Ambient Noise in the Challenger Deep

In December 2014, ambient noise measurements were made in the Challenger Deep, Mariana Trench from the surface to a depth of 9000 meters using 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 instrument recorded an 8 hour continuous ambient noise time series over the bandwidth 5 Hz to 30 kHz on four hydrophones mounted with vertical and horizontal spacing in an ‘L’ shaped configuration.  Environmental data was recorded on a CTD and 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.  While the instrument was at 8 km depth, a near-bottom implosive source of opportunity was recorded.  The direct arrival and multiple surface and bottom reflected arrivals were resolved.