Optimized inside-out magnetic resonance probe for soil moisture measuring in situ.

Abstract

We report a novel inside-out NMR (nuclear magnetic resonance) probe for the measurement of soil moisture. The probe consists of a dumbbell-shape magnet and an opposed-solenoid RF (radio frequency) coil. Optimization methods for the structure of the magnet and RF coil that maximize the SNR (signal-to-noise ratio) of NMR measurements are also described. The dumbbell-shape magnet consists of three cylindrical magnets in series whose magnetic field was calculated with analytic expression deduced by converting magnet to equivalent magnetization current on its cylindrical surface. Based on the analytic expression, a nonlinear optimization mathematical model was built to determine the optimal structure parameters automatically. The opposed-solenoid is a pair of reverse-connected solenoids and used as RF coil on inside-out NMR probe in this work. Its structure-parameter optimization was carried out based on FEM (finite element method) simulation, and UD (uniform design) was applied to increase the optimization efficiency. A prototype was designed and built consisting of a magnet with length of 100 mm and a diameter of 40 mm. An NMR-based soil moisture measuring experiment was conducted by this prototype, with NMR performed using the CPMG (Carr-Purcell-Meiboom-Gill) pulse sequence for soil sample in different moisture content. The T2 distribution spectrum reveals that there are two compartments of water in the soil sample: free water and bound water.