oa Cavern Detection In The Presence Of A High-Contrast Surface Layer

Various geophysical methods for cavern detection were tested over a known shallow<br>cavern (Cave of the Mounds in S. Central WI) where the geologic section was particularly<br>problematic for geophysical work. The physical properties contrasts of the weathered<br>shale/shale, and massive dolomite section were so great that the conventional use of several<br>geophysical methods failed. Seismic refraction, however, was useful for defining the shale<br>thickness and also showed that the shale-dolomite contact was irregular, with several meters<br>of local relief. A detailed gravity survey showed numerous small features which were<br>apparently due to the variability in the thickness of the low-density surface layer. No seismic<br>Head waves passed through the cave level because the cave is located well within the thick<br>dolomite unit; hence arrival time delays were not significant. Shallow reflection records<br>were overwhelmed by reverberations due to the 1:10 velocity contrast. GPR was unable to<br>reach the dolomite because of the 20 Ohm-m surface resistivity. Slant resistivity soundings<br>using the Bristow technique (pole-dipole array) was the only technique which showed the<br>presence of the 30 foot deep cave, but even this required drastic modification of the<br>reduction and plotting methods to accommodate the 1:40 contrast in layer resistivities. This<br>example illustrates how crucial it is to obtain physical properties information early, to assist<br>in the choice of the appropriate geophysical techniques or their modifications.