Peeping into our planet's depths

27 February 2002

Imagine being able to see some 10 kilometres down into the ground. What would you expect to see? Perhaps gaping subterranean caverns, twisting labyrinths, or a sunless sea — as featured in the classic adventure tale, Journey to the Centre of the Earth?

Why would we want to see what's down there in the first place? It's because the sunburnt surface of this land is ancient. Unlike most other countries, soil and debris thickly covers more than 85% of the Australian continent, making it hard to fathom what underlying rocks are really like.

But scientists at Geoscience Australia have this insight at their fingertips. All it takes is the help of the colourful map they've made to see the diverse environments and structures that exist kilometres beneath our very feet.

And what's more, the map was made without leaving a mark on the surface of the land.

Not even a scratch

It simply wouldn't be practical to dig or drill down through every hectare in Australia to see what lies beneath. Physically sampling deep-lying rocks is generally lengthy and expensive.

For instance, most underground mines only reach one or two kilometres below the surface. And it took overseas researchers 19 years to drill down roughly 12 kilometres for the world's deepest hole, the Kola Superdeep Borehole.

Luckily, a relatively quick and cost-effective method came to the rescue: mapping with aeromagnetics. The act of installing magnetic field detectors in small planes caused a quiet revolution.

Aeromagnetics — almost like mowing your lawn

Experts at Geoscience Australia can paint a picture of the magnetism of the Earth's crust.

How? They do this by flying small planes over the land in a series of equally-spaced parallel lines, similar to the to-and-fro pattern when mowing your lawn. The plane carries a magnetometer, which detects the Earth's total magnetic field. The information is captured digitally using an on-board computer.

A spectacular map is then generated by stitching together the results of hundreds of individual flights done at different times, by many generations of aircraft. The stitching is done at Geoscience Australia, and is quite mathematically complex. But it's all worth the effort.

It took about six million kilometres of surveys to cover the Australian landmass. If all of this had been flown continuously in a straight line, the plane would have circled the globe 225 times — a distance equivalent to 12 return trips to the moon.

Mini-magnets in the Earth's crust

So where does all of this magnetism come from? Our planet has a magnetic field which resembles an extremely powerful bar-magnet. Most of this magnetic field is generated deep within the Earth.

But there are small changes, or anomalies, in the Earth's magnetic field. These changes are caused by the rocks in the crust — the thin, brittle skin which envelops our planet and extends to depths of about 15 to 45 kilometres.

It's the tiny magnetic minerals found in rocks which are responsible for anomalies. The most common and most strongly magnetic mineral is called magnetite. Different rocks contain differing amounts of magnetite — and there are processes that can alter or destroy the magnetite a rock is 'born' with.

The magnetic anomaly seen in a region may be caused by several sources at various depths within the crust. It's similar to using a metal detector on the beach — when the detector goes crazy, the signal might be made by more than one diamond ring buried at different levels in the sand (if you're lucky).

'Sunlit' rainbow'

The map shows the magnetic anomalies created by the continental crust of Australia. The colour scheme uses the spectrum of white light, the rainbow, as the scale on the map. The colours towards the red end of the spectrum show high levels of magnetisation — with pale pinks being the highest of all — while the greens, blues and purples show low levels of magnetisation (see lower right-hand corner of the map for the scale).

The additional effect of 'sunlighting' helps to bring out the highs and lows, like looking at a map of hills and valleys. The important areas on the map are the steepest slopes — places where you climb up or down many colours over a short distance.

Some areas show a lot of detail, while others look ironed out. This is because finer detail shows up for rocks which are close to the surface. Smoother areas on the map tell us we are looking at deeper rocks buried by younger sediments, which can be a several kilometres thick. It's similar to seeing a smooth mountain range in the distance — we can't see the bumpiness of the tree tops.

Hidden secrets revealed

The map allows scientists at Geoscience Australia to divide the Australian landmass into eight major chunks, with some containing many smaller fragments. Each chunk of the Australian continent records something of its past evolution — and has a unique story to tell.

Together, the stories reveal how our country evolved from colliding bits and pieces of land.

Topic contact: Last updated: October 4, 2013