Sidescan Sonar: Mapping the Sea Floor

    Sonar - the use of reflected sound to navigate and to detect objects—is nothing new to whales and dolphins, or even bats.  However, it wasn't until the beginning of the 20th century that humans first started using this same strategy to explore the ocean environment. Sonar technology advanced during WWII when early sonar systems were developed to locate enemy submarines.  As sound technologies have improved over the last few decades, scientists are now capable of using sonar to produce high-quality maps showing details once unimagined on the seafloor.  

     

  • Sidescan Sonar “towfish” is usually towed behind ship
  • Maps seafloor terrain including surface sediment
  • Uses intensity of returning echo to determine slope and physical character of the seafloor
  • Creates grayscale images with darker areas corresponding to hard surfaces

Just as you would buy a good map before embarking on any family road trip, marine scientists today produce sonar maps of the ocean floor to locate major “landmarks” such as artificial reefs and platforms.  In the Louisiana artificial reef program, we are using sonar to document the exact location and condition of other artificial substrates such as charted wrecks, and also to determine the presence and condition of any natural habitats within the artificial reef planning area.  This information will help to guide future artificial reef deployments to maximize fisheries productivity and protect valuable natural habitats.  The figures above show an artificial reef and a natural reef from a recent survey of the Ship Shoal artificial reef planning area. 



With sidescan sonar, the strength or intensity of the returning acoustic signal is recorded. The intensity of the reflected echo gives scientists a hint as to what kind of sediment makes up the seafloor. A strong signal means the substrate is relatively hard; or contains a lot of texture.  Hard surfaces appear dark on the sidescan map and indicate a wreck, a platform (see figure), shell, coral or even shrimp mounds.  A weaker echo appears light on a sidescan map and indicates a soft or finer surface such as silt and sand. Sidescan surveys are typically conducted as you might mow the lawn.  Information from the individual passes or swaths can then be pieced together to form a complete map.  The width of the swath in the offshore sidescan surveys are typically 500m (approx 550yds).  In shallow waters like Lake Pontchartrain, the useful range decreases and a typical swath width is 150m (165 yds).  The figure to the right shows the impact of dredging operations on the seafloor of Lake Pontchartrain.