Ground penetrating radar (GPR) and electrical resistivity are two commonly used geophysical tools for locating voids.  GPR is the preferred method because the resolution is much better and profiles can be collected less expensively.  Radar signal, however, is readily absorbed by conductive material and thus the depth of penetration can be very limited.

We were asked to find voids in the Bunker Hill Gypsum deposits using GPR, but the penetration depth was not sufficient for the project goals.  Electrical resistivity was used instead and provided detailed, useful results.

A few sink holes and voids were known to exist within the deposit, but the resistivity revealed a much larger problem than had been expected.  Some of the voids showed up as highly resistive (red) air spaces, but others were water-filled conductive zones (blue).  The two Dipole-Dipole profiles shown below were processed to highlight different depth ranges and used different color scales seen in the figure.  The top panel shows a depth from 0-11 meters.  The bottom panel shows a depth from 0-45 meters.