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.

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