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Exploration Geophysics - Volume 28, Issue 1-2, 1997

Volume 28, Issue 1-2

Volume 28, Issue 1-2, 1997

  • Articles

      • Regional Surveys in Victoria – Experiences And Lessons

        Authors A. J. Willocks
        https://doi.org/10.1071/EG997307

        The Victorian Government, through the Department of Natural Resources and Environment, is in the final year of a 3-year, $16.5 million program, the Victorian Initiative for Minerals and Petroleum (VIMP) to support the state’s mineral and petroleum exploration industry. Projects being undertaken as part of the initiative include acquisition of airborne magnetic and radiometric data, gravity and regional seismic data, stratigraphic drilling, geophysical interpretation and geological mapping. Major airborne survey areas are in the northwestern part of the state, the eastern highlands and the Otway Basin.

        Since 1992, major changes have taken place in legislation, quality of available data and government customer focus. This encouragement from government has resulted in a significant increase in exploration in Victoria. However, many areas still remain under-explored. For the first time, the explorer now has access to high-quality detailed regional scale datasets to include in a modern exploration program and to help determine mineral exploration targets.

        The VIMP program has led to an increased level of sophistication in survey and product requirements. Major developments in data acquisition and processing have taken place in the airborne survey industry partly as a result of the VIMP surveys. Government and industry require contractors to carry out surveys to obtain high quality data, in a timely and cost-effective manner. In addition, Government requires regionally consistent data and marketable products with a long shelf life.

    • The Use of Constraints in Geophysical Tomographic Reconstruction

      Authors Jeanne Young
      https://doi.org/10.1071/EG997313

      A significant problem with geophysical tomography is that the image reconstruction problem does not usually have a unique solution, given the measured data. A number of tomograms can be consistent with the measured data because of the limited access available for taking measurements. It is necessary to introduce additional information into the reconstruction process in order to find a unique solution.

      A common approach is to choose the solution that optimises some objective function. However, such criteria (eg, minimum norm and maximum entropy) are not necessarily consistent with the geology. Other prior information is usually available which is consistent with the geology, much of which can be expressed in the form of constraints which define sets of images, called property sets. The intersection of the property sets contains feasible solutions. Set theoretic estimation finds solutions in this feasibility set which are consistent with the measured data and all prior information.

      A number of tomography algorithms used in geophysics fit into the category of set theoretic estimation techniques. This paper gives an introduction to some of the concepts that are common to these techniques and compares several reconstruction algorithms, using straight-ray synthetic data.

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