The Plymouth Sound test site

The test site has a row of objects, from bicycles to a diver's helmet, resting on the seabed. Two techniques - sidescan sonar and multibeam sonar - were used to see if these objects could be located and identified.

Side scan proved to be best at locating objects, as any projection above the seabed casts a long, distinctive shadow. Multibeam was better at identifying the object, however, as it provided a more accurate profile of the object.

The conclusion is that both techniques can play an important role in identifying and monitoring archaeological sites underwater.

Above: Interactive acoustic traces taken on the Plymouth Sound test site in 2005. It will take a few seconds for the traces to load.

Identifying the objects seen on an acoustic trace is very difficult even under ideal circumstances. The side scan trace provides a quick picture of the site, which is illuminated by a strong, directional acoustic signal from the fish. Upstanding objects reflect strongly and cast long shadows across the seabed.

Multibeam probes the sediment with a spectrum of acoustic pulses. The data is harder to interpret, but if we suspect that there is an object in a given area - from the side scan data, for example, then multibeam data in that area can often be refined to produce very detailed profiles.

Plymouth Sound overview.
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