b'John Denhams best of Exploration GeophysicsFeatureCentral Victoria between the geophysical interpretation and the geological map, the geophysical map also shows subsurface geological There is a different challenge mapping where access isfeatures on one map at 1:100 000 scale.usually good but Palaeozoic outcrop poor. The radiometric and magnetic data are first used to identify areas of potentialBy using the geophysical data to assist the mapping process by outcrop for further follow-up in the field. The geologicalfocusing the mapping effort, a time saving of about 40% was interpretation of geophysical data involves mapping unitsachieved on the Benambra 1:100 000 map area compared to the beneath cover, depth estimation and modelling of geologicaladjacent Murrindal sheet. It also provided a more accurate and units, identification of major faults and interpretation of thereliable product.overall structural framework. An example from the Heathcote Greenstone Belt (Fig. 4) uses the magnetic data to extend theMethodologygreenstones to the North as an antiformal thrust stack beneathIrrespective of the terrain and geology the GSV approach Quaternary cover (Edwards et al. 1998). to mapping requires that all of the geophysical features be explained in terms of geology. The key steps in this Eastern Victoria processare:In rugged areas, exposure is usually good, but access is oftenSurvey timing and specificationsextremely difficult. The geophysical images are used to determine where the mapping effort can best be directed byBefore mapping starts, magnetic/radiometric and DTM data are identifying areas of different lithologies or structural features.acquired with the best specifications affordable, preferably at Once identified on the ground, the geological boundaries areleast 200 m line spacing. We have found that 200 m line spacing extended using the units geophysical characteristics. Whileis usually required for mapping at 1:25 000 scale. The data need this means that most boundaries and structures are commonto be collected with as much lead time as possible.Figure 4.Grey scale first vertical derivative of total magnetic intensity and geological interpretation of basement geophysical features of portion of the Heathcote Greenstone Belt in Central Victoria showing an antiformal thrust stack beneath Cainozoic cover.49 PREVIEW JuNE 2020'