The absence of geologic materials associated with subsurface anomalies such as voids creates an abrupt and distinct change in material properties. These voids can be challenging to image directly using geophysical techniques, but their effect on surrounding materials may produce physical property variations that are detectable. This has been described as the "halo" effect, where changes in density, stress, fracturing, etc. may produce a geophysical anomaly larger than the actual target. The removal of geologic material causes an increase in effective stress as the load previously bome by the removed material is transferred to the sidewalls and roof of the void. Shear-wave velocity (V,) is directly related to stress and it is not unreasonable to suggest that localized changes in stress will result in V, variations. Examples from seismic surface wave and reflection studies showing detectable increases in V, coincident with zones of expected stress build up are presented.
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