Surface Nuclear Magnetic Resonance Measurements in Berlin - Proof-Of-Concept for Applying the Prepolarisation Technique in Urban Areas

Measurements of surface nuclear magnetic resonance (NMR) combined with a prepolarisation previous to the conventional spin excitation are expected to enable non-invasive soil moisture investigations even in urban areas with high environmental noise. To test this expectation, we conducted prepolarised surface NMR using a very small coil (figure-of-eight coil with a diameter of 0.5 m) on a water-filled basin in the city area of Berlin. Using an additional prepolarisation coil with a diameter of 2 m, the NMR signal amplitudes could be amplified by factors up to ten. These NMR signals from depths up to 1 m could be measured and quantified despite the high urban noise level in contrast to those without prepolarisation. Significant Earth’s magnetic field heterogeneities due to the reinforcement of a near-by building yielded a shortened relaxation time for the bulk water (about 40 ms) and correspondingly a broad distribution of Larmor frequencies. We conclude that for the general applicability of surface NMR in urban areas, field heterogeneity and corresponding off-resonance effects must be considered in the forward operator. Our future research will combine prepolarisation with adiabatic spin excitation and will include the exact spin dynamics in the forward calculation.