NEAR-SURFACE GEOLOGY AND SUBSIDENCE IMAGING USING COINCIDENT 3D SURFACE-PENETRATING RADAR, MULTI-COMPONENT SEISMIC REFLECTION, AND CROSS-HOLE RADAR TOMOGRAPHY

Multi-component seismic reflection, surface-penetrating radar and cross-hole radar data were acquired in an area where a highway had collapsed due to mine subsidence. These data were acquired to test the applicability of each method for near-surface geology and subsidence problems, and to identify locations in the study area having a high risk for future failure. Using P-wave data we imaged the top of the saturated overburden, and using S-wave data we located vertical offsets and horizontal disruptions in the bedrock that resulted from subsidence processes. However, seismic data did not provide useful information about the road fill or roadway. This was due to surface wave interference, a lack of data resolution and insufficient media impedance contrasts. Although the penetration depth of surface radar was limited by road fill signal attenuation, 3D volumes of these data enabled high resolution imaging of rebar disruptions, fracture zones, and slump features within the roadway. The spacing of available boreholes and surface refracted-wave interference limited the effectiveness of the cross-hole radar in the near-surface road fill. However, using velocity tomograms and amplitude information, zones of increased secondary porosity were mapped in the overburden and bedrock that resulted from subsidence. Through coincident analyses of these data sets, we identified many locations where bedrock, overburden, road fill materials and the roadway structure had been disrupted by mine-related subsidence processes. Subsequent to classifying locations as high risk for future failure, our geophysical interpretations were confirmed through exploratory drilling and probing.