Shallow Water In Presence Research Articles

Spatial and temporal sound fluctuations in shallow water in presence of internal soliton in transition areas.

Behavior of the sound field is considered in a 1–2‐h time interval in presence of train of internal soliton (IS) crossing an acoustic track. During this period moving ISs pass through several stages: approaching an acoustic track, consequent covering source (receiver) only, both source and receiver, receiver (source) only, and receding from an acoustic track. Correspondingly there are different regimes of interaction of the sound field with IS: horizontal reflection, capture of signals in horizontal waveguide, adiabatic variations, and transition areas between these regimes. So, rather complex spatial and temporal structure of the sound field takes place, including frequency and modal dependence of its parameters. Theoretical analysis of variations in the sound field on the basis of techniques of vertical modes and horizontal rays (PE in horizontal plane) is carried out, and estimation of feasibility of an experimental setup is presented. [Work was supported by CRDF.]

Phase fluctuations and horizontal refraction of low‐frequency sound signals in shallow water in presence of internal waves.

Fluctuations of the sound pulse phase front propagating approximately perpendicular to the direction of motion of a train of intensive internal waves in the experiment Shallow Water 2006 (SW06) are studied. Acoustic data received by a horizontal/vertical line array (WHOI‐Shark array) during 3 h are analyzed. Low‐frequency modulated signals with a carrying frequency of 300 Hz were transmitted, propagating approximately along coastal line at the source‐receiver distance of 20 km. During this time, a train of intense internal waves (Rosey) was moving toward the New Jersey coast. Internal waves were being monitored by the ship radars (aboard R/V Sharp and R/V Oceanus) and by the thermistor chains. It is shown that phase fluctuations of the acoustic pulses arise with the appearance of internal waves at the source‐receiver acoustic track and they correspond to horizontal refraction of the sound pulses. For the low‐frequency sound pulses, modal decomposition of received signals is used both for the processing of experimental data and for the theoretical analysis. Experimental data are compared with theoretical estimation of horizontal refraction. [Work was supported by ONR code 321OA and by CRDF, Grant BP3C10].

Time‐frequency analysis of mode coupling in shallow‐water sound propagation

Time‐frequency (TF) analysis for propagation of broadband sound pulses in shallow water in presence of perturbations of waveguides parameters, such as internal waves, temperature fronts, or bottom variability is considered. Due to waveguide dispersion one can construct a TF diagram for individual normal modes using Gabor transform for each received pulse. If we have a temporal sequence of propagating pulses then we can find a temporal sequence of TF diagrams. It is shown that in the presence of spatial and temporal waveguide perturbations (such as the sound‐speed profile in the water column or the sea bottom) structure of a TF diagram and its temporal behavior depends on waveguide parameters, on the source‐receiver positions, and on the character of mechanisms influencing the sound propagation, such as mode coupling, horizontal refraction, or adiabatic regime. Dependence of the TF diagram on waveguide variability in geotime provides information about temporal behavior of the perturbations. Structure of TF diagram is analyzed for a shallow‐water waveguide model as well as for the SWARM95 experimental data. [Work supported by ONR and RFBR.]

Low-frequency reverberation signal in shallow water in presence of internal waves

The reverberation of low-frequency (100–400 Hz) sound in shallow water is considered. It is supposed that reverberation signals are formed by summarizing from circle area (radius about 50 km) as result of sound field backscattering on bottom surface. The developed model of reverberation takes into account mode coupling caused by background internal waves in shallow water and depending on direction of backscattered signal. Influence of internal waves on space-time structure of reverberation signal is considered for two cases: single-modal and multi-modal. The computer simulation shows that internal waves lead to significant temporal fluctuations of reverberation signal in both cases. The results of computer simulation are compared with data of experimental research in the Barents Sea. [Work supported by RFBR, Grants 02-02-16509 and 03-05-64568.]

Sound scattering in shallow water in presence of internal solitons

Sound scattering by localized inhomogeneity (object) in a shallow-water waveguide is studied. Influence of internal solitons (IS) traveling in the sea, on the scattered field is considered. The problem is analyzed within the framework of theory of the sound scattering in the waveguide developed by the authors, where the scattering matrix is expressed through modal decomposition including the scattering amplitude of the object in free space. The peculiarity of the given work is the taking into account of additional modes conversion due to IS. The waveguide and IS are modeled on the basis of experimental data (sound-speed profile, bottom relief, IS parameters, etc.) collected in the Japan Sea. The inhomogeneity is selected in the form of soft spheroids with dimensions characteristic of gray whales (eschrichtius). Calculations of amplitude fluctuations of the low-frequency sound field at the receiving point are carried out for moving spheroid and different orientations of acoustic track with respect to direction of propagation of IS. It is shown that sound fluctuations have essentially different characteristics for longitudinal and transversal propagation of IS. [Work supported by RFBR, Grant No. 00-05-64752.]