b'John Denhams best of Exploration Geophysics FeatureRaised awareness of the application of these datasets forFigure 1 shows a part of an interpretation by Sands, in geological mapping and exploration has led to an increasedVandenBerg et al. (1995), of magnetic and radiometric demand for maps, images and interpretations. Advances indata for the GSV project in the Mt. Wellington area. The computer technology and image processing have meantinterpretation is in the old style with the geophysicist that the geophysical data are more readily presentableand geologists having worked in partial or complete and hence more accessible. The need in the 1990s for dataisolation. The result bore little resemblance to the geology integration has seen the advent of many different mapand was almost immediately discarded by the geologists combinations (such as potassium over digital terrain model).as unhelpful. The mapping proceeded with very little These are important first steps in visualising the data andreconciliation of the geophysical data or use of the getting geologists and geophysicists to work together,interpretation. As a result, many features in the geophysical but they stop short of fully integrating the geological anddata were not explained in the geological map. For instance, geophysical data. in one area, because of time constraints and access, the geologists used a single road traverse and previous This paper compares the old-style of geological mapping andmapping compilations to map a major greenstone unit. A far geophysical interpretation with the new approach developedmore reasonable outcrop pattern for this unit would have by the Geological Survey of Victoria (GSV). This new approachresulted by using the geophysical interpretation of this well aims to integrate the geological mapping with interpretationdefined magnetic unit. Neither geologists nor geophysicist of the geophysical data to produce a single compositeappreciated the significance of many of the responses understanding of the rocks and their relationships. The result isbecause of poor communication and the pre-conceived idea better geological maps in shorter times. that the geophysics was irrelevant to geological mapping. These published maps contain avoidable errors.Old-style interpretations and mapping The Bendoc and Murrindall 1:100 000 map areas in eastern Victoria were mapped before any detailed geophysical data Previously geological mappers have rarely regarded geophysicswere available. Parts of the area are in rugged country with as a fundamental tool, in stark contrast to, for instance, theirpoor access and can only be mapped by long foot traverses use of air photos. The reasons for this include poor quality,along creeks or ridges. Figure 2 shows a simplified geology unavailability or poor presentation of the geophysical data,of part of the Bendoc 1: 100 000 sheet and how the map was insufficient time for geologists to fully utilise the data in a busymodified using the new magnetic data. The Goonmirk Rocks mapping program, the lack of understanding of the strengthsGranodiorite has a pronounced tail, which the field mapping and weaknesses of the geophysical methods, and the commonmissed. The trends and continuity of faults can be clearly seen problem of poor correlation between the petrophysicalin the magnetic data but could not be determined accurately properties mapped in the geophysical data and the essentiallyby ground mapping. using geophysical data, if it had been visual properties measured by the geologist. available, would have significantly reduced the time spent The single biggest hurdle to having geologists consideron this mapping project and would have resulted in a more geophysics an essential mapping tool, however, has been pooraccurate map.data resolution. Regional datasets with 1500 m line spacedIsles, Valenta, and Cooke (1995) and Nash (1995) promoted airborne magnetic and radiometric data and 11 km spacedthe use of high resolution regional data sets to recognise gravity data, provide inadequate resolution of geologicalgeometries to enable the merging of structural geology features mapped at 1:25 000 scale for publication at 1:100 000and geophysical interpretation. This style of interpretation or 1:50 000 scales. Indeed, mapping at these scales, 400 m lineis now being commonly, although not universally, spacing is often inadequate and requires at least 200 m andaccepted as the preferred style. We have refined their sometimes 50 m line spacing, depending on the complexitytechniques and applied them to the production of regional of the rocks. The high resolution of the VIMP data provides thegeological maps.level of correlation between the geophysical and geological datasets necessary to convince the mappers of the usefulnessRealising that the geophysical data was necessary for of the geophysical data. The data need to be available beforemapping came to the geologists gradually. It started in commencing mapping and presented at appropriate scales to1994, when geologists mapping the Beaufort and Ballarat be used in the mapping programs. 1:100 000 map areas in central Victoria found that a Even when the data resolution was adequate, the previousCainozoic capping unit could be mapped using its distinctive interpretation style usually identified magnetic units, trendsradiometric signature (Cayley and McDonald 1995; Taylor and lineaments, gravity highs and lows and zones of higher oret al. 1996). This reduced the time required to complete lower radiometric response. This style of interpretation doesthe mapping by six months. The 400 m line spaced data not readily translate to the mapped geological units or help tofrom AGSO was crucial to this success. VandenBerg (1997) make geological maps. It only partially explains the geophysicalreported that while mapping in east Gippsland, the magnetic features and fails to explain the geophysical responses inand radiometric responses of the Snowy River Volcanics meaningful geological terms. As a result, the interpretationswere used to provide links between different outcrops of were often only given cursory consideration by the geologists,individual flows along strike. Again Cainozoic sediments and if the interpretation did not agree with the geologicalwere mapped using radiometric data with control obtained mapping model the geophysics was ignored. This style ofat easily accessible sites. By using the geophysical data to geophysical interpretation is still often presented in companysolve specific geological problems, the GSV took the first exploration licence reports and is typical of that previously usedtentative steps towards fully integrating the geophysical and for regional mapping. geological data in a mapping program.JuNE 2020 PREVIEW 46'