In a numerical study on the prepolarization switch-off performance, we investigated the theoretical achievable maximum signal enhancement in dependency on the particular switch-off characteristics. Thereby, the switch-off performance itself depends mainly on three parameters: 1) strength of the prepolarization field (Fig. 1a); 2) relative orientation between the Earth’s magnetic field and the prepolarization field (Fig. 1b) and 3) shape and duration of the prepolarization switch-off ramp (Fig. 2). The quality of the prepolarization switch-off for four different switch-off ramps is exemplarily shown in Fig. 2a-d. The so-called “adiabatic quality p” is unity (white regions) if the switch-off is adiabatic, meaning here ideal. With ever increasing magnitude of the prepolarization field and relative angle of orientation theta, the adiabatic quality decreases depending on the particular switch-off ramp. Note that due to the inhomogeneity of the prepolarization field an ideal switch-off is practically not achievable. Therefore, there will always be regions in the subsurface with a decreased adiabatic quality. Figure 2e+f shows the effect of an imperfect prepolarization switch-off on the MRS sounding curve for two different inclinations of the Earth’s magnetic field. Depending on ramp shape and duration, the losses in signal enhancement compared to an ideal switch-off can reach up to more than 80%. Non-consideration of this effect would directly result in an underestimation of the soil water content of similar magnitude.

After the numerical study, a validation experiment has been conducted to evaluate the developed numerical modelling code. The measurements were conducted using the NMR Midi (Radic Research) device adapted to provide prepolarisation pulses using a copper coil.

Finally, a very first realistic soil moisture experiment during n irrgiation experiment has been conducted. Again the NMR Midi (Radic Research) has been used.