Surface NMR measurement of proton relaxation times in medium to coarse-grained sand aquifer

Abstract

A surface NMR investigation of groundwater in the geomagnetic field is under study. To detect the surface NMR a wire loop with a diameter of about 100 m, being an antenna for both an exciting field source and the NMR signal receiver, is laid out on the ground. A sinusoidal current pulse with a rectangular envelope is passed through the loop to excite the NMR signal. The carrier frequency of the oscillating current in this pulse is equal to the Larmor frequency of protons in the Earth's magnetic field. The current amplitude is changed up to 200 amps and the pulse duration is fixed and is equal to 40 ms. The exciting pulse is followed by an induction emf signal caused by the Larmor nuclear precession in geomagnetic field. The relaxation times T 1, T 2, and T 2 ∗ were measured by the surface NMR for both groundwater in medium to coarse-grained sand at borehole and for bulk water under the ice surface of frozen lake. To determine T 1, a longitudinal interference in experiments with repeated pulses was measured. A sequence with equal period between equal excitation pulses was used. The relaxation times T 1, T 2, measured for bulk water under the ice of the Ob reservoir were 1.0 s and 0.7 s, respectively. To estimate an influence of dissolved oxygen T 1 of the same water at the same temperature was measured by lab NMR with and without pumping of oxygen. The relaxation time T 1 measured for water in the medium to coarse-grained sand is 0.65 s. The relaxation time T 2 estimated by spin echo sequence is found to be equal to 0.15 s. The relaxation time T 2 ∗ is found to be about 80 ms. This result contradicts published earlier phenomenological correlation between relaxation time T 2 ∗ and grain size of water-bearing rock. This could be as a result of unsound approach based on grain size or influence of paramagnetic impurities.