I have generally interested myself in the application of geophysical methods to the solution of geological problems. This has led to a number of studies in which my students and I have worked on geophysical data acquisition and interpretation, more recently using gravity, magnetics, and electromagnetics.
I have recently been working on modelling of electromagnetic responses in the time domain (TDEM).
Physical modelling work, with Andrew Boyd, produced a range of example responses for 1:5000 scale models of conductive targets in conductive hosts, for comparison with numerical modelling. This followed work done by and with several students to obtain example sets of type curves for simple downhole systems (with targets in air). Funding was being sought from ARC for further work of this kind in 1997, using different materials for the host, but the work was not funded (in part, 'tis said, because of the age of the applicant...)
Theoretical work, with Dr T.J. Lee, on the prediction of the TDEM response of a polarizable sphere has led to studies of analytical modelling methods for wider classes of models. Recently Lee and Thomas were working on simple analytical modelling of airborne responses, but the approach appears, after much work, to be invalid.
A new problem which seems, surprisingly, not to have been solved is the electromagnetic response of an anisotropic halfspace. It appears that only the case where the horizontal resistivity is different from the vertical resistivity has been tackled; the case where the resistivity is azimuthally dependent as well seems to have been overlooked. Or it is too hard!
Numerical modelling, in an industry-sponsored project, has shown some of the range of repsonses possible with three dimensional targets. Enjie Jing studied some of the methodologies involved in applying numerical-modelling algorithms, and produced some empirical techniques for discretization in EM studies in his Ph D work.
Recent gravity programs have involved mapping of the gravity field in central and western Victoria, contributing to an integrated geological mapping effort in these locations by the Geological Survey of Victoria.
Work by Suzanne Haydon used gravity to support the proposition that Cambrian greenstones in south of Ararat in western Victoria act as controlling structures, by showing the consistency in orientation of gravity features with magnetic and surface-mapped features.
Kevin Tucknott found evidence in Central Victoria for granite intrusions at depth near Heathcote and Costerfield, where both gold and base-metal mineralisation has been reported. The granite intrusions modelled from the gravity data seem to cut through the expected location of the Cambrian greenstones here, which is an interesting conclusion.
A program presently being written up saw the acquisition of gravity data near Casey, in Antarctica, by Suzanne Haydon. The aim was to establish a basic net of gravity stations to study and perhaps constrain uplift in this region, which geological evidence suggests could be occuring at the rate of metres per century.
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A project commenced with Rebecca Allen in 1995 intended to test the utility of the "sandwich model" for filtering near-surface basalt responses.
There are large areas of Victoria (for example, around Creswick) where a layer of basalt, tens of metres thick and generally less than two million years old, covers the surface. The magnetic properties of these rocks leads to significant short-wavelength responses being recorded by airborne magnetics programs. To locate resources in - or even construct models of - the older rocks beneath the basalts is severely inhibited.
The approach taken was to design Wiener filters to separate the contribution of defined layers, taking into account the magnetization distribution. The outcome is still to be tested fully, but forward modelling suggests that the separation is in fact unlikely to be successful, because the surface response dominates at all wavelengths.
In 1998 Trudi Hoogenboom and I looked at the effect of topography on magnetic responses, using data around a granite mountain near Melbourne. The resulting map, stripped of topographic effects, showed more clearly the zonation in the granite pluton.
In the same year Roger Hurren and I looked at the lateral variation in magnetisation in basalts, using La Trobe University's paleomagnetic facilities. We found that the direction of magnetisation varies unpredictably, although not randomly, in a way which does not seem to relate to any of the physical or chemical properties of the rock, while intensity does seem to be more dependent on the rock material locally. This variability is the source of the short-wavelength responses mentioned above, but the lack of an ordering mechanism is likely to continue to inhibit deterministic filter methods.
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