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

Abstract

Fractures in Earth's critical zone influence groundwater flow and storage and promote chemical weathering. Fractured materials are difficult to characterize on large spatial scales because they contain fractures that span a range of sizes, have complex spatial distributions, and are often inaccessible. Therefore, geophysical characterizations of the critical zone depend on the scale of measurements and on the response of the medium to impulses at that scale. Using P-wave velocities collected at two scales, we show that seismic velocities in the fractured bedrock layer of the critical zone are scale-dependent. The smaller-scale velocities, derived from sonic logs with a dominant wavelength of ~0.3 m, show substantial vertical and lateral heterogeneity in the fractured rock, with sonic velocities varying by 2,000 m/s over short lateral distances (~20 m), indicating strong spatial variations in fracture density. In contrast, the larger-scale velocities, derived from seismic refraction surveys with a dominant wavelength of ~50 m, are notably slower than the sonic velocities (a difference of ~3,000 m/s) and lack lateral heterogeneity. We show that this discrepancy is a consequence of contrasting measurement scales between the two methods; in other words, the contrast is not an artifact but rather information—the signature of a fractured medium (weathered/fractured bedrock) when probed at vastly different scales. We explore the sample volumes of each measurement and show that surface refraction velocities provide reliable estimates of critical zone thickness but are relatively insensitive to lateral changes in fracture density at scales of a few tens of meters. At depth, converging refraction and sonic velocities likely indicate the top of unweathered bedrock, indicative of material with similar fracture density across scales.

Symmetry in Hillslope Steepness and Saprolite Thickness Between Hillslopes With Opposing Aspects

“…Below the maximum vertical velocity gradient, the seismic velocity increased from ∼1,300 to 3,000 m/s on average at 26 m (SD 10 m) and 49 m (SD 11 m) below the ground surface on the SFS and NFS, respectively. The increase in the seismic velocity could be due to a gradually decreasing porosity (Figure 5; Flinchum et al., 2022; Gu et al., 2020). This transition most likely represents the transition from weathered bedrock (more competent than the saprolite above) to more pristine, low porosity material.…”

“…From our resulting velocity models described above, we calculated the vertical velocity gradient, defined as the change of P‐wave seismic velocity with depth. Maxima in vertical velocity gradients have been shown to correlate with a transition from highly disaggregated or weathered material to more pristine, low porosity bedrock (Flinchum et al., 2022). Thus, we used vertical velocity gradient profiles across our study transect to identify potential transitions in CZ structure (i.e., porosity, lithology).…”

Mapping Variations in Bedrock Weathering With Slope Aspect Under a Sedimentary Ridge‐Valley System Using Near‐Surface Geophysics and Drilling

“…Seismic refraction is a useful tool to determine broad scale subsurface structure by identifying transitions in P‐wave velocity (Vp) that can correspond to rock properties associated with weathering. However, seismic refraction is not expected to perfectly capture borehole‐inferred properties since it is sensitive to larger spatial scales (meter‐scale; Flinchum et al., 2022), whereas the borehole diameter is 6.35–12.7 cm and has cm‐level sampling resolution for some measurements (Pedrazas et al., 2021). Vp is a measurement of bulk material strength that depends on lithology, porosity, moisture content, and chemical weathering.…”

“…These measurements do not capture the total porosity which includes pore space associated with fractures, from processes such as gravity unloading and tectonic loading. On the other hand, seismic waves from near‐surface active source seismic surveys are generally sensitive to length scale in 10 s of meters (e.g., Flinchum et al., 2022). In order to estimate a total bulk porosity (Φ total ) that is reflective of fracture and matrix pore space, and to obtain porosity values on a broader spatial scale, we apply a rock physics model to our seismic refraction data (e.g., Gu et al., 2020; Hayes et al., 2019; Holbrook et al., 2014).…”

Catchment‐Scale Architecture of the Deep Critical Zone Revealed by Seismic Imaging

“…The depths of these interfaces have increasingly been determined by V P (Flinchum et al., 2022). However, when one of these interfaces lies in the vicinity of the water table, V P is also influenced by variations of subsurface water content that can bias the estimation of the corresponding interface depth (Pasquet, Bodet, Dhemaied, et al., 2015).…”