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9 August 2023 4 min read

Many of the accessible mineral deposits essential for Australia’s decarbonisation have already been discovered – and over the next few decades, finding more deposits of minerals including lithium, cobalt, zinc, copper and more will be crucial.

Seismologist Dr Erdinc Saygin is CSIRO’s Theme Leader for Geoscience Imaging in the Deep Earth Imaging FSP, and his work involves computational and observational research across a broad range of geophysics – a sector that is growing fast as worldwide demand for metals skyrockets during our global transition to renewable energy.

A thick sedimentary blanket of weathered rock (regolith) covers much of the Australian continent – so the next frontier for many vital ore deposits will lie deep beneath the surface.

Erdinc’s work with CSIRO’s Deep Earth Imaging Future Science Platform is developing new tools and methods to image deeper into the subsurface to help us locate these essential minerals.

Erdinc and former CSIRO postdoctoral research fellow Dr Yunfeng Chen (who is now assistant professor at Zhejiang University) have recently published in Nature Communications about the outcomes from the new technique they have developed to image the deep earth.

Their work involved integrating 30 years of data from existing, permanent seismic stations around Australia with passive seismic surveys conducted in different time windows.

“One of the biggest challenges in deep earth exploration is to see what’s hidden beneath – and we’ve used different techniques to do this, in what we now call geophysics, for over two hundred years,” says Erdinc.

“In Australia, the thick blanket of sediments adds an extra challenge.”

The team's innovative approach harnesses more information from existing data sets to produce sharper images, revealing critical information about what lies beneath the surface.

These high-resolution images have the potential to significantly advance our understanding of the subsurface, including possible mineral deposits.

The model will allow ‘depth slices’ at least 44 kilometres deep, showing an important seismic measurement, shear-wave velocity, which Erdinc and Yunfeng’s model have demonstrated can be used to show the location of large mineralisation zones.

A 3D rendering of the shear velocity model. The sedimentary basins are represented by an isosurface of 3.1 km/s. The four interfaces are, from the top to bottom, surface topography, upper-middle crust boundary, middle-lower crust boundary, and the Moho

Why understanding S-Wave speed helps us to map deep layers

The speed at which shear or secondary seismic waves (S-waves) travel through the layers of the Earth, can provide crucial insights into the composition, density, and structure of the Earth's subsurface layers.

These waves move through the Earth by shearing or shifting the rock in a direction perpendicular to the wave's path.

These waves have been used for decades to gather information about the Earth’s crust because their speed is influenced by the matter they pass through; S-waves slow down when passing through more fluid materials and speed up in denser, more solid areas.

Tracking the speed and movement of S-waves allows seismologists to create detailed models of the Earth's internal structures, including fault lines, mineral composition, and chambers containing oil, gas or magma.

And in the deep earth, because S-waves don't travel through liquids, their absence in certain regions helps confirm the existence of the Earth's liquid outer core.

Passive Seismic Exploration key to enhancing our image of the deep earth

Erdinc says that one of the challenges of deep-earth imaging is that the resolution of S-wave seismic velocity maps is too low to give sufficient information to have confidence about the specific make-up of parts of the deep earth.

The team developed a new passive seismic technique where geophones or sensors are placed on the ground to detect vibrations.

“These sensors listen to the rumblings of the earth, very much like a stethoscope for the Earth,” Erdinc says, adding that the sensors pick up vibrations from various sources such as ocean waves, storms, trains, and even nearby human activity.

Sophisticated mathematical techniques and physics-based algorithms are then applied to discern the origins and properties of these vibrations.

The beauty of this technique is that it makes it far easier to gather information, allowing a geophone to be used in a number of locations at different times.

“Data that we collect data at different times, say in 2010, 2015, and 2020, can then be combined to image the earth with unprecedented resolution,” he says.

“This is a big step forward because collecting data simultaneously is logistically challenging and expensive.”

Asking better questions

While the method does not directly predict where mineral deposits might be found, it provides a more comprehensive and detailed picture of the subsurface, allowing explorers to refine their exploration strategies, Erdinc says.

“What we offer is a map that sharpens the questions explorers ask. Instead of drilling or conducting geophysics at random locations, they might use our results to make more informed decisions about where to allocate resources,” he says.

However, the breakthrough didn't come easily.

The team had to overcome technological and computational hurdles, Erdinc explains.

“We used advanced physics methods and mathematical algorithms to do this work, and the task involves terabytes of data collected since the early 1990s.

Erdinc likens the results to a CT scan of the earth.

“We take these measurements and ask what kind of physical structure would be needed to create what we observed, which gives us our final image," he says.

He says that their recent 'heat-maps' of shear-wave seismic velocity in the top five kilometres of the crust shows a distinctive relationship between mineral deposits and higher velocity crust.

“The new shear-wave velocity model and the relationships we see with mineral deposits of many kinds, increases our confidence in this link and contributes to our understanding of the role of whole crustal architecture,” he says.

Erdinc says that their research, which is publicly available, is the result of years of collaboration between a range of institutions working together to advance our knowledge of the deep Earth.

“All of the data collection efforts by Geoscience Australia with AusArray and also GSWA with the WAArray, which is quite an ambitious program, are taking us in the right direction and will further improve our knowledge,” he says.

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