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-y) 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-r) while the water drained by the pump is the S-y. The transverse relaxation cutoff time (T-2C) is the length of time that partitions the total porosity measured by bNMR into S-r and S-y. When applying a calibrated T-2C, S-y equals the bNMR total porosity minus S-r; thus, a calibrated T-2C is required to determine S-y directly from NMR results. Based on laboratory experiments on sandstone cores, the default T-2C is 33 ms; however, its applicability to fractured granite aquifers is uncertain. The optimal T-2C based on our pumping test is 110 +/- 25 ms. Applying this calibrated T-2C on a saturated, A-type granite at our field site, we estimate the S-y to be 0.012 +/- 0.005 m(3) m(-3) which is significantly different from the S-y (0.021 +/- 0.005 m(3) m(-3)) estimate using the default T-2C of 33 ms. This S-y estimate falls within a range determined using traditional hydraulic testing at the same site. Using the conventional T-2C (33 ms) for fractured granite leads to an inaccurate S-y; therefore, it is essential to calibrate the bNMR T-2C for the local site conditions prior to estimating S-y.