SACLANTCEN, 19138 Viale S. Bartolomeo 400, La Spezia, Italy
Ten Swallow Floats were deployed in the Ionian Sea (Mediterranean) during the SACLANTCEN sea trail of June 1992. Each of the floats (property of the Marine Physical Laboratory---Scipps) contained three orthogonal particle velocity sensors, one hydrophone, and a compass, enabling measurements of vectorial acoustic intensity in the range of 0.5 to 25 Hz. The data compare well with other ambient noise data from heavy shipping traffic areas, and comparisons are made with published data from other sources. Because of a subduction front south of Crete, the area is rich in seismic events. Part of the earthquake energy is transmitted into the water, and the intensity measurements allow one to locate the areas that are particularly active. In fact, due to the relatively low acoustic attenuation in water, a Swallow Float seems to have a higher sensitivity than a land station to earthquakes occurring under water. Examples to support this hypothesis are shown. A transmitted 20-Hz tone allowed the causes of errors in bearing estimates to be determined.