Increasing the Signal-to-Noise Ratio of Field NMR (SNMR) Data by means of Pre-Polarization.
SNMR measurements in densely populated areas, such as those found almost everywhere in central Europe, often cause S/N ratios of less than 1. Time consuming stacking is an unavoidable component of almost every SNMR measurement despite fully developed strategies to suppress disturbance signals. The useful signal amplitude linearly scales with the water content and the strength of the local terrestrial magnetic field. If it were possible to fabricate an artificial static field to the scale of an area, then the useful signal amplitude would increase and a greater S/N ratio, and ultimately a higher advancement in measurement, would be achieved.
We have, for this reason, added a transmitter to our popular SNMR equipment (MRS-MIDI), for the pre-polarization (PP) of the soil. Within the 2 meter wide PP-loop, a field of 0.5 mT can be generated for several seconds. Through this, the nuclear magnetization of the water protons is raised in the top 1-2 meters up to one order of magnitude. The immediate subsequent “conventional” SNMR measurement delivers an up to ten times larger response signal (FID). However, after a short time, this increased magnetization is lost. A deciding factor here is the longitudinal relaxation time T1. Therefore, before every further SNMR measurement, the groundwater has to be pre-polarized again.
When exploring small depths of up to 2 m, this procedure proves to be considerably more effective and provides new fields of application for the SNMR, such as the examination of the soil water in the humus layer or moist zones in tunnels and mines. For larger exploration depths, higher field strengths and larger PP-loops are necessary. However, taking into account the current available technical means, the financial and logistical costs would be too high.