Focussing effect of an averaged three-electrode configuration

Geoelectrical measurements with multi-electrode arrays are performed to obtain an image of the subsurface resistivity structure. Conventional configurations like Wenner, dipole-dipole or pole-dipole yield pseudosections of apparent resistivity showing a more or less distorted image of the real structure. Consequently, inversion procedures have to be applied to transform the measured data into a realistic model. Practical experience has shown that a simple averaging of the apparent resistivities measured with a configuration of three collinear, equispaced electrodes in the forward (AMN) and in the backward mode (NMA) by changing the position of the current electrode A and the potential electrode N results in an improved pseudosection reflecting the main features of the subsurface. This configuration has proved to be an efficient tool for data acquisition combining sounding and profiling (Peschel, 1967). Based on this arrangement, a special continuous imaging technology was developed which has been successfully applied to engineering geological problems and at archaeological sites (Peschel and Lubenow, 1995). The available tools for modelling and inversion have been used to investigate the resolving .power ofthe averaged three-electrode configuration.