Aim of the project MoreSpin (Mobile Magnetic Resonance Sensor with supraconducting coil for prepolarization in the near surface soil) is to develop a mobile NMR instrument for detecting and mapping near-surface (2m) soil physical parameters. Supraconducting prepolarization coils and adiabadic excitation pulses will significantly improve signal quality.
Moisture stored in near surface soils is key for a manifold of ecological processes. The soil, being the interface between the atmosphere and the lithosphere, plays a crucial role for the transport of water and the dissolved substances therein. At present, there are no suitable direct methods to determine soil moisture content at the required spatial and temporal length and time scales. Not only are direct methods on the km scale disproportionately involved, furthermore, the only available geophysical methods on this length scale are indirect methods and therefore highly ambiguous. One nondestructive geophysical surface method that is well established is surface nuclear magnetic resonance or magnetic resonance sounding (MRS). However, standard MRS is not suitable to map (km scale) the near surface soil physical properties due to its lack of sensitivity in the first 2 m and the usually high expenditure of time needed for conducting measurements.
At this point, the developments of the MoreSpin project come into play. By using a superconducting coil for prepolarization (Px) together with adiabatic excitation pulses the near surface sensitivity is greatly increased. Furthermore, we aim to develop measurement strategies based on compact B-field receivers that a allow a detailed spatial resolution of the soil parameters in question.
In a first numerical study on the prepolarization switch-off performance, we could show that the theoretical achievable maximum signal enhancement strongly depends 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.
Grant no. MU 3318/4-1