b'Conferences and eventsNewsFigure 9.Garnet gneiss probability volume from 3D seismic data over the Tropicana deposit. Garnet gneiss distribution from drilling in black. Gold mineralisation from drilling in pink.Figure 8.MLEM 200m loop B field data (1Hz, 17ms) over the Nova-BollingerLaterite resource definition through geophysics, case deposit. studies from Ravensthorpe, WAThe use of geophysics in the post-discovery of the NovaChris Wijns (First Quantum Minerals)deposit The Ravensthorpe nickel laterite deposits in Western Australia are defined by different regolith layers that must be mined and Andrew Fitzpatrick (IGO Limited) processed separately. A surface caprock layer is waste material IGO Limited acquired the Nova-Bollinger deposit post- (and used as road base), beneath which are limonite and discovery in 2015, and subsequently began acquiringsaprolite layers that are sent to separate processing streams. As tenements across the Albany-Fraser Orogeny (AFO) forthe regolith grades into saprock below this, the mineral resource magmatic nickel sulphide exploration. This paper discussesfalls below an economic level. Within the limonite and saprolite, IGOs use of geophysical techniques from brownfield tofurther variations in material properties affect processing results, belt-wide scale greenfield exploration across the AFO.including during the beneficiation, or physical upgrading Such techniques include 3D reflection seismic across thestage. The boundaries between these layers are geometrically Nova mining lease, to belt-wide airborne and groundcomplicated, such that very close-spaced drilling would be EM, gravity and magnetic surveys. The rationale of using therequired to define them to an accuracy relevant to mining. In an different techniques is discussed along with their merits andeffort to mitigate wider drillhole spacing, different geophysical limitations in their use for exploring for such nickel systems.techniques have been trialled in attempts to trace boundaries between drill holes. Any technique must be rapid, with high spatial resolution, and preferably continuous, in order to cover Building 3D rock models from seismic at the Tropicanathe many kilometres of resource drilling and eventually be gold mine deployed in a mine pit during operation. Petrophysical logging serves as both a check on the surface geophysics and a potential Kevin Jarvis (HiSeis), Ockert Terblanche and Stephen Brownavenue for predicting material behaviour where multi-element (AngloGold Ashanti) geochemical assays alone have failed. Conductivity and gamma Seismic imaging technology involves the propagation oflogging, for example, characterise discrete zones that are not sound waves into the earth with the objective to imagerepresented in the geochemistry or visual logging. Active and changes in the subsurface. The sound waves are sensitive topassive seismic, electromagnetics, and ground-penetrating changes in rock impedance which is the product of velocityradar show variable success in mapping different interfaces.and density. The final images represent the boundariesGeophysics is a space filler between drill holes, between different rocks with the amplitude related to theChris Wijns.contrast in the impedance above and below the boundary. There is often both random and coherent noise in the data that further complicates how the seismic image relates to the subsurface. Understanding the seismic and what it reveals about the subsurface can be challenging. The conversion of the seismic into models of rock makes the results accessible to a larger audience. The key to making the conversion is to understand which rocks can be separated and to apply technology that can exploit all available data to build rock models. The technology chosen is geostatistical inversion, which is essentially geostatistical modelling with the addition of seismic data as an additional constraint. The geostatistical inversion exploits the detection limit of the seismic to build detailed rock models. Data from the Tropicana gold mine were selected for testing because it consists of a large set of deep drillhole data, several sonically logged drillholes, andFigure 10.Detailed constrained gravity model and regolith horizons at the good quality 3D seismic reflection data. Ravensthorpe mine.FEBRUARY 2022 PREVIEW 24'