Three-dimensional inversion of magnetotelluric data with static shifts

Abstract

The static shifts caused by small-scale shallow resistivity anomalies hamper the interpretation of magnetotelluric (MT) data. In this paper, an approximate three-dimensional inversion scheme is developed that inverts jointly for resistivities and static-shift values. The unknown static-shift parameters are incorporated into the inverse problem based upon an assumption that they are Gaussianly distributed. A staggered-grid finite-difference method is used to calculate the MT responses in three dimensions. The sensitivities of the MT responses to subsurface resistivity changes are calculated from the analytic solutions of the electric field for a homogeneous half-space model using the adjoint-equation approach. These approximate sensitivities are used throughout iterations, which results in a substantial saving in the computation time over the full inversions. A synthetic example indicates that the inversion scheme recovers the resistivity structure from the data contaminated heavily with static shifts.