Combined seismic and borehole investigation of the deep granite weathering structure—Santa Gracia Reserve case in Chile

Abstract

Imaging the critical zone at depth, where intact bedrock transforms into regolith, is critical in understanding the interaction between geological and biological processes. We acquired a 500 m‐long near‐surface seismic profile to investigate the weathering structure in the Santa Gracia National Reserve, Chile, which is located in a granitic environment in an arid climate. Data processing comprised the combination of two seismic approaches: (1) body wave tomography and (2) multichannel analysis of surface wave (MASW) with Bayesian inversion. This allowed us to derive P‐wave and S‐wave velocity models down to 90 and 70 m depth, respectively. By calibrating the seismic results with those from an 87 m‐deep borehole that is crossed by the profile. We identified the boundaries of saprolite, weathered bedrock, and bedrock. These divisions are indicated in the seismic velocity variations and refer to weathering effects at depth. The thereby determined weathering front in the borehole location can be traced down to 30 m depth. The modelled lateral extent of the weathering front, however, cannot be described by an established weathering front model. The discrepancies suggest a more complex interaction between different aspects such as precipitation and topography in controlling the weathering front depth.

Low Vp/Vs Values as an Indicator for Fractures in the Critical Zone

“…Other observations report a larger‐than‐expected range, including high V p /V s values greater than 4 (Flinchum et al., 2020; Pasquet et al., 2015a; Stümpel et al., 1984) or even as high as 20 (Salas‐Romero et al., 2021). There are reports of V p /V s values less than 1.4 in the CZ of Brazil (Trichandi et al., 2022), in dry soils (Salem, 2015), and in full waveform inversion results in the CZ of Pennsylvania (Liu et al., 2022). These observations raise an important question: what can anomalous V p /V s values tell us about physical and chemical processes that drive CZ evolution?…”