Exploiting Adiabatic Pulses With Prepolarization in Detection of Underground Nuclear Magnetic Resonant Signals

Abstract

During the excavation of underground tunnels and in ore mining, accidents related to water bursts occasionally occur. As the only technique used for the direct detection of groundwater, the nuclear magnetic resonant (NMR) method has advantages for the detection of disaster-inducing water flows. Unfortunately, the amplitudes of underground NMR (UNMR) signals are in the range of some tens of nanovolts ( $10^{-9}$ V) or even picovolts ( $10^{-12}$ V), and thus extremely susceptible to environmental noise. By increasing the macromagnetic moment of groundwater, both adiabatic pulses and prepolarization (PP) methods have been employed in surface NMR. However, when using either method, it is difficult to achieve substantial signal enhancements over large volumes. For maximum signal amplitudes, we integrated these two approaches and derived the forward formulas with adiabatic pulses under PP for UNMR. In comparison with existing methods, this new model can achieve high sensitivity and broad responses. (A 6-m antenna attains a $10^{-5}$ V signal level for a homogeneous subsurface with 0.2 $\text{m}^{3}/\text{m}^{3}$ water content.) Thus, better resolution could also be provided even in a high-noise place. Overall, the large NMR signals and high resolutions make the combination of adiabatic pulses with PP a valuable approach, which is expected to open up a new application for UNMR.