Using its novel rotating magnetic gradiometer system called GETMAG, the CSIRO team takes measurements around three separate axes so the full magnetic gradient tensor is determined. The depth, size and shape of a deposit can be visualised without resorting to the “suck it and see” approach typical of many conventional surveys.

GETMAG has applications in the coal industry in a similar way to normal magnetic surveys. But according to CSIRO research scientist Dr Phil Schmidt, the advantage that tensor gradiometry has over conventional surveying is the ability to determine depth and structure in a more direct way.

Also, if a magnetic survey is available and it is of high enough resolution, then it is possible to determine the gradient tensor from the normal survey. But it must be a high quality survey to enable quantitative deductions to be made from it.

“With normal magnetic anomaly devices you just get a bump, which doesn’t tell you much. With tensor measurements, the data is processed in such a way it gives you a lot more information, and our three-drum system even allows for error estimation,” Schmidt said.

At the heart of GETMAG lies a high temperature superconducting quantum interference device (SQUID) operating in liquid nitrogen. SQUIDs are extremely sensitive, able to detect subtle variations in the strength of a magnetic field.

The team tested its prototype over a magnetite deposit in eastern Australia, which has been thoroughly characterised through drilling and laboratory-based magnetic measurements.

“All determinable parameters of location, geometry and magnetisation were found to accord with directly measured properties [and] only a few tensor gradiometer stations were needed to extract the same information as a whole total magnetic intensity survey,” the team said.

The prototype was developed with the support of an industry consortium. “It demonstrates the sort of thing we can do,” Schmidt said. “The next stage is to get it to fly.”

Despite only testing the device about a metre off the ground, Schmidt is confident it will work when airborne.

“Theory tells us the magnetic gradient signal over most targets of interest will be strong enough to detect from an aircraft, and we know the instrument is sensitive enough; what we don’t know is how much compensation will be involved,” he said.

“Can we place the system in a stinger, or will it have to be in a bird dragged behind a helicopter, the way some electromagnetic surveys are done?”

As the team points out in its paper, there are also ground-based applications of its system, such as surveys for unexploded ordnance, mine-site geophysics, and a miniaturised version could be used down-hole.