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

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.