Seismic sensing of extremely-low-frequency sounds in coastal waters

A very compact, but quite lightweight digital, triaxial ocean-bottom seismometer has been developed to study ELF-acoustics (1 to 100 Hz) in shallow water by mounting three active, non orientation-privileged 1-Hz geophones in a flat container together with a compass, two tilt-meters, a thermistor, and the pertinent A/D electronics. Each geo-acoustic channel is sampled at 600 Hz and the floating-point conversion is performed with 12 + 2 bits, giving the large dynamic range of 120 dB needed to cope simultaneously with ambient noise and signals from nearby cw or explosive sources. The signals are then transmitted by a radio buoy to the shipboard acquisition and pre-processing systems. These sensor stations have been deployed to investigate infrasound propagation in coastal waters. Signals from small charges and LF-sources are analyzed, together with records of the noise background.