| The two-dimensional (2-D) magnetotelluric (MT) inverse problem still poses difficult challenges in spite of efforts to develop fast and efficient methods for its solution. In this work, a new approach based on regularization theory and the quasi-analytic calculation of the Frechet derivatives is presented. For the forward solution, a fast and efficient finite difference formulation to the solution of the MT equations in both transverse electric (TE) and transverse magnetic (TM) modes based on the balance method is used. The Frechet derivative matrix is obtained as a solution to simple forward and back substitution of the LU decomposed matrix of coefficients from the forward problem utilizing the principle of reciprocity. The magnetotelluric inverse problem is ill-posed. In order to constrain the solution to a set of acceptable models, Tikhonov regularization is applied based on the minimization of a parametric functional. The regularized cojugate gradient method is then utilized to minimize the parametric functional. Inversion results of a set of synthetic data and of a set of CSAMT data from Kennecott Exploration show that the method is fast, stable and produces geologically reasonable models. |
