SUBSURFACE INVESTIGATION OF PSEUDOKARST FEATURES IN EASTERN CONNECTICUT USING GROUND PENETRATING RADAR

This study examines ground penetrating radar (GPR) imaging of subsurface pseudokarst structure at three geologically differing sites in eastern Connecticut. All sites contain sink areas underlain by insoluble bedrock (gneissic or sandstone) with till and artificial fill overburden. Discussion with home owners suggests that the sinks are the result of using improper grading techniques that have caused persistent settling that required maintenance by owners. A total of 65 GPR profiles were collected using pulseEkko Pro GPR with 100 and 200 MHz antennae. Topographic control for profiles and three dimensional grids were surveyed using a Trimble VX spatial station. All GPR profiles were processed using a dewow filter, topographically correcting profiles, and applying spreading exponential gains. Ground velocities (averaging 0.09 m/ns) were determined by hyperbola fitting. Ground truth control was limited to profiles that traverse known obstacles including bedrock exposure that become buried by fill down-line. GPR records were examined to define reflectors, void boundaries, and to identify radar facies. Sink void boundaries and bedrock facies were more clearly identifiable at the sites underlain by gneissic bedrock than the site underlain by sandstone. The clearest indication of sink voids consisted of dish shaped reflectors with overlying complex facies that have varying amplitudes. Within sinkhole fill, GPR records display contorted to complex facies. Results from the sandstone site, despite being collected at 0.5 meter grid spacing and known locations of surface sinks, were the most difficult to interpret. Profiles from the sandstone site displayed 3.5 meters of penetration of contorted fill with varying amplitude and intensity with no clear reflector for the boundary of the sinks. Ambiguous facies at the sandstone site likely is caused by a high water table and more fine grained sediments in the overburden. Ongoing efforts seek to define the volume of sinks and fill using tightly spaced grids (X lines at 0.5 meter spacing) and software seek to develop three-dimensional visualization of the pseudokarst.