Delineation of Regolith Zones in Nickel Laterite using Electromagnetic Imaging: Ravensthorpe, Western Australia.

Nickel laterite is a product of intensive weathering processes of ultramafic rocks near the surface. The Ravensthorpe Nickel Operation has traditionally used boreholes spaced at 40 meters to delineate the major regolith zones of caprock, limonite, saprolite, and saprock. The high geometric complexity of the weathering profile leads to an inaccurate resource delineation by drilling alone. Loupe is a new ground system for transient or time-domain electromagnetic measurements suitable for resolving shallow electrical conductivity distribution. At Ravensthorpe, the technique was tested in 2019 to improve the spatial resolution of conductivity models and, consequently, to better distinguish different regolith zones. Appraisal of the Loupe system involved experimentation with different transmitter waveforms, assessment of diffusion depths and noise levels, and 1D inversions with and without constraining geological input. Interpretation of the inverted conductivity suggests that the Loupe data are imaging the caprock thickness and reproducing the thinner but continuous conductive zones defined by downhole logging within certain segments of the regolith profile. Below the caprock, the conductivity correlates with higher clay proportion, as defined in visual logging and reflected in higher Al content.