The Study of Local Distortion in Magnetotelluric Using Three-dimensional Modeling

Local distortion by small-scale structures has impeded the interpretations of the magnetotelluric method (MT) for several years. The developing of three-dimensional (3D) MT modeling makes it is possible for the study of the local diction and reliable imaging of the subsurface electrical conductivity structure. In this paper, several general 3D local distortion models are designed and model responses are computed by 3D modeling. The apparent resistivity curve of polarization mode with electric field perpendicular to the electrical interface, which is corresponding to TM mode in three-dimensional (2D), will upward shift at first and then downward shift when it across small conductive surface body. While crossing resistive body, the curve will downward shift at first and then upward shift. This phenomenon is associated with secondary electric field caused by the accumulative surface electric charges at the boundaries of the anomaly. In 3D model, the variation in horizontal slice of apparent resistivity and phase for YX mode and XY mode are approximate vertically symmetrical, which is consistent with the case that curves of polarization mode with electric field perpendicular to the electrical interface. When the survey line along the east-west direction, the YX mode of the 3D response corresponds to the TM mode in the 2D case. While the survey line along the south-north direction, the XY mode of the 3D response corresponds to the TM mode in the 2D case. The distortion on regional response caused by small conductive inhomogeneous are more severe than that of resistive inhomogeneous. The apparent resistivity and phase of both two polarization modes are obviously affected by local distortion of small conductive anomaly, while the distortion on TM mode response is slightly lesser than that on TE mode caused by electrical anomaly. Therefore, when two-dimensional inversion is used for MT data, it is preferable to fit the TM model data. At the position located directly over the small inhomogeneous, the difference between 3D distortion response and 2D corresponding regional response is much large. But this 3D distortion response can be equivalent to a 2D response. It is more important to pay more attention to this false 2D response caused by local distortion in MT interpretation.