Adiabatic pulses enhance surface nuclear magnetic resonance measurement and survey speed for groundwater investigations

ABSTRACTSurface nuclear magnetic resonance (surface NMR) is an extremely powerful tool for groundwater resource investigations. However, the technique suffers from an inherently low signal-to-noise ratio (S/N), which commonly necessitates extensive signal averaging, resulting in very long measurement times. Previous approaches to improve S/N and measurement efficiency have focused primarily on reducing noise, through hardware and processing advancements. We introduce a new and divergent approach to actually increase the signal amplitude by modifying the form of the transmitted pulse used to excite the groundwater signals. An on-resonance pulse, the only form of excitation pulse previously used in surface NMR, has a fixed frequency and induces coherent excitation over a narrow range of transmit field strengths. Given spatially inhomogeneous fields underlying the surface coil, an on-resonance pulse excites water, a limited volume of water, producing a similarly limited signal amplitude. An adiabatic pulse, ...