Void Detection Using Near-surface Seismic Methods

Detection of anomalies such as voids in the shallow subsurface using noninvasive geophysical techniques has proved to be challenging at best. Three near-surface seismic methods are introduced, including diffracted body waves, backscattered surface waves, and changes in reflection moveout velocities to detect voids directly or their effects on surrounding material properties using different parts of the wavefield. Examples are presented, including modeled and field data sets to demonstrate each technique. Body-wave diffractions were used to identify and locate man-made tunnels in multiple geologic settings. Variations in shear-wave reflection velocities are shown to correlate to changes in stress over known void locations; backscattered surface waves are shown to correlate with a known void location. Results of the studies show that the field data correlate well with the synthetic, and these methods show promise in furthering the ability to locate subsurface voids and their effects on the surrounding media.