Subsurface Weathering Revealed in Hillslope‐Integrated Porosity Distributions

Abstract

Subsurface weathering has traditionally been measured using cores and boreholes to quantify vertical variations in weathered material properties. However, these measurements are typically available at only a few, potentially unrepresentative points on hillslopes. Geophysical surveys, conversely, span many more points and, as shown here, can be used to obtain a representative, site‐integrated perspective on subsurface weathering. Our approach aggregates data from multiple seismic refraction surveys into a single frequency distribution of porosity and depth for the surveyed area. We calibrated the porosities at a site where cores are coincident with seismic refraction surveys. Modeled porosities from the survey data match measurements at the core locations but reveal a frequency distribution of porosity and depth that differs markedly from the cores. Our results highlight the value of using the site‐integrated perspective obtained from the geophysical data to quantify subsurface weathering and water‐holding capacity.

What Do P-Wave Velocities Tell Us About the Critical Zone?

“…While smaller-scale heterogeneity certainly exists beneath the lateral and vertical resolution of the refraction inversion, the velocity discrepancy itself is not a consequence of the inversion, but rather of the physics of wave propagation at different wavelengths. Understanding this large-scale averaging is particularly important at sites where velocities are used quantitatively to estimate porosity (Mota and Santos, 2010;Flinchum et al, 2018a;Callahan et al, 2020;Gu et al, 2020b) or strain (Hayes et al, 2019).…”

“…In areas with crystalline bedrock, we can assume that the porosity of intact bedrock is near zero, so that changes in the seismic velocity must be due to porosity generated through chemical and physical weathering processes (including fractures). The strong relationship between porosity and velocity has allowed for the estimation of porosity in the CZ from seismic refraction data (Holbrook et al, 2014;Flinchum et al, 2018a;Hayes et al, 2019;Callahan et al, 2020). Several rock physics models link porosity to velocity, including porous media models (Hashin and Shtrikman, 1963;Dvorkin and Nur, 1996;Dvorkin et al, 1999) and differential effective media (DEM) (Budiansky and Oconnell, 1976;Berryman et al, 2002a,b).…”

“…et Olyphant et al, 2016;Callahan et al, 2020;Moravec et al, 2020). Seismic refraction uses elastic energy to produce 2D images of P-wave velocity on the order of 10s−100s of meters per deployment.…”

Vadose Zone Thickness Limits Pore‐Fluid Pressure Rise in a Large, Slow‐Moving Earthflow

“…Recent research shows that there are predictable, lithologically controlled bedrock weathering patterns across landscapes (Riebe et al., 2017). Considerable research efforts have been directed toward quantifying these patterns in recent years, particularly as part of the U.S.‐based, NSF‐funded Critical Zone Observatories program and its successor the Critical Zone Collaborative Network (e.g., Callahan et al., 2020; Flinchum et al., 2018; Hahm et al., 2019; Holbrook et al., 2014; Leone et al., 2020; Moravec et al., 2020. ; Pedrazas et al., 2021).…”

Anisotropy and Microcrack Propagation Induced by Weathering, Regional Stresses and Topographic Stresses

“…The zone of weathered and fractured regolith between fresh bedrock and mobile soil is of wide interest. It is a common feature of terrestrial landscapes (Graham et al., 1994; Riebe et al., 2021) and it forms the thickest portion of the Critical Zone—the region between the top of the forest canopy and unweathered bedrock at depth—in many regions (Buss et al., 2008; Callahan et al., 2020; Flinchum et al., 2018; Holbrook et al., 2019; Pavich et al., 1989). The thickness and material properties of this weathered layer influence the pace of landscape evolution (Dixon et al., 2009), the capacity of landscapes to retain water through drought (Salve et al., 2012), and the degree and type of landsliding a given area is subject to (Watakabe & Matsushi, 2019).…”