Electromagnetic inversion using quasi-linear approximation

Three-dimensional electromagnetic inversion continues to be a challenging p roblem in electrical exploration. We have recently developed a new approach to the solution of this problem based on quasi-linear approximation of a f orward modeling operator. It generates a linear equation with respect to th e modified conductivity tensor, which is proportional to the reflectivity t ensor and the complex anomalous conductivity. We solved this linear equatio n by using the regularized conjugate gradient method. After determining a m odified conductivity tensor, we used the electrical reflectivity tensor to evaluate the anomalous conductivity. Thus, the developed inversion scheme r educes the original nonlinear inverse problem to a set of linear inverse pr oblems. The developed algorithm has been realized in computer code and test ed on synthetic 3-D EM data. The case histories include interpretation of a 3-D magnetotelluric survey conducted in Hokkaido, Japan, and the 3-D inver sion of the tensor controlled-source audio magnetotelluric data over the Su lphur Springs thermal area, Valles Caldera, New Mexico, U.S.A.