This is a progress report posted for discussion and planning purposes only. Contact the authors for updates prior to citing information contained in this report. Standard caveats regarding the use and interpretation of geophysical measurements or opinions apply.
Soil conductivity can be a useful geophysical property in the search for buried caverns, such as in the area near Indian Trail Caverns. In this search, we are using the Geonics, Ltd. EM34-3XL earth conductivity meter with data logger to measure the lateral variations of conductivity along profiles over known and presently unknown caverns. The EM34 transmits a low-frequency electromagnetic signal into the earth which a receiver antenna and electronic analyzer interpret as the apparent conductivity of the ground. Using the horizontal or the vertical dipole, as well as the three separation settings of the meter, we can utilize six possible settings for interrogating an area.
Graphs of the EM34 response for all six settings are shown in Figure 1 and Figure 2..
In the vertical dipole mode, the EM34 is most sensitive to variations about 4 meters below the surface at the 10 meter spacing, about 8 meters deep at the 20 meter spacing, and from 10 to 20 meters below the surface at the 40 meter spacing (Figure 1). In the horizontal dipole mode, the EM34 is most sensitive to new-surface changes (Figure 2).
Cumulative response functions (Figure 3 and Figure 4) can be used to calculate EM34 readings due to a horizontally layered earth model.
Reconnaissance measurements were made with the EM34 along 2 profiles, Profile 1 across known caverns and Profile 2 in an open field (Figure 5). Coordinates of profile endpoints were obtained with a Trimble GeoExplorer II GPS system. Field readings were processed using the CORS data from Detroit, available over the World Wide Web. The relative positions thus obtained are accurate to within approximately 1 meter. Fiberglass surveyors' tapes were used to measure intervals between readings along each profile.
Figure 6 shows a significant decrease in conductivity over the cavern located at meter 125 along this profile. The total conductivity of the ground is not important, but this lateral variation of conductivity is probably due to the large air-filled cave about 25 meters below the surface. These results show that the EM34 has definite potential for locating presently unknown caverns and sinkholes.
Figure 7 shows a conductivity increase between meter 100 and 120. The significance of variations between meter 150 and 200 is not clear.
Figure 8 shows an interesting "high-low-high" near 160 meters. This anomaly should be investigated with an electrical resistivity dipole-dipole profile.
The conductivity low at meter 195 in Figure 9 might indicate a void. Electrical resistivity should be used to investigate this possibility.
High conductivity at meter 180 in Figure 10 may indicate a shallow soil-filled depression in the bedrock. The 10 meter horizontal dipole "sees" material near the surface (Figure 2 and Figure 4) while the 20 meter vertical dipole "sees" a bit deeper (Figures 1 and Figure 3).
SUMMARY: These very low conductivities indicate that conditions are favorable for ground penetrating radar (GPR) - if one can be found. The significance of many relative highs and lows remains obscure. Additional EM34 profiles will be run during the next few weeks.