Three-dimensional inversion of magnetotelluric phase tensor data

Abstract

Recent increase in the application of 3-D inversion of magnetotelluric (MT) data is facilitated by the availability of several 3-D inversion codes, which led to improved interpretation of MT data. However, still the galvanic effects continue to pose problems in interpretation of the MT data. We have addressed this problem using the phase tensor (PT) concept and developed a scheme based on a modification of the sensitivity matrix in the 3-D inversion code of Siripunvaraporn et al., which enables us to directly invert the phase tensor elements. We have used this modified code for PT inversion of MT data and evaluated its efficacy in reducing the galvanic effects through a few examples of inversion of synthetic data and its application on real data. The synthetic model study suggests that the prior model (mo) setting is important in retrieving the true model. Comparison of results obtained from conventional impedance inversion and the proposed PT inversion method suggests that, even in the presence of the galvanic distortion (due to near-surface checkerboard anomalies in our case), the new inversion algorithm reliably retrieves the regional conductivity structure when the prior model or regional resistivity value level can be estimated with sufficient accuracy. Data errors were propagated to the PT elements using delta method while inverting the real data from USArray. The PT results compare very well with those from full tensor inversion published earlier, signifying the efficacy of this new inversion scheme.