Total Magnetic Intensity (TMI) Grid of Australia with Variable Reduction to Pole (VRTP) - sixth edition

Brief 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.
A variable reduction to Pole is aimed at locating magnetic anomalies exactly above their source bodies and without any distortion.
The 2015 Total magnetic Intensity (TMI) grid of Australia with variable reduction to pole (VRTP) 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. The VRTP processing followed a differential reduction to pole calculation up to 5th order polynomial. Magnetic inclination and declination were derived from the IGRF-11 geomagnetic reference model using a data representative date of January 2005 and elevation 300 m.

Lineage

The variable reduction to Pole (VRTP) grid of Australia is processed from the 2015 TMI grid of the Australian region. The TMI grid is the sixth edition magnetic grid 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 fifth edition was released in 2010 data from 41 new surveys have been added to the database, acquired mainly by the State and Territory Geological Surveys. It is estimated that 31,500,000 line-kilometres of survey data were acquired to produce the grid data, 4,500,000 line-kilometres more than for the previous edition.
Matching of the grids in the database 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. 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.
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.
The original grid was converted from ERMapper (.ers) format to netCDF4_classic format using GDAL1.11.1. The main purpose of this conversion is to enable access to the data by relevant open source tools and software. The netCDF grid was created on 2016-03-29.
References
Cooper, G.R.J., Cowan, D.R., 2005. Differential reduction to the pole, Computers & Geosciences, Vol. 31, Issue 8, pp. 989-999.
Finlay, C.C., Maus, S., Beggan, C.D., Bondar, T. N., Chambodut, A., Chernova, T.A., Chulliat, A., Golovkov, V.P., Hamilton, B., Hamoudi, M., Holme, R., Hulot, G., Kuang, W., Langlais, B., Lesur, V., Lowes, F.J. Lühr, H., Macmillan, S., Mandea, M., McLean, S., Manoj, C., Menvielle, M., Michaelis, I., Olsen, N., Rauberg, J., Rother,M., Sabaka, T.J., Tangborn, A., Tøffner-Clausen, L., Thébault, E., Thomson, A.W.P., Wardinski, I., Wei1, Z. and Zvereva, T.I., 2010. International Geomagnetic Reference Field: the eleventh generation, Geophysical Journal International, vol. 183, pp. 1216-1230.
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.
20170906 NetCDF file restructured to be indexed Southward-positive for improved performance and interoperability