application is usually based on the analysis of the fundamental mode only, whereas the surface waves as experienced during field surveys often are mutimodal in nature with higher modes dominating at high frequencies. This domination inevitably limits the analyzable bandwidth of the fundamental mode at the corresponding frequency band. In addition, the phase velocity range of higher modes often overlaps with that of the fundamental mode, making the conventional pie-slice f-k filtering ineffective for the purpose of filtering higher modes only. Two approaches are introduced. One implements in the frequency-wave number (f-k) space by redefining the rejection zone as narrow and curved (bow-slice) instead of the familiar pie-shaped zone. The other accomplishes the necessary filtering by utilizing the frequency-variant linear move out (FV-LMO) correction made from the dispersion information of the higher mode previously analyzed. These two approaches can also be useful during reflection processing.

                              Full Paper    PAR-02-02.PDF 418KB      

PAR-02-01

TIME BREAK CORRECTION IN MULTICHANNEL SIMULATION WITH ONE RECEIVER (MSOR)

Abstract

Recent investigations in the seismic evaluation of pavement systems indicates that the multichannel approach is indispensable. This is because of the complicated seismic phenomenon that originates from the unique seismic setting of a pavement system. A true multichannel survey would be a formidable task that would require an expensive multichannel (e.g., 48 channel) recording device and so-many receivers with complicated wiring deployed in a small area on the pavement. Instead, the multichannel simulation with one receiver (MSOR) approach can produce a simulated multichannel record by using only one receiver and a single (or two) channel recording device readily available for various types of engineering measurements. For this approach to be an effective alternative, a consistent timing of wave generation at each impact is the most critical condition to be met. Considering the necessary accuracy of tens of microseconds to deal with seismic waves in the range of kilohertz, it seems that a certain degree of inconsistency in time break can always occur in spite of a carefully designed timing mechanism. For a given inconsistency, extraction of the dispersion curve for surface waves is adversely affected most in the high frequencies and least in the low frequencies, making the lower frequencies still useful. This low-frequency dispersion information is then used to construct an impulsive surface wave event that should align perfectly at zero time if there were no time break inconsistency. The appropriate amount of time break correction can therefore be assessed from the amount of misalignment. The correction procedure may continue in an iterative manner because the extractable bandwidth of the dispersion curve would extend after each correction.

                              Full Paper    PAR-02-01.PDF 249KB     

PAR-01-05

Seismic Characterization of Geotechnical Sites By Multichannel Analysis of Surface Waves (MASW) Method

Abstract

Shear wave velocity (Vs) information of a soil site is one of the critical parameters in soil dynamics and earthquake engineering. Although it can be obtained from a shear-wave seismic survey either at the surface or through a borehole method, the surface-wave method is an excellent alternative. It utilizes the dispersion of Rayleigh waves that usually take more than two thirds of total seismic energy generated by an impact seismic source at the surface.  This indicates a relative easiness in the field survey and sometimes also during the post-acquisition processing steps.  Although because of this reason the method appears highly attractive, complications, however, may arise as with all other types of seismic method. Source-generated body waves (e.g., direct and refracted compressional waves), higher modes of Rayleigh waves, random ambient noise (e.g., cultural noise), and coherent arrivals scattered from surface objects (e.g., from building foundations) all interfere with the signal, the planar fundamental-mode Rayleigh waves propagating directly from source. A multichannel seismic method similar to the one long used in the oil industry  provides a highly effective quality control during both acquisition and processing stages. In addition, a 2-D pattern-recognition processing technique has an extraordinary capability to differentiate signal from noise even under a severe noise contamination. This seismic method is applied to various soil-site projects to produce 1-D and 2-D Vs maps. Compressional wave velocity (Vp) is one of the valuable by-products obtainable from the same data set used for Vs analysis through a different processing step.

                              Full Paper    PAR-01-05.PDF 517KB 

PAR-01-04

OFFSET AND RESOLUTION OF DISPERSION CURVE IN MULTICHANNEL ANALYSIS OF SURFACE WAVES (MASW)

Summary

Influence of offset-related parameters on the resolution of dispersion curve in multichannel analysis of surface waves (MASW) surveys is described from the theoretical perspective of the dispersion curve imaging method used during a normal implementation of MASW. The examined parameters include total number of channels (or traces), closest-to-source offset, receiver spacing, and total length of the receiver spread. The influences of different phase velocities and frequencies are also briefly described. It is shown that a larger total receiver spread length is always preferred to produce a higher resolution. This means that with a given number of channels available a greater receiver spacing is preferred as long as it does not cause a spatial aliasing problem. This shows that with MASW method the general notion that more channels are always better can be misleading.

                              Full Paper    PAR-01-04.PDF 212KB

PAR-01-03

SEISMIC INVESTIGATION OF PAVEMENTS BY MASW METHOD —GEOPHONE APPROACH

Summary

A feasibility test of the multichannel approach to seismic investigation of a pavement system is described. This test followed the procedure normally taken in the multichannel analysis of surface waves (MASW) method by using geophones and a light (8-oz) hammer source. A wavefield transformation of recorded multichannel data shows a strong fundamental-mode dispersion curve image in the frequency range of 30-600 Hz with normal (30-50 Hz) and reverse (50-600 Hz) trends. However, the transformation shows that this fundamental mode disappears quite abruptly and higher modes start to dominate in the higher frequencies up to 2000 Hz. Simultaneous recording of both vertical and horizontal components of seismic wavefields facilitates identification of seismic events. In order to record the horizontally travelling direct (or possibly guided) P-wave event in the uppermost layer, it seems critical to use horizontal phones with longitudinal orientation. Results of test indicate that for an investigation focused into the uppermost layers of a pavement system it is essential to use a different acquisition system that can deal with much higher (> 2000 Hz) frequencies. In addition, complicated and unique elastic properties of pavement systems call for an inter-disciplinary study to develop an effective multichannel seismic method for this area of application.

                              Full Paper    PAR-01-03.PDF 744KB

PAR-01-02

Multichannel analysis of surface waves (MASW) for pavement: Feasibility test

Summary

A feasibility test of the multichannel approach to seismic investigation of a pavement system is described. This test followed the procedure normally taken in the multichannel analysis of surface waves (MASW) method by using geophones and a light (8-oz) hammer source. A wavefield transformation of recorded multichannel data shows a strong fundamental-mode dispersion curve image in the frequency range of 30-600 Hz with normal (30-50 Hz) and reverse (50-600 Hz) trends. However, the transformation shows that this fundamental mode disappears quite abruptly and higher modes start to dominate in the higher frequencies up to 2000 Hz. Simultaneous recording of both vertical and horizontal components of seismic wavefields facilitates identification of seismic events. In order to record the horizontally travelling direct (or possibly guided) P-wave event in the uppermost layer, it seems critical to use horizontal phones with longitudinal orientation. Results of test indicate that for an investigation focused into the uppermost layers of a pavement system it is essential to use a different acquisition system that can handle much higher (> 2000 Hz) frequencies.  In addition, complicated and unique elastic properties of pavement systems call for an inter-disciplinary study to develop an effective multichannel seismic method for this area of application.

                              Full Paper    PAR-01-02 203KB

PAR-01-01

Higher Modes of Surface Waves over Unconsolidated Sediments

ABSTRACT

Behavior of higher-mode surface waves is examined through field experiments of multichannel analysis of surface waves (MASW) method at several unconsolidated sediments in the Fraser River Delta, near Vancouver, British Columbia, Canada. The behavior is examined in association with two parameters: shear-wave velocity (Vs) structure in the near-surface (< 30 m) sediments and distance of receiver spread from the source. Results indicate that energy of higher modes tends to become more significant as the source distance becomes greater. They reveal that the dominance may also be related to a Vs structure: a greater dominance as Vs changes little with depth, or Vs has an overall low value, or a combination. The dependency on the source distance is observed to be stronger than that on the Vs structure. Attempts are made to explain the dependency by referring to one or a combination of three factors: attenuation, the near-field effects, and the intrinsic nature of surface waves. Inclusion of higher mode during a surface wave measurement for near-surface (<30 m) application can be either an advantage or a disadvantage, depending on the specific type of application and the method used during the data acquisition and processing steps.

                              Full Paper    PAR-01-01.PDF 252KB

PAR-00-03

MULTICHANNEL SEISMIC SURFACE-WAVE METHODS FOR GEOTECHNICAL APPLICATIONS

ABSTRACT

Unlike other seismic methods (e.g., reflection and refraction), the surface wave method has advantages in several respects. First, the field survey is easiest because of the strong nature of surface-wave energy that can be generated by using a simple impact source (e.g., a sledgehammer) and by following simple field logistics. Second, the data-processing step is usually so simple that it does not require highlyexperienced personnel for reliable determination of optimum processing parameters. This also indicates the potential for full automation of the entire processing step. Third, surface waves respond most effectively to various types of near-surface anomalies that are common targets of geotechnical investigation.  Because of all these merits, the chance of a successful survey is usually much higher with the surface wave method than with other seismic methods when dealing with detection of near-surface anomalies.  When surface waves are utilized to deduce a near-surface shear-wave velocity (vs ) profile (vs versus depth), the analysis relies on the accurate calculation of phase velocities for the horizontally travelling fundamental-mode Rayleigh wave. In spite of the dominance of surface waves on acquired seismic data, interference by noise energy sometimes inhibits the reliability of calculated phase velocities when the whole wave field is inverted. The degree the noise contaminates the dispersion curve, and ultimately the inverted shear wave velocity profile, is dependent on frequency as well as distance from the source.  Multi-channel recording permits effective identification and isolation of these noise types according to their distinctive trace-to-trace coherency in arrival time and amplitude. Decomposition of a multichannel record into a time variable-frequency format, similar to an uncorrelated Vibroseis record, allows each frequency component to be separately and continuously displayed. This unique display format allows contamination by coherent noise to be examined in both frequency and offset space. Separation of these components permits real-time adjustments during acquisition and subsequent processing steps to maximize the signal-to-noise. Multichannel recording permits faster surveying of large areas throughout a broad depth range with only one or a few measurements without significant changes in field configuration.  The methodology of multichannel analysis of surface waves (MASW) is outlined. Effectiveness of the method being applied to near-surface targets that are commonly encountered in geotechnical projects is illustrated through some selected field examples. The examples are 1-D vs profile and 2-D vs imaging methods of MASW.                             

Full Paper    PAR-00-03.PDF 345KB

PAR-00-02

Multichannel analysis of underwater surface waves near Vancouver, B.C., Canada

Summary

Surface (Scholte) waves acquired during underwater seismic surveys with hydrophone arrays are analyzed using the multichannel analysis of surface waves (MASW) method to construct shear-wave velocity (Vs) profiles for the upper 40-m of water-bottom sediments in the Fraser River delta area, near Vancouver, British Columbia, Canada. Shear wave profiles are obtained using the Rayleigh-wave inversion method (based on the multimodal dispersion curves) since theory suggests the difference in phase velocities between the two types of surface waves (i.e., the Scholte vs. the Rayleigh waves) is minor and usually falls below the uncertainty of the measurement. Vs profiles calculated from dispersion analysis are compared with measured Vs profiles available from nearby land boreholes (within a few hundred meters of the underwater sites). The comparison shows MASW values are in good agreement with the overall trend of borehole values, but lower in general by about ten percent. This shift seems to be attributable to the water-bottom sediments being softer and their density being greater (at depths < 5 m) than sediments on land. Simple multichannel processing (surgical mute) seems critical to suppress the influence of the strong broad-band channel waves trapped in the water layer before extracting the dispersion curve.

                              Full Paper    PAR-00-02.PDF 176KB

PAR-00-01

DETECTION OF HIGHER MODE SURFACE WAVES OVER UNCONSOLIDATED SEDIMENTS BY THE MSAW METHOD

SUMMARY

In engineering application of surface waves it is critically important to accurately extract the fundamental mode dispersion curve. Among several factors that may adversely affect the extraction is the existence of higher modes with significant amount of energy. A calculated phase velocity can be an average of the fundamental and the higher-modes phase velocities or it can be the phase velocity of a specific higher mode, depending upon the specific method used for the application, unless the higher modes are properly handled during the data acquisition and processing steps. Therefore, it will have a practical value to observe the higher mode generation through field experiments and examine for any parameter that can be controlled during data acquisition.  A higher mode (the first overtone) of high frequency (5–30 Hz) surface waves was observed by using the multi-channel analysis of surface waves (MASW) method at three boreholes located in unconsolidated sediments in the Fraser River Delta, near Vancouver, British Columbia. Each site has a unique near-surface shear (S)-wave velocity (Vs) structure as verified from downhole Vs measurements. The relative dominance of higher mode energy is examined in association with source distance as well as Vs structure. Our examination indicates that energy of higher modes tends to become more significant as the source distance becomes greater. It also reveals that the dominance may be related to a Vs structure: a greater dominance as Vs changes little with depth, or Vs has an overall low value, or a combination. The dependency on the source distance is observed to be stronger than that on the Vs structure. Attempts are made to explain the dependency by referring to one or a combination of three factors: attenuation, the near-field effects, and the intrinsic nature of surface waves. Inclusion of higher mode during a surface wave measurement for near-surface (<30 m) application can be either an advantage or a disadvantage, depending on the specific type of application and the method used during the data acquisition and processing steps. It is, therefore, important to recognize through field observations those conditions both favorable and unfavorable to the generation of higher modes of high-frequency surface waves.

                              Full Paper    PAR-00-01.PDF 240KB

PAR-99-04

Multichannel analysis of surface waves

ABSTRACT

The frequency-dependent properties of Rayleigh-type surface waves can be utilized for imaging and characterizing the shallow subsurface. Most surface-wave analysis relies on the accurate calculation of phase velocities for the horizontally traveling fundamental-mode Rayleigh wave acquired by stepping out a pair of receivers at intervals based on calculated ground roll wavelengths. Interference by coherent source-generated noise inhibits the reliability of shear-wave velocities determined through inversion of the whole wave field. Among these nonplanar, nonfundamental-mode Rayleigh waves (noise) are body waves, scattered and nonsource-generated surface waves, and higher-mode surface waves. The degree to which each of these types of noise contaminates the dispersion curve and, ultimately, the inverted shear-wave velocity profile is dependent on frequency as well as distance from the source.  Multichannel recording permits effective identification and isolation of noise according to distinctive traceto-trace coherency in arrival time and amplitude. An added advantage is the speed and redundancy of the measurement process. Decomposition of a multichannel record into a time variable-frequency format, similar to an uncorrelated Vibroseis record, permits analysis and display of each frequency component in a unique and continuous format. Coherent noise contamination can then be examined and its effects appraised in both frequency and offset space. Separation of frequency components permits real-time maximization of the S/N ratio during acquisition and subsequent processing steps.  Linear separation of each ground roll frequency component allows calculation of phase velocities by simply measuring the linear slope of each frequency component.  Breaks in coherent surface-wave arrivals, observable on the decomposed record, can be compensated for during acquisition and processing. Multichannel recording permits single-measurement surveying of a broad depth range, high levels of redundancy with a single field con-figuration, and the ability to adjust the offset, effectively reducing random or nonlinear noise introduced during recording.  Amultichannel shot gather decomposed into a sweptfrequency record allows the fast generation of an accurate dispersion curve. The accuracy of dispersion curves determined using this method is proven through field comparisons of the inverted shear-wave velocity (vs) pro-file with a downhole vs profile.

                              Full Paper    PAR-99-04.PDF 1.38MB

PAR-99-03

Higher mode observation by the MASW method

Summary

A higher mode (the first overtone) of high frequency (5–30 Hz) surface waves was observed by using the multi-channel analysis of surface waves (MASW) method at three boreholes located in unconsolidated sediments in the Fraser River Delta, near Vancouver, British Columbia. Each site has a unique near-surface shear (S)-wave velocity (Vs) structure as verified from downhole Vs measurements. The relative dominance of higher mode energy is examined in association with source distance as well as Vs structure. Our examination indicates that energy of higher modes tends to become more significant as the source distance becomes greater. It also reveals that the dominance may be related to a Vs structure: a greater dominance as Vs changes little with depth, or Vs has an overall low value, or a combination. The dependency on the source distance is observed to be stronger than that on the Vs structure.  Attempts are made to explain the dependency by referring to one or a combination of three factors: attenuation, the nearfield effects, and the intrinsic nature of surface waves. Inclusion of higher mode during a surface wave measurement for near-surface (<30 m) application can be either an advantage or a disadvantage, depending on the specific type of application and the method used during the data acquisition and processing steps. It is, therefore, important to recognize through field observations those conditions both favorable and unfavorable to the generation of higher modes of high-frequency surface waves.                             

Full Paper    PAR-99-03.PDF 171KB

PAR-99-02

MULTIMODAL ANALYSIS OF HIGH FREQUENCY SURFACE WAVES

ABSTRACT

Surface waves on a multi-channel record are converted directly into images of multimode dispersion curves through a simple wavefield transformation method. Pre-existing multichannel processing methods require preparation of a shot gather with exceptionally large number of traces that cover wide range of source-to-receiver offsets for a reliable separation of different modes. The method described here constructs high-resolution images of dispersion curves with relatively small number of traces. This method is best suited for near-surface engineering project where surface coverage of a shot gather is often limited to near-source locations and higher-mode surface waves can be often generated with significant amount of energy. Performance of the method is illustrated through tests using both real and synthetic data.

                              Full Paper    PAR-99-02.PDF 145KB

PAR-99-01

DETECTION OF NEAR-SURFACE VOIDS USING SURFACE WAVE

ABSTRACT

Ground roll is displayed, on an uncorrelated field record obtained using a monotonic sweep, in increasing or decreasing order of frequency with each frequency well separated from all others. Phase velocity and attenuation characteristics of each frequency contain the average elastic property of near-surface materials down to approximately half the wavelength.  Uncorrelated field record, therefore, by itself can be associated with a two-dimensional display of the change in near-surface elastic property. Through the redundancy in data acquisition and a simple data processing step, the uncorrelated field records can be transformed into a stacked section that can be correlated directly to image of the change in elastic property of near-surface materials with respect to a certain reference location. This method can be effectively used to detect near-surface anomalies of various kinds.

                              Full Paper    PAR-99-01.PDF 135KB

PAR-98-03

Imaging dispersion curves of surface waves on multi-channel record

Summary

Real and synthetic data verifies the wavefield transformation method described here converts surface waves on a shot gather directly into images of multi-mode dispersion curves.  Pre-existing multi-channel processing methods require preparation of a shot gather with exceptionally large number of traces that cover wide range of source-to-receiver offsets for a reliable separation of different modes. This method constructs high-resolution images of dispersion curves with relatively small number of traces. The method is best suited for near-surface engineering project where surface coverage of a shot gather is often limited to near-source locations and higher-mode surface waves can be often generated with significant amount of energy.

                              Full Paper    PAR-98-03.PDF 138KB

PAR-98-02

Ground roll as a tool to image near-surface anomaly

Summary

On an uncorrelated field record obtained using a monotonic sweep, ground roll is displayed in increasing or decreasing order of frequency with each frequency well separated from all others. Phase velocity and attenuation characteristics of each frequency contain the average elastic property of nearsurface materials down to approximately half the wavelength.  An uncorrelated field record, therefore, by itself can be associated with a two-dimensional display of the change in near-surface elastic property. Through the redundancy in data acquisition and a simple data processing step, the uncorrelated field records can be transformed into a stacked section that can be correlated directly to the image of the change in elastic property of near-surface materials. This method can be effectively used to detect near-surface anomalies of various kinds.

                              Full Paper    PAR-98-02.PDF 119KB

PAR-96-01

Multi-channel analysis of surface waves using Vibroseis (MASWV)

Summary

On uncorrelated Vibroseis shot gathers, each frequency component of ground roll is represented with a unique slope as a function of arrival time and sweep function with excellent isolation from other components. The calculation of phase velocity becomes a simple matter of measuring the slope of each different frequency using an appropriate coherency measure. Possible contamination by coherent noise can readily be determined for each frequency by visual inspection. Any change in field configuration or extra efforts during data processing steps can be immediately designated. The multi-channel measurement method allows averaging and therefore effective reduction in any random noise introduced during recording. Therefore, the dispersion curve is constructed in a fast, accurate, and fully automated manner. Qualitative information about near-surface conditions can also be inferred from visual inspection, making it possible to detect near-surface anomalies.  Multi-channel analysis of surface waves using Vibroseis (MASWV) has advantages over the Spectral Analysis of Surface Waves (SASW) method which employs only two receivers with an impact source. First of all, because of the high spectral integrity of acquired data, a high degree of accuracy can be placed on the results of the method. Furthermore, the method is much faster and less labor-intensive than SASW since only a single or a few recorded shot gathers are usually necessary to produce a well behaved dispersion curve.

                              Full Paper    PAR-96-01.PDF 144KB

PAR-95-01

Characterization of Geotechnical Sites by Multi-Channel Analysis of Surface Waves (MCASW)

ABSTRACT

Evaluating stiffness of near-surface materials has been one of the critically important tasks in many civil engineering works. It is the main goal of geotechnical characterization. The so-called deflection-response method evaluates the stiffness by measuring stress-strain behavior of the materials caused by static or dynamic load. This method, however, evaluates the overall stiffness and the stiffness variation with depth cannot be obtained.  Furthermore, evaluation of a large-area geotechnical site by this method can be time-consuming, expensive, and damaging to many surface points of the site.  Wave-propagation method, on the other hand, measures seismic velocities at different depths and stiffness profile (stiffness change with depth) can be obtained from the measured velocity data. The stiffness profile is often expressed by shear-wave (S-wave) velocity change with depth because S-wave velocity is proportional to the shear modulus that is a direct indicator of stiffness. The crosshole and downhole method measures the seismic velocity by placing sources and receivers (geophones) at different depths in a borehole. Requirement of borehole installation makes this method also time-consuming, expensive, and damaging to the sites. Spectral-Analysis-of-Surface-Waves (SASW) method places both source and receivers at the surface, and records horizontally-propagating surface waves. Based upon the theory of surface-wave dispersion, the seismic velocities at different depths are calculated by analyzing the recorded surface-wave data. This method can be nondestructive to the sites. However, because only two receivers are used, the method requires multiple measurements with different field setups and, therefore, the method often becomes timeconsuming and labor-intensive. Furthermore, the inclusion of noise wavefields cannot be handled properly, and this may cause the results by this method inaccurate.  When multi-channel recording method is employed during the measurement of surface-waves, there are several benefits. First, usually single measurement is enough because multiple number (twelve or more) of receivers are used. Second, noise inclusion can be detected by coherency checking on the multi-channel data and handled properly so that it does not decrease the accuracy of the result.  Third, various kinds of multi-channel processing techniques can be applied to filter unwanted noise wavefields and also to analyze the surface-wavefields more accurately and efficiently. In this way, the accuracy of the result by the method can be significantly improved. Fourth, the entire system of source, receivers, and recording-processing device can be tied into one unit, and the unit can be pulled by a small vehicle, making the survey speed very fast. In all these senses, multi-channel recordingof surface waves is best suited for a routine method for geotechnical characterization in most of civil engineering works.

                        Full Paper    PAR-95-01.PDF 235KB

RYD-02-02

BRANCHING OF DISPERSION CURVES IN SURFACE WAVE TESTING OF PAVEMENTS

Abstract

When a set of simulated multichannel seismic records acquired over pavement is processed by a 2-D wavefield transformation technique normally used in the Multichannel Analysis of Surface Waves (MASW) method, several branches are observed in the dispersion-curve image constructed from the transformation. Some investigators theoretically anticipated this branching phenomenon a few decades ago in connection with discontinuities in a layer model that has decreasing stiffness with depth. Although this phenomenon has long been speculated about during measurements with a conventional two-receiver approach, sometimes by attributing the results to several other possible causes like higher modes, an objective observation confirming its link to the predicted theory was never made. The dispersion curve image shows several frequency-phase velocity branches that match fairly well with the discontinuities in the dispersion curve predicted by theory.  With a case study and numerical modeling we discuss a new approach that can yield thickness and stiffness of layers in a pavement system simply from the characteristics of this branching phenomenon. These determinations can be made without going through the normal procedure of dispersion curve analysis followed by inversion for the shear wave velocity (Vs) profile.

                              Full Paper    RYD-02-02.PDF 1MB

RYD-02-01

PORTABLE SEISMIC ACQUISTION SYSTEM (PSAS) FOR PAVEMENT MASW

Abstract

A seismic method (e.g., surface- or body-wave method) has been often used in pavement engineering to evaluate such critical constructional parameters as the E-modulus and Poisson’s ratio. Conventional method usually uses one or two-channel recording device (e.g., dynamic signal analyzer) for data acquisition whose cost is by no means trivial. In addition, recent applications with the multichannel approach have drastically improved the effectiveness of the seismic method in general and proven a greater potential of the method than ever. A true multichannel approach, however, would require a multichannel recording device (e.g., a 48-channel seismograph) and so-many accelerometers deployed simultaneously. The high-cost aspect of seismic method would make this otherwise-effective method excluded from consideration during the early stage of project planning. Instead, we propose a cheap, compact, and convenient seismic system that can be used with either conventional or multichannel approach. This Portable Seismic Acquisition System (PSAS) consists of a laptop computer, one or two accelerometers, and a hammer. A 16-bit PC-card (PCMCI bus) readily available nowadays is equipped into the computer as a data acquisition board. With this system, the multichannel measurement is simulated through repetitive generation of seismic waves along a linear survey line at different distances from the receiver fixed at a surface point. Data can be processed directly in the field on the same portable computer, only seconds after data acquisition. In combination with the robust dispersion curve analysis by the multichannel approach, this system creates new possibilities for seismic non-destructive testing (NDT) of pavements with on site evaluation. Data acquisition flow chart, signal conditioning, triggering, and other key features of the system are explained.  We also present a case of evaluating pavement concrete thickness, E-modulus, and Poisson’s ratio directly in the field using the proposed system.

                              Full Paper    RYD-02-01.PDF 132KB

RYD-01-01

HIGH FREQUENCY MASW FOR NON-DESTRUCTIVE TESTING OF PAVEMENTS—ACCELEROMETER APPROACH

Abstract

The dispersive nature of surface waves in pavement systems is imaged through a multichannel approach using one accelerometer as receiver and multiple shot points. The image obtained from a wavefield transformation method shows multimodal dispersion curves up to 10 kHz.  We present results from a simplified MASW data acquisition method applied to a pavement surface. The method can simulate an arbitrary number of channels. The sensor separation can be chosen arbitrarily small. In these experiments, the upper frequency limit is 10 kHz, which can be increased by the exchange of one sensor. The method is tested by one manual and one automated procedure. Both rely on source-receiver reciprocity. The automated procedure is regarded as necessary when a large number of channels is combined with a small sensor separation. The manual method will not provide the necessary accuracy and endurance for that kind of measurement, but is promising for less complicated setups. We present recommendations for high frequency measurements on pavements.  In the subsequent data processing, we follow the procedure of multichannel analysis of surface waves - MASW. It has recently been developed as a geophysical method for near-surface investigation. We demonstrate that the MASW technique can identify detailed aspects of the high frequency total wave-field of both surface and body-wave events.  Results of dispersion curve extraction indicate that higher modes of surface waves are dominating at depths associated with the transition between the asphalt and the base layer.  However, the deviation from the fundamental mode is not large because all modes are converging in an asymptotic manner with increasing frequency.  The study indicates that the MASW method is a fast and cost efficient method for measuring pavement stiffness parameters.

                              Full Paper    RYD-01-01.PDF 250KB

XIA-03-01

Inversion of high frequency surface waves with fundamental and higher modes

Abstract

The phase velocity of Rayleigh-waves of a layered earth model is a function of frequency and four groups of earth parameters: compressional (P)-wave velocity, shear (S)-wave velocity, density, and thickness of layers. For the fundamental mode of Rayleigh waves, analysis of the Jacobian matrix for high frequencies (2–40 Hz) provides a measure of dispersion curve sensitivity to earth model parameters. S-wave velocities are the dominant influence of the four earth model parameters.  This thesis is true for higher modes of high frequency Rayleigh waves as well. Our numerical modeling by analysis of the Jacobian matrix supports at least two quite exciting higher mode properties. First, for fundamental and higher mode Rayleigh wave data with the same wavelength, higher modes can ‘‘see’’ deeper than the fundamental mode. Second, higher mode data can increase the resolution of the inverted S-wave velocities. Real world examples show that the inversion process can be stabilized and resolution of the S-wave velocity model can be improved when simultaneously inverting the fundamental and higher mode data.

                              Full Paper    XIA-03-01.PDF 349KB

XIA-02-05

Determining Q of near-surface materials from Rayleigh waves

Abstract

High-frequency (z2 Hz) Rayleigh wave phase velocities can be inverted to shear (S)-wave velocities for a layered earth model up to 30 m below the ground surface in many settings. Given S-wave velocity (VS), compressional (P)-wave velocity (VP), and Rayleigh wave phase velocities, it is feasible to solve for P-wave quality factor QP and S-wave quality factor QS  in a layered earth model by inverting Rayleigh wave attenuation coefficients. Model results demonstrate the plausibility of inverting QS from Rayleigh wave attenuation coefficients. Contributions to the Rayleigh wave attenuation coefficients from QP cannot be  ignored when Vs/VP reaches 0.45, which is not uncommon in near-surface settings. It is possible to invert QP from Rayleigh wave attenuation coefficients in some geological setting, a concept that differs from the common perception that Rayleigh wave attenuation coefficients are always far less sensitive to QP than to QS. Sixty-channel surface wave data were acquired in an Arizona desert. For a 10-layer model with a thickness of over 20 m, the data were first inverted to obtain S-wave velocities by the multichannel analysis of surface waves (MASW) method and then quality factors were determined by inverting attenuation coefficients.

                              Full Paper    XIA-02-05.PDF 325KB

XIA-02-04

A pitfall in shallow shear-wave refraction surveying

Abstract

The shallow shear-wave refraction method works successfully in an area with a series of horizontal layers. However, complex near-surface geology may not fit into the assumption of a series of horizontal layers. That a plane SH-wave undergoes wave-type conversion along an interface in an area of nonhorizontal layers is theoretically inevitable. One real example shows that the shallow shear-wave refraction method provides velocities of a converted wave rather than an SH-wave. Moreover, it is impossible to identify the converted wave by refraction data itself. As most geophysical engineering firms have a limited resources, an additional P-wave refraction survey is necessary to verify if velocities calculated from a shear-wave refraction survey are velocities of converted waves. The alternative at this time may be the surface wave method, which can provide reliable S-wave velocities, even in an area of velocity inversion (a higher velocity layer underlain by a lower velocity layer).

                              Full Paper    XIA-02-04.PDF 625KB

XIA-02-03

Utilization of high-frequency Rayleigh waves in near-surface geophysics

Summary

The shear (S)-wave velocity of near-surface materials (such as soil, rocks, and pavement) and its effect on seismic wave propagation are of fundamental interest in many groundwater, engineering, and environmental studies. The Kansas Geological Survey conducted a three-phase research project to estimate near-surface S-wave velocity from ground roll: acquisition of ground roll, creation of algorithms that extract Rayleigh wave dispersion curves from ground roll, and development of algorithms to obtain near-surface S-wave velocity profiles. The research project has been expanded to analyze 1) the effects of higher modes on inversion of surface waves and investigation depth and 2) the feasibility of estimation of near-surface quality factor (Q) by inverting attenuation coefficients of Rayleigh waves. This paper summarizes these research results.

                              Full Paper    XIA-02-03.PDF 63KB

XIA-02-02

USING SURFACE WAVE METHOD TO DEFINE A SINKHOLE IMPACT AREA IN A NOISY ENVIRONMENT

Abstract

A sinkhole developed at Calvert Cliffs Nuclear Power Plant, Maryland in early 2001. To prevent damage to nearby structures, the sinkhole was quickly filled with dirt (approximately 40 tons). However, the plant had an immediate need to determine if more underground voids existed. The location of the sinkhole was over a groundwater drainage system pipe buried at an elevation of +3 feet (reference is to Chesapeake Bay level). Grade in the sinkhole area is +45 feet. The subsurface drain system is designed to lower the local water table from approximately +20 feet above Bay level to +10 feet. The subsurface drain system is connected to the top of the condenser cooling water discharge conduit at an elevation of -4 feet. The cause of the sinkhole was a subsurface drain pipe that collapsed due to saltwater corrosion of the corrugated metal pipe. The inflow/outflow of sea water and ground water flow caused dirt to be removed from the area where the pipe collapsed.  A high-frequency surface-wave survey was conducted to define the sinkhole impact area. Five surface-wave lines were acquired with limited resources: a 24-channel seismograph with a hammer and an aluminum plate as a source. Although the surface-wave survey at Calvert Cliffs Nuclear Power Plant was conducted at a noise level 50-100 times higher than the normal environment for a shallow seismic survey, the shear (S)-wave velocity field calculated from surface-wave data delineated a possible sinkhole impact area. The S-wave velocity field showed chimney-shaped low-velocity anomalies that were directly related to the sinkhole. Based on S-wave velocity field maps, a potential sinkhole impact area was tentatively defined. S-wave velocity field maps also revealed, depending on the acquisition geometry, one side of the water tunnel of the power plant.

                              Full Paper    XIA-02-02.PDF 867KB

XIA-02-01

Comparing shear-wave velocity profiles inverted from multichannel surface wave with borehole measurements 

Abstract

Recent field tests illustrate the accuracy and consistency of calculating near-surface shear (S)-wave velocities using multichannel analysis of surface waves (MASW). S-wave velocity profiles (S-wave velocity vs. depth) derived from MASW compared favorably to direct borehole measurements at sites in Kansas, British Columbia, and Wyoming. Effects of changing the total number of recording channels, sampling interval, source offset, and receiver spacing on the inverted S-wave velocity were studied at a test site in Lawrence, Kansas. On the average, the difference between MASW calculated Vs and borehole measured Vs in eight wells along the Fraser River in Vancouver, Canada was less than 15%. One of the eight wells was a blind test well with the calculated overall difference between MASW and borehole measurements less than 9%. No systematic differences were observed in derived Vs values from any of the eight test sites. Surface wave analysis performed on surface data from Wyoming provided S-wave velocities in near-surface materials. Velocity profiles from MASW were confirmed by measurements based on suspension log analysis.

                              Full Paper    XIA-02-01.PDF 1.29MB

XIA-01-02

INVERSION OF PHASE VELOCITIES OF SURFACE WAVES WITH HIGH MODES

Abstract

Elastic properties of near-surface materials and their effects on seismic wave propagation are of undamental interest in groundwater, engineering, and environmental studies. As an example, Imai and Tonouchi (1982) studied compressional (P) and shear (S) wave velocities in an embankment, and also in alluvial, diluvial, and Tertiary layers, showing that S-wave velocities (Vs) in such deposits correspond to the N-value (Vs = 97.0× N0.314), an index value of formation hardness used in soil mechanics and foundation engineering. S-wave velocity is also used to determine “stiffness,” one of the key earth properties in construction engineering. The S-wave velocity profile, a function of depth, can be derived from inverting the phase velocity of the surface (Rayleigh and/or Love) wave. The Rayleigh-wave phase velocity of a layered earth model is a function of frequency and four groups of earth parameters: P-wave velocity, S-wave velocity, density, and thickness of layers.  Estimations of S-wave velocity from ground roll, a particular type of Rayleigh wave, have for the most part focused almost exclusively on the fundamental mode of Rayleigh waves. The technique developed to determine the shear wave velocity from the fundamental mode using multichannel recording consists of: 1) acquisition of wide band ground roll using a multichannel recording system; 2) creation of efficient and accurate algorithms to extract Rayleigh-wave dispersion curves from ground roll using a basic, robust, and pseudo-automated processing sequence (Park et al., 1999); and 3) development of stable and efficient inversion algorithms to obtain S-wave velocity profiles (Xia et al., 1999). This technique has also been successfully applied to map a bed rock interface and voids based on S-wave velocity anomalies (Miller et al., 1999). Empirically it has been shown that higher mode energy tends to become more dominant as the source-offset distance becomes larger. In some cases, the shorter wavelength components of the fundamental mode Rayleigh waves are obscured by more dominant higher modes of Rayleigh waves in a higher frequency range, making analysis of higher mode data essential.  Analysis of the Jacobian matrix for high frequencies (5-40 Hz) provides a measure of higher-mode dispersion curve sensitivity to earth model parameters. S-wave velocity is the dominant influence of the four earth model parameters in higher-mode dispersion curves. Modeling results demonstrate at least two unique and useful properties of higher modes. First, we know that the penetrating depth of surface waves is limited by a wavelength of a surface-wave component. For fundamental and higher mode Rayleigh wave data with the same wavelength, higher modes penetrate deeper (longer than the wavelength) than the fundamental mode (normally shorter than the wavelength). For the layer model (Xia et al., 1999), in order to “see” a depth of 17 m, a wavelength of 12.3 m is required for fundamental mode data. For the second-mode data, however, a component with a wavelength of 10.9 m can penetrate a depth of 17 m. For the third-mode data, a component with a wavelength of only 6 m can “see” a depth of 17 m. We concluded that high-mode Rayleigh-wave data can “see” deeper in comparison to the same wavelength components of the fundamental mode Rayleigh-wave data.  Second, an inversion with higher mode data increases the resolution of the inverted S-wave velocities. Figure 1 shows the difference in phase velocities calculated from two S-wave velocity models. Although one model contains more than 100% relative error at 6 m and 7 m, the standard deviation between the fundamental mode phase velocities from these two models is only 4.6 m/s. Thus, the inversion process will not guarantee to choose the true model at a 4.6 m/s error level. However, because the standard deviations are 33.5 m/s for the second mode (solid squares with a solid line in Figure 1) and 27.3 m/s for the third mode (solid triangles with a dashed line), an inversion with highmode data will only be allowed to choose the true model so that a stabilized inversion is achieved and resolution is improved. The larger difference in higher modes suggests that higher modes are more sensitive to the changes in S-wave velocities than is the fundamental mode. Inverted S-wave velocity profiles of real world examples are comparable to direct borehole measurements showing improved resolution and accuracy of inverted S-wave velocity profiles using high-mode data.

                              Full Paper    XIA-01-02.PDF 80KB

XIA-01-01

Feasibility of determining Q of near-surface materials from Rayleigh waves

Summary

High-frequency (5–35 Hz) Rayleigh waves phase velocities can be inverted to shear (S)-wave velocities for a layered earth model up to 30 meters below ground surface in many settings (Xia et al., 1999 and Park et al., 1999). Given S-wave velocity, compressional (P)-wave velocity, and Rayleigh wave phase velocities, it is feasible to solve for P-wave quality factor QP and S-wave quality factor QS in the layered earth model by inverting Rayleigh wave attenuation coefficients. Model results demonstrate the plausibility of inverting QS from Rayleigh wave attenuation coefficients. Contributions to the Rayleigh wave attenuation coefficients from QP cannot be ignored when Vs/Vp reaches 0.45, which is not uncommon in near-surface settings. It is possible to invert QP from Rayleigh wave attenuation coefficients in some unconsolidated materials, which is a concept that differs from the common perception that Rayleigh wave attenuation coefficients are always far less sensitive to QP than to QS (Mitchell, 1975).

                              Full Paper    XIA-01-01.PDF 74KB

 

XIA-00-03

Comparing Shear-Wave Velocity Profiles from MASW with Borehole Measurements in Unconsolidated Sediments,  Fraser River Delta, B.C., Canada

ABSTRACT

Recent field tests illustrate the accuracy and consistency of estimating near-surface shear (S) wave velocities calculated using Multi-channel Analysis of Surface Wave (MASW). To evaluate the technique in a variety of near-surface conditions and through a wide range of velocities, MASW-derived S-wave velocity profiles (S-wave velocity vs. depth) were statistically compared to S-wave velocity profiles measured in seven boreholes in the unconsolidated sediments of the Fraser River delta, near Vancouver, B.C., Canada. An overall difference of approximately 15 percent was observed between these two uniquely determined sets of S-wave velocities from the seven well locations. A blind test of the standalone accuracy of MASW was conducted at an eighth well. For this blind test, S-wave velocity measurements made in and interpreted from the borehole were not made available during MASW data processing. Differences between S-wave velocities using MASW from those measured in the blind test borehole averaged nine percent. No systematic differences between these results were observed in data from any of the eight test sites. The MASW method provided reliable S-wave velocity profiles from 2 to 30 meters below the ground surface at some sites in the Fraser delta.

                              Full Paper    XIA-00-03.PDF 1.14MB

XIA-00-02

Advantages of calculating shear-wave velocity from surface waves with higher modes

Summary

The Rayleigh-wave phase velocity of a layered earth model is a function of frequency and four groups of earth parameters: compressional (P)-wave velocity, shear (S)-wave velocity, density, and thickness of layers. For the fundamental mode of Rayleigh waves, analysis of the Jacobian matrix for high frequencies (5-40 Hz) provides a measure of dispersion curve sensitivity to earth model parameters. S-wave velocities are the dominant influence of the four earth model parameters. With the lack of sensitivity of the Rayleigh wave to P-wave velocities and densities, estimations of these parameters can be made for a layered earth model such that dispersive data vary predominantly with S-wave velocities (Xia et al., 1999a). This thesis is valid for higher modes of Rayleigh waves as well. Experimental analysis indicates that energy of higher modes tends to become more dominant as the source distance becomes larger (Park et al., 1999a). In some cases, higher mode data are necessary since shorter wavelength components of fundamental mode Rayleigh waves are obscured by these higher frequency data where higher modes of Rayleigh waves dominate. As well, our modeling results demonstrate at least two quite exciting higher mode properties. First, for fundamental and higher mode Rayleigh wave data with the same wavelength, higher modes can “see” deeper (longer than the wavelength) than fundamental modes (normally shorter than the wavelength). Second, higher mode data can increase the resolution of the inverted S-wave velocities. A much better S-wave velocity picture can be produced from inversion of surface wave data if higher-mode data are included. Real world examples show how resolution can be improved.

                              Full Paper    XIA-00-02.PDF 267KB

XIA-00-01

CONSTRUCTION OF 2-D VERTICAL SHEAR-WAVE VELOCITY FIELD BY THE MULTICHANNEL ANALYSIS OF SURFACE WAVE TECHNIQUE

ABSTRACT

We present a method that utilizes the Multichannel Analysis of Surface Waves (MASW) technique and a standard common depth point (CDP) roll-along acquisition format similar to conventional petroleum exploration seismic data acquisition to construct a vertical section of the near-surface shear (S)-wave velocity field. A one-dimensional (1-D) S-wave velocity vs. depth plot is obtained by inverting phase velocities using the MASW technique. This 1-D profile appears to be most representative of the materials directly below the middle of a geophone spread. Multiple 1-D plots of S-wave velocity vs. depth are generated as the source and receivers roll along a survey line. A two-dimensional (2-D) vertical cross-section of S-wave velocity can be constructed by contouring grids produced by combining all the 1-D S-wave velocity profiles that are a function of the middle point of geophone spread (x) and depth (z). The combination of inverting the phase velocity for S-wave velocity and the standard CDP roll-along acquisition format makes this a very effective and time-efficient method of imaging two-dimensional Swave velocity along a survey line. There are several advantages that make this method attractive in real world applications. 1. The method focuses on high-frequency ( = 2 Hz) ground roll to provide a 2-D near-surface S-wave velocity map and to detect targets significantly shallower than feasible with other acoustic techniques. 2. That ground roll, which is acquired by the multichannel acquisition method, has a high signal-to-noise ratio, allowing 2-D images to be obtained in extremely noisy environments. 3. The method uses the standard CDP roll-along acquisition format, which provides an efficient way to acquire large quantities of broadband surface wave data along a line. 4. The method utilizes the redundancy of the standard CDP roll-along acquisition format so that it not only provides a reliable way to verify inverted S-wave velocities, it also reduces the ambiguity of inverted S-wave velocities. 5. A 2-D display of S-wave velocity can be produced easily and quickly by contouring the inverted S-wave velocity to provide a map of the S-wave velocity field. 6. 2-D data processing techniques, such as regression analysis, could easily be applied to a vertical S-wave velocity section to enhance local anomalies (gas or oil fields, voids, tunnels, etc.). More than five thousand shots of MASW data have been acquired and processed producing more than forty vertical near-surface S-wave velocity sections since 1997. Four real world examples demonstrate the usage of the method.

                              Full Paper    XIA-00-01.PDF 969KB

XIA-99-04

Estimation of near-surface shear-wave velocity by inversion of Rayleigh waves

ABSTRACT

The shear-wave (S-wave) velocity of near-surface materials (soil, rocks, pavement) and its effect on seismicwave propagation are of fundamental interest in many groundwater, engineering, and environmental studies.  Rayleigh-wave phase velocity of a layered-earth model is a function of frequency and four groups of earth properties: P-wave velocity, S-wave velocity, density, and thickness of layers. Analysis of the Jacobian matrix provides a measure of dispersion-curve sensitivity to earth properties. S-wave velocities are the dominant in.uence on a dispersion curve in a high-frequency range (>5 Hz) followed by layer thickness. An iterative solution technique to the weighted equation proved very effective in the high-frequency range when using the Levenberg–Marquardt and singular-value decomposition techniques. Convergence of the weighted solution is guaranteed through selection of the damping factor using the Levenberg–Marquardt method. Synthetic examples demonstrated calculation ef.ciency and stability of inverse procedures.We verify our method using borehole S-wave velocity measurements.                             

Full Paper    XIA-99-04.PDF 813KB

XIA-99-03

Evaluation of the MASW technique in unconsolidated sediments

Summary

Shear (S) wave velocities derived from the MASW (multi-channel analysis of surface wave) technique and borehole measurements at seven well locations in unconsolidated sediments of the Fraser River Delta are compared. The overall difference between these two sets of S-wave velocities is about 15%. S-wave velocities from the MASW technique at an additional location are also obtained and await comparison with borehole measurements.

                              Full Paper    XIA-99-03.PDF 91.5KB

XIA-99-02

A pitfall in shallow shear-wave refraction surveying

Summary

The shallow shear-wave refraction method works successfully in an area with a series of horizontal layers. However, complex near-surface geology may not fit into the assumption of a series of horizontal layers. That a plane SH wave undergoes wave-type conversion along an interface in an area of non-horizontal layers is theoretically inevitable. One real example shows that the shallow shear-wave refraction method provides velocities of a converted wave rather than an SH wave. Moreover, it is impossible to identify the converted wave by refraction data itself. To verify if velocities calculated from a shear-wave refraction survey are velocities of converted waves, an additional P-wave refraction survey is necessary. The best alternative at this time is MASW, which can provide reliable S-wave velocities, even in an area of velocity inversion (a higher velocity layer underlain by a lower velocity layer).

                              Full Paper    XIA-99-02.PDF 199KB

XIA-99-01

CONFIGURATION OF NEAR-SURFACE SHEAR-WAVE VELOCITY BY INVERTING SURFACE WAVE

ABSTRACT

The shear (S)-wave velocity of near-surface materials (such as soil, rocks, and pavement) and its effect on seismic wave propagation are of fundamental interest in many groundwater, engineering, and environmental studies. Ground roll is a Rayleigh-type surface wave that travels along or near the surface of the ground. Rayleigh wave phase velocity of a layered earth model is a function of frequency and four groups of earth parameters: S-wave velocity, P-wave velocity, density, and thickness of layers. Analysis of Jacobian matrix in a high frequency range (5- 30 Hz) provides a measure of sensitivity of dispersion curves to earth model parameters. S-wave velocities are the dominant influence of the four earth model parameters. With the lack of sensitivity of the Rayleigh wave to P-wave velocities and densities, estimations of near-surface S-wave velocities can be made from high frequency Rayleigh wave for a layered earth model. An iterative technique applied to a weighted equation proved very effective when using the Levenberg-Marquardt method and singular value decomposition techniques. The convergence of the weighted damping solution is guaranteed through selection of the damping factor of the Levenberg-Marquardt method. Three real world examples are presented in this paper. The first and second examples demonstrate the sensitivity of inverted S-wave velocities to their initial values, the stability of the inversion procedure, and/or accuracy of the inverted results. The third example illustrates the combination of a standard CDP (common depth point) roll-along acquisition format with inverting surface wave one shot gather by one shot gather to generate a cross section of S-wave velocity. The inverted S-wave velocities are confirmed by borehole data.

                              Full Paper    XIA-99-01.PDF 127KB

XIA-98-01

Construction of Vertical Section of Near-Surface Shear-Wave Velocity from Ground Roll

Summary

The shear (S)-wave velocity of near-surface materials (such as soil, rocks, and pavement) and its effect on seismic wave propagation are of fundamental interest in many groundwater, engineering, and environmental studies. Ground roll is a Rayleightype surface wave that travels along or near the surface of the ground. Rayleigh wave phase velocity of a layered earth model is a function of frequency and four groups of earth parameters: P-wave velocity, S-wave velocity, density, and thickness of layers.  Analysis of Jacobian matrix in a high frequency range provides a measure of sensitivity of dispersion curves to earth model parameters. S-wave velocities are the dominant influence of the four earth model parameters. With the lack of sensitivity of the Rayleigh wave to P-wave velocities and densities, estimations of these parameters can be made for a layered earth model resulting in dispersion data that vary predominantly with S-wave velocities. An iterative technique to a weighted equation proved very effective when using the Levenberg-Marquardt method and singular value decomposition techniques. The convergence of the weighted damping solution is guaranteed through selection of the damping factor of the Levenberg-Marquardt method. One vertical section of near-surface shear-wave velocity is presented and the depth to bedrock is verified by well data.

                              Full Paper    XIA-98-01.PDF 64.7KB

XIA-97-01

Estimation of shear wave velocity in a compressible Gibson half-space by inverting Rayleigh wave phase velocity

Summary

Shear wave velocities in a compressible Gibson half-space (a non-layered earth model) are estimated by inverting Rayleigh wave phase velocity. An analytical dispersion law of Rayleigh-type waves in a compressible Gibson half-space is in an algebraic form (Vardoulakis and Verttos, 1988), which makes our inversion processing extremely simple and fast. The convergence of the weighted damping solution is guaranteed through selection of the damping factor using the Levenberg-Marquardt method (L-M) (Marquardt, 1963). Calculation efficiency is achieved by reconstructing a weighted damping solution using the singular value decomposition (SVD) techniques (Golub and Reinsch, 1970). One real example is presented and verified by borehole S-wave velocity measurements. Results of this example are also compared with results of the layered-earth model (Xia et al., in review).

                              Full Paper    XIA-97-01.PDF 608KB

KGS-01-19

Feasibility of using the MASW method to define sinkhole impact area at Calvert Cliffs Nuclear Power Plant, Maryland

Abstract

A sinkhole developed at Calvert Cliffs Nuclear Power Plant, Maryland, early this year.  Approximately 700 tons of soil were used to fill in the sinkhole to prevent further damage. A high-frequency surface-wave survey was conducted in hopes of defining the sinkhole impact area. Acquisition of surface-wave data was in an extremely noisy environment. The S-wave velocity field calculated from surface-wave data showed chimney-shaped low-velocity anomalies that were directly related to the sinkhole. Based on S-wave velocity field maps, a potential sinkhole impact area was tentatively defined. S-wave velocity field maps also revealed, depending on the acquisition geometry, one side of the water tunnel of the power plant.

                              Full Paper KGS-01-19.PDF 1.07MB          

KGS-00-75

Detecting fracture-related voids and abandoned lead/zinc mines and appraising the subsidence potential near Baxter Springs, Kansas

Summary

Historical lead/zinc mining activities have left surface scars and underground hazards across a portion of southeastern Kansas, southwestern Missouri, and northeastern Oklahoma, known as the Tri-State Lead/Zinc Mining District. Fractures and voids in otherwise competent near-surface rock layers can pose a stability risk to overlying surface structure. Confident detection and delineation of voids or open fractures prior to surface expression permits the evaluation and possible reduction in risk such features pose to people and property. Surface growth associated with a sinkhole located within 100 ft of State Line Road and a gas metering station south of Baxter Springs, Kansas, raised concerns for public safety and prompted an extensive drilling and geophysical investigation. The mine workings of interest ranged in depth from 100 to over 150 ft below ground surface and are in an area where Mississippian Limestone acts as both host and roof rock. Overlying the limestone is a soil and clay layer varying in thickness from 10 to 20 ft. Subsidence in this area has a historical precedent dating back prior to the mid-twentieth century.Abandoned lead/zinc mines (drifts) known from mine maps to pass beneath State Line Road and generally coincident with the sinkhole of concern were the primary targets of this study and were suggested to embody a risk to surface structures in this area. Extensive drilling in the immediate area provided excellent 1-D ground truth but lacked the lateral resolution necessary to map the subsurface extent of fracture zones and voids. Surface wave imaging detected lateral variations in rock rigidity at depths and locations generally consistent with mine maps and drillencountered sediment-filled and open fractures. Stable voids, regardless of origin, represent no risk to public safety; however, if stable voids begin to grow and surpass a size or roof span that can be supported by its roof rock, subsidence will occur. At the present time, science provides no method to confidently predict subsidence rates, extent, and volume. However, by using geophysical methods such as surface wave imaging as a monitoring tool, iterative “snap shots” of the subsurface allow discrimination of change and estimates of current subsidence rates.  Surface wave analysis using MASW allowed relatively quick and accurate (in relation to other geophysical methods) acoustic images to be produced of the upper 50 ft to 150 ft.  Considering the sensitivity of surface wave propagation to shear wave properties, changes in the phase velocity of surface waves is directly proportional to changes in rock stiffness or rigidity.  Abrupt changes in shear wave velocity will occur at the boundary between voids (water or rubble filled) and consolidated rock. As well, fractured or highly altered zones where competent rock is broken up and replaced by unconsolidated sediments and/or fluids will produce distinctive changes in the shear wave velocity field. Very localized anomalous features detected at depths of over 100 ft in two locations at this site were generally consistent with the geometry of drifts suspected to pass under the lines. Images produced as a result of this study clearly indicate that rock layers beneath the profile lines possess localized lateral discontinuities in rock properties.  With no obvious connectivity existing between these structurally weak zones, it is not feasible to suggest the orientation and potential commonality of these features.  Interpretation of surface wave and drill data was complementary in depth and extent of void areas, but unique in terms of resolution. The single snap shot in time provided by the surface wave technique provides a relatively clear picture of subsurface rock strengths, but without a prolonged study evaluating change over time it is not possible with either data set (drilling or subsurface) to confidently discern the long-term risk of collapse at a specific spot. It is reasonable to suggest, based on both data sets, that a sizeable thickness (> 10 ft) of competent limestone separates the bedrock surface and the shallowest of the void/fracture areas encountered by either study. Two areas have been identified on surface wave data where some strain exists above void/fracture zones. It is not clear in this setting how much of a strain gradient is indicative of failure versus being consistent with a long-term sustainable load. An analogy would be the strain present between bridge pillars. A bridge is designed to span a specific distance and support a given load. This load will manifest itself as increased strain between the bridge supports.  Based on the data presently available, there is no way to confidently predict long-term subsidence rates or location along these profiles. With only a single snap shot in time, it is not possible to detect vertical migration. Considering the small size of the anomalous features and the thickness of overlying competent rock, returning to the site in six to nine months and acquiring identical surface wave data will provide a measure of change in rock properties (if any exists). This study was successful in developing an accurate shallow subsurface image consistent with the ground truth provided by drill data. The feasibility of this technique to delineate lateral changes in shear wave velocity and its relationship to drill data was evaluated.

                              Full Paper    KGS-00-75.PDF 3.29MB          

KGS-00-50

Potential of MASW to delineate fractures in the Winterset Limestone at the Johnson County Landfill, Kansas

Summary

The shear wave velocity as a function of depth, calculated using the Multi-channel Analysis of Surface Waves (MASW) method (Park et al., 1999; Xia et al., 1999), provided a reliable measure of shear wave velocity well within the empirically determined level of accuracy the method possesses (Xia et al., 2000). Tests designed specifically to evaluate the potential of MASW to discriminate moderate changes in fracture permeability at the Johnson County Landfill (JCL) suggest moderate differences in yield such as observed between MW-I and MW-K cannot be confidently detected using this method (Figure 1). Depths of investigation on the five profiles acquired during phase 1 testing on this study ranged from a few feet below ground surface (BGS) to as much as 100 ft BGS in some cases. Discrepancies between shear-wave velocity profiles measured at these two sites are attributable to localized changes in near-surface conditions, differences in near-surface material, and variability of surface noise sources, but not as a result of changes in rock properties in this very uniform, cyclic limestone/shale geologic setting.  Considering the characteristics of these two test sites, the MASW method provided representative measurements, consistent with ground truth and well within the expected accuracy and repeatability range.  Drilling has provided evidence of differences in permeability but no measurable differences in rock properties. With the subtle difference expected at this site, based on the drill data, it is not surprising MASW did not provide the necessary resolution to be used as a reconnaissance tool for locating high permeability fracture lineaments at this site. When comparing this study to previous investigations using this surface wave imaging technology at sites across the country (Miller and Xia, 1999a; Miller and Xia, 1999b, Miller et al., 1999), the inability to detect or discern a unique velocity pattern consistent with the proposed fracture patterns is not surprising.  It also brings up the possibility that they do not exist. There are a variety of reasons that could explain the small difference in yield at these two monitor wells, fractures is only one.  Calculating the shear wave velocity field from surface wave arrivals can generally be accomplished with a high degree of accuracy regardless of cultural noise or obstacles. Data for this study were acquired in and around areas with significant amounts of high amplitude, low frequency noise. Care was taken to insure no data carried artifacts related to surface features and all data were acquired with special attention placed on the spread location relative to surface materials and structures. Comparisons of data characteristics recorded from geophones with steel baseplates to those with spikes revealed no significant difference in wavetrain properties or calculated dispersion curves. Of particular concern during data acquisition was geophone placement in the parking lot, where coupling of the steel plates would vary significantly from station to station. Unlike recording concerns prominent when using other types of acoustic waves, surface waves seem to have only limited dependence on changes in receiver coupling. Non-source noise recorded on surface wave data reduces the quality of the dispersion curve but does not usually prevent an accurate and robust inversion unless the noise is excessive (i.e., equivalent in amplitude and with the same seismic source-to-receiver orientation).  MASW provides shear wave velocity profiles accurately (15%) representing average shear wave velocities for a particular subsurface volume (Xia et al., 2000). Velocities measured during this study ranged from just over 900 ft/sec to around 4000 ft/sec. A localized change of over 1100 ft/sec (100%) in the very near-surface material was observed at site 1 between the parking lot site and the shale site behind the Johnny on the Spot warehouse. This change has been correlated to the fill material present beneath the parking lot not behind the warehouse.  General differences of around 15 to 20% were observed between site 1 and site 2 at the depth of the Winterset Limestone (around 100 or so ft). These differences are not sufficiently above the accuracy of the method to be interpreted to represent changes in geology. There does not seem to be a characteristic or property of the shear wave velocity field uniquely distinguishable or diagnostic of fractures in at the depth of the Winterset Limestone. If these fractures really exist, the apparent lack of sensitivity could be related to their size or lateral extent relative to the wavelength of the groundroll (horizontal resolution) or it could be related to size of the fractures and density (vertical resolution). With the limited evidence supporting fracturing as the source of the increased permeability and the extremely small differences in yield, it is unlikely, based on the seismic data in conjunction with supporting borehole data that has been incorporated into the seismic study, that significant differences in fracture permeability exist between MW-I and MW-K.  Interpreting changes in lithology with this technique has routinely involved correlating high velocity gradients and measured velocities to ground truth. Velocity fields at these two sites have not provided distinct enough changes in velocity at a vertical resolution sufficient to allow correlation to the borehole-defined geology. Only at a depth BGS of around 25 to 30 ft does a strong change in velocity occur at both sites. This is likely related to lithology, but due to the cyclic nature of the rocks at this site it is beyond the vertical resolution of these data to determine exactly which unit this change correlates to. There are strong indications that the borehole lithology can be grossly correlated to the shear wave velocity profile. The gradational nature of the shear wave velocity field makes interpreting and correlating unique layers to borehole geology a bit speculative at these two sites.

                              Full Paper    KGS-00-50.PDF 856KB          

KGS-00-25

Multichannel analysis of surface wave theory and applications

(A Power-Point File)

Presented by Jianghai Xia at China University of Geosciences, Wuhan, PRC, Chengdu University of Technology, Chengdu, PRC, China University of Geosciences, Beijing, PRC, North China Institute of Water Conservancy and Hydroelectric Power, Zhengzhou, PRC, duing June 5, 2000 – June 15, 2000;

Prepared by Jianghai Xia, Richard D. Miller, and Choon B. Park

                              Full Paper    KGS-00-25.PDF 8.05MB   

 

 

 

KGS-00-04

MASW to Investigate Anomalous Near-Surface Materials at the Indian Refinery in Lawrenceville, Illinois

Summary

The shear wave velocity field, calculated using the Multi-channel Analysis of Surface Waves (MASW) method (Park et al., 1999; Xia et al., 1999), and disturbances observed in the groundroll wavetrain transformations (phase and amplitude) were used to help identify variability in the lateral continuity of near-surface layers beneath asphalt roads within the Lawrenceville Refinery, settling ponds in and around the refinery property, surface remediated burial pits, and containment berms (Figure 1). Depths of investigation extended from a few feet below ground surface (BGS) to as much as 100 ft BGS on line 5. Anomalies interpreted within the soft sediments along all five lines are related to lateral changes in material properties. Confirmation and/or enhancement of interpretations of these data will require subsurface sampling. When comparing results from this study to previous investigations using this surface wave imaging technology at sites around the country (Miller and Xia, 1999a; Miller and Xia, 1999b; Miller et

al., 1999), it is interesting to note that in most cases interpreted zones of anomalous material possess a unique velocity pattern somewhat universally consistent with all features of the same kind (subsidence, fractures, bedrock, etc.). In most cases, low velocity closures within the shear wave velocity field are indicative of “anomalies.” Each profile collected in association with this feasibility test at the Lawrenceville Refinery was designed with a specific imaging objective in mind. The success of each profile can only be determined by a drilling program specifically designed to evaluate these findings.  Calculating the shear wave velocity field from surface wave arrivals can generally be accomplished with a high degree of accuracy regardless of cultural noise or obstacles. Data for this study were acquired in and around areas with occupied personal residences, railroad noise, site demolition activities, and normal industrial background noise (60 Hz, running motors, vehicle activity, etc.). A wide range of surface conditions required adaptations be made to optimize source and receiver coupling to cement and asphalt. Care was taken to insure no data carried artifacts related to surface features or background noise and that all data were acquired with special attention placed on the spread location relative to surface materials and structures.  Comparisons of data characteristics recorded from geophones with steel baseplates to those with spikes revealed no significant difference in wavetrain properties or calculated dispersion curves.  Unlike recording concerns prominent when using other types of acoustic waves, surface waves seem to have only limited dependence on changes in receiver coupling. Non-source noise recorded on surface wave data reduces the quality of the dispersion curve but does not usually prevent an accurate and robust inversion.  MASW provides shear wave velocity profiles that accurately (15%) represent average shear wave velocities for a particular subsurface volume (Xia et al., 2000). Velocities measured during this study ranged from just over 200 ft/sec to around 2500 ft/sec. Localized changes of over 700 ft/sec (300%) across distances less than 10 ft were common around areas with observed structural damage. Velocity inversions laterally consistent over significant distances are evident within the upper 10 ft along most lines and likely relate to stiffer clays or partially cemented sediments in close proximity to sand or gravel zones. The sensitivity of shear wave velocities to changes in sediment makeup within this alluvial setting allowed even subtle changes in nearsurface material properties to be identified on 2-D cross-sections. Uniquely locating localized zones of anomalous subsurface sediments (fill, sludge, or rubble) and/or objects (such as pipes, trenches, or old landfill materials) was possible with data from lines 1, 2, 3, and 4. Localized zones of lower velocity material can easily be picked out on lines 1 and 4. Line 3 provides a glimpse at an erosional surface beneath the base of the lower velocity fill materials. It takes very little imagination to interpret the well-defined low velocity zones extending down to depths of almost 10 ft at burial pits interpreted along line 4. Line 5 presents a reasonable depiction of the dike and relatively coherent sediments from the base of the dike down to about 100 ft BGS.  Bedrock or a significant increase in velocity is evident at depth on lines 1, 2, 3, and 5. Each type of target imaged during this survey possesses a unique signature with each of the different imaging methods used. Interpreting these data requires incorporation of drilling, borehole measurements, and other geophysical soundings.  Interpreting changes in lithology with this technique has routinely involved correlating high velocity gradients and measured velocities to ground truth. Velocity fields along these five profiles possess relatively uniform increases in shear wave velocity from the surface to the maximum depth of the survey. Anomalous (relative to surrounding materials) high and low velocity closures, likely indicative of extreme lateral variability in material properties or foreign materials, are evident within the unconsolidated sediments along most of the lines. Several localized changes in shear wave velocity are strong candidates for drill investigation. Coherent layers (bedding, changes in lithology, structural features, etc.) were interpreted based on velocity gradients, consistent changes in velocity contours, and overall velocity trends. Several possible explanations exist for each of these velocity phenomena. With the inherent nonuniqueness of these data, precisely located borings would be necessary along each of these lines to increase the confidence and/or modify these interpretations.  Large velocity gradients in the shear wave velocity field are likely indicative of changes in lithology (i.e., alluvial/glacial contacts, alluvial/bedrock, glacial/bedrock), while localized lateral changes (contour closures) in the shear wave velocity within the unconsolidated section were considered evidence of infilling or altered native earth. Mapping the surface of lithologic contacts using shear wave velocity data combined with drill data will result in a significantly higher resolution subsurface map than grid style drilling alone at this site. Advantages of mapping variations in the shallow stratigraphy with the shear wave velocity field calculated from surface waves using MASW include sensitivity to velocity inversions, ease of generating and propagating surface wave energy in comparison to body wave energy, being oblivious to cultural noise (mechanical or electrical), and sensitivity to lateral changes in velocity.

                              Full Paper    KGS-00-04.PDF 4.52MB   

KGS-99-33

MASW to Investigate Subsidence in the Tampa, Florida Area

Summary

The shear wave velocity field, calculated using the Multi-channel Analysis of Surface Waves (MASW) method (Park et al., 1999; Xia et al., 1999), helped identify variability in the lateral continuity of near-surface layers in and around surface structures with damage that appears to be related to non-uniform earth settling in the Tampa, Florida area (Figure 1). Depths of investigation extended from a few feet below ground surface (BGS) to as much as 100 ft BGS in some cases. Anomalies interpreted in “bedrock” near all six structures studied could be related to subsidence effects observed at the ground surface. Previous drilling and resistivity investigations at these six sites has not conclusively identified a geologic/structural situation that could be definitively correlated to sinkhole activity. When comparing this study to previous investigations using this surface wave imaging technology at sites around the country (Miller and Xia, 1999a; Miller and Xia, 1999b), it is interesting to note the inability to detect or discern a unique velocity pattern consistent with all subsidence features. In most cases, an anomalously low velocity within bedrock was detected and proposed to be indicative of dissolution and the subsidence implied by change in the shallow low velocity zone. Extrapolations between the “bedrock” features and the affected area on or near the ground surface required speculation as to the connectivity of the features interpreted to be subsidence related within the “unconsolidated” sediments. In each of the six areas studied, subsurface variations in shear wave velocity patterns above the interpreted/inferred bedrock surface were interpreted that could be associated with the subsidence responsible for structural damage.  Calculating the shear wave velocity field from surface wave arrivals can generally be accomplished with a high degree of accuracy regardless of cultural noise or obstacles. Data for this study were acquired in and around occupied personal residences requiring adaptations be made for cement and asphalt as well as for limitations and restrictions in generating acoustic energy inside houses, around swimming pools, and through gardens. Care was taken to insure no data carried artifacts related to surface features and all data were acquired with special attention placed on the spread location relative to surface materials and structures. Comparisons of data characteristics recorded from geophones with steel baseplates to those with spikes revealed no significant difference in wavetrain properties or calculated dispersion curves. Of particular concern during data acquisition was geophone placement inside houses where floor coverings ranged from carpet to ceramic tile to cement. Unlike recording concerns prominent when using other types of acoustic waves, surface waves seem to have only limited dependence on changes in receiver coupling. Non-source noise recorded on surface wave data reduces the quality of the dispersion curve but does not usually prevent an accurate and robust inversion.  MASW provides shear wave velocity profiles accurately (15%) representing average shear wave velocities for a particular subsurface volume (Xia et al., in review). Velocities measured during this study ranged from just over 100 ft/sec to around 2100 ft/sec. Localized changes of over 1300 ft/sec (400%) across distances less than 10 ft were common around areas with observed structural damage. Velocity inversions are prominent within the upper 20 ft and likely relate to stiffer clays or partially cemented sediments in close proximity to sand or gravel zones. The sensitivity of shear wave velocities to changes in sediment makeup correlates to differing degrees with other imaging techniques and/or observed differences in material composition.  There does not seem to be a unique characteristic or property of the shear wave velocity field diagnostic of subsidence activity at all the Tampa sites studied. Interpreting these data requires incorporation of drilling, borehole measurements, and other geophysical soundings.  Data acquired for this study, in general, have relative gradational vertical and horizontal variations in shear wave velocity when compared to data from other sites with noted subsidence activities. This apparent lack of sensitivity could be unique to the structural properties of the sediments in this area (subsidence results in only subtle changes in stiffness/rigidity), resolution limitations of the method, or the subsidence mechanics in the Tampa, Florida area. Sediments described in drill cuttings and correlated to blow counts seem to be generally consistent with velocity profiles and associated gradients.  Interpreting changes in lithology with this technique has routinely involved correlating high velocity gradients and measured velocities to ground truth. Velocity fields at most of the six Tampa sites possess distinguishable velocity inversions at or around 10 ft to 20 ft BGS with large velocity gradients immediately above and below. High velocity closures, likely indicative of extreme lateral variability in sediment cementation, appear sporadically within the unconsolidated portion of the section. These localized changes in cementation will strongly influence the linearity and preferred subsidence migration path from the bedrock to ground surface. Interpreting the bedrock surface based solely on the high velocity gradient could not be done with confidence using only the shear wave data. The gradational nature of the shear wave velocity field makes interpreting the surface of bedrock a bit speculative across most of the lines. However, incorporating drill information with interpretations of laterally continuous high velocity gradients makes it possible to trace the top of bedrock beneath most sites. The roughness of the bedrock surface strongly influences the gradational nature of the velocity field in proximity to bedrock. This roughness is suggestive of dissolution or erosional activity.  Large velocity gradients in the shear wave velocity field near the expected depth of bedrock (stiffer layers seeming more laterally continuous) were used as diagnostic of the bedrock surface, while localized lateral decreases in the shear wave velocity below the bedrock surface were considered characteristic of fracture zones or dissolution features. The depth-to-bedrock interpreted using shear wave velocity data and drilling logs possesses significantly higher resolution than drilling alone. Advantages of mapping variations in the shallow stratigraphy with the shear wave velocity field calculated from surface waves using MASW include a disregard for velocity inversions, ease of generating and propagating surface wave energy in comparison to body wave energy, being oblivious to cultural noise (mechanical or electrical), and sensitivity to lateral changes in velocity.

                              Full Paper    KGS-99-33.PDF 2.45MB   

KGS-99-09

Using MASW to Map Bedrock in Olathe, Kansas

Summary

The shear wave velocity field, calculated using the Multi-channel Analysis of Surface Waves (MASW) method (Park et al., 1999; Xia et al., in press) was used to map the bedrock surface at depths of 6 to 23 ft and identify potential fracture zones within bedrock at a site in Olathe, Kansas. Preliminary analysis of this site’s hydrologic characteristics, based primarily on borehole data, suggested fractures and/or an unmapped buried stream channel could be influencing fluid movement along the drill-defined bedrock surface. Since topographic variations on the surface of bedrock can influence the transport and eventual fate of contaminants introduced at or near the ground surface, determining the nature and location of anomalous bedrock was critical to hydrologic characterization of this site.  High velocity gradients within the shear wave velocity field were used as diagnostic of the bedrock surface, while localized lateral decreases in the shear wave velocity below the bedrock surface were considered characteristic of fracture zones or erosional channels. Calculating the shear wave velocity field from surface wave arrivals can generally be accomplished with a high degree of accuracy regardless of cultural noise. The insensitivity of MASW to cultural obstacles and noise was demonstrated at this site (e.g., approx 220,000 yd2 asphalt parking lot, electrical and mechanical noise from nearby industrial facilities, traffic noise from the adjacent highway, exploratory drilling on the asphalt parking lot, and aircraft noise). The depth-tobedrock map produced using only shear wave velocity data possesses significantly higher resolution than maps produced using drilling alone. There is less than 1 ft of difference in the depthto-bedrock interpreted from surface wave data versus the depths determined through drilling.  Geophones used for this study were equipped with steel baseplates. Advantages of mapping the bedrock surface with the shear wave velocity field calculated from surface waves include the insensitivity of MASW to velocity inversions, ease of generating and propagating surface wave energy in comparison to body wave energy, and its sensitivity to lateral changes in velocity.  Localized anomalies were observed on lines 2 and 4 that are likely representative of anomalies in the bedrock. Anomalously low velocities observed on the western portion of the east-west lines is very abrupt and localized, suggesting a zone of either fractures, a relatively shear-sided channel, or a fault. Based on the seismic data alone it is not possible to determine which of these scenarios is correct. Speculating from the character and apparent vertical extent of this feature, it is more likely a fault or fracture than a channel. The physical dimensions of this feature would require a channel to be over 20 ft deep and 30 ft wide with vertical side walls.  A channel with these dimensions is possible, of course, but would not be expected in this setting.  The general topographic trend of the bedrock surface suggests a north dip with an apparent change in the material composition of the shallow bedrock at a localized low in the bedrock surface. Bedrock lows are present at the north end of lines 1 and 3. Based on the drop in shear wave velocities, these lows are also areas with either increased weathered bedrock near the contact between unconsolidated material and bedrock or the bedrock material changes slightly (i.e., limestone to shale or possibly the shale limestone thickens). Lines 2 and 4 possess a low velocity bedrock material near their eastern ends. It appears to transition from high velocity to low velocity very subtly on line 2 while on line 4 the feature is very abrupt and similar in character to the fault/fracture/channel interpreted on the western end of both these lines. Bedrock toward the eastern ends of lines 2 and 4 can be interpreted to be either flat or maybe slightly shallower than near the building on the west.  The depth-to-bedrock contour map produced using both drilling and seismic data is significantly more detailed and represents a closer approximation to the real bedrock surface than either drilling or seismic data could have produced alone. The match between drilling and seismic is excellent. Improved resolution on the surface of the bedrock provides insight into the texture of bedrock and permits identification and appraisal of short wavelength variations in the bedrock surface. The goals and objectives of this survey were met.

                              Full Paper    KGS-99-09.PDF 1.86MB  

KGS-98-58

Comparison of Shear Wave Velocities from MASW Technique and Borehole Measurements in Unconsolidated Sediments of the Fraser River Delta

ABSTRACT

Shear (S) wave velocities derived from the MASW technique and borehole measurements at seven well locations in unconsolidated sediments of the Fraser River Delta are compared. The overall difference between these two sets of S-wave velocities is about 15%. S-wave velocities from the MASW technique at an additional location are also obtained and await comparison with borehole measurements.

                              Full Paper    KGS-98-58.PDF 277KB   

KGS-97-80

Summary Report on Surface-Wave Project at Kansas Geological Survey (KGS)

(A Document of graphical contents only)

                        Full Paper    KGS-97-80.PDF 542KB    

KGS-97-10

Multi-channel analysis of surface waves (MASW)-“A summary report of technical aspects, experimental results, and perspective”

Introduction

When seismic waves are generated at or near the surface of earth, both body (P and S)- and surface (e.g., Rayleigh, Love, etc.)-waves are generated. Body waves propagate through the whole body of the earth, whereas surface waves along (ore near) the surface of the earth. If vertical seismic sources are used, the type of generated surface-waves is Rayleigh waves, morecommonly called ground roll in seismic surveys. In exploration and engineering seismology, ground roll has been treated as the most troublesome noise masking the useful body wavefields. Most of efforts have been made to attenuate it through data acquisition and processing techniques (Lombardi, 1955; Anstey, 1986; Knapp, 1986).  Surface-waves, however, have dispersion property that body-waves lack. This property is that different wavelength has different penetration depth and, therefore, propagates with different velocity. Therefore, by analyzing the dispersion of surface waves, one can obtain near-surface velocity profile. It is the shear (S)-wave velocity (vs ) profile that is obtained by analyzing ground roll. The entire procedure of the analysis consists of three steps: acquisition of dispersive ground roll data, construction of the dispersion curve, and backcalculation (inversion) of S-wave velocities from the constructed dispersion curve. The accurate construction of the dispersion curve is the most critical because the final product of vs profile has the accuracy that depends solely upon the accuracy of the dispersion curve.  In all kinds of surface seismic surveys using vertical sources, ground roll takes more than two thirds of total generated seismic energy and usually appears with the most prominence on the multi-channel records. Therefore, generation and recording of ground roll is easiest among all other types of seismic waves.  Shear (S)-wave velocity (vs ) profile obtained from the ground-roll analysis can find many useful applications. First of all, because the shear-wave velocity is directly proportional to shear modulus that is in turn a direct indicator of stiffness (or rigidity) of material, the vs profile can be used as a near-surface stiffness profile, evaluation of which, in geotechnical engineering, has been one of the critically important tasks. The so-called deflection-response method (Lytton et al., 1975) in that area evaluates the stiffness by measuring stress-strain behavior of the materials caused by static or dynamic load. This method, however, measures the overall stiffness of a site and each individual layers cannot be measured. Furthermore, evaluation of a large-area geotechnical site by this method can be time-consuming, expensive, and even damaging to many surface points. The crosshole and downhole method of wave-propagation to measure the stiffness can also be time-consuming, expensive, and destructive to the site because it requires drilling of borehole(s) and installation of receivers (and sources) in borehole(s). The stiffness profile from the ground-roll analysis, on the other hand, can provide the stiffness of each subsurface layers in nondestructive and fast fashion.  The vs profile obtained from the ground-roll analysis can be used to detect (or resolve) nearsurface features that are hard to be detected even by the high-resolution reflection method that has been used increasingly since 1980 in engineering, environmental, and groundwater projects.  The high-resolution reflection method often fails to be effective in such cases as when the near-surface target is beyond resolution limit (e.g., vertically or near-vertically fractured or collapsed zone) or when the target is underlain by (or embedded in) highly heterogeneous and absorptive materials (e.g., mapping the interface between the bedrock and overburden that consists of highly absorptive and heterogeneous materials). In these cases, the target may cause a characteristic dispersion property of ground roll. Therefore, it is possible in these cases not only to detect but also to resolve the targets on the calculated vs profile. Continuous survey of ground roll can make it possible to image near-surface heterogeneity caused by the anomalous zones through proper design of field technique and effective handling of obtained surface wave data.  In early 1980’s, a wave-propagation method to generate the near-surface vs profile, called Spectral Analysis of Surface Waves (SASW), was introduced (Nazarian and Stokoe, 1983) that makes use of spectral analysis of ground roll generated from impact sources like hammers on the surface. The method has been widely and effectively used in many geotechnical engineering projects. Usually only two receivers (geophones) are used and the spacings between the receivers and receiver and source are changed many times to cover the desired range of investigation depth. Because of this necessity of the repeated tests, it usually takes several hours to complete the whole procedure at one test site.  Multi-Channel Analysis of Surface Waves (MASW) is a fast method of evaluating near-surface vs profile because the entire range of investigation depth is covered by one or a few generation of ground roll without changing receiver configuration. Furthermore, the inclusion of noise wavefields such as body waves (direct, refracted, reflected, and air waves) (Sanchez-Salinero et al., 1987) and reflected (Sheu et al., 1988) and higher-modes (Gucunski and Woods, 1991) ground roll can be identified by their different coherency in arrival times on a multi-channel record and can be handled properly by various kinds of multi-channel data processing techniques to improve the accuracy of the results from the analysis. Among all types of these body waves is the strong first arrivals (refraction events) most troublesome. Inclusion of nonplanar Rayleigh waves (Sanchez-Salinero et al., 1987) that can lead to errors can also be identified and handled properly during the analysis. As a whole, the multi-channel recording method makes the quality control during data acquisition and processing steps more effective than by SASW method.  On a Vibroseis uncorrelated record, all the characteristics of ground roll can be identified on the level of each single frequency component because each individual frequency component is represented in isolation with other components. This makes the quality control even more efficient. Furthermore, an accurate and fast coherency measure, called Cross-Correlation of Stacked Amplitudes with Sweep (CCSAS) (Park et al., 1996b) can be used to construct the dispersion curve.

                              Full Paper    KGS-99-10.PDF  435KB 

 

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