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.

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