Several mechanisms have been proposed by which an incident acoustic wave could be coupled into the ocean floor at angles below the compressional critical angle. Recent observations of acoustic transmissions from water into sediment with arrival times and amplitudes inconsistent with the refractive compressional path have been interpreted as the excitation of a Biot slow wave in the sediment. Another hypothesis attributes the observed signals to scattering at the rough water-sediment interface. A third entails scattering of evanescent waves by volume inhomogeneities in the sediment. The existence of multiple hypotheses, each of which could account for the received energy, invited further investigation. A series of well-controlled laboratory measurements were made and compared with the results of a first-order perturbation theory model in order to evaluate the accuracy of the rough interface hypothesis. The experimental measurements of transmission through adjacent flat and rough interfaces show good agreement with the results of the numerical model, giving clear evidence of subcritical penetration in an environment incompatible with the requirements of the Biot slow wave and volume scattering hypotheses.
Passive sonar depends on signals of opportunity to detect, track and localize targets. These signals are typically detected and then tracked using Kalman filter-type signal followers. Target motion analysis (TMA) is then used to estimate the target's range and, from this, its position, course and speed. The accuracy of TMA is strongly dependent on the duration of the available track. Initiating a second tracker in reverse time at the time of detection can reduce or eliminate the delay between target detection and localization. A detection and tracking system for a passive sonar using a towed array receiver is described and an example ofreverse-time tracking using real data is provided. Reverse-time tracking is able to significantly increase the amount of track data that can be extracted from already available data, highlighting the need for improved data fusion. Potential improvements to this enhanced system through track association are discussed.
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