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

Brady A Flinchum,W Steven Holbrook,Bradley J Carr

doi:10.3389/frwa.2021.772185

Brady A Flinchum, W Steven Holbrook + Show 1 more

Open Access

https://doi.org/10.3389/frwa.2021.772185

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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.

Highlights

Interactions among Earth’s atmosphere, biosphere, and hydrosphere drive chemical and physical processes that, over geologic time, convert bedrock into soil
The optical borehole image (OBI) logs and sonic logs based on both seismic velocities from the four deep (> 50 m) boreholes, all located within 70 m of each other, show strong lateral heterogeneity in the fractured rock directly below the casing (Figure 5)
Our results show that the refraction and sonic velocities converge at depths where the fracture density is uniform, consistent with the idea of a protolith that is defined by a small, uniform porosity

Summary

Introduction

Interactions among Earth’s atmosphere, biosphere, and hydrosphere drive chemical and physical processes that, over geologic time, convert bedrock into soil. These processes create and maintain a structure that extends from Earth’s surface vegetation to subsurface depths where bedrock is no longer physically or chemically altered by near-surface processes. This thin region, which supports virtually all terrestrial life, is referred to as Earth’s critical zone (CZ) (Richter et al, 2009; Brantley et al, 2017; Riebe et al, 2017). 2015; Rempe and Dietrich, 2018), the generation of porosity (Graham et al, 1997; Navarre-Sitchler et al, 2015; Hayes et al, 2019), and water movement in landscapes (Salve et al, 2012; Brooks et al, 2015; Lovill et al, 2018)

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