Electrical and electromagnetic (EM) methods have been successfully used to map subsurface electrical conductivity distribution for exploration of mineral resources, ground water and ground water contamination, and general study in earth crust. Electrical methods have been efficiently applied to map resistivity (inverse of conductivity) distribution, which in turn has been used to estimate water saturation and to delineate subsurface geology. Along with the resistivity method, induced polarization (IP) has become an important tool in investigating subsurface phenomena involving various chemical contaminants and minerals often associated with clay. Recent development in high-frequency controlled-source magnetotelluric (CSMT) methods allows us to investigate near-surface electrical conductivity distribution with improved resolution. Ground penetrating radar (GPR) has been successfully used to map subsurface structures in great detail when condition is right.
To successfully obtain high-resolution images of the shallow subsurface, including the determination of various buried waste forms, and the monitoring and verification applications, wide-band, densely sampled data is essential. Historically, EM induction techniques have been used with GPR and in many circumstances this combination is successful in locating buried objects. Quantitative estimates of the subsurface conductivity or dielectric distribution have been limited due in part to the lack of interpretational software. In electrically conductive environments, such as those commonly found in environmental characterization studies, the minimum depth of investigation for traditional EM sounding methods is often more than a few meters. GPR systems, however, often investigate no more than the top meter or so, especially when clay minerals are present in the soil. Hence, in many cases, there is a gap in our detection and thus resolving capability between the lower limit of GPR and the upper limit of traditional EM. Bridging this gap is essential to the characterization of buried waste, detecting contaminant plumes, monitoring and verifying remediation activities, and imaging other environmental and engineering targets located in the shallow subsurface. To provide this bridge we have been investigating a frequency band between traditional EM and GPR - 100 kHz to 50 MHz. The method of analysis is the EM impedance approach requiring the ratio of electric to magnetic fields. In an effort to further develop borehole resistivity methods, we are investigating a capacitive electric field measurement technique. Most drill holes for engineering and hydrologic study are cased with plastic, so traditional galvanic measurement using grounded electrodes is not possible. The success of this approach depends on finding the range of frequencies high enough to yield electric fields capacitively, but low enough to yield negligible EM induction.