Key points
- Shaped like the Australian coastline, the Australia Magnetic Anomaly is a concealed magnetic geological feature beneath the Northern Territory.
- High resolution aeromagnetic survey data can reveal hidden geological structures beneath the surface.
- Open-access geoscience data continues to drive mineral exploration and discovery across Australia's underexplored terrains.
Deep beneath the sweeping landscapes of the Northern Territory lies one of Australia’s most remarkable geophysical features: the Australia Magnetic Anomaly. From above, its outline uncannily echoes the shape of the Australian coastline. But its appeal goes far beyond this visual coincidence.
For geoscientists, it represents a powerful example of how magnetic data can illuminate the unseen world beneath our feet, revealing structures, stories and geological histories completely hidden from the surface.
Now, using high-resolution aeromagnetic data and sophisticated modelling techniques, a CSIRO research team led by Dr Clive Foss has peeled back the layers of this vast region. Their work reconstructs the architecture of buried rocks and exposes geological features that traditional surface mapping could never detect. And all without digging!
Magnetics offer a window into buried landscapes
Australia has long relied on aeromagnetic surveying as one of its most valuable exploration tools. Aircraft fitted with magnometers (instruments that measure magnetic fields) sweep back and forth across the landscape, measuring tiny changes in Earth’s magnetic field. Different rocks create different magnetic signals, allowing scientists to ‘see’ what lies below ground.
Magnetic signals detected from rock reveal clues about their formation and geological history. Induced magnetisation refers to the magnetic behaviour of buried rocks induced by the magnetic field measured from aeromagnetic data. This helps scientists estimate how strongly they are magnetised and how they are arranged below the surface. Remanent magnetisation is the permanent ‘magnetic memory’ locked into rocks when they formed, recording the direction of Earth’s magnetic field at that time which can differ markedly from today’s field.
By measuring subtle variations in Earth’s magnetic field caused by magnetic minerals, particularly in basalts and altered volcanic rocks, scientists can detect faults, folds, basin structures and the pathways through which minerals may have once travelled.
In regions like the Northern Territory, where sediment and weathering often conceal the true nature of the subsurface, magnetic data becomes a powerful method for seeing through the earth.
The Australia Magnetic Anomaly was imaged using regional aeromagnetic data collected during the Northern Territory Government’s Bonney Well Survey, flown at 400 metre line spacing. This open-access dataset, curated by Geoscience Australia in a national geoscience database, provided the foundation for CSIRO’s advanced modelling of the phenomenon.
Sharper data reveals clearer geology
Earlier maps of the region were hampered by artefacts; visual distortions that blurred important details, particularly where magnetic structure aligned closely to flightlines.
“My colleague, Dr Aaron Davis, created an innovative gridding algorithm which refined the dataset and produced cleaner, more consistent images,” said Dr Foss.
With this improved clarity, the team was able to identify features long hidden from view: distinct geological boundaries, subtle magnetic layers and magnetic signatures revealing the shapes and orientations of buried structures.
“Magnetic data allows us to see through the ground and understand geological architecture that would otherwise remain completely hidden,” Dr Foss explained.
“By improving how we process and model these datasets, we can extract more geological information than ever before.”
Applying advanced analytical techniques to model the area, the team produced a map that could identify the tops of magnetic bodies with unprecedented precision.
Together, these insights provide a much clearer understanding of the region’s structural evolution, hinting at the pathways that may have influenced mineral systems over geological time.
What lies beneath an ancient sea
Although most of the Australia Anomaly is buried, parts of its western margin host rare windows into the past through outcrops of the Paleoproterozoic Hatches Creek Formation.
These rocks tell a story of sandstones forming ridges and elevated terrain, interspersed with volcanic layers rich in magnetic minerals. The sandstones were laid down in shallow seas and river deltas, while the volcanic material originated from eruptions through fissures and vents over 1.5 billion years ago.
Over time, these formations were folded, refolded and reshaped, leaving a deep and complex geological tapestry that the new magnetic models have begun to unravel.
Interpreting magnetic anomalies - those distinctive disruptions in Earth’s magnetic field - is both an art and a science which Dr Foss and his team are mastering.
“Estimating the depth of the anomaly’s source and deciphering the direction of magnetisation can be complicated by remanent signatures preserved since the rocks formed,’” said Dr Foss.
“Australia’s shifting tectonic position, combined with periodic reversals of Earth’s magnetic field, means that remanent magnetisation often points in unexpected directions, demanding expert interpretation.”
Unlocking Australian resources through open data
Australian geoscientists have a wealth of available information available through open access datasets from State and Territory Geological Surveys.
Airborne magnetic surveys, flown at low altitudes and structured in dense grids, now cover much of the continent. Once expensive and difficult to obtain, these datasets have been freely available since the 1990s, enabling researchers, companies and students around the world to work with high-quality geophysical information.
The value of this open data is unmistakable. Companies discover new mineral deposits, governments benefit from increased exploration and researchers innovate by developing new analytical techniques. Other nations are now adopting similar open‑data policies, following Australia’s lead.
Dr Foss is an expert author of Exploration Magnetics: Theory and Practice which provides a comprehensive review of the application of magnetisation and magnetic field studies to mineral exploration. It is available as a free eBook or hard copy from CSIRO Publishing.