Geophysical investigation to support characterisation of structurally controlled groundwater flow into an open pit mine

Abstract

Efficient dewatering operations depend on reliable predictions of expected inflow and likely water level behaviour. These predictions stem from the conceptual understanding of the hydrogeology of the area, which itself is derived from studies into the aquifer extents, hydraulic parameterisation and connectivity with adjacent aquifer units. Due to their isolated nature, structural controls on these inputs to the conceptual understanding are amongst the most challenging to determine. An investigation was undertaken on structurally controlled flow into an open pit iron ore mine in the Pilbara region of Western Australia. Bore yields, groundwater salinity and water level behaviour of in-pit bores unmistakably indicated flows in excess of normal aquifer throughflow. Airborne magnetic data allowed regional identification of a potential lineament which was confirmed with high density grade control drilling results. These indicated the presence of a trough-like mineralised feature likely to enhance connection to a regional aquifer system down dip of the iron ore body, or with a fractured rock aquifer beneath the ore body. A detailed ground magnetic survey was conducted along strike from the interpreted lineament, drastically improving on the aeromagnetic data, further validating the conceptualisation of the structure and providing greater accuracy with which to target future hydrogeological works around the current pit. Subsequent quantitative magnetic modelling constrained with close spaced drilling information and downhole magnetic susceptibility logging, increased the understanding of the characteristic basement magnetic response, and encourages the use of the magnetic method for local hydrogeological studies.