Insights into subsurface properties from sonic core drilling in the Keep River region, East Kimberley, northern Australia

In 2018, as part of the Australian Government-funded Exploring for the Future program, Geoscience Australia drilled fifteen sonic boreholes in the Keep River region in north-western Northern Territory. The boreholes averaged 49 m in depth with 99% core recovery, and were completed as piezometers. Boreholes were geophysically logged (induction, spectral gamma and nuclear magnetic resonance) and sampled for hydrochemistry; the data from these measurements facilitating the calibration and validation of satellite and airborne geophysical measurements. Recovered core was immediately extruded into polythene tubes on site, sealed, and refrigerated. At a nearby field laboratory core was split, transferred to core trays, measured, geologically logged, and duplicate sub-sampled at ~30 cm intervals. Sub-sampled material was stored in sealed vials, and refrigerated to ensure suitability for hydrochemical analysis. One of each sub-sample pairs was hydraulic pressed to acquire pore water samples, which were rapidly analysed for pH and EC. The remaining half core was air dried and scanned using a Hylogger multispectral system. The refrigerated pore water and core samples were kept for further study, including magnetic susceptibility, grainsize, palaeontology, and hydrochemistry. The sonic drilling technique combined with our workflow has many advantages over conventional rotary drilling, including: • High core recovery. • Recovery of uncontaminated sub-samples, including pore fluids, for further study. • Clean bore construction and ease of completing bores as piezometers. The results of the drilling program analyses have enabled discrimination of Ord River palaeovalley sediments from underlying weathered Paleozoic basin sediments, with valley fill up to 50 m thick. Facies analysis of sonic core permitted discrimination of fluvio and fluvio-estuarine sediments. Porewater salinity in the unconfined water table zone varies widely, and increases up to an order of magnitude with depth. The core will also facilitate experiments to determine salt mobility within a range of material grainsizes, which is vital for determining salinity hazard. Presented at the 2019 Australasian Groundwater Conference (AGC 2019)