b'John Denhams best of Exploration Geophysics FeatureNew-style mapping Northwest VictoriaThe new high-resolution geophysical data from the VIMPWhere access is good, but the Palaeozoic geology is fully program has fundamentally changed the way geologicalcovered by Cainozoic sediments, mapping both basement mapping is carried out in the GSV. Sufficiently detailed dataand cover depend on geophysical interpretation and (200 m line spacing in eastern Victoria) acquired prior to theprevious drilling. No new surface geological mapping is geological mapping allows a fully integrated interpretation thatcarried out. For example, Moore (1996, 1997) has interpreted uses all the geophysical and geological data to produce mapsthe geophysical data in a geological context, mapping that better reflect geological reality. units from outcrop in the South into areas of cover in the Horsham and Ouyen 1:250 000 maps. Figure 3 shows The mapping process varies according to the geology andMoores 1997 interpretation of the basement features from access conditions, but mapping teams include a geophysicistthe geophysical data. The units have been traced over a as an active contributor during mapping and data synthesis.significant distance under the cover rocks and their structural When interpreting the data, every geophysical response isrelationships determined. This geological looking map is assumed to have either a geological or man-made source.very different from the old style of interpretation. These new Each geophysical unit and structure is assigned geologicalinterpretations rely heavily upon the geological, as well as meaning consistent with the geophysical and geologicalgeophysical, skills of the interpreter and discussions with facts. The interpreted stratigraphy, structure, geologicalgeologists who are comfortable with geophysical data to history and tectonic framework must be geologically andconstruct a plausible geological and structural framework. geophysically plausible. TheGSV tested Moores (1996) interpretation of the Horsham Mapping 1:250 000 map area with a seventeen hole drilling program (Maher et al. 1997; Moore & Maher, 1998) which confirmed its Depending on access and geology, the mapping processesgeological accuracy.used in Victoria fall into one of three categories with two styles of products. In the Northwest, where surface mapping is aHigh resolution data with appropriate interpretations low priority, the interpretation and report identify basementhave attracted explorers to the Murray Basin in Victoria. and surficial features at 1:250 000 scale. In the priority surfaceThis increased exploration activity has led to the discovery mapping areas of central and eastern Victoria, the productsof significant near- surface mineral sand deposits and a are a report, geological maps at I :50 000 scale and a 1: 100 000developing interest in the basement greenstone rocks for base scale geological interpretation of geophysical features map. metals and gold.Figure 3.Simplified geological interpretation of basement geophysical features for the Ouyen 1:250 000 map area, Victoria. The background is a grey scale first vertical derivative of total magnetic intensity.JuNE 2020 PREVIEW 48'