Nuclear Magnetic Resonance: Applications for Groundwater Exploration in Queensland

Abstract

Nuclear Magnetic Resonance (NMR), also known as Proton Magnetic Resonance, is observed when the magnetic dipole moments of single protons resonate in a magnetic field with a frequency proportional to the strength of that field. Typically, an artificial magnetic field is used to align the proton moments in a direction different from the ambient field and terminated abruptly, after which the resonant frequency can be observed with a decaying amplitude. This effect is exploited in proton precession magnetometers, as well as in medical imaging. The magnetic field of a large electric current in a wire loop on the surface of the earth will excite protons in the subsurface (usually in the form of water) in the same way. The relaxation of the protons can be observed and used to determine water content as a function of depth. The aquifer at WMC?s Phosphate Hill Mine in northwest Queensland is associated with a siliceous facies of both the Monastery Creek Phosphate and Lower Siltstone Members of the Beetle Creek Formation with a maximum thickness of 60-80m, and is confined by the overlying Inca Shale. The groundwater is contained in fine joints, bedding partings and interstices of sandy beds, and the water table depth is about 30m below the surface throughout the area. In this article we examine the results of NMR measurements using 100m loops to delineate the aquifer and determine its hydrogeological parameters. The test indicated an excellent correlation between the results of one dimensional layer inversions of NMR results data and the known aquifer geometry. The inversion utilised the amplitude E0 of the proton relaxation field after turn-off of the excitation (directly indicative of the water content), the decay time constant T2* of the relaxation field (related to pore size), and phase shift j0 of the relaxation field with respect to the excitation current in the loop (related to resistivity). The top of the aquifer and the water content were both defined within known parameters from drilling, while the base of the aquifer was inferred to be consistent with current estimates of its position in almost 50% of the locations.