Total Magnetic Intensity (TMI) Grid of Australia 2019 - seventh edition - 80 m cell size

Full description

Total magnetic intensity (TMI) data measures variations in the intensity of the Earth magnetic field caused by the contrasting content of rock-forming minerals in the Earth crust. Magnetic anomalies can be either positive (field stronger than normal) or negative (field weaker) depending on the susceptibility of the rock.
The 2019 Total magnetic Intensity (TMI) grid of Australia has a grid cell size of ~3 seconds of arc (approximately 80 m). This grid only includes airborne-derived TMI data for onshore and near-offshore continental areas.
Since the sixth edition was released in 2015, data from 234 new surveys have been added to the database, acquired mainly by the State and Territory Geological Surveys. The new grid was derived from a 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), which serves as a baseline to constrain long wavelengths in the final grid. It is estimated that 33 500 000 line-kilometres of survey data were acquired to produce the 2019 grid data, about 2 000 000 line-kilometres more than for the previous edition.

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Maintenance and Update Frequency: notPlanned

Statement: The 2019 magnetic grid of the Australian region is the seventh edition with a cell size of ~3 seconds of arc (approximately 80 m). This grid only includes airborne-derived TMI data for onshore and near-offshore continental areas. Since the sixth edition was released in 2015, data from 234 new surveys have been added to the database, acquired mainly by the State and Territory Geological Surveys. It is estimated that 33 500 000 line-kilometres of survey data 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. 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. 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. 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.