ASSESSING AZIMUTHAL SEISMIC FIRST-ARRIVAL TOMOGRAPHY (ASFT) TO QUANTIFY GEOMETRY OF MULTIPLE SETS OF SHALLOW SUBVERTICAL BEDROCK FRACTURES FOR HYDROGEOLOGIC STUDIES

Over the past decade there has been an increasing focus on prevention and remediation of contaminant plumes that are entering groundwater through fractured bedrock. Groundwater resources require proper management to remain resilient in regions with growing populations. Current methods for characterizing subsurface groundwater flow in fractured bedrock are expensive, invasive, and/or imprecise, due to the inherent spatial heterogeneity that is typically shorter in wavelength that well spacings. We are in the early stages of testing a new technique—azimuthal seismic first-arrival tomography (ASFT)—that provides a cheaper, less invasive, and more robust method to potentially assess critical pathways of contaminant flow in fractured bedrock at field scales. We propose that ASFT will provide an accurate representation of fracture-pathway orientations through azimuthally anisotropic seismic-velocity propagation anomalies. Previous studies indicate seismic P-waves travel faster parallel to fractures and slower perpendicular to fractures. Thus, azimuthal travel-time mapping across multiple sets of subvertical fractures should yield a superimposed azimuthal seismic velocity plot where each high-velocity orientation is parallel to the azimuth of dominant fracture sets. This experiment is conducted at a ~500 m² site on an exposed bedrock outcrop having one known major fracture set and two minor fracture sets with different azimuths. The objective is to present the effectiveness and viability of using ASFT for groundwater flow evaluation in regions with more complex bedrock fractures.