Steady‐State Surface NMR for Mapping of Groundwater

Groundwater is a critical freshwater source for billions of people (Klee, 2013), but resources are increasingly stressed by climate change, pollution, and overexploitation (Frappart & Ramillien, 2018, https://doi.org/10.3390/rs10060829). Locating groundwater sources and monitoring their sustainable use is a difficult task. One method that shows great promise is surface nuclear magnetic resonance (NMR), as it is the only technique capable of non‐invasive direct measurements of subsurface water content and pore‐space properties. Unfortunately, surface NMR often suffers from low‐amplitude signals that limit mapping speeds and may prohibit measurements in noisy environments. We demonstrate a steady‐state scheme for surface NMR, enabled by recent breakthroughs in transmitter capabilities and numerical modeling that delivers orders of magnitude signal enhancements. The new technique is a highly efficient measurement that is both narrowband and utilizes the full measurement interval in contrast to traditional approaches. Our results demonstrate that high‐fidelity groundwater measurements are now possible in hitherto inaccessible areas. Plain Language Summary Surface nuclear magnetic resonance (NMR) is a non‐invasive technique that allows one to locate groundwater and quantify its abundance. The technique shows great promise to help enhance understanding of groundwater systems, but commonly suffers from low signal qualities due to low‐amplitude signals and high noise levels. To address the signal quality issues, a novel data acquisition strategy for surface NMR is presented, which involves measuring the signal produced by long pulse trains composed of identical repeated pulses. The proposed scheme's advantage over traditional workflows are an order of magnitude increase in stacking rates, and a dramatic reduction of noise bandwidths. The presented scheme shows great potential to expand the conditions where surface NMR can be employed. Key Points Steady‐state surface nuclear magnetic resonance offers significant signal enhancements compared to traditional approaches Steady‐state processing enables a significant reduction of the noise bandwidth