Abstract

In 1962, Varian proposed the use of NMR to prospect for water in deep gravel layers [1]. This technique has since been improved for use at depths down to 100m [2]. We are interested in applying a similar technique to extract spatial information of the water content in soil nearer the surface of the earth. A first step in determining the feasibility of such a technique is to study the NMR signal from the water in soil. This work concentrates on the contribution on the contribution to the NMR signal from the spin-lattice relaxation rate, R1. R1 studies of soils and synthetic soils at a single high frequency have been performed [3,4]. This study focuses on R1 values as a function of magnetic field strength B0, where B0 is less than 280 mT.

Surface soil samples were collected from five locations in the USA. The particle size distribution in the samples was determined by optical microscopy. Samples were hydrated with HPLC water to the point of saturation for at least six months. A Spinmaster-FFC 2000 (Stelar srl, Mede, Italy) fast field cycling NMR relaxometer was used to obtain 16-point relaxation curves at 20 magnetic field values (B0) between 0.2 and 280 mT for the five soil samples. Relaxation curves were fit with the monoexponential (ME) and biexponential (BE) functions with an offset. Some of the curves clearly displayed BE behavior, while others displayed only ME behavior. The BE behavior is perhaps indicative of a structured water component, experiencing the influence of the surface of the soil particles, and a free water component, influenced to a lesser extent by the soil particles. The calculated R1 values displayed a decreasing trend with increasing B0, and an approximately linear relationship as a function of log(B0) over the entire B0 range. The decreasing R1 with increasing B0 indicates that when designing a field-based MRI system, it will be preferable to use larger B0 values for more favorable relaxation rates. Assuming the spin-spin relaxation rate, R2, will be greater than R1, many of the soils will have a structured water signal component that will be unobservable with a moderate cost detection system associated with a portable field based MRI system. Relaxograms and correlations with soil particle distribution will be presented.