This Magnetic Anomaly Map of Australia, Seventh Edition, 2019 TMI RTP Greyscale Image includes variable reduction to the pole (VRTP) of airborne-derived Total Magnetic Intensity (TMI) data for onshore and near off-shore continental areas (Poudjom Djomani et al., 2019). The Magnetic Anomaly Map of Australia, Seventh Edition, 2019 under project number 1324 was produced with a grid cell size of 0.00083 (approximately 80m), given in units of nT.

It is estimated that 33 500 000 line-kilometres of survey data collected by State and Territory geological surveys and Geoscience Australia were acquired to produce the 2019 grid, about 2 000 000 line-kilometres more than for the previous edition. The 2019 magnetic grid was derived from a complete re-levelling of the national magnetic grid database. The survey grids were levelled to each other, and to the Australia Wide Airborne Geophysical Survey (AWAGS) (Milligan et al., 2009), which serves as a baseline to constrain long wavelengths in the final grid. The levelling and grid-merging procedure was described in detail in Minty et al. (2003). The new 2019 map compilation is comprised of a merge of 1059 survey grids. The addition of almost a decade?s worth of new high-quality surveys adds significantly to the 2010 and 2015 versions of the map.

Further processing was applied to the original TMI grid to reduce the magnetic effect of the Portland Smelter Complex (PSC). The anomaly associated with the complex can have erroneous effects on products created using Fourier domain processing of the grid. A subset of the TMI grid covering the area of the PSC was cut for easy processing. Filling of the grid was done in MATLAB using function 'inpaintn' from Garcia (2020), based on Wang et al. (2012). This procedure resulted in an anomaly of +100nT in the region of the PSC. Further, a cosine averaging filter was applied to the PSC and its edges to remove the short wavelength anomalies which may lead to misinterpretation of the data. The processed subset grid was then overlain onto the national TMI grid as a final product.

The data has had a variable reduction to the pole applied to centre the magnetic anomaly over the magnetised body. The VRTP processing followed a differential reduction to pole calculation up to 5th order polynomial. Magnetic inclination and declination were derived from the IGRF-15 geomagnetic reference model using a data representative date and elevation representative of the survey. The image shows the magnetic response of subsurface features with contrasting magnetic susceptibilities and can also be used to locate structural features such as dykes. Details of the specifications of individual airborne surveys can be found in the Fourteenth Edition of the Index of Airborne Geophysical Surveys (Percival, 2014). This Index is also available online at http://pid.geoscience.gov.au/dataset/79134.

References: Garcia, D., 2020. Inpaint over missing data in 1-D, 2-D, 3-D, ... nd arrays.

https://www.mathworks.com/matlabcentral/fileexchange/27994-inpaint-over-missing-data-in-1-d-2-d-3-d-nd-arrays), MATLAB Central File Exchange.

Intrepid Geophysics http://intrepid-geophysics.com,

Milligan, P.R., Minty, B.R.S., Richardson, M. and Franklin, R., 2009. The Australia-wide Airborne Geophysical Survey accurate continental magnetic coverage. Preview, No. 138, p. 1-128.

Minty, B.R.S., Milligan, P.R., Luyendyk, A.P.J. and Mackey, T., 2003. Merging airborne magnetic surveys into continental-scale compilations. Geophysics, 68 (3), 988-995.

Percival, P.J., 2014. Index of airborne geophysical surveys (Fourteenth Edition).

Poudjom Djomani, Y., Minty, B.R.S., Hutchens, M., Lane, R.J.L. 2019, Total Magnetic Intensity (TMI) Grid of Australia 2019 - seventh edition - 80 m cell size. Geoscience Australia, Canberra. http://dx.doi.org/10.26186/5e9cf3f2c0f1d, eCat record number 131505;

Wang, G., Garcia, D., Liu, Y., de Jeu, R. and Dolman, A.J., 2012, A three-dimensional gap filling method for large geophysical datasets: Application to global satellite soil moisture observations: Environmental Modelling Software, 30, 139-142, doi: 10.1016/j.envsoft.2011.10.015.