Improving Surface-NMR Estimates of Nuclear-spin Relaxation (T1)

Surface nuclear magnetic resonance (NMR) is a useful tool for hydrological investigations of shallow aquifers. An important parameter is the NMR relaxation time T1, from which information on pore structure or even hydraulic conductivity can be inferred under favourable circumstances. T1 data are conventionally acquired using a scheme that involves two sequential pulses of electromagnetic energy, the second of which is phase-shifted by pi relative to the first. We have discovered that variations of the excitation field with distance from the transmitter introduce a significant bias to conventional estimates of T1. Here, we propose a novel yet simple modification to the conventional scheme that is theoretically capable of resolving this problem. The proposed scheme comprises a conventional double-pulse sequence followed by an additional double-pulse sequence in which the 2nd pulse is in-phase with the 1st pulse. Subtracting the voltage signals measured during the two double-pulse sequences eliminates the bias. This strategy of continuously cycling the phase of the 2nd pulse between pi and 0 in sequential double-pulse experiments and then subtracting the resulting voltages is a highly promising step towards recording more reliable T1 data under general field conditions.