Groundwater NMR in conductive water

Abstract

A surface method of groundwater prospecting using nuclear magnetic resonance (NMR) in the Earth’s magnetic field is under study. The technique is employed for hydrogeological surveys down to a depth of about 100 m. The advantage of this method is that an NMR signal can be observed only in the presence of groundwater. A circular wire loop with a diameter of 100 m is laid out on the ground to excite and receive the NMR signal. An oscillating current with a rectangular pulse‐shape is passed through the loop, with the carrier‐frequency being equal to the proton‐resonance frequency in the Earth’s field. The excitation pulse is followed by a nuclear induction emf caused by the free Larmor precession in the Earth’s field. Of practical importance is the effect of the electrical conductivity of the ground on a groundwater NMR survey. Finite‐ground conductivity can result in induced currents that can screen the NMR signal. The calculations of NMR signals are based on the transformation of Maxwell’s equations in terms of magnetic Hertz potentials through use of the reciprocity principle. Groundwater NMR is measured with an instrument designed at the Institute of Chemical Kinetics and Combustion, Russian Academy of Science, Novosibirsk. Experiments were conducted in the Altay region of Russia. Both NMR‐signal amplitude and phase, were measured and compared with the calculated results for horizontally stratified media. Borehole logs and vertical‐resistivity profiles were also used for evaluation of results. The conductivity is shown to affect both phase and amplitude of the NMR signal at resistivities of a few to a few tens of ohm‐m depending on the depth of the water‐saturated layers. There is good agreement between calculated and experimental data. It is also established that the measurements of only NMR amplitude and phase are not sufficient for determining groundwater salinity.