Characterizing aquifers when using magnetic resonance sounding in a heterogeneous geomagnetic field

It has previously been reported that the heterogeneity of the geomagnetic field disturbs the currently-measured free induction decay signal of magnetic resonance sounding (MRS). To overcome the limitation of MRS in a non-homogeneous geomagnetic field, we adapted the spin-echo methodology usually used at the laboratory scale and in boreholes. We present examples of measurements carried out in a sandy aquifer in southern India. The 15-25 m thick sand deposit overlays a gneissic basement. Two sources of geomagnetic field heterogeneity have been identified at this site, both affecting the geomagnetic field within the sandy aquifer: the gneissic bedrock and an intruded dyke into the bedrock. Spin-echo and free induction decay signals have been recorded at six locations. We found that the groundwater content, the thickness of the saturated aquifer and its transmissivity calculated with free induction decay measurements are underestimated compared to those derived from spin-echo measurements. The closer to the dyke the higher the underestimation. Time-domain electromagnetic measurements indicate that the aquifer is rather homogeneous at the site scale, as suggested by spin-echo results. We also found that a small heterogeneity of the geomagnetic field can go unnoticed, thus leading to an unknown mis-estimate of aquifer properties when using free induction decay measurements. Thus spin-echo measurements can be used to improve the accuracy of aquifer characterization when using MRS in geological contexts where geomagnetic field heterogeneity exists.