BORNER, F., GRUHNE, M. and SCHON, J. 1993. Contamination indications derived from electrical properties in the low frequency range. Geophysical Prospecting 41,83-98.Electrical measurements are an important and integrated component of geophysical investigations connected with environmental problems. As a result of an analysis of the electrical conductivity, basic experiments on sandstones at frequencies below 10 kHz show that the complex behaviour of conductivity is caused exclusively by a complex interface conductivity. Its value is determined mainly by the internal rock interface to porosity ratio, the composition of the pore fluid and connected matrix-water interactions resulting in a specific microstructure of the interface. Therefore, it can be expected that the interface region of a soil or rock material is very sensitive to changes in composition caused by contamination. Contaminated sandstone and clay samples were investigated using a low-frequency measurement system. The investigations are directed at the influence of different contaminants and their concentration. Results show that the complex electrical conductivity (real and imaginary parts) is influenced by properties of the pore-filling contaminant. This influence results in a change of the level of both parts and the shape of their frequency dependence. The imaginary part in particular seems to provide important secondary information ; in some cases this part alone allows a differentiation of the various contaminants. The different behaviour of various rock types shows that the effects observed are the result of interactions between pore fluid properties and the internal pore surface structure.
Complex electrical conductivity tomography has been applied to monitor hydraulic experiments in a research facility for in-situ remediation of contaminated soils and aquifers. Arrays of electrodes were installed in a flume where infiltration experiments with non-aqueous phase liquids (NAPL) were performed. Data sets of about 500 configurations have been acquired with a computer-controlled equipment for spectral induced polarization. The tomographic algorithm used for interpretation is based on the simultaneous iterative reconstruction technique (SIRT). As in other inversion techniques, the Jacobian or sensitivity matrix has to be known. The three-dimensional forward modelling is performed by the ftite difference method using complex values of conductivity and potential. The resulting tomographic images show the distribution of complex conductivity inside the tanks. They reflect both the spatial variability of soil and aquifer properties and changes between different stages of the experiments. These images were successfully used to determine the state of the multiphase system consisting of silicate matrix, water, NAPL and air. 745 Downloaded 06/21/16 to 128.210.126.199. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/ 746 Downloaded 06/21/16 to 128.210.126.199. Redistribution subject to SEG license or copyright; see Terms of Use at http://library.seg.org/
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