Measurements of spectral induced polarization for environmental purposes

Abstract

Hydraulic permeability is one of the most important parameters for the evaluation of sediments relevant to environmental and hydrogeological problems. Up to now, permeability could be determined only by time-consuming and expensive methods like pumping tests or sampling and laboratory investigations. The results are confined to few locations, and they depend on the scale of the investigation method. Measurements on rock samples in a laboratory can differ significantly from well test results. Geophysical measurements are performed on different scales from high resolution measurements in boreholes up to large-scale soundings. Variations in permeability are mainly caused by varying grain size and by changes in porosity. A decrease of average grain diameter results in an increasing internal surface area. Petrophysical investigations have shown a reliable correlation between the imaginary part of electrical conductivity and the porespace-related internal surface. The formation resistivity factor, which is related to porosity, can be determined by geoelectrical measurements if the electrical conductivity of the pore fluid is known. The internal surface area and the formation factor are the only two parameters used by a Kozeny-Carman-like equation to evaluate the permeability or hydraulic conductivity for the investigated representative volume. Complex electrical conductivity is determined by measurements of induced polarization in the frequency domain. Frequencies below 10 Hz are used to avoid electromagnetic coupling. The permeability values determined by electrical measurements in boreholes can well be compared with those derived from the grain size distribution of samples. The same algorithm can be applied to evaluate the hydraulic conductivity of subsurface layers by complex resistivity soundings. The high sensitivity of the imaginary conductivity component to changes at the internal surface may be used as an indicator for contaminations.