Side-scan sonar

During the Titanic mission, the French researchers used a new kind of sonar known as side-scanning sonar. Ordinary sonar surveys a narrow field that must be carefully mapped out in advance. Side-scanning sonar eliminates this drawback, by picking up signals at a diagonal. 

Side scan sonar record showing landform bounded by escarpments enclosing an area of displaced blocks. (Data from Prior, D.B., and Coleman, J.M., Marine Geotech., 3,37-60,1978.)... 

SONAR allows the imaging of the seafloor and obstacles on it. An example of a side-scan sonogram mosaic of a mudslide is shown in Figure 3.5 (Prior and Coleman, 1978,1981). The soimd waves are emitted by a source and reflected off the seafloor and travel back to a receiver. The sound waves are produced by either active or passive systems. Active systems send out sound pulses. Passive systems monitor sounds emitted by the objects they are tracking. Ship-borne SONARs are active systems that are very powerful and can image seafloors and objects up to several kilometers from the source. The frequencies used in these systems determine their depth capabilities. A summary of some of the available side-scan sonar systems that are commercially available is presented in Table 3.8. 

Newton, R.S., Seibold, E., and Werner, F. 1973. Facies distribution patterns on the Spanish Sahara continental shelf mapped with side-scan sonar. "Meteor" Forsch. Ergebnisse, C15 55-77. 

Simm, R.W., and Kidd, R.B. 1984. Submarine debris flow deposits detected by large-range side-scan sonar 1000 km from source. Geo-Marine Letters, 3 13-16. 

Rock Intrusive in sedimentary rocks side scan sonar, echo sormding Magnetic... 

A survey vessel operated by the Geological Survey of Sweden surveyed the area where the scuttled ships ought to be found. A side scan sonar was used. Nineteen objects were found and 16 were believed to be wrecks or parts of wrecks. They were accurately positioned. 

Sub-Bottom-Profiler This system is another multi-frequency acoustic system which is comparable to a seismic monitoring system of low seabed penetration capability. The acoustic emissions are frequency-modulated and the pulse rate is adjustable to suit the local requirements. The echoed signal can be recorded and processed into a display-form similar to the side-scan-sonar. It produces information about the thickness of sedimentation and can plot the contour of the seabed below the sedimentation layer. 

Side scan sonar is used to locate and identify man-made objects (shipwrecks, pipelines, cables, debris, etc.) on the seafloor and determine sediment and rock characteristics of the seafloor. The side scan sonar provides a sonogram of the seafloor that appears similar to a continuous photographic strip (Figure 7.13). A mosaic of the seafloor can be provided by overlapping the coverage of adjacent survey lines. 

FIGURE 7.13 Side-scan sonar image of river bottom and rock-protected river side-slope. 

Obstructions can be detected either by side scan sonar or with a magnetometer. The side scan sonar cannot identify buried objects. A magnetometer can only detect metal objects which cause a magnetic anomaly. Therefore, one can never be sure whether the subsurface is free of obstacles. Shallow geophysical methods can be helpful for this. 

Acoustic (high frequency sound pulses) o Side Scan Sonar (SSS)... 

The side scan sonar emits fan-shaped acoustic pulses down towards the seabed across a wide angle perpendicular to the path of the ship and is used to efliciently create an image of large areas of the sea bottom. It consists of a series of transducers, fitted in a fish-shaped vehicle (Figure B.37) that is towed behind or attached to the hull of a ship a few metres above the seabed. 

Objects and obstacles located on the seabed give a sharp reflection of these pulses and create an acoustic shadow on their lee side. Thus, an image is obtained similar to an aerial photo at sunset, where shadows are accentuated, so that protrusions can be detected and an estimation of their height can be made. An absolute height measurement cannot be performed with the side scan sonar, for this echo sounding or multi-beam techniques are required. 

The side scan sonar gives a clear image of irregularities in a wide swath on both sides of the ships path. However, there is always a trade-off between resolution of the image and coverage (swath), which can be 25 to 300 m. The side scan with a... 

Figure B.37 Retrieving a side scan sonar device (Photo NOAA).

The side scan sonar images can also be used to get an indication of the nature of the superficial soil layers, see Figure B.38 and Figure B.39. 

The multibeam is sometimes referred to as Swath echo sounder. The multibeam is in principle equal to the side scan sonar and the sub-bottom profiler, except that... 

Figure B.38 Side scan sonar image indicating the presence of gravel and rock.

An advantage of the multibeam sonar compared to the side scan sonar is that the multibeam gives depth information (accuracy 0.05 to 0.10 m in shallow water) over the entire scanned area instead of only the area directly under the side scan sonar fish . However, the range of the multibeam is smaller than that of a side scan and the acoustic image is much less detailed. 

Side scan sonar 100-500 kHz Wide, 5-15 X water depth 0.10 m Im No Moderate... 

---