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Summary Small-scale investigations of the unsaturated zone has gained increasing interest in the hydro-geophysical research community. The technique of surface-NMR (SNMR), while providing valuable data on water content and permeability in the saturated zone, has difficulties to reliably detect and interpret signals from partly-saturated soils due to low signal amplitudes. Recently, SNMR using prepolarization (SNMR-PP) has gained interest to overcome this limitation. Here, a strong prepolarization field enhances the SNMR signal of coils with a footprint of below 1m up to a level that promises to enable soil moisture measurements in the upper two meters of the subsurface in the near future. We present the first SNMR-PP measurements on a real soil and demonstrate the general feasibility of this technique to qualitatively and quantitatively detect soil moisture. Our soil moisture measurements are validated by independent time domain reflectometry (TDR) data.

Summary Since several years, surface nuclear magnetic resonance (SNMR) is a well-established method for the hydrogeological characterization of the subsurface up to depths of 150m. When used with small surface loops of only a few square meters, SNMR suffers from low signal-to-noise ratios. This limits the methods applicability in urban areas or areas with low water content (vadose zone). Recently, the SNMR method has been extended by the application of prepolarization (PP) pulses prior to the classical spin excitation, to amplify the measured response signal. Depending on the applied PP-current, the amplification can reach up to two orders of magnitude very close to the PP-loop. This theoretically achievable amplification crucially depends on the assumption of perfect adiabatic switch-off of the corresponding PP-pulse. To study the effect of non-adiabatic switch-off, which is most likely always the case in practical applications, we implement the full spin dynamics simulation of the PP switch-off into the SNMR forward modelling. We show, that depending on the shape and duration of the PP-switch-off ramp, the resulting sounding curves can have decreased signal amplitudes of up to 45%. Neglecting this effect would yield an underestimation of the corresponding subsurface water content of similar magnitude.

Summary 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.