Echo is a side scan sonar tow sled, an instrument that uses sound to create images of the sea floor and objects on it. Echo is towed 50-100 meters above the sea floor and emits high-pitched "chirps" of sound diagonally through the water.
Sound bounces back to Echo from objects such as rocks, depressions or shipwrecks, creating a scanned picture of the sea floor as the vehicle progresses along. The images created look very much as though the surface was visually viewed from above, with lighting from very low and to the side. The sound bounces off only one side of any object, and generates long "shadows." The lengths and shapes of the shadows can reveal the shape and height of the objects.
Because sound waves travel much farther than light underwater, they can be used to search large areas of the sea floor much more quickly than lights and cameras. Echo uses two sound frequencies, 100 and 400 kilohertz. Higher frequencies result in images of better resolution, but lower frequencies can "see" at greater ranges from the tow sled. The two frequencies complement each other well. First, a large area is surveyed with the low frequency. Then, when an interesting target is located, investigators can go back and pass close to the target to obtain a higher resolution image.
Like Hercules, Echo is neutrally buoyant. The complete Echo system includes a "depressor," a heavy weight that is attached to the bottom of the main fiber-optic cable. A fixed length of neutrally buoyant cable attaches the Echo tow sled to the depressor. In this configuration, the depressor absorbs the motions of the ship, allowing Echo to be towed along in a straight, undisturbed line to minimize distortions of the acoustic images.