Feasibility study on prepolarized surface nuclear magnetic resonance for soil moisture measurements

Abstract

In the past few years, small-scale (2 m) prepolarized surface nuclear magnetic resonance (SNMR) has gained increasing interest in the research community. As recent studies demonstrated, the application of a strong prepolarization field enhances the SNMR signal of coils with a footprint <1 m 2 up to a level that even enables investigations in urban areas. In particular, it is expected that this noninvasive method provides the soil moisture distribution in the upper 2 m of the subsurface in the near future. However, until now all field experiments have been carried out on water reservoirs only, in an approach to test and implement this rather new technique into the field of SNMR applications. We present the first prepolarized SNMR measurement on a real soil and demonstrate the general feasibility of this technique to qualitatively and quantitatively detect soil moisture in the upper first 0.5 m. Our soil moisture measurements are validated by independent time domain reflectometry data. To complement the field experiments with numerical simulations, we adapted the underlying SNMR spin dynamics simulations and account for prepolarization switch-off effects in the forward modeling of the SNMR excitation.This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

First Measurements of Surface Nuclear Magnetic Resonance Signals Without an Oscillating Excitation Pulse – Exploiting Non‐Adiabatic Prepolarization Switch‐Off

“…As demonstrated by Hiller et al (2020), this switch-off ramp produces non-adiabatic regions below the PP-loop considering the B-field orientations and PP-field strengths of our setup. For effective noise cancellation, we used a three-channel reference antenna which was installed about 2 m next to the pool (see also Costabel, 2019;Hiller et al, 2021). We use two different receiver (Rx) loop configurations.…”

“…After an on‐time of 5 s (blue), the PP‐field is switched‐off within 1 ms (green). The effective PP dead time in our setup is 10.5 ms, because the device we are using is originally not designed to be used without an oscillating excitation pulse and its measurement protocol enforces certain timings that need to be obeyed (Hiller et al., 2021). We expect that the effective PP dead time can be shortened to the length of the PP switch‐off ramp, that is, few ms, in an optimized device.…”

“…The pool has a diameter of 5.4 m and at its maximum, the height of the water body inside the pool was about 0.9 m with an accuracy of about ±0.01 m. To conduct SNMR-PP measurements at different water levels, the water was lowered in steps of about 0.05 m to a maximum distance between the platform and the water surface of 0.25 m. The pool was installed on the Test Site for Technical Safety of the Federal Institute for Materials Research and Testing in Horstwalde, about 70 km south of Berlin (Germany). A detailed description of the site and measurement conditions can be found in Hiller et al (2021). In this study, we use a device which is optimized for shallow investigations at depths not larger than 10 m (Radic, 2009;Radic & Lehmann-Horn, 2011).…”

“…(2018) that show a signal amplification of up to five fold at a depth of about 1 m when using a measurement setup with a PP‐loop diameter of 2 m, Hiller et al. (2021) have additionally shown the feasibility of SNMR‐PP to measure reliable signals from the uppermost few decimeters of the soil. In addition, while not being a classical geophysical SNMR‐PP application in terms of soil moisture characterization, the detection of oil under sea ice from a mobile, helicopter‐borne NMR device also makes use of PP to amplify the measured NMR signals (Altobelli et al., 2019; Conradi et al., 2018).…”

“…This effect is expected to be even more pronounced in the vadose zone, where the pores are generally not fully saturated and therefore exhibit very short relaxation times. This was demonstrated by Hiller et al (2021), where the authors used supporting laboratory NMR data to determine the amplitude loss due to the effective PP dead time to be about 60%. One possibility to overcome the effective PP dead time limitation may be the development of future SNMR-PP devices with more advanced electronic components that enable an operation at higher excitation pulse currents and hence, shorter excitation pulses and wait times.…”