Casting Sunlight on the Deep Heat Sources with Magnetotelluric Geophysical Imaging
The eastern Great Basin of western Utah has long been known as a high heat flow region containing young volcanic rocks and several producing hydrothermal systems. The Utah FORGE Enhanced Geothermal Systems project seeks to advance technology to extract the heat in the huge volumes of hot rock underground, identified or strongly suspected, that do not currently possess adequate permeability. Resolving the underlying mechanisms that have heated such volumes should not only help characterize the magnitude of an individual resource but also create a pathfinder for discovering other systems, both EGS and hydrothermal.
Attempts to “see” inside the Earth to reveal its structures and physical state commonly rely on geophysical techniques. With those, signals of various types are propagated downward using either man-made or natural sources that are meant to scatter from subsurface objects of interest to be recorded at the surface for transformation to images of the interior. The methodology is related to more familiar medical CT-scans of the human body, or to non-invasive tomographic testing of structures, such as concrete.
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The eastern Great Basin of western Utah has long been known as a high heat flow region containing young volcanic rocks and several producing hydrothermal systems. The Utah FORGE Enhanced Geothermal Systems project seeks to advance technology to extract the heat in the huge volumes of hot rock underground, identified or strongly suspected, that do not currently possess adequate permeability. Resolving the underlying mechanisms that have heated such volumes should not only help characterize the magnitude of an individual resource but also create a pathfinder for discovering other systems, both EGS and hydrothermal.
Attempts to “see” inside the Earth to reveal its structures and physical state commonly rely on geophysical techniques. With those, signals of various types are propagated downward using either man-made or natural sources that are meant to scatter from subsurface objects of interest to be recorded at the surface for transformation to images of the interior. The methodology is related to more familiar medical CT-scans of the human body, or to non-invasive tomographic testing of structures, such as concrete.
MT Method
The application of the magnetotelluric, or “MT” method at Utah FORGE enhances our fundamental understanding about heat transfer and fluid flow from shallow to deep levels in the crust. MT exploits naturally occurring electromagnetic waves as sources for imaging the Earth’s subsurface electrical resistivity. Anomalous volumes of low resistivity in turn can indicate thermal fluid pathways or heat sources generating high temperature rock. These EM waves are produced ultimately by the sun, whose energy drives the weather producing lightening bolts and whose solar wind produces global geomagnetic field variations, both of which can be treated as broadcast antennae. The waves penetrate downward diffusively, scattering back to the surface from resistivity structures for recording and analysis.
New data from an array of 120 MT recording sites were acquired over and around the project area. They were merged with prior recordings to achieve a combined view of Utah FORGE and Roosevelt Hot Springs structure and the Quaternary volcanic system of the Mineral Mountains (figure 1). Resistivity tomograms, or computed images of resistivity from the observed EM data, were rendered using an algorithm created in-house, based on the finite element engineering method. An east-west cross-sectional view through the fully 3-D resistivity model volume appears in figure 2. It is presented at the scale of the whole crust in this area as pertinent mechanisms of heat and fluid transport are believed to be operating at that scale.
Directly beneath the Utah FORGE site, the low-resistivity warm (yellow-orange) colors represent the shallow sequence of alluvium deposits, whereas at deeper levels the high-resistivity cool (purple-dark blue) represent tight basement rocks made of granitoid and gneiss that are ideal for EGS development. At a much deeper level (20-kilometers depth or more), a broad tabular zone of low resistivity (yellow-orange colors) is seen that likely represents recent intrusion or “underplating” by magma such as has occasionally erupted at the surface along the north-south length of western Utah.
Protruding upward from that large volume is a narrowing zone of moderately lower resistivity that focuses near-surface to Roosevelt Hot Springs. This likely represents the conduit for heat and fluids in the hydrothermal system that supplies the Blundell power plant east of Utah FORGE. The connection through the entire thickness of the crust provides an explanation for evidence of a magma source region which, over geologic time, heated a large volume of crystalline rock beneath the Utah FORGE site. Other such zones and pathways for energy exploitation could be sought using the MT technique throughout the eastern Great Basin.