Clearing explosives is a major operation and removing the deadly residue of over a century of warfare is a never ending task. The problem is that before you can remove explosives you have to find them. That isn’t always easy – especially underwater, so Australia’s Commonwealth Scientific and Industrial Research Organisation (CSIRO) has developed a new sensor that uses high-temperature planar gradiometers to seek out explosives in the sea.

The new sensor is being developed on behalf of the the United States Strategic Environmental Research and Development Program (SERDP) and the US-based research organization Sky Research. It’s purpose is to help clear 10 million acres (four million hectares) of US coastline that are contaminated with explosives. These are often corroded and leaking, so they’re an environmental as well as a safety issue.

The CSIRO sensor (Image: CSIRO)

The sensor is a form of magnetometer, which is a device for measuring local magnetic fields. It’s used in geophysical surveying, archaeology and by the military for mine-hunting and submarine detection. The principle is very simple. You have two sensors that measure the intensity of a magnetic field and with a pair of them you can measure the gradient of the field between them. Move the sensors over an area and you can build up a complete map of the magnetic landscape. Obviously, a magnetometer can detect metal, but it can also detect less obvious things like baked clay, old fire pits and explosives.

The trouble is, there are a lot of places where magnetometers have difficulty working. The sea is one of these. Not only is there the effect of the Earth’s magnetic field to contend with, but the sea is like a huge wet-cell battery filled with all sorts of electrical fields produced by the salt water, waves and other conditions.

Planar gradiometer schematic (CSIRO)

The CSIRO sensor is a superconducting quantum interference device (SQUID). According to a paper in the OCEANS 2010 IEEE – Sydney, it consists of six planar gradiometers (a form of magnetometer), located on the faces of a hexagonal pyramid that is cooled with liquid nitrogen and towed underwater. The gradiometers themselves are what are called “flip-chips.” These are highly sensitive semiconductors, which would be a problem with all those background fields, but the CSIRO sensor uses feedback loops to cancel out the effect of the Earth’s magnetic field and seawater conditions.

The result of this is a sensor that can locate small amounts of explosives more accurately and return information on their make up. It works by either passing over an area or when inserted into a borehole and is useful for not only pinpointing explosives, but also for prospecting for minerals on the ocean floor.

The CISRO sensor has been tested in a stationary laboratory environment and also while in motion over test targets. The next phase will involve underwater tests to see how it copes with a marine environment.


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