Sidescan Technology

SIDE SCAN SONAR and the GULF OF MEXICO MARINE DEBRIS PROJECT

Sonar (SOund NAvigation and Ranging) is used in hydrographic surveys to “take a picture” of the ocean floor by using sound. Throughout the Gulf of Mexico Marine Debris Project, the type of sonar being used is side scan sonar (SSS). The side scan sonar unit, often referred to as a “fish”, is attached to the vessel by a line and towed while it sends out pulses of sound. Unlike ordinary sonar which transmits sound only straight down, the side scan sonar transmits sound into the water at an angle, in a fan-like shape which covers a greater area. The sound reaches the seafloor and bounces back to the side scan sonar unit where it is recorded as shades of black and white, depending upon the intensity of the returned signal. The intensity is determined by the density of the object hit by the sound. Harder surfaces like rocks, metal objects, and even oyster shells have stronger returns (meaning they reflect more of the sound wave than they absorb) and are recorded as light-colored objects on the SSS image. Soft surfaces such as mud or silt have weak returns because they absorb much of the sound energy and are recorded as shades of grey to black.


The image on the Left shows Hull-mounted multibeam sonar (left) and towed side scan sonar “fish” (right). The image on the Right shows how different features reflect the sonar differently to create an image.

The National Oceanic and Atmospheric Administration (NOAA) Office of Coast Survey uses side scan sonar extensively, both for charting and for detection of submerged objects that may interfere with vessel traffic. When developing navigational charts, the areas surveyed receive 200% coverage, meaning the entire area is surveyed twice to make sure that no object is missed. When the goal is to cover as much area as possible in a short amount of time, the survey covers an area only once, or with 100% coverage. The tradeoff is that quality of the images suffers, and there is a higher chance that some items may be missed. This process is similar to mowing your lawn – if you overlap a little bit, you can be more confident you have cut all the grass and not missed any. Using 200% coverage also produces a higher quality image that looks more like a photograph.


The side scan image below shows ship containers and a sunken barge. The vertical black line in the middle shows the path of the “fish” that is towed under the vessel.

In addition to side scan sonar, the survey vessel has other specialized equipment to determine the depth of the water (bathymetry). Accurate global positioning system (GPS) units also provide exact location at all times. It is very important to know the exact position of the vessel and also where any identified objects are located, because these positions could be different.




A new technology that has been used during the Gulf of Mexico Marine Debris Project is the use of an Autonomous Survey Vessel (ASV). The ASV is larger than the “fish”, moves on its own power, and can be deployed from shore, a pier, or vessel. It can also survey for up to 18 hours while traveling at 3 knots. The vehicle can navigate with the aid of acoustic buoys or can be programmed independently with GPS coordinates. The vehicle has a maximum operating depth of 100 meters, but it is used primarily for shallow water surveys (less than 50 meters). Just like the side scan sonar imagery from the “fish”, the ASV is used to detect and map submerged wrecks, rock, and other objects which pose a hazard to navigation for commercial and recreational vessels. Below is an image of an ASV.

The following links provide additional information on side scan sonar:

NOAA Office of Coast Survey
http://chartmaker.ncd.noaa.gov/HSD/wrecks.html