Borehole Nuclear Magnetic Resonance Estimation of Specific Yield in a Fractured Granite Aquifer.

In this study, we introduce a novel field-based method to estimate specific yield (S) 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 (S) while the water drained by the pump is the S. The transverse relaxation cutoff time (T) is the length of time that partitions the total porosity measured by bNMR into S and S. When applying a calibrated T, S equals the bNMR total porosity minus S; thus, a calibrated T is required to determine S directly from NMR results. Based on laboratory experiments on sandstone cores, the default T is 33 ms; however, its applicability to fractured granite aquifers is uncertain. The optimal T based on our pumping test is 110 ± 25 ms. Applying this calibrated T on a saturated, A-type granite at our field site, we estimate the S to be 0.012 ± 0.005 m m which is significantly different from the S (0.021 ± 0.005 m m) estimate using the default T of 33 ms. This S estimate falls within a range determined using traditional hydraulic testing at the same site. Using the conventional T (33 ms) for fractured granite leads to an inaccurate S; therefore, it is essential to calibrate the bNMR T for the local site conditions prior to estimating S.