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Supporting Data Repository for paper for Negative Poisson Ratios Determined from Vp/Vs Ratios are a Possible Indicator for Microcracks in the Critical Zone
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| Created: | May 11, 2023 at 2:50 p.m. | |
| Last updated: | Jan 17, 2024 at 2:29 p.m. | |
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| Sharing Status: | Public |
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Abstract
This is supporting data for "Low Vp/Vs Values as an Indicator for Fractures in the Critical Zone":
Poisson's ratio for earth materials is usually assumed to be positive (Vp/Vs >1.4). However, this assumption may not be valid in the critical zone because near Earth’s surface effective pressures are low (< 1 MPa), porosity has a wide range (0-60%), there are significant texture changes (e.g. unconsolidated versus fractured media), and saturation ranges from 0 to 100%. We present P-wave (Vp) and S-wave (Vs) velocities from seismic refraction profiles collected in weathered crystalline environments in South Carolina and Wyoming. Our data show that ~20% of the subsurface has negative Poisson’s ratios (Vp/Vs values < 1.4), a conclusion supported by borehole sonic logs. The low Vp/Vs values are confined to the fractured bedrock and saprolite. Our data support the hypothesis that weathering-generated microcracks can produce a negative Poisson’s ratio and that Vp/Vs values can thus provide insight into important critical zone weathering processes.
Plain Language Summary:
When a material is squeezed, the ratio between the change in height and width is described by an elastic parameter called Poisson's ratio. Most earth materials have a positive Poisson ratio, meaning the material will expand when squeezed (e.g. Playdough or wet sand). Materials with a negative Poisson’s ratio rarely occurs naturally and will shrink in all directions when squeezed. Cork is a common material with a Poisson’s ratio of approximately zero. Cork is ideal for bottling wine because its width does not change when pushing it into the bottle's narrow neck. Here we use surface-based measurements to quantify Poisson’s ratio from P-wave (Vp) and S-wave (Vs) velocities in the top 50 m of Earth’s surface. Our results show an unexpected result—material in the CZ has a negative Poisson’s ratio. We believe this unexpected behavior is caused by the combination of low effective pressures and small and irregular cracks created during rocks' transformation into soil. The cracks have a greater impact on the material’s ability to resist compression. At the same time, most of the rock is still coherent and thus only experiences a minimal loss of shear strength.
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ReadMe.txt
Data Directory: Blair Wallis Profile: BW.zip — This file contains all of the raw data and stacked data for the BW seismic refraction presented in Figure 2g-i Once unziped, there is a file called 072613-seismicNotes.txt, which contains the notes taken in the field. Raw - THis has three more directories inside, each with seg2 (.dat) files for each shot. In each one of those directories is converted .segy files. The files were converted using seismic unix, and the bash script is in the folder. The data were staked in the field for this data set, so a single file was collected per shot. Raw_SH-NorthSwing — Direction number 1 with the s-wave source Raw_SH-SouthSwing — Direction number 2 with the s-wave source Raw_vertical — vertical geophones with vertical source. Stacked_SH — this is the “stacked” shots where we subtracted direction 1 from direction 2 to kill the P-waves. SCP Line 1 There is a FieldNotes.txt file, which is the notes taken in the field. This line was different from BW in that we recorded vertical and horizontal ground motions at the same time. Thus, each shot file has 96*2 channels. Raw — This has all of the field’s seg2 (.dat) files. P-wave -> StackedSegy -> This has the final stacked vertical phones extracted and stacked from the raw data. S-wave -> StackedSegy -> This has the final stacked vertical phones extracted and stacked from the raw data. Stacked, in this case, means subtracted. The Python scripts used to stack are also in this directory. SCP Line 2 There is a Notes.txt file, which is the notes taken in the field. This line was similar to BW in that we shot with 96 horizontal phones and then switched to vertical ones. Thus each shot has 96 channels. P-wave -> Raw - has the original seg2 (.dat) files P-wave -> Segy - is the converted segy files P-wave -> Stacked — are the final stacked segy files used to pick S-wave -> Raw - has the original seg2 (.dat) files S-wave -> Segy - is the converted segy files S-wave -> Stacked — are the final stacked segy files used to pick. Again, stacked here is subtracted. Inversions.zip This folder contains all of the model runs, the picks, the topography, and the scripts used to run the inversions.
Credits
Funding Agencies
This resource was created using funding from the following sources:
| Agency Name | Award Title | Award Number |
|---|---|---|
| National Science Foundation | Water in a Changing West: The Wyoming Center for Environmental Hydrology and Geophysics | 1208909 |
| National Science Foundation | Collaborative Research: Network Cluster: Bedrock controls on the deep critical zone, landscapes, and ecosystems | 2012227 |
How to Cite
This resource is shared under the Creative Commons Attribution CC BY.
http://creativecommons.org/licenses/by/4.0/
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