Microearthquake Hypocenter-Location and Focal-Mechanism Inversions for the EGS Collab Project: A Synthetic Study

Abstract

Multiple U.S. national laboratories and universities are conducting collaborative research under the EGS Collab project supported by the U.S. Department of Energy, to understand the fracture creation and imaging during fracturing in enhanced geothermal systems. Microearthquake hypocenter locations and focal mechanisms will be used to monitor hydraulic fracturing growth and characterization at the EGS Collab experimental site at the Stanford Underground Research Facility using geophones in multiple monitoring wells. To design a geophone network for cost-effective passive seismic monitoring, we conduct numerical simulations to study the relationship between geophone distributions and standard deviation errors of microearthquake hypocenter locations, and that between geophone distributions and focal-mechanism inversion errors. Our results indicate that microearthquake hypocenter locations and focal mechanisms can be reasonably well reconstructed for the EGS Collab Experiment 1 using the current design of six monitoring wells, including four fracture-parallel monitoring wells and two orthogonal wells. Eight geophones evenly distributed in four parallel monitoring wells or twelve geophones in all six monitoring wells are required for hypocenter location, and twelve geophones evenly distributed in six wells or sixteen geophones in four wells are needed for focal-mechanism inversion.