Imaging Near‐Surface Bedrock Fracturing and Fluid Pathways Using Seismic Attenuation and Velocity From Viscoacoustic Full‐Waveform Inversion of Refraction Data

Abstract

Seismic attenuation provides sensitivity to fracture density, fluid saturation, and pore geometry that complements velocity, enabling quantitative imaging of fracture architecture and associated fluid‐pathways in near‐surface bedrock. Yet high‐resolution attenuation models remain rare in refraction studies due to amplitude‐fidelity and modeling challenges. We present the first such model, using viscoacoustic full‐waveform inversion (‐FWI) of active‐source seismic refraction data acquired at the Garner Run field site within the Susquehanna Shale Hills Critical Zone Observatory (Pennsylvania). To preserve amplitude fidelity under the viscoacoustic approximation, we process the data to remove unaccounted‐for effects and convert the modeled pressure wavefields into vertical particle velocity. Our ‐FWI method simultaneously inverts for P‐wave velocity and attenuation, and we interpret the results using rock‐physics modeling to estimate porosity and fluid saturation across a range of permeabilities. The results reveal: (a) a low‐velocity zone beneath the valley, consistent with thick colluvial fill; (b) localized high‐velocity zones within weathered sandstone beneath the hillslopes, interpreted as less‐fractured regions; (c) strong attenuation anomalies down to ∼20 m depth beneath the hillslopes, linked to high porosity and partially water‐saturated pores in the vadose zone; and (d) low‐attenuation anomalies in the colluvium and valley fill, mapped to low‐water‐saturation zones. These results demonstrate that ‐FWI of refraction data provides quantitative, meter‐scale constraints on the architecture and physical state of fractured bedrock and their fluid‐pathways in the near surface.