High-Frequency Ambient Noise as Background to Deep Ocean Transient Signal Detection

Ambient noise measured in the deep ocean is studied in the context of a search for high frequency transient signals. The spectral shape of the noise is found to be very stable for an extensive data set collected over several months from 49 hydrophones mounted near the bottom of the ocean at ∼1600 m depth. The slopes of the ambient noise spectra above 15 kHz are found to roll-off faster than the -6 dB/octave seen in Knudsen spectra. A model including frequency-dependent absorption at large depth is found to fit the data well up to 25 kHz. For a subset of the data a correlation is observed between the overall noise level and the wind intensity. These measurements confirm that ambient noise, even at large depths, is still modeled well over a large frequency band by a uniform distribution of surface sources related to wind conditions.