Numerical Study of Prepolarized Surface Nuclear Magnetic Resonance in the Vadose Zone

Abstract

Surface nuclear magnetic resonance (SNMR) is a geophysical technique for water exploration in the shallow subsurface. To investigate the vadose zone, it is necessary to increase the method sensitivity; therefore we developed a new approach for SNMR experiments. Adapted from laboratory applications, an artificial static magnetic prepolarizing (Px) field is superimposed on the Earth's magnetic field to enhance the equilibrium magnetization of the pore water and thus to increase the SNMR sounding signals. We numerically investigated the basic feasibility of this approach by modeling the relevant static and oscillating magnetic fields and resulting SNMR responses. Configuration parameters, e.g., current intensities, loop sizes, and layouts, were optimized by numerical simulations. The most feasible field layouts comprised two Px loops symmetrically arrayed with respect to the transmitter/receiver (Tx/Rx) antennae or a single Px loop with its radius approximately three times larger than the respective Tx/Rx loops, thus yielding a 10‐fold increase of the recorded signals compared with conventional SNMR. Eventually, we simulated synthetic sounding curves obtained from a water body layer at various depths and thicknesses, concluding that in the shallowest subsurface, the use of two loops to enhance the signal is more effective than a single antenna, but the results deteriorate for deeper water layers.