b'John Denhams best of Exploration Geophysics FeatureIn western and central Victoria where access and terrainfrom the geophysical data, such as granite boundaries, are used allow, semi-regional (1.5 km station spacing) gravity surveysto show unit extents beyond the outcrop contacts.are carried out to provide a further dataset to help map the structures and granites, particularly subsurface bodies. The geological interpretation of geophysical features map uses boundaries from the mapping and the geophysical data to Preliminary interpretation interpret surface and subsurface geophysical features. These interpretation maps have the geological interpretation draped The project team discuss the geophysical data in the context ofover a magnetic image surrounded by other enhancements of the known geology and to identify geological problems to bethe data, modelled and schematic cross sections, a stratigraphic resolved. A preliminary interpretation is made using the existinglegend with description of responses and other reference data.geology and any rock property measurements and drilling asThe maps are complex and present many challenges in their a guide. This process identifies potential rock boundaries andpreparation, especially in the conversions between digital structures. Maps showing the areas requiring field checking,and hard copy versions. Currently, interpretation line work presented over a topographic overlay, are taken into the field byand mapping line work are presented on separate stable each of the team members. topographic bases for digitising. This process has problems, Before mapping starts, mapping scale images (1 :25 000) ofincluding unavailability of digital line work during the TMI, 1 VD, ternary radiometric and DTM are prepared. Theinterpretation phase, labour intensive transfer of line work maps show the same road and stream base as used during thefrom images to stable bases, ensuring geophysical and geological mapping. geological boundaries are coincident, and avoiding duplication of digitising. Digitising the preliminary interpretations for Field mapping use during the mapping process and use of on-screen image During this phase the geophysicist is in the field with theinterpretation and digitising process have been tried with geologists. Their role is to define the geophysical responselimited success. The mapped geological boundaries are not of the rock units and follow up unexplained or unmappedavailable to the geophysicist in digital form during any part of responses. They work with the geologists on areas ofthe interpretation process. Clearly the synthesising and hard geophysical interest, making sure that the team identifiescopy generating part of the process needs to be improved. We the responses of each rock unit in the area. The mappingneed to be able to interpret onto images at mapping scale in scale images are used in the field to match responses witha digital environment, with all relevant data available digitally, rock type. Field mappers are aware of the geophysicaland have the resulting line work compatible with the drafting properties and interpreted geology while doing theirsoftware. This is one of the borders still to be crossed.ground traverses. Magnetic susceptibility measurements areThe drafting process has become more complicated as more routinely recorded against GPS location and rock type, andcomplex maps are produced. Each interpretation map has a compiled into a project database. This awareness encouragesgeological interpretation as a transparent overlay generally thorough ground checking of areas with properties differenton a TMI derivative image. The map surrounds have images of from background. The geophysicist is on site to discuss thegravity, radiometrics, and digital terrain model often with either geophysical responses and influence geological decisionssurface geology or interpretation overlays.as they happen. Traverses are planned into areas where the geophysics indicates contacts, structures and unusual rockTo ensure consistently high quality interpretations and types. Significant time savings can be made by using thepresentation, these maps are subject to the same technical geophysics to focus the mapping effort. and editorial peer reviews as the geological maps. To assist this process we have documented the key elements of the Agreed interpretation interpretation maps including line type hierarchy, symbols and stratigraphic legend components (Slater 1997). This is an Where the preliminary interpretation and outcrop match, theevolving process, with each new map presenting a new range of interpreted boundary is used to map boundaries betweenpresentation issues.accessible control points. Discussion and geophysical modelling may be required to generate new ideas where questionsThe map reports, or explanatory notes, now include integrated have been raised. Through discussion, both the geologicaldescriptions of the various rock units. The geophysical map and geophysical interpretation converge to agreedproperties and features are described along with the more solutions that incorporate the best of each approach. The leveltraditional properties such as lithology, distribution, thickness, of interpretation attempted may raise more questions thanthin section descriptions and age. The reports are written by answers but also provides more accuracy, understanding andup to five authors and integrate the various contributions confidence in the resultant maps. into relevant sections. Not having separate chapters for the geophysics makes editing more complex.PresentationPresenting the results of this synthesis provides more challengesDiscussionto the mappers, interpreters, managers and cartographers. The products are presented as geological maps and geologicalThe need for geologists and geophysicists to work together has interpretation of geophysical features maps, with the processbeen recognised for a long time, often without success. Within aiming to ensure consistency between the two. the Geological Survey of Victoria significant progress has been made towards making it happen. Intelligent use of the detailed The geological maps show boundaries and structuresmagnetic and radiometric data and their interpretation has interpreted from both the geological mapping, geophysicalimproved the mapping process by reducing the time involved and data and aerial photographs. Subsurface features interpretedsignificantly increasing the quality and reliability of the geological JuNE 2020 PREVIEW 50'