Adiabatic Pulses Enhance Speed and Sensitivity of Geophysical Surface NMR Measurements for Groundwater Investigations

Abstract

We present a new approach to improve the sensitivity and efficiency of geophysical surface nuclear magnetic resonance (NMR) measurements. An extremely powerful tool in groundwater investigations, surface NMR inherently has a relatively low signal-to-noise ratio (SNR), which sometimes necessitates long survey times for signal averaging. In pursuit of faster survey speeds, we show that replacing the standard on-resonance excitation pulse with an adiabatic, frequency-swept pulse can provide significant increases in the NMR signal amplitude. This increase results from the fact that adiabatic pulses can excite larger volumes of groundwater more efficiently than conventional pulses. Using numerical simulations and full-scale field experiments, we show that adiabatic pulses can provide a factor of ~3 increase in signal, and suggest other advantages for groundwater imaging. The signal increase alone allows for data of equivalent SNR to be acquired in a fraction of the time required for conventional on-resonance pulses. Ultimately these improvements can allow surface NMR to be exploited in an expanding range of applications.