Depth‐profiling ambient noise in the deep ocean.

Abstract

Deep Sound is an untethered instrument platform designed to free‐fall from the sea surface to a preassigned depth, at which point a burn wire releases a weight, allowing the system to return to the surface under buoyancy. The descent and ascent rate is 0.6 m/s. A Vitrovex glass sphere houses lithium‐ion batteries and a suite of microprocessor‐controlled electronics for data acquisition, data storage, power management, and system control. Outside the sphere, several hydrophones are arranged in vertical and horizontal configurations, a CTD returns environmental data, and motion sensors monitor pitch, roll, and yaw. Data may be downloaded, and the batteries may be recharged, via throughputs in the sphere. The hydrophones, with a bandwidth of 30 kHz, are rated to a depth exceeding 11 km, and the sphere itself has a depth‐rating of 9 km. The system made three descents in the Philippine Sea in May 2009, to depths of 5100, 5500 and 6000 m; and in November 2009, two descents were made in the Mariana Trench to a depth of 9000 m. On all these deployments, ambient‐noise time‐series were recorded continuously, yielding the power spectrum and vertical coherence of the noise as functions of depth. [Research supported by ONR.]