Ambient noise in the Mariana Trench.

Abstract

In November 2009, ambient noise measurements were made in the Mariana Trench from the surface to a depth of 9000 m using the instrument platform Deep Sound. Deep Sound is a free-falling acoustic recorder designed to descend from the ocean’s surface to a pre-assigned depth where it drops an iron weight and returns to the surface under its own buoyancy. The ascent and descent rate is 0.6 m/s, resulting in an 8 deployment time to 9 km. The instrument recorded the continuous ambient noise time series over the bandwidth 5 Hz–30 kHz on four hydrophones mounted with vertical and horizontal spacings. Environmental data were recorded on a CTD and were used to calculate the local sound speed. Power spectra of the ambient noise were calculated as a function of depth, while vertical and horizontal coherence were calculated and used to infer information on the directionality of the noise field. The spectral levels of ambient noise over the measured range of frequency were found to increase with depth. [Work supported by ONR and the UC Ship fund.]