Sounding out a town’s gold secrets

The search for gold has inspired people for generations and has resulted in numerous expeditions to far-flung areas of the world.

It has never been an easy task, and over time it has become even harder as the easier deposits of the elusive treasure have been exhausted. This has created a need for more advanced exploration methods.

One such advancement has recently been used in Kalgoorlie in Western Australia under the instruction of David Reed, who holds numerous tenements under the city streets alongside Kalgoorlie’s famous Super Pit, which has generated millions of ounces of gold.

The technique employed in David Reed’s most recent exploration is called seismic reflection, using sound energy to create a map of the underground.

This is done by concentrating a controlled source of sound energy into the ground. Microphones on the surface pick up the resulting echoes as the sound bounces off different structures underground.

For HiSeis, the company behind the exploration technique in Kalgoorlie, the source of sound is created by a large sophisticated vehicle called a VibroSeis truck.

With some minerals, seismic reflection is able to work as a direct detector and identify exactly what is underneath the surface. For others, such as gold, it acts more like a proxy to give an indication of where to drill.

“It’s all about improving your drill success rate and it only needs to improve by a few per cent to make the net present value (NPV) on the project tens, if not hundreds of millions of dollars more attractive,” HiSeis Chief Executive Officer Joe Dwyer said.

There are numerous advantages to using seismic technology to explore underground. One of the main ones is its ability to search at depth without losing resolution. Mr Dwyer said the technology was capable of providing an imaging up to 60km in depth and had been used by GeoScience Australia to understand the deep crustal architecture of Australia.

“They have done 1000km lines through the middle of Australia as seismic is very modular in that everything is very mobile, you can keep moving and make the line longer or shorter, depending on the situation,” Mr Dwyer said.

In the recent situation in Kalgoorlie, a line of eight to 10kms was used due to the restrictions surrounding available tenements, the road layouts and the geology of interest.

Another key bonus with using seismic reflection is the amount of data it can yield. Mr Dwyer said terrabytes of data were being retrieved every day.

“The data has now come back into the processing house in our head office and our processing department will start to make meaning of it in a geological sense, which could take anywhere between two and three months,” he said.

“The next phase is to interpret that data against some geology.”

Although there are definite advantages to using seismic power as an exploration technique, it is not a common method, according to Mr Dwyer. As present, HiSeis is the only commercial provider of seismic technology for minerals globally.

Since its inception around a decade ago, the company has performed over 60 seismic projects for mining companies around the world. Although some of these were conducted in built-up areas, most were around operational mines. The project in Kalgoorlie was the first through a large urban environment.

“The uniqueness of this project was it was done through a city of 20,000-30,000 people,” Mr Dwyer said.

“The beauty of seismic is we move around infrastructure. We don’t need perfectly straight lines or the exact dimensions on spacings as we can move around and design the survey around the infrastructure available.

“And it’s quick. We did this survey in less than two weeks through Kalgoorlie and then we were out.

“If you’re drilling you can sometimes be sitting in one spot for many weeks, if not months, and you only get a pin-prick of information in that drill and then you need to move around and keep on going.”

Due to the speed, manoeuvrability and accessibility of seismic energy, Mr Dwyer said the technique could well be considered by tenement holders in other old historic mining areas around Australia.

“If you look around Australia and the other mining towns like Bendigo and Ballarat, there’s historic mining right underneath there so this might pique people’s interest as it’s a non-intrusive methodology to image underneath the town,” Mr Dwyer said.

“Nothing has to be moved and there’s very little disruption compared to other exploration techniques.”

Image: The VibroSeis truck.

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