Development of a source discriminant for T waves recorded by seismometers located on oceanic atolls. This figure compares the amplitude of the signal, measured as the maximum of the envelope of ground velocity recorded by the station, with its duration defined at 1/3 of the maximum. Various symbols denote different kinds of acoustic sources. Note that earthquakes and explosions are well separated; the only signals not identifiable are volcanic events; however, their occurrence in swarms makes them easy to identify under operational conditions.