Oceanic ambient noise as a background to acoustic neutrino detection

Ambient noise measured in the deep ocean is studied in the context of a search for signals from ultrahigh-energy cosmic ray neutrinos. The spectral shape of the noise at the relevant high frequencies 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 attributing the source to a uniform distribution of surface noise that includes frequency-dependent absorption at large depth is found to fit the data well up to 25 kHz. This depth-dependent model should therefore be used in analysis methods of acoustic neutrino pulse detection that require the expected noise spectra.