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Shallow Fault Location and Characterization in Complexly Deformed Zones Using Novel 3D Gravity Tomography: Examples From Dam Construction and Geothermal Assessment in Colorado

Session: Advances in Upper Crustal Geophysical Characterization [Poster]

Type: Poster

Date: 4/21/2021

Presentation Time: 11:30 AM Pacific

Description: 

Shallow geophysics is often called upon to identify faults and determine their orientation, sense of slip, and depth range. Numerous seismic methods can achieve these goals, yet 3D imaging is often cost- or time-prohibitive. Gravity is comparatively affordable and can rapidly locate faults but typically has poor depth sensitivity when using sparse data. We have developed a suite of 3D microgravity tomography methods, including forward, inverse, gradient-/Euler Deconvolution-based, and wavenumber-domain elements. We first present results from a 40-km² dam and reservoir development project in a heavily faulted intermontane basin (South Park, CO) underlain by complex volcanic assemblages and Paleozoic evaporites. Gravity tomography provided the basis for reservoir layout, dam alignment, and pre-construction by quickly imaging structure to 3 km depth and capturing ~100-m scale shallow features, identifying multiple generations of cross-cutting faults buried by 100s of meters of mixed volcanics, and delineating evaporite zones to be avoided. Six seismic reflection lines were also acquired to bolster resolution at dam foundations, and gravity images compare favorably with seismic results. Moving on to a previously unmapped geothermal prospect in southwestern Colorado, gradient-based methods delineate a complex shallow fault network, and stress analyses determine which are suitably oriented to serve as geothermal conduits. Existing methods, however, could not map fault interactions at depth nor judge fault connectivity with basement heat sources. Joint analysis of 3D tomography and regional well data reveals that several previously unidentified faults may reach basement, while known structures peter out in the sedimentary cover. These newly identified faults provide targets for drilling, aqueous geochemistry, and higher-resolution geophysics as the next step toward geothermal viability. Both sites are also exceptionally scenic, so we will have some nice photographs to accompany the science in this presentation.

Presenting Author: Will Levandowski

Student Presenter: No

Authors