An investigation of factors affecting the depths of steady-state surface NMR signals

Recent developments in surface nuclear magnetic resonance (NMR) data transmit schemes, called steady-state measurements, involve acquisition of the NMR signal during a train of closely spaced identical pulses, and show great promise to enhance the measurement’s signal-to-noise ratio. The steady-state signal displays a complex dependence on a range of experimental parameters, such as the strength of the individual pulse and the separation between the pulses, as well as subsurface parameters, including the relaxation times controlling the time-dependence of the NMR signals. It is imperative to understand the role that each of these parameters plays in controlling the depth of origin for steady-state signals so as to enable an optimization of a suite of measurements consisting to the fewest possible measurements delivering satisfactory resolution. A range of synthetic studies are conducted to gain insights into controls on steady-state signals’ depths of origin. Relaxation times, duty cycle, pulse train timing are all observed to play strong controls on the signal’s depth, in addition to the pulse duration and current strength. Discussion of whether high-duty cycle steady-state sequences may enhance depth penetration is given, along with the presentation of a field data set composed on a non-traditional depth sounding approach, where repetition times are varied to encode depth sensitivity.