3-D Full-tensor Magnetotelluric Analysis of Coso Geothermal Field

Abstract

Previous analysis of 100+ magnetotelluric (MT) soundings from the Coso Geothermal Field, CA revealed important correlations with known fault locations, injection and production well intervals, and zones of drilling losses (Newman et al., 2008). However, this 3-D inverse modeling effort considered only the off-diagonal elements of the impedance tensor, and initialized from interpolated two-dimensional (2-D) TM results. Phase data in the Zyx component of the impedance tensor exhibited unique out-of-quadrant behavior that could not be reproduced with this inversion workflow. Recent experience modeling resistivity in complex 3-D environments using MT shows that it is necessary to analyze the full impedance tensor, because analysis using only the principal component tensor data (the off-diagonal components) can produce artifacts in the imaging process. Here we show how full-tensor 3-D inversion, initializing from a homogeneous 30 Ohm-m half-space, retrieves a better-fitting model than previous work, and in particular is able to explain the anomalous Zyx phase trend. Furthermore, this result does not make simplifying assumptions about dimensionality and directionality of the Earth.