b'2.5D AEM inversionFeatureFigure 17.Kevista: Cross sections along Line 30415 and 30295 of the log conductivity 2.5D (top) and CDI (bottom) 300 m depth slicesalong lines 30415 and 30295 of the 2.5D and CDI 300 m depthby geochemical follow up of a bullseye magnetic anomaly. slices are shown in Figures 17. A consistent linear stretch (logThe first massive sulphide drill hole intersection occurred in conductivity, 1 to 3 mS/m) has been applied to all images. The1974, and mining commenced in 1983, (Schmidt 1989). The pant-leg artefacts at the edges of conductive features on the CDIElura Mine was purchased by CBH Resources Ltd in 2003, and sections and some deeper conductors produce quite coherentrenamed the Endeavor Mine. The mine is currently in Care and conductivity trends in the CDI depth slice that could easily beMaintenance.mistaken for geological structure.The modelling work flow was as follows:Kevitsa takeawayDepth slices generated by 2D gridding of the 2.5D inversion1. Published material from the Proceedings of the Elura log conductivity cross sections are better than verticalSymposium, Sydney 1980 (Emerson 1980), was used to build sections for defining geology and geological structuresa 3D model of the massive sulphide orebody. The papers in (formation boundaries and faults) mapped on a horizontalthe proceedings provide a well described set of mineralised plane. These products also facilitate the integratedrock units and their resistivities on a series of cross sections interpretation of AEM, magnetics, gravity and surfaceand level plans that allowed an accurate model of the upper geology. 500 m of the deposit to be generated.2. The 3D model was built in GeoModeller, which provides In addition, multiple depth slices can be used to build apart of the user interface and visualisation engine for more complete 3D picture of the geology. The 2D griddingMoksha.methodology is faster and easier to control than full 3D3. The model units and assigned properties were exported to gridding, which can be plagued by base level shifts betweena 2D section mesh which was used to generate the finite sections when the inversion nears the depth of investigation.element mesh used in the 2.5D AEM modelling process.The 2D depth slices are easily de-corrugated to remove4. The mesh resolution was optimised for the size and scale thiseffect. of the problem to ensure that an accurate electrical model response could be calculated.This type of enhancement is not achievable for a 1D inversion5. The mesh was of variable resolution and was adapted to when there are strong lateral discontinuities in the geo- accurately define the geometry of the smallest features to be electrical section. For example, pant-leg artefacts, expectedresolved.in geological scenarios such as Kevitsa, could create falseThe main massive sulphide lens at Elura is in the form of a structural features. steeply dipping pipe with X, Y, Z dimensions of 60 x 150 x 500 m. Elura, New South Wales, Australia This pipe lies beneath a layer of conductive regolith 100 m thick. This regolith is what limits AEM detection. Weathering of Forward modelling can be used to test the ability of AEMthe orebody has resulted in the formation of a gossan, which systems to detect ore bodies of interest beneath conductivehas a small surface outcrop (Section 5730N, Figure 18).cover and, further, to inform choices about the best and/ The sections and plans in Figures 18 and 19 show the general or most cost-effective survey system for solving a particulargeometry and mineral zonation of the orebody. The core of the exploration problem. orebody consists of massive pyrite and pyrrhotite that is highly In this example the Moksha software was used to testconductive. The ore units, resistivities and forward model mesh whether the NRG Xcite system (or similar helicopter AEMdimensions are summarised in the forward model mesh and ore systems) could have detected the Elura Cu-Pb-Zn-Ag massivetype property legend, Figure 20.sulphide orebody. The 2.5D forward model has been calculated for both X and Z The Elura orebody was discovered in 1973 by the Electrolyticcomponents using system noise estimated from the recent NRG Zinc Company of Australasia Ltd prior to the availability ofXcite Cobar regional survey flown by the Geological Survey of modern lower frequency AEM systems. The discovery was madeNew South Wales in collaboration with Geoscience Australia. AUGUST 2020 PREVIEW 42'