There are several echo-detection sonar methods used in marine research, i.e. multibeam, side-scan, and sediment sonar, each with their specific purpose. The results obtained by these various methods can be combined to provide an even more complete picture.
Multibeam sonar produces an image of the seabed landscape
Multibeam sonar is used to map the depth of water and the shape of the seafloor. The data produced by this method provides an accurate three-dimensional view of the bottom landscape, as well as information on the bottom quality and hardness of the seabed. Based on such echo sounding data, it is possible to estimate, for example, the stoniness and roughness of the seabed or the natural habitats of an area.
The multibeam sonar system consists of a sensor, a central processing unit, and a motion detector. The sensor transmits a fan-shaped beam of sound signals towards the seafloor and registers the reflected return signals from the bottom. Any errors created by the movement or swaying of the vessel are corrected by the motion detector.
The accuracy of the sounding is greatly influenced by the speed at which the sound pulses progress through the various layers of seawater of differing temperatures. Indeed, the speed of the sound propagation must be measured often during the sounding. Some devices continuously measure the speed of sound and automatically account for localised speed differences.
Multibeam sonar provides data from a wide strip of the seafloor
With the help of multi-beam sonar, it is possible to map extensive areas. This sounding method provides very accurate depth information not only along the vessel's course but also in a wide band on either side of the vessel.
The width of the strip to be created can be changed by adjusting the opening angle of the sensor. Although the width of the band to be sounded may be up to twelve times the water depth, the measurement accuracy at the edge of such a wide strip will then be poor.
Man-made traces show up in multibeam scan images
Multibeam sonar can also be used to investigate the traces caused by man. Images produced from this sounding method show the various marks left on the seabed due to dredging, spoil-dumping, or even bottom trawling. Besides, different structures, such as cables, as well as water and sewage pipes, can also be distinguished from such pictures.
multibeam sounding can be used to search for objects that have sunk to the seafloor, such as aeroplanes, snowmobiles, or cars. It can also be used to map shipwrecks and other marine archaeological sites.
Nevertheless, this method is not as accurate a side-scan sonar. Indeed, to achieve greater accuracy, modern multibeam sonars also have a side-scan echo feature.
The marks of sea gravel extraction can be seen in the image produced by multi-beam sonar. The location is situated in the Soratonttu area off Helsinki and measures approximately 1.5 x 1.5 km in area. A suction dredger has been used for gravel removal; traces of the suction hose appear in the bottom sediments as high-definition pits. In the aspect ratio, the depth has been magnified about six times.
Side-scan sonar produces an acoustic silhouette of the seabed
A side-scan echo sounder provides an acoustic silhouette of the seabed surface. The signal returning from the bottom registers strongly when reflected from a hard surface, such as rock or boulder. Weaker echoes are received from soft sediments, such as clay or mud.
The path of the returning sonar signal is also affected by seafloor formations. For example, boulders or pier structures leave behind an acoustic shadow. The height of these structures from the seabed can be calculated from the length of such shadows. The result is a picture that closely resembles an aerial image.
The side-scan sonar provides a very detailed and accurate view of the surface structure and composition of the seabed, as well as any objects upon it. This method is widely used, for example, for locating wrecks and mapping the seabed.
Side-scanning is carried out from a moving vessel
The hardware of the side-scan sonar consists of a transceiver unit, a central unit, as well as a supply cable and associated winches and remote controls. The transceiver unit is called a “fish” and is lowered into the water and towed behind the vessel. The towing depth is selected according to the location and conditions.
The image produced by side-scan includes areas located on either side of the ship's course line. However, the bottom strip directly below the vessel is not shown.
Side-scanning the seabed with a Metsähallitus research boat near Jussarö Island, Raasepori.
Sediment sonar mapping extends below the surface of the bottom sediments
Although side-scan sonar also gives information on the quality of the bottom sediment, it does so only superficially. Instead, sediment sonar is required to study the deeper sediment layers. This method can be used to determine the thickness and internal structures of different sediment layers.
Sediment echo sounding works best with soft sediment bottoms. For example, in clay and silt deposits, it can penetrate up to several tens of metres. By contrast, the sonar signal does not penetrate harder bottom deposits, such as sand and gravel. Nevertheless, the sediment echo sounding image shows the boundary between the hard and soft deposits, including information about how stony the sediments are.
Echo sounding provides a continuous view of the area under study, i.e. a so-called bottom profile, which with current technology, has a vertical resolution accuracy of five to ten centimetres.
Sound pulses reflected from the seafloor turn into images
In sediment echo sounding, the transducer is mounted on the bottom of the survey vessel. From there, it sends short bursts of sound to the seabed. Signal penetration is affected by both the frequency of the signal, as well as the type of bottom sediment; the best penetration occurs with low frequencies and soft sediments. Sonar echoes that reflect from the bottom are received by the sensor mounted on the ship’s hull.
Before the sound pulse is transmitted, it is magnified by an amplifier. The duration and shape of the pulse can also be adjusted.
The strength and return time of the reflected sonar signals vary according to what kind of sediment stratification the sound strikes. The final sounding image is produced by a computer programme. It converts the return signal strengths into degrees of darkness and the return time in milliseconds to depth metres. This provides a continuous profile from the seabed corresponding to the vessel's course.
An echo profile produced from sediment echo sounding.
Echo sounding data is used for many purposes
The information obtained from sonar methods can be supplemented by taking sediment samples from the seabed. The echo images can be used to ensure that a sampling area is suitable for a given purpose or that it contains exactly the type of seabed that is to be studied.
A significant advantage of sonar methods is that they have much higher coverage than more traditional sampling. Sediment sampling provides information from only one point, whereas sonar methods provide uninterrupted regional knowledge of the seabed conditions. For example, sonar data can be used to create geological maps of the seabed or even to map seabed habitats.
There is a particular need for echo sounding information when planning marine utilisation, such as sea sand retrieval, dredge-spoil dumping, or building structures on the seafloor. Echo sounding profiles have even been used to investigate ancient earthquakes!