Numerical modeling of electromagnetic coupling effects for phase correction of EIT borehole measurements

The electrical impedance tomography (EIT), combined with the capability of spectral induced polarization (SIP) has recently become important in the field of geophysics. To characterize weakly polarizable soils and sediments with EIT high phase accuracy is required. Usually long electrode cables with large electrode spacing are used for the borehole measurements, however, this leads to undesired electromagnetic coupling effects. Within the cable inductive coupling between the double wire pairs for current injection and potential measurement could be observed. Furthermore, capacitive coupling appeared between the electrically conductive shield of the cable and electrically conductive soil. Both coupling effects can cause large phase errors depending on the electrical conductivity of subsurface and measured transfer impedance. In order to correct the phase errors both coupling effects were modeled by using discrete mutual inductance and capacitance, whose electrical properties were measured in the laboratory and described for subsequent correction. The correction method was tested successfully on the basis of measurements under controlled conditions in a water-filled rain barrel. First results show that this correction method allows high phase accuracy in the frequency range of a few mHz up to greater than 1kHz