Surface NMR with Very Long Transmitter Pulses

Since the invention of surface NMR in the 1980’ies, the method has been the subject of much research due to the method’s direct sensitivity to groundwater. While new steady-state methods show great potential for dramatic increases in signal-to-noise ratio and mapping speeds, the stable workhorse in the surface NMR community is still the measurement of the free induction decay (FID). An FID measurement is carried out by pulsing a resonant current in a coil placed at the surface. The current generates a spatially varying electromagnetic field in the subsurface, which perturbs the nuclear spins of hydrogen nuclei in water molecules. After the pulse, the spins return to equilibrium while emitting a measurable signal with an amplitude proportional to the number of excited nuclei and NMR relaxation times controlled by the subsurface constituents. By pulsing with different current strengths and inverting the results, depth profiles of water content and relaxation times are retrieved. The common understanding in the surface NMR world is that the transmitter pulse should be much shorter than the NMR relaxation times to avoid relaxation of the nuclear spins during the pulse. The rule of thumb being that the pulse duration should not exceed about one-fifth of the NMR relaxation times. In this paper we present analytic, numerical, and experimental evidence that this need not be so. We solve the full Bloch equations in the long pulse limit and show that a stationary state is obtained where the continuous excitation balances the continuous relaxation and analyze the dependence of pulse current strength and NMR relaxation times. After the long pulse is terminated, the relaxation from the stationary state to equilibrium can be measured completely on par with standard FID experiments. The numerical analysis suggests that the long-pulse concept can be used to increase the depth of investigation. We give experimental proof for the long-pulse concept and specifically we measure NMR signals with a 200 ms relaxation time following excitation with a 3 s long pulse.