First Measurements of Surface Nuclear Magnetic Resonance Signals in a Grounded Bipole

Surface nuclear magnetic resonance (surface NMR) soundings are geophysical techniques that offer direct detection of groundwater. Ordinary surface NMR soundings are achieved with a wire loop that acts as both transmitter and receiver. We extend the capability of the technique by using a grounded electrical bipole as the measurement sensor. We provide the first successful measurements of surface NMR signals taken with a grounded electrode pair on a beach outside Perth, Western Australia. Simple changes to existing equations are sufficient to provide forward models for the changes in measurement technique, and the resulting groundwater models are consistent with coincident loop soundings. Our result opens the field for novel sounding techniques of surface NMR signals that could have broad impact on near-surface groundwater investigations. Plain Language Summary Surface nuclear magnetic resonance (surface NMR) is a geophysical technique that can directly measure the presence of water in the subsurface. An NMR sounding typically uses a large loop to generate a pulse that excites water into a higher energy level. As the water relaxes back to the equilibrium state, it releases energy that can be detected in the loop. Bipoles are often used in geophysics to determine the ground's electrical structure. Our work uses bipoles to measure groundwater. This has the advantage that the bipoles could be used for both resistivity sounding and surface NMR measurements in the same deployment. We have successfully measured surface NMR signals using a bipole at a beach near Perth, Western Australia. We used a closed loop as the transmitter, and the electrodes were steel pegs. The signals we recorded can only be explained by the presence of groundwater. The groundwater models generated from the bipole data are consistent with models from the traditional technique. Our measurements are the first demonstration that bipoles can be used to detect water in the subsurface. This has the potential to open up an entirely new technique for groundwater detection and exploration.