Updated 3D resistivity model of the Tres Vírgenes geothermal field (Mexico) from magnetotelluric data: Implications for geothermal exploration and development

Abstract

A case study is presented involving the 3D inversion of a legacy magnetotelluric (MT) dataset from the Tres Vírgenes Volcanic Complex (TVVC), Baja California Sur, Mexico, which is a significant geothermal energy resource with an installed capacity of 10 MWe. The MT dataset consists of 76 stations acquired between 1992 and 1999 (bandwidth: 0.003 – 100 Hz), and was previously analyzed using 1D, 2D, and 3D approaches, revealing a layered resistivity structure. However, significant uncertainty surrounds the capability of this dataset to resolve deep resistivity structures associated with the magmatic system. This study applies a 3D inversion approach that incorporates topography and bathymetry and inversion tests to assess the sensitivity of the model features to the data. Well-logs and laboratory measurements of resistivity were used to constrain the interpretation. The integrated methodology enabled a more accurate delineation of the resistivity structure of the TVVC, mapping an extensive conductor primarily associated with a brine-saturated smectite clay cap (10 – 60 % clay fraction, 150 – 200 ◦C). Internal variability within the conductor suggests the presence of a small sill with andesitic mush (30 – 85 % melt fraction) north of El Azufre Volcano, possibly associated with sulphate-type hydrothermal activity along the El Azufre Fault Zone. Moreover, a zone of lower resistivity to the north of the main conductor suggests a possible intrusion of seawater that remains isolated from the main hydrothermal systems in TVVC. Sensitivity tests indicate that the dataset lacks the resolution required to image deep bodies beneath the main conductor; consequently, deep conductors modelled in previous studies are likely inversion artifacts, and are unrelated to the deep magmatic system. These findings have the potential to improve the delineation of the geothermal system boundaries, and provide insights for future geothermal and magnetotelluric exploration.