Acoustic Technologies –
Acoustic tools: Principles and equipment

Sidescan sonar

Sidescan sonar uses high-frequency sound pulses that are bounced off the seafloor to create an image of morphology and differences in seabed texture and substrate types. Typically a sidescan sonar consists of two transducers commonly mounted in a towed body or 'fish'. Each transducer generates a fan-shaped sound pulse perpendicular to the vessel track. After the sound pulse emitted by the transducers arrives at the seafloor, some of the sound is reflected back to the transducer and some is reflected away. As the sonar moves along the track the recorded sound reflections create an acoustic image of the seafloor.

Multibeam sonar is a common offshore surveying tool that uses multiple sound signals to detect the seafloor. As it sends out multiple beams it is able to map a swath of the seabed under the ship in contrast to single beam sonar. Generally, the multibeam sonar transducer is mounted rigidly to the hull of the survey vessel and its position can be calculated very accurately. With the capability to cover 100 per cent of the seafloor, multibeam sonars are becoming the industry standard for hydrographic and geological applications.

Acoustic sub-bottom profiling systems are used to determine physical properties of the seafloor and to image geological information a few metres below the seafloor. The systems have been widely adopted by marine researchers because of their ability to collect data rapidly and non-intrusively. Sub-bottom profilers are usually comprised of single channel source that sends sound pulses into the shallow sub-seafloor sediments. The sound pulses bounce off different rocks according to differences in their acoustic impedance (hardness). The different times taken for this signal to be returned and recorded by the sub-bottom profiler indicate where the rocks are below the seafloor.