1Geological Survey of New South Wales, Department of Regional NSW, Maitland, NSW, Australia
The Geological Survey of New South Wales (GSNSW) has released an updated suite of magnetic grids and imagery which improve the existing images through the inclusion of high-resolution, open-file company data. The foundation of this product comprises over 60 regional government surveys flown at 200 to 400 m line-spacings. The addition of about 150 private company surveys flown at 50 to 200 m line-spacings increases the resolution of the grids in these areas. The higher-resolution surveys also allow the data to be gridded with a 25 m cell size rather than the previous 50 m, which provides better definition of anomalies.
The project commenced with the quality assurance (QA) of about 650 airborne magnetic surveys flown in New South Wales since the late 1950s. An algorithm was derived to quantitatively assess each survey on the merits of the survey metadata. The algorithm applied a weighted score to various parameters such as line-spacing, flight height, sampling interval, and survey area. A baseline score was calculated for the previous statewide merge based on the regional government surveys, which became the cut-off for the company data. All surveys falling below that score were excluded. A final manual QA of the remaining surveys was performed to ensure only the best data filtered through the algorithm before being included in the new merges.
The boundaries of each survey were placed into a shapefile and where any overlap occurred, the survey with the lower score from the algorithm was clipped using the boundaries of the higher scoring survey. An external buffer of 500 m was added to the survey boundaries, creating a small overlap to allow the software to align features when merging. After clipping, all surveys were re-sampled and re-projected into a unified grid cell size and projection. A base layer of total magnetic intensity (TMI) for the merge was created using the regional government surveys, which were levelled to the Australia-wide 70 km spaced magnetic traverses (AWAGS) to retain the long spatial wavelengths. Holes were clipped into this grid and buffered to the shapes of the company surveys. The final merge with the company data was then performed to create the high resolution statewide TMI grid.
The TMI grid became the foundation to create a series of enhancements. Reduction to the Pole, First and Second Vertical Derivative, and Tilt Angle filters were applied to the TMI data, which were then reprojected into a suite of projections relevant for NSW. The final high-resolution imagery is now available on MinView, the GSNSW web-based data portal. The improvements are immediately visible, with far higher resolution of magnetic anomalies across the state, especially in regions with extensive company surveys. The additional resolution provides precise definition of geophysical signatures and geological structures at all scales and will lead to improved image interpretation for geological mapping and exploration targeting.
Sam Matthews works as a geophysicist within the Geological Survey of NSW. He earnt his PhD from Macquarie University studying the geophysical signatures of sequestered CO2 and is now responsible for maintaining and enhancing all geophysical data within NSW.