Three‐dimensional inversion of MT and ZTEM data

Abstract

ZTEM is an airborne electromagnetic survey in which the vertical magnetic field from natural sources is recorded. The data are transfer functions that relate the local vertical field to orthogonal horizontal fields measured at a reference station on the ground. The transfer functions depend on frequency and provide information about the 3D conductivity structure of the Earth. Since a 1D conductivity structure produces no vertical magnetic fields, the ZTEM technique is not very sensitive to the background conductivity. In order to increase sensitivity to the background conductivity, and greatly improve the depth of investigation, MT and ZTEM data can both be collected. The combination of sparse MT data, with the economical and rapid spatial acquisition of airborne ZTEM data, creates a cost effective exploration technique that can map large‐scale structures at depths that are difficult to image with other techniques. We develop a Gauss‐Newton algorithm to jointly invert ZTEM and MT data. The algorithm is applied to a synthetic model and to a field example from the Reese River geothermal property in Nevada.