VALIDATING NMR DERIVED EFFECTIVE AND TOTAL POROSITIES FOR GROUNDWATER-RESOURCE EVALUATION IN NEAR-SURFACE (<100 M) SEDIMENTS

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Kok Piang Tan1, Ken Lawrie1, Ross S. Brodie1 and Jared Abraham2 Geoscience Australia, Canberra ACT, Australia Kokpiang.Tan@ga.gov.au 1 Geoscience Australia, Canberra ACT, Australia 2. XRI Geophysics, Denver, USA The Broken Hill Managed Aquifer Recharge (BHMAR) project in western New South Wales, Australia, aimed to determine the potential groundwater resource and identify managed aquifer recharge targets. One of the key parameters for confidently deriving groundwater resource volume is the effective porosity of the aquifer. In this study, borehole NMR measurements provide mobile- and total-water contents that are respectively equivalent to effective and total porosity of the sediment. The following laboratory methods and results were used to substantiate these relationships. NMR data were classified into five hydraulic texture classes ranging from clay to (gravely) coarse sand using the sedimentary core, geophysical, mineralogical, and hyperspectral logs. In selected zones of the project-target semi-confined aquifer and its overlying confining aquitard, a total of 26 cored sediments encapsulated in tubing were analysed for water retention capacity at various matric potentials and the total porosity, using liquid nitrogen injection method. The NMR mobile water contents for the fine sand, medium sand and (gravely) coarse sand best reflect the water lost at 10 kPa and 70 kPa matric potentials. In a soil environment, these conditions represent water held in the macropores that readily flows under gravity between saturation and field capacity. There is a wide range of NMR total water within each texture class, and the median values for fine sand, medium sand and (gravely) coarse sand are consistently lower than the porosities determined using the nitrogen injection method. There are consistencies between porosity determined at 200 kPa matric potential and the nitrogen injection method for medium and coarse sand, whereas for fine sand NMR total water is comparable to those at 200 kPa matric potential. The determination that the NMR mobile water is representative of the effective porosity allowed the groundwater resource in the BHMAR study area to be determined.