Volume 128, Issue 4, October 2010
Index of content:
- UNDERWATER SOUND 
128(2010); http://dx.doi.org/10.1121/1.3483720View Description Hide Description
This paper presents the first results of an imaging technique that measures the geoacoustic structure of a seafloor in shallow water areas. The devices used were a broadband (100 Hz–6 kHz) acoustic source towed by a ship and a vertical array. Among all the acoustic paths existing in the water column, two are used: the direct one and the seabed-reflected one, the latter being composed of the reflections from the seafloor’s surface as well as that from each buried layer. Due to the good time resolution of the signal and to the short range configuration, the reflected signal can be modeled as a sum of contributions coming from image sources relative to the seabed layers. The seabed geometry and the sound speed profile can then be recovered with the detection and localization of these image sources. The map of the image sources is obtained by a function that combines back-propagation of signals and knowledge of the emitted pulse. The thickness and sound-speed of each layer is finally obtained by a position analysis of the image sources. The results obtained by this data-driven algorithm on both at-sea and synthetic data are satisfactory.
128(2010); http://dx.doi.org/10.1121/1.3479544View Description Hide Description
A simulation using linear maximal sequence (m-sequence) transmissions demonstrated the potential for detecting and tracking near-surface targets. In this simulation a low power, omnidirectional source and four omnidirectional hydrophones were arbitrarily located in water approximately 4 m deep. The simulated −15 dB target traveled at 1.5 kn. Using “channel digit response” processing and “block zeroing,” the direct arrival, multipaths, clutter and reverberation were rejected. (The block zeroing process is very different from other methods typically labeled as “cancellation.”) With the improved signal-to-interference ratio, a simple probability based algorithm demonstrated tracking at source-target distances of 250 m, the maximum range investigated.