Volume 105, Issue 3, March 1999
Index of content:
- UNDERWATER SOUND 
105(1999); http://dx.doi.org/10.1121/1.426699View Description Hide Description
Acoustic propagation along the Hawaiian–Aleutian path at low frequency (65 Hz) for the acoustic thermometry of ocean climate (ATOC) has been simulated using the implicit finite difference parabolic equation (IFD-PE). This simulation is based upon the Semtner–Chervin model with one-half-degree resolution. The adiabaticity has been assessed, and it was found that significant mode coupling takes place at the sub-arctic front area (44.8 ° N). Due to the range-dependent double channel, two characteristic ranges were identified in this area: (i) the modal degeneration range and (ii) the modal scattering range At range mode m and mode n (usually, are degenerated, the modal wave numbers of mode m and mode n become equal, and mode n then takes the shape of mode m. Therefore, there is no real repopulation (modal scattering), but a modal name change takes place at this range. The modal name change can cause a miscalculation of the adiabatic travel time. At range however, real repopulation does take place. Numerical simulations illustrate that and are separated by about 10 km at lower modes and are merged at higher modes.
105(1999); http://dx.doi.org/10.1121/1.426740View Description Hide Description
A second phase-conjugation experiment was conducted in the Mediterranean Sea in May 1997 extending the results of the earlier time-reversal mirror experiment [Kuperman et al., J. Acoust. Soc. Am. 103, 25–40 (1998)]. New results reported here include (1) extending the range of focus from the earlier result of 6 km out to 30 km, (2) verifying a new technique to refocus at ranges other than that of the probe source [Song et al., J. Acoust. Soc. Am. 103, 3234–3240 (1998)], and (3) demonstrating that probe-source pulses up to 1 week old can be refocused successfully.
105(1999); http://dx.doi.org/10.1121/1.426700View Description Hide Description
A new wave theory model providing consistent modeling of seabed insonification, three-dimensional target scattering, and rough seabed reverberation has been used to investigate the spatial and temporal characteristics of the multistatic scattering and reverberation from rippled, shallow water seabeds. It is shown that the highly polarized—close to monochromatic—spectral characteristics of ripple fields are associated with a reverberation environment which is highly sensitive to both the frequency and insonification aspect relative to the ripples. The study suggests that significant gains in detection performance for buried objects can be achieved by band-limiting the processing to frequencies below an environmentally dependent “cut-off” frequency. The study also confirms theoretically the intuitive advantage of insonifying the seabed along the ripple direction to reduce monostatic reverberation.
105(1999); http://dx.doi.org/10.1121/1.426702View Description Hide Description
This paper develops two new models for ambient noise processes which exist in the deep ocean. The first model, termed the Kronecker autoregressive model, extends existing ideas regarding the use of linear models on uniform line arrays. The paper shows that the novel parametrization accurately models physical noise processes for uniformly sampled volumetric arrays. A new proof is presented which shows that for the case of purely sinusoidal signals, the Kronecker form provides an exact model, suggesting its use for colored noise sources. The second model uses a form of the spherical harmonic expansion to develop an original linear matricial model. Additionally, this paper presents a new algorithm needed for computing the weighting coefficients involved with this model.