This grant supports acquisition of equipment to measure the magnetic susceptibility of prepared laboratory samples and core samples. Specific equipment to be acquired includes a Bartington Magnetic Susceptibility Meter, Bartington Dual Frequency Sensor, Bartington Core Logging Sensor, ASC manual core track, and a digital tape measure. The equipment will complement existing proton nuclear magnetic resonance instruments in the PIs lab at Stanford. The equipment will support investigations of the relationship between NMR measured spin-spin relaxation time constants of Hydrogen (1H) nuclei stimulated using standard well logging NMR tools and those measured with surface NMR (SNMR) field instruments and their relationship with the hydraulic conductivity (K) of subsurface materials in the saturated zone of uncosolidated aquifers. Down hole NMR logging has long been used in the petroleum industry to estimate the permeability of formations and the theoretical basis for the relationship between the relaxation time constant measured and pore throat length scale is well established. In contrast, the relationship between the relaxation time constant measured by SNMR and K is poorly understood but the linkage between the SNMR measured relaxation time constant and that measured down hole is related to the magnetic susceptibility of magnetic minerals in the target horizon. Should the PI be able to develop an algorithm for relating SNMR measurements to K there could be wide spread applications for rapid assessment of groundwater aquifers with implications for water resource management. The equipment would immediately supported PI funded NSF/EAR research that addresses the possibility of using SNMR for estimating K.
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Nuclear magnetic resonance is a powerful technique for sensing water in a porous material and estimating the properties of the material that control the storage and flow of water. Based upon the same principles as MRI (which is used to image inside the human body), there are surface NMR systems used to image inside Earth, seeing to depths of about 100 m below the surface, and logging NMR systems used to make measurements in water wells. The surface NMR measurement is impacted by the magnetic properties of Earth materials. The acquisition of a system to measure the magnetic susceptibility allows us to better understand this effect, so that we can more accurately use surface NMR to describe groundwater aquifers. One of the logging NMR measurements is also impacted by magnetic susceptibility. The strength of the internal magnetic field, in the pore space of the sampled materials, must be known to accurately interpret the acquired data. It is typically assumed that the internal field is equivalent to the externally-applied magnetic field, but we have found that the two can be significantly different. This can lead to inaccuracies in the information derived from the NMR measurement. Through our research we are exploring ways of using measurements of magnetic susceptibility to more accurately estimate the magnitude of the internal field.
