Three-Dimensional Inversion of Magnetotelluric Data on a PC: Methodology and Applications to the Coso Geothermal Field

Abstract

We describe here efforts in technology development to invert magnetotelluric (MT) data collected in geothermal settings for three-dimensional resistivity models using desktop PC’s or small clusters. A finite difference scheme is utilized for the forward problem, with various options to compute the parameter Jacobians, and parameter step estimates are defined using an explicit Gauss-Newton step. The paper examines mesh and inversion parameter design suitable for MT data collected on the east flank of the Coso geothermal field by using synthetic models based on existing ensembles of two-dimensional inversions. The algorithm design can include static shifts at each MT site as additional parameters solved in the inversion. Initial inversion results for the Coso data set qualitatively resemble previous models from 2-D inversion stitches and from massively parallel 3-D inversion, although improvements are still sought in fit and convergence. At present, run times on modern serial desktops are 2-3 days, with the great majority of time being spent on the parameter Jacobians using reciprocity. An important developmental goal is to speed Jacobian calculation through alternate means such as a good approximation using integral equations.