What Do P-Wave Velocities Tell Us About the Critical Zone?
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 inacc...
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| Format: | Article |
| Language: | English |
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Frontiers Media S.A.
2022-01-01
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| Series: | Frontiers in Water |
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| Online Access: | https://www.frontiersin.org/articles/10.3389/frwa.2021.772185/full |
| _version_ | 1798034065849319424 |
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| author | Brady A. Flinchum W. Steven Holbrook Bradley J. Carr |
| author_facet | Brady A. Flinchum W. Steven Holbrook Bradley J. Carr |
| author_sort | Brady A. Flinchum |
| collection | DOAJ |
| description | 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. |
| first_indexed | 2024-04-11T20:39:06Z |
| format | Article |
| id | doaj.art-9f4d593b82a442f68047a1fd1a4e5aa5 |
| institution | Directory Open Access Journal |
| issn | 2624-9375 |
| language | English |
| last_indexed | 2024-04-11T20:39:06Z |
| publishDate | 2022-01-01 |
| publisher | Frontiers Media S.A. |
| record_format | Article |
| series | Frontiers in Water |
| spelling | doaj.art-9f4d593b82a442f68047a1fd1a4e5aa52022-12-22T04:04:16ZengFrontiers Media S.A.Frontiers in Water2624-93752022-01-01310.3389/frwa.2021.772185772185What Do P-Wave Velocities Tell Us About the Critical Zone?Brady A. Flinchum0W. Steven Holbrook1Bradley J. Carr2Clemson University, Environmental Engineering and Earth Sciences, Clemson, SC, United StatesDepartment of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA, United StatesDepartment of Geology and Geophysics, University of Wyoming, Laramie, WY, United StatesFractures 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.https://www.frontiersin.org/articles/10.3389/frwa.2021.772185/fullcritical zoneseismic refractionvelocitiescritical zone architecturegeophysicssonic velocities |
| spellingShingle | Brady A. Flinchum W. Steven Holbrook Bradley J. Carr What Do P-Wave Velocities Tell Us About the Critical Zone? Frontiers in Water critical zone seismic refraction velocities critical zone architecture geophysics sonic velocities |
| title | What Do P-Wave Velocities Tell Us About the Critical Zone? |
| title_full | What Do P-Wave Velocities Tell Us About the Critical Zone? |
| title_fullStr | What Do P-Wave Velocities Tell Us About the Critical Zone? |
| title_full_unstemmed | What Do P-Wave Velocities Tell Us About the Critical Zone? |
| title_short | What Do P-Wave Velocities Tell Us About the Critical Zone? |
| title_sort | what do p wave velocities tell us about the critical zone |
| topic | critical zone seismic refraction velocities critical zone architecture geophysics sonic velocities |
| url | https://www.frontiersin.org/articles/10.3389/frwa.2021.772185/full |
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