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).

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