First Measurements of Surface Nuclear Magnetic Resonance Signals Without an Oscillating Excitation Pulse - Exploiting Non-Adiabatic Prepolarization Switch-Off

Recently, small-scale surface nuclear magnetic resonance (SNMR) measurements with a footprint of about a few square meters have gained interest in the soil geophysics community as they directly provide water content and pore geometry information. Here, the application of strong prepolarization (PP) fields enables the detection of water in the vadose zone. Introducing PP into SNMR shifts the reference for the decay of the SNMR signal from the excitation pulse to the end of the PP, effectively increasing the instrumental dead time, which is already too long to measure short signals associated with unsaturated fine-grained soil. In an approach to overcome this limitation, we present the first measurements of SNMR signals from a water reservoir without an oscillating excitation pulse. Instead, we use the non-adiabatic, that is, imperfect, switch-off of the PP-field as an effective excitation mechanism. We complement our field experiments with numerical simulations of the corresponding SNMR spin dynamics.