Sonar is used to find objects on the bottom of the ocean. Side scan sonar is one method used to look at the detail of the ocean floor. Side scan sonar instruments are towed behind ships and often called towfish or tow vehicles.
The instrument sends out a sonar signal in pulses at right angles to the direction the ship is moving (so it is "looking" sideways and down). The sonar signal is concentrated in a narrow band on both sides of the tow vehicle (See the diagram below).
Some of the sound sent out by the side scan sonar reflects off the seafloor and returns to the tow vehicle. The tow vehicle has sensitive hydrophones (also called receivers) which receive the returning sound. The signals from the hydrophone are sent to the ship for processing and an image is made showing the strength of the returned sound over the area the tow vehicle was sending the sound. The signal is often sent to a graphic recorder which makes an image. The darker parts or the image represent greater echo strength.
The image below shows a typical side scan sonar trace. The middle of the image is the path of the tow vehicle. There is a black line (or two black lines) right in the middle of the image. This is actually the sound pulse sent out from the instrument which is immediately heard by the hydrophone on the instrument. The blank white space, moving out from the black lines, is the time it takes the sound to travel through the water. The first echo from the seafloor (or sometimes from the sea surface) is the next mark. Then echos from the seafloor and objects at greater and greater distances from the tow vehicle.
Side scan sonar is very sensitive and can measure features on the ocean bottom smaller than 1 centimeter (less than 1/2 an inch). Typical uses of side scan sonar include: looking for objects on the seafloor (sunken ships, pipelines, downed aircraft, lost cargo), detailed mapping of the seafloor, investigation of seafloor properties (grain size, etc) and looking at special features on the seafloor like underwater volcanos.