A Reconnaissance Electrical Resistivity Survey Over Carroll Cave,
Camden County, Missouri
(unpublished report submitted to the Carroll Cave Conservancy)
University of Missouri-Rolla
A reconnaissance electrical resistivity survey was conducted over portions of known cave passage of Carroll Cave in Camden County, Missouri, in late summer 2003. The purpose of this reconnaissance study was to determine the applicability of the electrical resistivity geophysical method in surficial exploration for subsurface voids. This study is of interest to the Carroll Cave Conservancy (CCC) and the Carroll Cave Survey (CCS) because if this geophysical technique was found to be useful, it may be utilized by the CCS in the future for surficial exploration of unknown cave passages.
The electrical resistivity method consists of the injection of electrical current into the subsurface by current electrodes, and measurement of the change in voltage by potential electrodes. The change in voltage measured by the potential electrodes allows for the resistivity of the subsurface materials to be calculated. The resistivity of a material is simply the inverse of the conductivity. In general, relatively low resistivity areas (high conductivity) are represented as shades of blue, intermediate resistivity areas are represented by yellow and green, and relatively high resistivity areas (low conductivity) are represented as shades of orange and red. Some geologic materials that may have a relatively higher resistivity include, for example: air-filled voids, sands and gravels, and bedrock. Geologic materials that may have a relatively low resistivity include, for example: water filled voids, clays and silts. A full description of the electrical resistivity method can be found in any geophysics text (i.e. Burger, 1992).
The locations of the electrical resistivity survey lines were determined by using a map provided by the CCC. Both line length and site accessibility were the overriding factors in the determination of line location. Three, 320 m long lines were setup over the survey area (figure 1). The electrode spacing for all lines was 8 m. A total of four electrical resistivity surveys were conducted over the three resistivity lines, and are labeled CCER1, CCER2, CCER3, and CCER5 (figure 1). CCER1 and CCER2 both covered the same resistivity line, but two different electrical resistivity arrays were used. The dipole-dipole array was used for CCER1, CCER3, and CCER5. The pole-dipole array was used for CCER2, in order to determine the maximum depth of penetration from our electrical resistivity surveying tool. It should be noted that the pole-dipole method, while having the greatest depth penetration, has a lower spatial resolution.
Electrical resistivity measurements were collected using the University of Missouri-Rolla’s Super Sting™ R8 IP, Earth Resistivity/IP Meter manufactured by Advanced Geosciences, Inc. (AGI). The electrical resistivity data were transferred from the Super Sting™ R8 IP, Earth Resistivity/IP Meter to a computer by a data transfer serial cable using the AGI Sting/Swift Administrator software program. AGI Sting/Swift Administrator was used to convert the resistivity data into .dat format for use with the RES2DINV software program. RES2DINV inversion produces a 2-D subsurface model from the acquired apparent resistivity data.
The RES2DINV software program was used to invert the resistivity data and create a 2-D subsurface model from the acquired apparent resistivity data. It should be noted that the color scales on the apparent resistivity pseudosections were not normalized relative to sections in which they were compared to. (RES2DINV software program is freely available off the web).
Results and Discussion
The results of the electrical resistivity survey are shown in figure 2, as electrical resistivity pseudo-depth sections. In general, all of the sections show a top layer of relatively low resistivity to a maximum depth of about 14 m, and under this layer there is a general trend of increasing resistivity with depth. The upper, more conductive layer is interpreted to soil overburden. The general trend of increasing resistivity with depth is interpreted to be the gradation from weathered bedrock to more intact bedrock.
CCER1 shows an area of relatively high resistivity from about 20 m deep to total depth, located from about 50 m to 150 m along the distance of the profile. CCER2 shows the same area of relatively high resistivity from about 20 m deep to total depth. CCER3 shows a small area of relatively high resistivity approximately 12 m deep, located at about 150 m along the distance of the profile. CCER3 also shows a relatively high resistivity layer that seems to be inclined along the bottom of the profile. CCER5 shows two distinct areas of relatively high resistivity, the first of which is located from about 32 m to 64 m along the length of the profile, from a depth of about 12 m and extending to 32 m. The second area of relatively high resistivity is located from about 128 m to 192 m along the length of the profile, from a depth of about 12 m to total depth.
The areas of relatively high resistivity discussed above are suspected to be subsurface air filled voids, owing to the relatively high resistivity values (i.e. >1000 Ohm.m). Each pseudosection was put on top of the map of the line locations to compare the areas of known cave passage to the areas of high resistivity found in each pseudosection (figures 3-6). Unfortunately, the observed relatively high resistivity values did not correlate well with the portions of the lines known to be above cave passages. This may be due to a number of factors, one of which is the complexity of fracturing within dolomite bedrock.
Further electrical resistivity work should be conducted over Carroll Cave, along with several other geophysical techniques to determine which technique is best suited to delineate subsurface voids.
Burger, H.R., 1992, Exploration geophysics of the shallow subsurface, Prentice Hall, New Jersey.