Evaluation of the Influence of 2-D Electrical Resistivity on Magnetic Resonance Sounding

Abstract

Magnetic resonance sounding (MRS) or surface nuclear magnetic resonance (SNMR) is used for groundwater exploration and aquifer characterization. The method performs NMR measurements with large surface coils (diameter range of [Formula: see text]) to measure the induced decay of the spin of protons of water molecules. The main property of interest is water content. For a reliable analysis of the measurement, the electrical resistivity in the subsurface must be taken into account. The extent of resistivity influence on MRS depends on the size of the coil. So far, the influence of the resistivity has been considered only for stratified 1-D subsurfaces. We explore the influence of 2-D resisitivity models on the MRS signal by comparing the signal calculated for 2-D models to the signal calculated for layered 1-D models. The results indicate that a 1-D approximation is valid if the midpoint of the loop is at least one diameter away from the 2-D structure, or if the extension of the 2-D structure is twice as wide as the loop diameter. For cases in which the 1-D layer approximation is invalid, the substitution of a 2-D resistivity with a 1-D equivalent layer model has been found impossible because of the different 2-D kernel functions and corresponding sensitivities.