Combining Magnetotelluric, Dc Resistivity, and Time-Domain Electromagnetic Data in Geothermal Exploration: An Example from the M’Deek Geothermal Field in Western British Columbia, Canada

Abstract

The M’Deek geothermal field is an extensional, fracture-controlled hydrothermal system located in western British Columbia, Canada. The system has surface expressions of hot springs and pockmarks. Heat is derived from past subduction on the west coast of North America.Electric and electromagnetic (EM) geophysical methods are commonly used in the exploration for geothermal resources due to their sensitivity to the presence of both fluids and regions of hydrothermal alteration. In the initial stages of exploration, time-domain electromagnetics (TDEM) and direct-current (DC) resistivity methods were used to measure the near-surface resistivity structure. These datasets were inverted to obtain resistivity models, which showed a ~100 m thick near-surface conductor. However, both of these methods have a limited depth of exploration. To image the deeper structure of the geothermal field, broadband magnetotelluric (MT) data were acquired in 2020 and 2022. The TDEM and DC inversion models were incorporated into the starting models for the MT inversion in order to improve the resolution of the near-surface conductor.The preferred 3D MT inversion resistivity model of the M’Deek geothermal field shows a sub-vertical low resistivity feature at depths between 500 m and 5 km. This feature was interpreted to be the fault system, which acts as a conduit for the thermal waters that supply the hot springs. The fault system is the target for ongoing geothermal exploration. Sensitivity analysis showed that the fault had an estimated width of 400-1000 m, a strike between S and S45°W and a porosity of 10-20%.

Keywords: Fault-hosted, Convection, Electromagnetics, Magnetotellurics, Time-Domain, Direct-Current