SONAR

Mapping the Sea Floor

Topographic maps are used to show the elevations of formations on land. Bathymetric maps show the elevations of formations under the sea. Topographic and bathymetric maps can show the elevations by using colors for different elevations or by using contour lines where each line represents a particular elevation.

The first bathymetric maps of the sea floor were made using a line with a weight attached. The weight was dropped from a ship and the line was measured when the weight hit the bottom. This was a very slow, and not very accurate, method.

During World War II, the methods used to map the sea floor advanced rapidly as the U.S. military worked to improve submarine warfare and protect the U.S. from enemy submarines. Sonar (Sound Navigation Range) developed as a much more accurate and less time consuming mapping method. Sonar devices send sound waves to the sea floor and measure the time it takes for the sound to be reflected and return to the surface. From the times, depth can be determined.

This expedition is using both multibeam sonar and side-scan sonar systems.

The multibeam sonar is attached to the hull, near the middle of the ship. It emits sound waves of different wave lengths and bathymetric maps can be developed from the information it receives.

The side-scan sonar system is a device towed behind the ship and scans the sea floor to the sides and directly below it. The actual device being towed is called the fish. The pilots "fly" the fish from the same control room where they direct the activities of Jason. There are no cameras-the system works using computers that receive information from the side-scan sonar.

The side-scan sonar device used on this expedition is called DSL-120. DSL for Deep Sea Laboratory at Wood's Hole Institute of Oceanography and the 120 kilohertz sound frequency it uses. Wood's Hole Institute of Oceanography (WHOI - pronounced "who ee") also operates and maintains Jason II.

DSL-120 side scan sonar

 

The side-scan sonar system uses transducers that both send and receive the sound signals. The system not only measures the time it takes the wave length to travel but also the amplitude, or intensity, of the return. This provides information about the composition of the surface of the sea floor. A hard sea floor is very reflective so the sound waves bounce back with a high amplitude of return. Softer sea floors, such as sediment, absorb the sound and less is reflected.

Side-scan sonar is very high tech. Using a sound frequency of 120 kHz means that it must work close to the bottom. Called phase difference sonar, it can actually measure the precise spot on the sound wave that is received. This makes for really accurate mapping.

When the side scan sonar system is deployed to a depth of 3000 meters, it takes about two hours to reach the bottom. The pilot flies the fish at about 100 meters over the sea floor. The deepest this DSL-120 has traveled was 5700 meters but at that depth the syntactic foam began to crush.