SUMMARYHorizon mapping using magnetic data was conducted over a part of the Western Papuan Fold Belt, in an area of rugged terrain, where the geological structures are of relatively low complexity. Energy spectral analysis was used to detect magnetic susceptibility contrasts that were laterally merged to form magnetic interfaces corresponding to horizons derived from seismic and well data.Numerous magnetic interfaces were detected corresponding to: magnetic layers within the Darai Limestone, top of Ieru Formation, intra-Ieru and deeper intra-sedimentary boundaries. These mapped sedimentary surfaces form an anticlinal structure which plunges towards the south-east. A major thrust fault, mapped from magnetic data using automatic curve matching, truncates the anticline in the south-west. Sedimentary magnetic layers were mapped on both sides of this fault. The results obtained from the interpretation of the magnetic data are consistent with structures mapped from seismic and well data.

High resolution magnetic data has successfully been used to map the top and base of coal measures, related faults and fracture patterns across the Red Hill area in the northern Bowen Basin, eastern Australia. A horizon mapping technique, based on energy spectral analysis (ESA), was used to detect magnetic susceptibility contrasts that were laterally merged to form two magnetic interfaces, corresponding to the top of the Fair Hill Formation and base of the Goonyella Middle Seam in the Permian Moranbah coal measures. The depth estimates were made at stations on a regular mesh of 400 m x 400 m and at nine wells. The final detailed mapping of both horizons was constructed from spectral depth estimates on a regular mesh of 100 m x 100 m. Major faults, associated structures and fractures were mapped in 3D from the magnetic data by using automatic curve matching (ACM). This technique was applied to detect single magnetic anomalies produced by inter-sedimentary sources. These results were used to interpret magnetic lineaments in, above and below the coal measures to produce a rock fabric model. The results obtained from the interpretation of the magnetic data are consistent with structures mapped from existing seismic and well data. The mapped rock fabric was also confirmed by well results to be a fair representation of the open fracture set, which has an orientation that is consistent with the existing stress regime.

Horizon mapping using magnetic data was conducted over a part of the Western Papuan Fold Belt, in an area of rugged terrain, where the geological structures are of relatively low complexity. Energy spectral analysis was used to detect magnetic susceptibility contrasts that were laterally merged to form magnetic interfaces corresponding to horizons derived from seismic and well data. Numerous magnetic interfaces were detected corresponding to: magnetic layers within the Darai Limestone, top of Ieru Formation, intra-Ieru and deeper intra-sedimentary boundaries. These mapped sedimentary surfaces form an anticlinal structure which plunges towards the southeast. A major thrust fault, mapped from magnetic data using automatic curve matching, truncates the anticline in the southwest. Sedimentary magnetic layers were mapped on both sides of this fault. The results obtained from the interpretation of the magnetic data are consistent with structures mapped from seismic and well data.

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Energy spectral analysis techniques have been applied to magnetic and gravity data acquired across the Olympic Dam cratonic area in Australia and sedimentary basins along the Equatorial Margin of Brazil. Analysis has been conducted along two Deep Seismic Sounding lines (DSS) acquired by Geoscience Australia. There is a good correlation between interfaces found in this analysis and structures interpreted from the seismic data. Interpretation of gravity data using energy spectral analysis along the DSS survey lines show a number of deep crustal structures are evident, including the Moho which was detected using gravity data, while similar analysis of the magnetic data show indications of the Curie isotherm. In addition, the analysis was extended away from the seismic lines to detect many deep crustal horizons and structures at considerable distances from the DSS lines. The results obtained from energy spectral analysis across this area in Australia encouraged the application of this technique on the Equatorial Margin of Brazil, where the potential field data is of much lower resolution. This suggests that a much wider application of this approach could be highly valuable to investigate the deep structure under other sedimentary basins and to assist heat flow studies.