3D inversion of MT data from northern Mexico for geothermal exploration using TEM data as constraints

Abstract

San Felipe's area has geothermal surface manifestations such as hot springs and high temperature in drinking water wells. As a first stage of the geothermal exploration program, a regional geophysical survey has been carried out where 17 MT (Magnetotelluric) and TEM (Transient Electromagnetic) soundings were recorded along 4 different profiles. Both techniques were applied in the same spots and the TEM data are used to correct for static shift in MT data. Transient responses were measured from 1 to 70 ms, using a 50 x 50m2 single-loop configuration. MT signals were recorded with Metronix systems in the period range from 0.001 to 100 s. A robust processing scheme is applied to the MT time series and estimated Transfer Functions are appraised. The dimensionality analysis of MT data suggests a 1D and 3D subsurface structure. Thereby, a three-dimensional inversion of MT data is later carried out. The information obtained from the inversion modeling of TEM data is incorporated into the 3D MT inversion scheme. Thus, unconstrained and constrained inverse models are achieved and appraised. The preferred 3D inversion model is correlated to geological information of the survey area and a conductive structure is interpreted as a sedimentary basin at its shallow part and as a fault zone with geothermal fluids at depths >1.5 km.