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Withdrawn: Dense Nodal Ambient Noise Tomography for Geothermal Prospecting at Aargau, Switzerland

The availability of detailed geophysical and geological subsurface models is critical during the geothermal prospection phase to reduce drilling risk and to help predict induced seismicity hazards. Traditional 3D seismic reflection surveys using active sources are expensive and logistically demanding, especially near urban centers. Thanks to the recent commercial availability of nodal geophones, passive seismic imaging techniques such as ambient noise tomography (ANT) are a promising, affordable alternative for reliable subsurface prospecting and monitoring. In ANT, surface waves are typically extracted from empirical Green’s functions retrieved via cross-correlation of the recorded ambient noise field between every station pair. Surface wave dispersion curves are then inverted for 3D velocity structure of shear waves (Vs), which are particularly sensitive to fluid-rich and fractured rocks.

The northern part of the canton Aargau in Switzerland is currently being investigated as a potential geothermal site due to a high heat flow anomaly potentially linked to hydrothermal circulation along faults bounding the Permo-Carboniferous trough crossing the area. During Winter 2020-2021, we deployed a dense nodal network of 210 SmartSolo geophones in a circular configuration with a diameter of ~25 km and recording for ~30 days. Thanks to the 3-component data, we retrieve the 9 cross-components of the Normalized Cross-Correlation Functions. We point out the challenges encountered and solutions found for automatic picking of dispersion curves from the very large number (~43 k) of Frequency time ANalysis (FTAN) images. Dispersion curves are inverted for 2D Rayleigh wave group velocity maps, which are then inverted themselves for 3D Vs structure. To benchmark the performance of ANT, our model is compared against available seismic reflection surveys. Our study shows the potential of ambient noise imaging acquired with dense nodal networks as a promising early prospection tool for the exploration of the velocity structure of the upper crust in regions of interest for geothermal energy.

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Presentation Time: Pacific

Presenting Author: Genevieve Savard

Student Presenter: No

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