Side-scan sonar is a category of sonar system used to create an image of large areas of the seafloor efficiently. This tool is used to map the seabed for a wide variety of purposes, including creating nautical charts and detecting and identifying underwater objects and bathymetric features. Side-scan sonar imagery is also a commonly used tool to detect debris and other obstructions on the seafloor that may be hazardous to shipping or to seafloor installations for subsea field development. In addition, the status of pipelines and cables on the seafloor can be investigated using side-scan sonar. Side-scan data are frequently acquired along with bathymetric soundings and sub-bottom profiler data, thus providing a glimpse of the shallow structure of the seabed.
Side-scan imaging surveys use sonar to efficiently create a precise acoustic image of large areas of the seafloor. Side-scan sonar survey uses transducers that emit fan-shaped acoustic pulses down toward the seafloor across a wide angle perpendicular to the path of the sensor through the water. Side-scan sonar transducers may be towed from a surface vessel or mounted on the ship’s hull. The intensity of the acoustic reflections from the seafloor of this fan-shaped beam is recorded in a series of cross-track slices. When stitched together along the direction of motion, these slices form an image of the sea bottom within the beam’s swath (coverage width) or slant range. Image resolution is a function of pulse length; the shorter the transmit pulse length, the better the target resolution. Generally, the pulse length is selected to optimize resolution and range as the shorter the pulse length, the less the slant range of the side scan image. Side-scan sonar surveys are made with two transducers to cover both sides. The transducers are either contained in one hull-mounted package or within a tow fish and pulled by a tow cable to get closer to the bottom target of interest.
Side-scan sonar surveys have several advantages. They are commonly available at a relatively low cost, and portable units can be easily deployed on small boats or ships of opportunity. This tool is used for mapping images of the seabed for a wide variety of purposes, including detection and identification of underwater objects and seafloor features.
Side-scan may be used to conduct surveys for maritime archeology; in conjunction with seafloor samples, it can provide an understanding of the differences in material and texture type of the seabed surface. Side-scan sonar imagery is also a commonly used tool to detect debris items and other obstructions on the seafloor that amy be hazardous to shipping or to seafloor installations by the oil and gas industry. In addition, the status of pipelines and cables on the seafloor can be investigated using side-scan sonar. Side-scan data are frequently acquired along with hydrographic and sediment imaging data, thus providing a glimpse of the shallow structure of the seabed. Side-scan sonar survey is also used for fisheries research, dredging operations, environmental studies, and military applications, including mine detection.
Rather than produce a quantitative set of lowest elevations, side scan sonar surveys generate a qualitative suite of underwater imagery. For projects where a snapshot “picture” of the bottom is necessary, side-scan sonar surveys can be an excellent choice to identify bottom features such as submerged debris, remnant derelict infrastructure, exposed bottom geology, hazards to navigation, and shipwrecks. Side-scan sonar surveys can also be used to monitor ongoing progress for sensitive underwater construction projects such as a protective mattress, rip-rap, and structural placement.