Index of content:
Volume 115, Issue 4, April 2004
- UNDERWATER SOUND 
115(2004); http://dx.doi.org/10.1121/1.1650014View Description Hide Description
Sounds received in the Gulf of Alaska at 3115 km from the ATOC/NPAL source at Kauai (75 Hz, 0.027-s resolution, bottom-mounted) are compared with acoustic and oceanographic models. Unlike data collected at stationary SOSUS arrays, these data come from a towed horizontal array at 372-m depth of military origin. A plausible identification of the acoustic reception is made despite the fact that only one transmission is collected and sound interacts with the bottom near the source. The similarity between the modeled and measured impulse response here may be useful for understanding the signals between this same source and the NPAL array near southern California. The plausible identification of sound from the horizontal array here appears to point toward the feasibility of using other military platforms of opportunity besides SOSUS to study acoustic propagation and possibly map climatic changes in temperature by means of tomography.
115(2004); http://dx.doi.org/10.1121/1.1645854View Description Hide Description
Spectral factorization is shown to restore the phase of an incoherent layered sediment reflection coefficient so that its Fourier transform is the minimum phase impulse response at each angle. The method requires the reflection coefficient to be known over a range of frequencies and the grazing angles in question to be above critical. It is developed here in the context of another recently established technique for extracting the seabed’s plane wavereflection coefficient from ambient noise data measured on a moored or drifting vertical array (VLA). Thus it offers the possibility of sub-bottom profiling from a single platform with no sound source. Limitations of the phase restoration method are discussed and, using modeled data, comparisons are made between the “true” impulse response derived from the known complex reflection coefficient and the result of applying spectral factorization to the absolute value of the reflection coefficient. The method is also demonstrated on experimental reflection loss inferred from ambient noise measurements at three moored VLA sites and one VLA drift track in the Mediterranean Sea. Sub-bottom profiles (impulse response versus position) are shown for the drift track demonstrating that one can indeed survey with only a single directional receiver. The technique appears to perform well when compared with other profiling techniques and published results.
Acoustic remote sensing of swimbladder orientation and species mix in the oreo population on the Chatham Rise115(2004); http://dx.doi.org/10.1121/1.1649998View Description Hide Description
A method for combining in situmeasurements and theoretical swimbladder-derived estimates of target strength of the deep-water fish, black and smooth oreos, is described. The technique uses Monte Carlo simulation and yields fish length–target strength relationships suitable for use in estimating biomass from echo integration acoustic surveys. The relationships are derived from estimates of the mean and standard deviation of the tilt angle distributions of the wild fish generated by the method. The relationships may also be used to estimate proportions of the two oreo species in the wild. The mean tilt angle of black oreos in the wild was about 10° with a standard deviation of 8°. For smooth oreos it was close to zero with a standard deviation of about 4°. The target strength relationships derived for biomass estimation purposes were and where L is the fish length and and are the target strengths of black and smooth oreos respectively.
115(2004); http://dx.doi.org/10.1121/1.1649737View Description Hide Description
Reverberation from rough ocean boundaries often degrades the performance of active sonar systems in the ocean. The focusing capability of the time-reversal method provides a new approach to this problem. A time-reversal mirror (TRM) focuses acoustic energy on a target enhancing the target echo while shadowing the boundaries below and above the focus in a waveguide, thereby reducing reverberation. The resulting echo-to-reverberation enhancement has been demonstrated experimentally using a time-reversal mirror in the 3–4 kHz band in shallow water.