Three-Dimensional Inversion of Magnetotelluric Data over the Coso Geothermal Field, Using a PC

Abstract

Magnetotelluric (MT) data from 101 tensor stations over the East Flank of the Coso geothermal field, southeastern California, were inverted on a PC using a 3-D Gauss-Newton regularization algorithm based on a staggered-grid, finite difference forward problem and jacobians. Static shifts at each MT site can be included as additional parameters and solved for simultaneously. Recent modifications to our algorithm include the addition of an LU solver to calculate the model parameter update, to reduce storage. The inversion for the Coso data set was started from a 22 ohm-m half-space and results qualitatively resemble models from 2-D transverse magnetic (TM) inversions, and from massively parallel 3-D inversion by other workers. In particular, we resolve a steeply west-dipping conductor under the western East Flank tentatively correlated with a zone of high-temperature ionic fluids. Implementation on desktop serial PC’s is an attempt to widen the potential user base. Run times for the Coso data set on a 3.4 GHz desktop are 2-3 days, with the greatest amount of run-time taken up in computing the jacobians explicitly using reciprocity. Continuing efforts are being made in storage efficiencies, in speeding the jacobian computations, and improved parameter weighting.