Gary R. Olhoeft and James H. Scott
U.S. Geological Survey, Denver, CO
ABSTRACT
Electrical properties of materials are determined by the motion of charge or charged particles in response to a force. Frequency-independent electrical resistivity is a result of simple charge transport with no complicating particle interactions beyond the level of simple scattering. Electrical properties become frequency dependent when moving charge accumulates near energy barriers in a process termed electrical polarization. If the energy barriers are small relative to the energy of the applied driving force, current density will be linearly proportional to electric field strength. Nonlinear complex resistivity (NLCR) results when large energy barriers relative to the applied energy cause a breakdown in this linear proportionality. In rocks, the two principal mechanisms of charge transport across such large barriers are oxidation-reduction reactions and ion exchange processes, both commonly occurring at water-rock interfaces. From the specific electrical potentials (indicative of energy levels) at which reactions are observed to occur, the types of reaction, reaction products, and reaction chemistries may be inferred. The asymmetry and
harmonic content of the potential waveform compared to the driving current waveform indicate whether oxidation-reduction or cation exchange is the dominant mechanism. If sufficient current is injected into the water-rock system, the water will breakdown into hydrogen and oxygen, producing a known set of potentials which may be used to calibrate the remaining wave-form to estimate ground water Eh and pH. The results are not necessarily unique, but the combination of frequency dependence, harmonic distortion, waveform asymmetry, and inflection of the electrical potential all aid in placing limits on the geochemistry of the water and the rock as well as the
equilibrium state of the water-rock system. Such limits may be used to tailor solution mining operations to changing conditions within an ore body, to monitor the growth of pore-blocking alteration products in geothermal production wells, and to monitor or measure ground geochemistry in general (including changes caused by the presence of organic materials).
Borehole measurements in the Sulfur Gulch GP-1 drill hole near Delta, Colorado have confirmed that the phenomena expected theoretically and found experimentally in the laboratory are observable in situ. The GP-1 drill hole passes through nearly 80 meters of clean sand, pyritized sand, coal, shale, carbonaceous matter, and clay. The nonlinear complex resistivity logs were included in a suite of standard logs comprised of normal and Wenner resistivity, SP, 1P, caliper, density, neutron-neutron, and natural gamma ray. The NLCR data were presented in forms equivalent to conventional resistivity, 1P and SP (duplicating logs with conventional tools) and in total harmonic distortion, individual harmonic distortion and asymmetry parameters.