Ocean Ecology |
 Click image to enlarge. Sidescan sonars use a swath of sound to esonify the seafloor. They differ from single-beam sounders in that they have a much larger footprint shape.  Click image to enlarge. However, like single-beam systems, they generate two main types of data, depth (or bathymetry) and backscatter. Traditionally, side scan sonars provide better backscatter data than depth data. They are often used to create a wide, often almost photo realistic image of the seabed using backscatter intensity.  Click image to enlarge.  Raw waterfall image (plot of backscatter intensity) from a sidescan sonar. Click image to enlarge. (Information derived from Acoustic Techniques for Seabed Classification (2005) by J D Penrose, P J W Siwabessy, A Gavrilov, I Parnum, L J Hamilton, A Bickers, B Brooke, D A Ryan and P Kennedy - see link below) Sidescan sonars are used to map the seabed for a wide variety of purposes, including creation of nautical charts and the detection and identification of underwater objects and bathymetric features. They are often used to conduct surveys for marine archaeology, and assist in locating and identifying underwater anthropogenic artifacts. In conjunction with seafloor samples, sidescan sonar can be used to classify the seabed based on material and texture type. Sidescan sonar imagery is also a commonly used tool to detect debris items and other obstructions on the seafloor that may 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 sidescan sonar. Sidescan data are frequently acquired with bathymetric soundings and sub-bottom data, thus providing a glimpse of the shallow structure of the seabed. Sidescan sonar is also used for fisheries research, dredging operations, and environmental studies. Sidescan uses a sonar device that emits fan-shaped pulses down toward the seafloor in a direction perpendicular to the path of the sensor through the water. The sensor may be towed from a surface vessel (e.g., mounted on a "towfish") or mounted on the ship's hull. The intensity of the acoustic reflections of the beam from the seafloor 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 swath (coverage width) of the beam. The sound frequencies used in sidescan sonar usually range from 100 to 500 kHz; higher frequencies yield better resolution but less range. Ocean Ecology has developed a sidescan sonar towfish based on the Humminbird 997c SI unit.  Ocean Ecology's towfish. This unit has a traditional downward-looking sonar combined with a side imaging sonar. Specifications of downward-looking sonar:
Specifications of HDSI (high-definition side imaging) transducer (best for shallow water use):
Specifications of SI (side imaging) transducer (best for deep water use):
Power output of unit:
Shown below is a diagram of the beam orientation and coverage for the HDSI transducer at -10 dB.  Sidescan beam angles. Sidescan images are processed using either the Mini Image Processing System (MIPS), an open source image processing software developed by the United States Geological Survey (USGS) or MB-System, an open source image processing software developed by the Monterey Bay Aquarium Research Institute (MBARI) (see link below). An example of a processed and mosaiced image using the Humminbird side imaging sonar and MIPS is shown below:  Click image to enlarge. The same data can be viewed using MB-System. Shown below is a sidescan plot of the data from MB-System.  Click image to enlarge. The above image consists of sidescan swaths plotted in the order that they were taken (e.g., more recent swaths overly older swaths). MB-System can also be used to generate a true mosaic image where the overlapping data points are averaged (see below).  Click image to enlarge. Although the mosaic image generated by MB-system has lost some of the detail shown in the MIPS mosaic image, this processing technique retains z (or depth) values, and thus the image can be projected in 3D (see below).  Click image to enlarge. This technique also maintains the projection information in the file, so that the mosaic can be easily projected into a mapping system such as Google Earth without requiring new georeferencing to be done (see below).  Click image to enlarge. |
| Web Design - Sea Storm | Site Navigation | Updated: October 27, 2011 |
