The International Hydrographic Organization defines hydrography as “the branch of applied science which deals with the measurement and description of the physical features of the navigable portion of the earth’s surface [seas] and adjoining coastal areas, with special reference to their use for the purpose of navigation.”
Hydrographic surveying “looks” into the ocean to see what the sea floor looks like.
Hydrographic surveys are used to measure the depth and bottom configuration of water bodies. We use the data to update nautical charts and develop hydrographic models; increasingly, the hydrographic data is used for multiple purposes, including pre and post dredging operations.
Surveys can also determine sea floor material (i.e. sand, mud, rock), which is important for anchoring, dredging, structure construction, pipeline and cable routing, and fisheries habitat.
Survey vessels primarily use side scan and multi-beam sonar. Sonar uses sound waves to find and identify objects in the water and to determine water depth. Some vessels may use single beam echo sounders.
Side Scan Sonar
Side scan sonar is a specialized system for detecting objects on the seafloor. Most side scan systems cannot provide depth information.
Like other sonars, a side scan transmits sound energy and analyses the return signal (echo) that has bounced off the seafloor or other objects. Side scan sonar typically consists of three basic components: tow-fish, transmission cable, and topside processing unit.
In a side scan, the transmitted energy is formed into the shape of a fan that sweeps the seafloor from directly under the tow-fish to either side, typically to a distance of 100 meters. The strength of the return echo is continuously recorded, creating a “picture” of the ocean bottom. For example, objects that protrude from the bottom create a light area (strong return) and shadows from these objects are dark areas (little or no return) as in the image at left above, or vice versa, depending on operator preference.
Side scan sonar is typically used in conjunction with a single beam or multi-beam sonar system to meet full bottom coverage specifications for hydrographic surveys.