Resistivity Inversion of Magnetic Resonance Sounding – Assessment of Sensitivity and Reliability

The Magnetic Resonance Sounding (MRS) or Surface Nuclear Magnetic Resonance (SNMR) method is used for groundwater exploration and aquifer characterization. MRS provides useful information about water content and hydraulic conductivity of the aquifer. The excitation field of the NMR experiment is generated by a transmitter loop placed on the surface. Due to the electromagnetic attenuation, the magnetic excitation field depends on the resistivity of the subsurface. Therefore, the resistivity has to be taken into account when inverting the MRS data. Neglecting the resistivity causes a biased estimate of the water content distribution. The used resistivity information can be given by a priori information or, as newly introduced in the presented inversion scheme here, it can be obtained during the inversion process. Having also the resistivity along with the water content distribution as inversion result, MRS gives an information about the salinity of the aquifer and thereby, the quality of the aquifer can be estimated. This is of utmost interest for the further hydrogeological interpretation. This specific information cannot be achieved by geoelectrics alone, because of the nonuniqueness in resistivity concerning water content and salinity.