Importance of understanding bottom-up control when characterizing geothermal systems

Methods designed to identify favorable areas for geothermal resources have traditionally been focused on near-surface information, namely data that can be compiled into a 2D map. However, these methods fail to account for the third dimension: depth. As a result, they do not incorporate deep crustal and mantle features like heat sources. Geophysical methods with multi-scale capabilities, such as magnetotellurics (MT), provide tools to image deeper structures and bottom-up control on location of near surface hydrothermal systems in 3D. This study briefly demonstrates the advantage of understanding bottom-up control on hydrothermal systems to aid assessment and characterization. A regional 3D electrical resistivity model of the Great Basin is developed from MT data that images the near surface down to the mantle. From the 3D model, electrical conductance (depth integrated electrical conductivity) maps are created at logical depth intervals to identify anomalies. The conductance maps image discrete zones of high-conductance between 15-20 km depth indicative of fluid collection at the brittle-ductile transition; high-conductance zones around Moho depths (30-50 km) suggestive of partial melt; high-conductance zones in the upper mantle indicative of higher temperature and larger melt fraction, and low-conductance zones indicative of lithospheric material descending in the mantle. One anomalous zone of low-conductance in the mantle is under the north central Nevada, suggesting vertical mantle flow transport of heat to the crust related to sinking lithospheric material.