b'2.5D AEM inversionFeatureFigure 9.Quamby/Dugald River 3D perspective view of AEM 2.5D inversion results.The inductive-only inversion in the left side panel does notenhancements accurately when flight lines are spaced close fit the negative late time IP pull down, (evident in Channelsenough to adequately map across line continuity.31-45 in Figure 8), but does in the joint inversion on the right (as denoted by the red arrows). A chargeability section is alsoThe geology of the Kevitsa mine area, as it was known in generated from the joint inversion as shown in the panel below2009 (FQML 2009), is shown in Figure 10. The extent of the the noise map. VTEM survey is shown over a 2020 Google Earth image in Figure 11. The survey was flown in 2009 prior to mine To summarise, a 3D perspective view of the 2.5D inversionconstruction.results for the full set of eight lines from this area is shown in Figure 9. A 50 m conductivity depth slice generated from 2.5D inversion of the VTEM survey lines flown from East to West is shown in Quamby/Dugald River takeaway Figure 12. The correlation of the depth slice with the geological The distinct anomalies apparent in the 2.5D sections providemap is very good, and the inversion clearly maps the phyllitic clear drilling targets for rapid strategic decision making.rocks within the mafic intrusive complex. The correlation is Whilst the VTEMTMPlus survey was conducted at a 2 km lineemphasised by overlaying the geological structure boundaries spacing, conductivity imaging by the 2.5D inversion softwareon the inversion depth slice, see Figure 13. There is also a demonstrates an ability to correctly identify the geometry (dips)significant correlation between the airborne magnetics reduced of structurally complex exploration targets. to pole (RTP) Total Magnetic Intensity (TMI) and the geological structure, see Figures 14 and 15. The 2.5D inversion depth slice Kevitsa, Finland more clearly defines the geological structure in this instance, with the TMI adding some detail in the more magnetics units as The Kevitsa VTEM survey data was provided to Intrepid by Firstmight be expected. The correlation with the geological map is Quantum Minerals Ltd. (FQML) prior to the Kevitsa mine beingnot surprising since there is little surface exposure in this area, sold to Boliden in June 2016. and the map had been interpreted from the existing geophysics This example illustrates the ability of the 2.5D inversion processand some drilling.to generate 2D depth slices or 2D level plans at constantThis type of enhancement is not achievable for a 1D inversion elevation to highlight geological structure. These productswhen there are strong lateral discontinuities in the geo-electrical enable direct comparison of conductivity with 2D maps ofsection. For example, pant-leg artefacts, expected in geological other geophysical data such as magnetics and gravity, as well asscenarios such as Kevitsa, could create false structural features maps of the surface geology. The inversion resolves resistivitywhich would mar the interpretability of the depth slice. 2.5D and and conductivity contrasts very well, and can produce theseCDI 300 m depth slices are shown in Figures 16. Cross sections 37 PREVIEW AUGUST 2020'