Underwater Imagery by Mapping Sonar

Abstract

This will be a discussion of an instrument capable of sea floor mapping with sound waves in a similar fashion to which side looking radar maps the land with radio waves from an airplane. The instrument used is a long range side scan sonar. One developed specifically for geologic mapping of the sea bottom. This particular instrument was developed by Institut Francais du Petrole; the French governmental petroleum research company. It is capable of producing imagery as far as 1/2 mile on each side of a fish towed behind a ship. Figure 1 is a picture of a fish. It is 13 feet long and weighs 2000 pounds. In it are mounted transducers that emit sound and receive the echoes from the ocean floor. One pulse is emitted each second. The frequency used is 37-1/2 kilocycles, well out of the range of petroleum seismic sources. Because a much higher frequency is used, the instrument is compatible with marine seismic techniques. The fish can be towed at speeds of up to 10 knots, although the slower speeds of seismic operations are more ideal. Figure 2 shows the way in which sound is emitted and received. Sound is emitted in a vertical beam that is 1-1/2 degrees wide on each side of the fish. The vertical sweep can be adjusted from 10 to 30 degrees. In addition to being able to adjust the sweep angle, the look angle of the sweep can also be pointed downward or more horizontally to compensate for changes in water depths. This particular instrument is unique among sonars in that its range is 750 meters, or approximately 1/2 mile. For geologic interpretation and mapping, the range of the sonar determines the amount of bottom that is mapped with one pass, i. e. the cost per unit area. The geologic interpretability of sonar records is a direct function of the amount of the bottom that is seen; that is its range. Ideally, the fish is kept at a constant height above sea bottom that is, 50 meters, or 150 feet. The speed of the ship and the length of the towing cable determine the depth of the fish. By pulling the fish 150 feet above the bottom, a scan of 1500 feet of the bottom is obtained on each side, with a 1500 foot blank area beneath the fish. The range of the instrument is sufficient to allow excellent interpretation from a traverse. Interpretational abilities are enhanced greatly by other information, such as simultaneous sparker profiles. For this reason, the side scan is almost always used in conjunction with some type of vertical profiling.