| Abstract |
In this study, we introduce a novel field-based method to estimate specific yield (Sy ) in fractured, low-porosity granite aquifers using borehole nuclear magnetic resonance (bNMR). This method requires collecting a bNMR survey immediately following a pump test, which dewaters the near-borehole fractures. The residual water content measured from bNMR is interpreted as "bound" and represents the specific retention (Sr ) while the water drained by the pump is the Sy . The transverse relaxation cutoff time (T2C ) is the length of time that partitions the total porosity measured by bNMR into Sr and Sy . When applying a calibrated T2C , Sy equals the bNMR total porosity minus Sr ; thus, a calibrated T2C is required to determine Sy directly from NMR results. Based on laboratory experiments on sandstone cores, the default T2C is 33 ms; however, its applicability to fractured granite aquifers is uncertain. The optimal T2C based on our pumping test is 110 ± 25 ms. Applying this calibrated T2C on a saturated, A-type granite at our field site, we estimate the Sy to be 0.012 ± 0.005 m3 m-3 which is significantly different from the Sy (0.021 ± 0.005 m3 m-3 ) estimate using the default T2C of 33 ms. This Sy estimate falls within a range determined using traditional hydraulic testing at the same site. Using the conventional T2C (33 ms) for fractured granite leads to an inaccurate Sy ; therefore, it is essential to calibrate the bNMR T2C for the local site conditions prior to estimating Sy . |
| Authors |
Stephanie N. Phillips   , Bradley J. Carr , Ye Zhang , B. A. Flinchum , Shuangpo Ren
|
| Journal Info |
Wiley-Blackwell | Groundwater
|
| Publication Date |
11/16/2023 |
| ISSN |
0017-467X |
Type |
article |
| Open Access |
closed
|
| DOI |
https://doi.org/10.1111/gwat.13374 |
| Keywords |
Nuclear Magnetic Resonance (Score: 0.528081) , Surface NMR (Score: 0.514151) , Fractured Geological Media (Score: 0.510923)
|
