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

Abstract

Surface 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, one of many pulse forms used for medical imaging and chemical spectroscopy, modulates pulse frequency and provides excitation over a much larger range of transmit field amplitudes. Numerical simulations of surface NMR with adiabatic pulses demonstrate almost a factor of three improvement in the peak signal amplitude compared to an on-resonance pulse. Simulations also show that a single measurement using an adiabatic pulse with high transmit current provides sensitivity to water over a wide range of depth. In contrast, multiple on-resonance measurements using a range of transmit currents are required to span sensitivity over a similar range of depths. Numerical simulation results are validated by the first field experiments comparing on-resonance and adiabatic pulses. We have considered how improvements in S/N can be used for dramatically improved measurement speed and how other advantages of adiabatic pulses may more generally be used to enhance surface NMR measurements.