Application of mohr circle impedance tensor decomposition to inhibit the distortion caused by shallow inhomogeneity

摘要

The observing impedance tensors, in magnetotelluric(MT) measurement, contain disturbed information in the regions with complex geologic structure, and transverse electrical inhomogeneities in shallow earth cause signal distortions. Considering the structure feature of the observing region, the Mohr circle decomposition method is applied to rotate the electric observing axes and the magnetic observing axes independently, and make impedance tensor matrix with 2-D characteristics from one with 3-D characteristics. Informations of local structure and region structure are analysed separately by the rotation angles of electric horizon component and magnetic horizon component. The study show that the galvanic distortions are removed effectively. After rotating the observing impedance tensors with Mohr circle decomposition method, the continuity of geoelectric information and the layer structure of the observing region are indicated perfectly. And characters of the local structure and the region structure are expressed efficiently, especially the discontiguous low resistivity distortions alone 3000Hz and the distortions that locate at 650 meters, 750 meters and 920 meters are removed, and the interface of igneous rock and clastic rock at the location of 750 meters is expressed more clearly. These results suggest the Mohr circle decomposition method can successfully remove the distortions by inhomogeneity in shallow earth and improve analysis more accuracy.