Mine of information

Naval mine-hunting operations could be made easier with an acoustic sensing system that can provide a detailed model of the seabed, including information such as sediment composition.

A project led by Prof Dick Simons at Hollland's Delft University aims to replace expensive in situ sensors with a remote sensing technique that uses sonar technology to gather information about the top few centimeters of the seabed. It will also be able to classify the sediment, distinguishing mud, clay, fine sand, coarse sand and gravel, which is not possible in existing commercial sensor systems.

'In general navy operations, such as mine-hunting, you will have a higher probability of success if you know the seabed. Advance warning of whether it is clay or coarse sand, at least by acoustic means, lets you know how the mines behave on the seabed and how they can be distinguished from it,' said Simons.

This information may also have safety benefits for navy personnel. 'In principle, it will help them excavate the mine safely,' said Simons. 'If you have a very soft seabed, such as clay or mud, you know the mine will be buried. That is not possible if it is a very hard, coarse-grained seabed, because the mine would just sit on top.'

Two versions of the system are being developed and they have been designed to operate in 'shallow' water, by which Simons means the continental shelf — up to 200m in depth.

One platform will use ship-borne instruments, such as a hull-mounted sonar system and commercially-available multi-beam echo sounding technology.

'The data processing is carried out on the ship, and it will be combined with other information from space, radar systems from satellites or laser altimetry systems from aircraft. This is external information with which you can compare or complement your measurements,' said Simons.

The university team will concentrate mainly on this sensor, while a remote, drifting version is being developed in collaboration with Universite Libre de Bruxelles.

'Both sensors are acoustic, but the remote buoys — sensors operated from the ship — are able to drift freely. Depending on the circumstances, they work two to five kilometres away from the ship,' said Simons.

Simply designed, the buoy has an antenna that transmits radio and GPS signals to the ship, where the data is then processed. There are also underwater microphones, known as hydrophones, attached to underside of the buoy to pick up underwater acoustics. Simons claimed the remote sensing technique would have a number of advantages over conventional in situ methods.

'With ship-borne sensors, you can cover a very large area while the ship is sailing without having to recover instruments all the time,' he said. In situ sensors are buried in the seabed to take measurements, and the problem is that the ship has to be very close to them to work. They are continually having to be recovered and put somewhere else, which can get quite expensive and time consuming.'

Simons also said that the remote device could have even more benefits compared to the ship-borne system, once the challenge of developing a more complicated algorithm to process the data is overcome. 'The coverage will be greater because you would get information over the distance between a ship and a remote device. 'You can also receive data from the seabed in shallower waters, as a buoy could be placed where ships cannot go.'

The range at which the systems can operate will be determined by the underwater conditions, which is why Simons and his team have carried out tests in real-life conditions as well as controlled environments.

'We have already done one experiment in the Gulf of La Spezia, off the coast of northern Italy. I can imagine we will do more, not necessarily there, but it is vital to this project that we use field data,' said Simons.

He expects to have a commercially available system within five years.