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The ocean sub-bottom seismometer (OSS IV) is one of the quietest seismic stations in the world at frequencies between 4 and 15 Hz. Noise levels of 1 0 1 2 m /Hz are observed above 4 Hz. Noise at frequencies above 5 Hz appears to be caused by system noise during quiet periods, but is dominated by storms at other times. Ships, whales, and earthquakes also add to the noise. The equivalent acoustic noise level varies between 60 and 70 dB re 1 μPa, about 10 to 20 dB quieter than the ocean. Signals generated in the ocean can be observed out to long distances, with propagation loss on the order of 120 dB (referenced to a source level at 1 m) at 100 km. Signals generated from earthquakes are observed about once per hour with signal-to-noise ratios and fidelity considerably improved over those obtained from ocean bottom seismometers. Bottom loss measurements indicate that acoustic signal amplitudes decrease by slightly less than a factor of two with every reflection from the ocean bottom at pre-critical angles.

OSS IV: Noise Levels, Signal-to-Noise Ratios, and Noise Sources