DOI: 10.4225/25/561c7cfa2f230
title: Total Magnetic Intensity (TMI) Image of Australia with Variable Reduction to Pole (VRTP) greyscale

description: Total Magnetic Intensity (TMI) Anomaly Image of Australia with Variable Reduction to Pole (VRTP)

The file is an image created from  magmap_v6_2015_VRTP  a Variable Reduction to Pole TMI grid of the Australian region with a grid cell spacing of ~3 seconds of arc (approximately 80 m).  This image only includes airborne-derived TMI data for onshore and near-offshore continental areas.

Matching of the grids for the 6th Edition TMI anomaly grid of Australia was achieved using a program called Gridmerge, which was originally developed within Geoscience Australia and has now been commercialised. This program was used to merge 41 new surveys to the 5th Edition Total Magnetic Intensity Anomaly Grid of Australia (Milligan et al., 2010). The 5th Edition merged 795 individual grids to create the compilation and to constrain long wavelengths, an independent data set, the Australia-wide Airborne Geophysical Survey (AWAGS) airborne magnetic data, was used to control the base levels of those survey grids which overlapped the AWAGS data (Milligan et al., 2009). As the 5th Edition was used as a base grid for the Gridmerge operation the new 6th Edition is essentially levelled to AWAGS. 

Details of the specifications of individual airborne surveys can be found in the Fourteenth Edition of the Index of Airborne Geophysical Surveys (Percival, 2014), which is included with the grid as a PDF document called magmap_V6_2015.pdf. This Index is also available online at http://www.ga.gov.au/metadata-gateway/metadata/record/gcat_f3ad4f15-96bc-0cf3-e044-00144fdd4fa6/Index+of+airborne+geophysical+surveys%3A+14th+edition. Further up to date information about individual surveys can also be obtained online from the Airborne Surveys Database at http://www.ga.gov.au/oracle/argus/.

The VRTP processing followed Cooper and Cowan�s (2005) differential reduction to pole up to 5th order polynomial. Magnetic inclination and declination were derived from the IGRF-11 geomagnetic reference model (Finlay et al., 2010) using a data representative date of January 2005 and elevation 300 m. 


Grid downloads:  The VRTP Magnetic Map of Australia grid can be downloaded using the Geophysical Archive Data Delivery System (GADDS) on the Australian Government's Geoscience Portal at http://www.geoscience.gov.au/gadds


File size:  At full resolution, the VRTP Magnetic Map of Australia grid has 41882 rows and 50591 columns and has a file size of approximately 8.3 Gb in ERMapper format.  Note that, because of GADDS file size limits, it is not possible for clients to download very large areas of the grid at full resolution. Clients wishing to do so should contact Geoscience Australia to make special arrangements to have the complete grid dataset provided on a user-supplied portable hard drive.
 


Milligan, P.R., Franklin, R., Minty, B.R.S., Richardson, L.M. and Percival, P.J., 2010. Magnetic Anomaly Map of Australia (Fifth Edition), 1:5 000 000 scale, Geoscience Australia, Canberra.

Percival, P.J., 2014. Index of airborne geophysical surveys (Fourteenth Edition). Geoscience Australia Record 2014/014.