1. A free-floating device consisting of a hydrophone and radio-transmitting antenna. Used in marine refraction surveys and extended profile work for detecting energy from a distant shot and radioing the information to the recording ship; see Figure S-14. A sonobuoy is a free-floating buoy which is usually simply thrown off a recording ship. Once in the water, seawater activates the buoy’s batteries, one or more hydrophones drop (are suspended) below the buoy, and a radio antenna is extended upward into the air. As the ship travels away from the buoy, firing charges (or other energy source) as it goes, the seismic arrivals are received by the hydrophones and transmitted to the ship, where they are recorded and timed. The distance from the energy source to the sonobuoy can be determined by the arrival time of the wave which travels directly through the water. The buoy is expendable and sinks itself after a certain time, the cost of the buoy usually being smaller than the cost of retrieving it. 2. A buoy which automatically transmits a radio signal when triggered by a water-borne sonic signal; used in positioning. 3. Military passive-receiver/transmitter for detecting underwater movement.