Curtin researchers are finding ways to combine seismic and electromagnetic data to generate highly detailed pictures of the subsurface. These pictures are then used by mineral explorers searching for new mega-deposits of gold, copper, lead, zinc and other commodities at varying depths below the surface.
For the past century, the search for oil and gas has relied primarily on seismic reflection. More recently, they have deployed electromagnetic methods in deep ocean settings. Seismic reflection involves the creation and measurement of seismic vibrations in the earth as a means to map the earth’s subsurface, while electromagnetic methods, such as magnetotellurics (MT), measure distribution of electrical and magnetic fields in grids of sensors to reconstruct electrical properties in the earth, and in doing so, gain insight to what lies beneath.
Vastly different, each technique has been treated quite independently. However, according to Curtin Associate Professor Brett Harris, there are clever ways to combine the technologies to achieve an improved representation of subsurface geology with important application in hydrocarbon industries, groundwater, hydrothermal and minerals industries. These “cooperative inversion” methods will be needed as Australia explorers transition to searching at much greater depths for new massive tier one mineral deposits.
“Seismic is tremendous in that it’s accurate and it can usually identify boundaries of a subsurface layer or even mineral deposit,” Harris explains.
“The problem with electromagnetic technology is that it gives a low-resolution image of the subsurface. So instead of seeing the presence of ore minerals in an area, you just see this big hairy blob.
“However, when you couple the power of the seismic with the electromagnetism, you can locate a target in a specific subsurface geological horizon.”
Harris is the leader of the Joint Inversion of 3D Seismic and MT Data project run by the Deep Exploration Technologies Cooperative Research Centre (DET CRC). DET CRC was established under the Australian Government to devise more successful, cost effective and safe ways to drill, analyse and target deep mineral deposits.
Harris and his team – comprising fellow Curtin researchers Dr Andrew Pethick, Professor Anton Kepic, Dr Ralf Schaa and Curtin PhD students Van Anh Cuong Le and Duy Thong Kieu – are investigating ways of combining the seismic and MT techniques to produce accurate high resolution geology for deep exploration targeting.
The key is to repeatedly compare the field data with data simulated over a detailed numerical model of the subsurface until an acceptable match is achieved – a process known as inversion. This process is extremely computer intensive and so the team have turned to one of the fastest computers around, the Cray Cascade Magnus supercomputer in the Pawsey Supercomputing Centre in Perth, to run their inversions.
Harris says that implementation of the process is technically onerous requiring specific expertise, which is why his team are also striving to automate cooperative inversion. This will mean that data beamed in from the field can automatically be converted to a subsurface image.
Comparison of MT field and final computed model data, acquired from a survey site in Nevada, USA, across nine inversion strategies. The blue diamond line represents the distribution of the real data. (Reproduced from the team’s research paper.)
“The reason we’re pushing automation is because the process of converting field data to a subsurface image is heavily dependent on the ability of the people processing it,” says Harris.
“Instead, we’re suggesting that once you have set up your instruments, you can send back the data straight away and update your understanding of the subsurface almost in real time.”
To test, Harris and his team conceived multiple cooperative inversion strategies and compared them at a location in the Carlin gold deposit district in Nevada, USA – one of the world’s largest concentrations of gold.
(a) The approximate survey location in the USA, with (b) a 3D migrated seismic image that shows the location of each MT station (the blue points), the specific location of Station 320 (used to collect the comparative MT data above) and two significant boreholes. (Reproduced from the team’s research paper.)
After receiving their results, the team compared outcomes for drill-hole data and concluded that their strategies provided dramatically improved detail and resolution when compared with standard methods.
Harris notes this is infinitely valuable for deep mineral exploration.
“It’s not something people outside the resources industry understand, but the vast majority of exploration doesn’t end up in an economic deposit,” notes Harris.
“These strategies are impactful because we can now produce a high-resolution image of the subsurface that we didn’t have before.”
Harris and his team are now working towards releasing the findings from the next stage in their research, where they are classifying seismic reflection data based on the macroscopic texture – the joints, faults and metamorphic processes – of the subsurface rock to help develop even clearer images of the subsurface.
If you would like to know more about cooperative inversion of seismic and MT data, read the team’s research paper.