Abstract
During the last decade several attempts have been made to quantitatively characterize aquifers from magnetic resonance sounding (MRS) results. However a reliable relationship between MRS water content and hydrogeological storage-related parameters has never been reached, and estimate of aquifer transmissivity from MRS results is not always reliable. In some cases theses failures can be explained by the heterogeneity of the geomagnetic field that corrupts the currently measured free induction decay (FID) signal of MRS. To overcome the limitation of MRS in non-homogeneous geomagnetic field, we adapted the spin-echo (SE) methodology routinely used for laboratory measurements. We present examples of sounding carried out in a sandy aquifer of Southern India where both SE and FID signals have been recorded. We found that the volumetric groundwater content estimated from SE measurements is higher than that derived from FID measurements (from 20% to 70%). Results of a groundwater model are in good agreement with SE measurements. Analysis of our results shows that underestimate of groundwater content and poor estimate of aquifer transmissivity could easily take place when performing FID measurements because small heterogeneity of the geomagnetic field can be unnoticed. Thus spin-echo measurements could be a way for improving aquifer characterization in many geological contexts.
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