AN AIRBORNE metal detector is helping Australian scientists to detect saltwater below ground before it ruins the farmland above. Armed with this information, farmers can plant trees wherever the salt level is rising. Trees keep the salt at bay because they suck up water as they grow and lower the water table.
Decades of irrigation and deforestation have swelled the reserves of water lying under much of Australia鈥檚 farmland. And as the water rises, it carries salt to the surface. Already, 2 million hectares have been degraded by salinity, according to the Murray-Darling Basin Commission 鈥� the government body looking at the problem. Salt damage costs the country A$200 million (拢100 million) a year in lost production, according to the government (鈥淎ustralia鈥檚 growing disaster鈥�, New 杏吧原创, 29 July 1995).
The new salt detection system is called Saltmap. It was designed by World Geoscience Corporation (WGC) in Perth, the Cooperative Research Centre for Australian Mineral Exploration Technology, and the CSIRO, Australia鈥檚 national research organisation. The system is adapted from metal detectors used by geologists to hunt for ore deposits. Like the metal detectors, Saltmap generates electromagnetic fields and picks up a response from the ground.
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An alternating current passes through a copper cable strung around the body of the plane. This generates a pulsating electromagnetic field which induces secondary electrical currents in the ground wherever there is a good conductor, such as saltwater.
This secondary field is measured by a torpedo-shaped antenna, which trails behind the aircraft. Data from the antenna are analysed by an onboard computer. The size of the induced current indicates the concentration of salt, while shallower salty layers return signals more rapidly and contain more higher-frequency components. With one flyby, the system can detect saline water lying between 5 and 30 metres underground.
The idea of an airborne salt detector is not new, but Guy Roberts of WGC says that Saltmap provides more detail than earlier systems. 鈥淲e can see at shallower depths and discriminate several layers of water,鈥� he says.
According to Lindsay Nothrop, director of government鈥檚 Community Landcare Initiatives, the technique works best when surveying relatively flat areas. This is the kind of terrain where salinity most often causes a problem. 鈥淎lthough the system is not applicable everywhere, it鈥檚 still a cheap reconnaissance tool to find saline hot spots in many areas,鈥� says Nothrop. (see Diagram)
