Elastic Characterization at Utah FORGE: P-wave Tomography and VSP Subsurface Imaging

Nakata, Nori, Vasco, Don W., Shi, Peidong, Bai, Tong, Lanza, Federica, Dyer, Ben, Chen, Coral, Park, Sea-Eun and Qiu, Hongrui 2023. Elastic Characterization at Utah FORGE: P-wave Tomography and VSP Subsurface Imaging. Presented at: 48th Workshop on Geothermal Reservoir Engineering, California, USA, 6-8 February 2023. Proceedings 48th Workshop on Geothermal Reservoir Engineering.

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Abstract

For understanding subsurface geology, fault locations, and fracture distribution, detailed subsurface structure information, such as velocity models and reflectivity images, is essential. The velocity models can be used for finding accurate earthquake locations and characterization. We use 3D seismic survey data collected in 2018 and a walkaway Vertical Seismic Profile (VSP) survey performed in 2022 to refine the elastic model at the FORGE EGS site, Utah. We apply travel-time tomography for the 3D seismic survey, which contains nearly 1100 vibroseis shots and 1700 receivers. To increase the picking accuracy, we develop and test two approaches: a machine-learning-based (ML) picker and a frequency-dependent travel-time picker. For the ML picker, we combine the EQTransformer algorithm with cross-correlation to pick P-wave travel times. Although the EQTransformer is designed for earthquake waveforms, it can still pick accurate travel-times of active-seismic data compared to other ML pickers. The frequency-dependent picker is a new approach applied to the waveforms before correlating the vibroseis sweeps. In the waveforms, the sweep signals are recorded as linear upsweep signals. We use a time-frequency analysis to find the linear trend, which corresponds to the arrival time of the wave. Both methods match reasonably well with some differences, which will be discussed. Then we apply a tomographic inversion with the Eikonal solver to estimate the subsurface velocities with higher resolution than the model developed for migration. The VSP survey was recorded after the 2022 April stimulation. The survey contains 106 vibroseis shot points, two DAS systems in nearby boreholes (78-32A and 78-32B), and two geophone systems (58-32 and 78-32B). Direct P and S waves are clearly observed with some reflections. Geophones generally have a higher signal-to-noise ratio than DAS, although DAS has much better spatial sampling. We apply reflection imaging using reverse-time migration, which reveals several reflectors that are imaged by previous studies, but we find the necessity of updating the velocity model for migration because of poor focusing

Item Type:	Conference or Workshop Item (Paper)
Status:	Published
Schools:	Earth and Environmental Sciences
Last Modified:	03 Dec 2024 14:52
URI:	https://orca.cardiff.ac.uk/id/eprint/174074

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