Ambient Seismic Noise Tomography Within the Floridan Aquifer System, Santa Fe River-Sink Rise, Florida, US

This study utilized ambient seismic noise tomography to investigate correlations of hydrogeological systems with heterogeneous porosity with the internal velocity structure of karst aquifers using the Floridan Aquifer System (FAS) as an example. We conducted a seismic signal analysis with 30 days of vertical component seismic data acquired within the Santa Fe River Sink-Rise system (Sink-Rise System), using power spectral density estimates and frequency-wavenumber analysis to identify any noise source biases. Empirical Green's functions were derived through phase cross-correlations and phase-weighted stacking. The functions allowed extraction of group velocity dispersion measurements, which were inverted to generate 2D tomographic images at various periods. Tomographic images show multiple layers with varying group velocities that aligned with known hydrogeological features of the FAS, including its base, low-porosity and low-permeability dolostone semiconfining units characterized by elevated velocities, and zones of elevated porosity from karstification with reduced seismic velocities. Our results show how ambient seismic noise can be used to evaluate aquifer physical properties, identify unknown features, and identify the location and continuity of aquifer units. We used ambient seismic noise to evaluate the speed of seismic waves in the subsurface to understand hydrologic properties of the rocks within a karst aquifer. We analyzed seismic data collected over 30 days in the Santa Fe River Sink-Rise system in north-central Florida. The seismic signal sources originate from ocean waves, anthropogenic activity, and weather. Using cross-correlation techniques, we measured the time lag of the seismic signals as they traveled between seismic station pairs, from which we obtained surface wave (Rayleigh) velocity. Finally, inversions of the velocity measurements provide 2D images of the subsurface at various depths that show alignment of several seismic velocity layers with aquifer units. Dolostone semiconfining units are indicative of elevated seismic velocities, which correspond with low porosity, while lower seismic velocities correspond with porous karstic zones. This study shows the utility of ambient seismic noise to evaluate heterogeneous hydrologic systems and improves our understanding of the depth, thickness, and extent of the aquifer units. Seismic tomography reveals the internal velocity structure of the Floridan Aquifer System and its correlation with hydrogeological units Seismic velocities provide details on aquifer relationships, semiconfining units, unknown features, and the degree of karstification This study demonstrates the broader applicability of seismic tomography in other aquifer systems