The global shift to a carbon-free energy system is set to drive a huge increase in the demand for rare or limited earth minerals. These are essential for manufacturing a wide range of green technologies including electric vehicle batteries, the magnets inside wind turbines, and copper electricity cables. This presents an urgent need to locate new sustainable sources of these elements.
A potentially rich source of these metals could be underground brines: highly concentrated salt solutions that can accumulate in the Earth’s crust. However, these brines can be challenging to locate as they typically accumulate kilometres under the surface. Accurately pinpointing the position of these deposits is important for reducing the risks and environmental impacts associated with drilling to extract these metal-rich brines.
In this new study, led by the University of Oxford’s Department of Earth Sciences, researchers developed a novel approach which proved capable of mapping both the location and composition of subsurface fluids. For the first time, this combined two different geological measurements: seismic attenuation tomography and seismic anisotropy.
Lead researcher Dr Thomas Hudson (Department of Earth Sciences, University of Oxford) explained: ‘Seismic attenuation measures the loss of energy of a seismic wave as it propagates through a medium. Seismic anisotropy, meanwhile, measures how the speed of seismic waves varies depending on the direction that the wave is travelling. Attenuation tomography images where fluids are (a little like a CT scan in a hospital) and whether the rock is partially saturated (contains gas) or fully saturated (no gas). Seismic anisotropy then tells us how the fluids travel and accumulate along faults.’
The researchers tested the combined approach on Uturuncu: a dormant volcano in the Bolivian Andes which last erupted 250,000 years ago. Dr Hudson said: ‘Uturuncu was an ideal system to test our approach to map liquids and gases because it sits atop the Altiplano-Puna Magma Body, the largest active magma body on Earth. This drives a source of hot, metal-rich fluids that ascend from deep beneath Uturuncu towards the surface. Uturuncu is at the edge of the Atacama desert, so the surrounding crust is particularly dry, enhancing images of any fluid-rich crust.’
The seismic attenuation and anisotropy measurements were taken from a catalogue of data from 1356 earthquakes, which had been captured between April 2010 and October 2012 by a network of 33 seismometers in the vicinity of Uturuncu. Together, the two techniques produced a rough map indicating whether the underlying crust was partly or fully saturated with fluids, at approximately kilometre resolution.
Dr Hudson said: ‘In combination, these techniques produced a high-resolution map identifying what fluids can be found where in this system. In particular, we can identify with precision where brines – concentrated salt solutions- are and whether they contain carbon dioxide (i.e. are “sparkling”) or not (i.e. “still”). If carbon dioxide bubbles through them, then this tells us that the system is still active, and presumably still accumulating metals, whereas if the brines are still, then the system can be treated as stable, i.e. not still actively accumulating minerals.’
‘The results are exciting because these brines are rich in metals that are critical for the green energy transition. Our new method will hopefully provide foundations for derisking the mineral exploration process, which could lead to mining brines becoming commercially viable.’
More widely, the Department of Earth Sciences is approaching the challenge of the green energy transition and achieving net zero through various research projects. These include investigating a number of novel sources of energy in addition to geothermal brines such as hydrogen; strategies for geological sequestration of carbon dioxide; and techniques for recovering critical metals from a wide variety of natural and waste fluids.
‘It is the interdisciplinary nature of the subject of Earth Sciences that puts us in a unique position to find solutions to these Earth resource challenges’ added co-author Professor Jon Blundy (Department of Earth Sciences). ‘By applying and combining our understanding of geological, geochemical, geophysical, and biogeological systems we can find new resources and new ways of recovering them with minimal environmental cost.’
The paper ‘Hydrothermal fluids and where to find them: Using seismic attenuation and anisotropy to map fluids beneath Uturuncu volcano, Bolivia’ has been published in Geophysical Research Letters.
The study also involved researchers from the University of Bristol, Cornell University, USA, and Michigan State University, USA.