Australia turns up the heat on heat flow data
A Google Earth image of global heat flow. The Australian Plate is a significant piece of the global puzzle. From: Lucazeau, F. (2019). Analysis and Mapping of an Updated Terrestrial Heat Flow Data Set. Geochemistry, Geophysics, Geosystems, 20(8), 4001-4024. doi:10.1029/2019gc008389
Heat flow data provide us with unique insights about how the Earth moves; from the churning interior to the rise of mountains and the jostling of tectonic plates. To help understand our future on this dynamic planet AuScope is enabling NCRIS to develop breakthrough heat flow research infrastructure so that Australia can lead and connect the world over the next decade.
Heat flow: a global process
Heat flow is the movement of heat energy from Earth’s core to its surface, and is derived mostly from the cooling of the Earth’s core along with radioactive heat generation in the upper crust. Plate tectonics, volcanism, epeirogenic motions, hotspots, basin formation, episodes of mountain building, igneous intrusions, metamorphism — all are consequences of the flow of heat within the Earth at the mantle and lithosphere scale.
From core to crust
Geophysical and petrophysical measurements are used to track the thermal state of the Earth. Heat, as a form of energy, spontaneously flows from regions of higher temperature to lower temperature according to the Second Law of Thermodynamics—that is, from the interior to the exterior of the Earth. The First Law of Thermodynamics ( Conservation of Energy) ensures that measurements made in the shallow sections of the crust can be confidently extrapolated to deeper regions.
As heat permeates outward through the crust, local heat flow patterns are affected by ore bodies, hydrothermal systems, large faults, diapirs, major disconformities, groundwater motion and other phenomena. Long-term changes in surface conditions brought about by erosion, deposition, inundation and changes in surface temperature and land use also affect the rates of heat flow across the surface boundary.
Heat and how to track it
Heat flow cannot be directly measured, but is instead calculated as the product of temperature gradient and thermal conductivity. Primary heat flow data are, therefore, temperatures directly measured in drill holes, and thermal conductivity measured on recovered rock chips or samples of core. Secondary data include the thermal diffusivity, heat capacity, and heat generating potential of rocks (which control how heat flow might vary through time), and other data from which lithospheric temperature or rock properties might be inferred (e.g. xenolith chemistry, seismic wave speed, electrical resistivity, and others.)
Heat flow across Australia
Heat flow data have revealed, for example, a broad band of relatively high heat flow through the central and eastern parts of the Australian continent, which points to mantle-scale processes and/or fundamental differences in the age, thickness or composition of the lithosphere. At a smaller scale, radioactive decay of uranium within the Olympic Dam ore body generates heat at a rate detectable as a surface heat flow anomaly centred over the ore body.
Elsewhere, an absence of elevated heat flow in the Newer Volcanics Province of South Australia and Victoria constrains the range of possible mechanisms responsible for the volcanism; and an observation of increasing heat flow with depth in a West Gippsland drill hole constrained estimates of land surface heating over the past century.
“Heat flow data feed into a broad range of research disciplines but require specialist equipment and expertise to collect.”
Surface heat flow across Australia gridded from about 1000 data points. The striking pattern of high versus low heat flow points to mantle-scale processes and/or fundamental differences in the age, thickness or composition of the lithosphere. Image: Hot Dry Rocks Pty Ltd.
Tracking heat flow matters
Temperature-dependent, heat-generating and heat-transport processes in the Earth cannot be confidently investigated without heat flow data. Heat flow data provide the most direct knowledge of the current temperatures and sources of heat in the Earth. This knowledge, in turn, provides the only measurable boundary condition for reconstructions of past thermal conditions.
Reliable heat flow data, therefore, are essential and irreplaceable for testing the predictions of sophisticated lithospheric modelling packages such as Underworld; for refining models of electrical resistivity and seismic velocity produced by the Earth Imaging and Sounding Program; for constraining basin histories in applications such as PyBasin; for potentially detecting the heat of Iron Oxide Copper Gold (IOCG) deposits from the surface prior to expensive appraisal drilling; for inferring changes in surface temperature over recent centuries; and for investigating any number of other basic and applied geoscience questions.
The challenge
Until now, Australia has been challenged in its ability to generate high quality heat flow maps because of sparse available data. Available data are clustered around sedimentary basins, leaving algorithms to predict values between data points that are sometimes many hundreds of kilometres apart.
The best way to improve the consistency and accuracy of heat flow maps is to increase the number of high quality heat flow determinations, especially in regions currently lacking data. NCRIS, through AuScope, is enabling such work.
A collaborative response
The AuScope Subsurface Observatory offers research infrastructure for determining heat flow, including drill hole temperature sensors and laboratory instruments for measuring thermal rock properties. In an AuScope funded pilot program, the University of Melbourne is currently using AuScope equipment in collaboration with Geoscience Australia, CSIRO Energy, and state geological surveys to collect heat flow data from new locations in parallel with the MinEx CRC National Drilling Initiative. The program has already collected valuable new data from the South Nicholson and East Tennant regions of the Northern Territory in 2020, and will cover the Delamerian region of South Australia in 2021.
Next steps for Australia
AuScope’s Five-Year Investment Plan recognises heat flow as a principal element of its integrated infrastructure system, known as the Downward Looking Telescope. The plan provides for significant investment to modernise and enhance Australia’s digital and physical infrastructure for measuring, visualising, interpreting and delivering heat flow data for the research community.
It is currently and will continue to bring Australia’s heat flow research community together, provide a new digital repository for national heat flow data, and enable new capabilities for regional heat flow mapping, drill hole heat flow determinations, research into aquifer thermal energy storage, seafloor heat flow determinations, and field and laboratory measurements of rock thermal properties.
“Ultimately, this new national capability will enable researchers to address a wide range of research problems by producing high quality heat flow maps on demand using state-of-the-art equipment.”
CSIRO Energy personnel using an NCRIS enabled fibre optic sensor (via AuScope’s Subsurface Observatory) to measure the temperature in a MinEx CRC National Drilling Initiative drill hole in the East Tennant region of the Northern Territory in December 2020. Image: Dr Ludovic Ricard
Linking globally
As the custodian of the Global Heat Flow Database (GHFD), the International Heat Flow Commission (IHFC) published a once-in-a-generation update to the structure of the GHFD in January 2021 and launched a major five-year International Lithosphere Program (ILP) project in May 2021 to recompile decades of legacy data into the new structure.
AuScope joined the ILP Project as a founding partner to ensure that NCRIS-enabled platforms such as the AuScope Virtual Research Environment openly deliver a comprehensive set of quality-assured Australian heat flow data to the world according to the new global standard.
“Australia is, in fact, set to lead the world in the provision of quality heat flow data over the coming decade.”
AuScope is connecting Australia’s heat flow research community with the world through the ILP’s Global Heat Flow Data Assessment Project led by the International Heat Flow Commission (IHFC). Image: Screenshot from http://www.ihfc-iugg.org/products/global-heat-flow-database/assessment-project
AUTHOR
Dr Graeme Beardsmore
FURTHER READING
Earth Imaging and Sounding Program