Bottom Reverberation In Shallow Water Research Articles

Review of long-range bottom reverberation in shallow water

Theoretical models of long range bottom reverberation in shallow water are reviewed. Firstly, several classical models are summarized based on the empirical scattering function and physical scattering mechanism. Then, the development of theoretical model and experimental measurement about bi-static shallow water reverberation are outlined in the second part. During summarization, the researching work of the group from HEU is also introduced. Bi-static reverberation in range dependent waveguide is the emphasis of their research, and the decaying rule of reverberation intensity in wedged environment is presented in paper.

The horizontal correlation of long range bottom reverberation in shallow water with inclined sea floor

The performance of active sonar system is seriously influenced by bottom reverberation in shallow water waveguide. It is important to understand the horizontal correlation of bottom reverberation for active towed-array processing techniques in shallow sea. However, little work had been done for the research on horizontal correlation of distant bottom reverberation. In this paper, a coupled mode reverberation model was applied for the horizontal correlation, and it was investigated as a function of receiving position, time and frequency. Calculations show that transverse correlation is greater than the longitudinal correlation in horizontal space for distant bottom reverberation. The adiabatic mode solution is introduced to derive the mathematic mode for horizontal correlation in the range-dependent waveguide with varying depth and the numerical results indicate that the influence of inclined sea floor on the horizontal correlation should be considered.

Propagation theory for bi-static long-range bottom reverberation in shallow water waveguide

Bi-static long-range bottom reverberation in shallow water is investigated, and the impulse response function is derived using the linear theory of sound channel. For the case of almost layered medium, the adiabatic mode solution is introduced for the derivation of transfer function. The reverberation intensity is calculated for the range-dependent waveguide, and then both the intensities and the decaying rules are compared with their corresponding counterparts in different wedged sea areas. It is shown that the propagation effect on intensity of distant bottom reverberation can not be neglected for the bi-static active sonar in shallow water waveguide.

Method of coupled mode for long-range bottom reverberation

The theory of coupled mode is used for modeling the long-range bottom reverberation in shallow water caused by bottom roughness. The distant bottom reverberation level and spatial coherence of impulsive source are both derived. The results agree with those from the classical reverberation model, and are compared with the experimental data. The influence of source bandwidth and the distance between sources and receivers on the intensity of bottom reverberation are particularly discussed. The method is shown to be available for both the mono- and the bi-static cases.

Distant bottom reverberation in shallow water

The method of coupled mode is introduced for investigation of bi-static distant bottom reverberation of impulsive source in shallow water, which will not contradict with principle of reciprocity in all cases. And the method of multi-pole for directional source is also introduced. It shows that in case of layered medium, intensity of bi-static bottom reverberation will decease according to the cubic power of receiving time t, and the transverse spatial correlation of bottom reverberation is a little greater than longitudinal correlation for equal separation of receivers, and both vary in form with the receiving time.

Low-frequency bottom reverberation in shallow water

The results of measuring the parameters of bottom reverberation in a shallow-water sea with the use of vertically developed phased transmitting and receiving systems are presented. The mode content of reverberation and its temporal variation in the case of a pulsed irradiation of the medium are investigated. A numerical model of bottom reverberation is developed on the basis of the mode description of the field and the analysis of experimental data. The model makes it possible to calculate the main parameters of bottom reverberation. It can be used in solving the problems of geoacoustic inversion for estimating the characteristics of sedimentary rock.

Using Triplet Arrays for Broadband Reverberation Analysis and Inversions

Some directional characteristics of observed clutter and reverberation are presented using line arrays with cardioid elements. It is shown that the cardioid arrays break the left/right ambiguity for reverberation sources above ~600-700 Hz. Data analysis uses a cardioid beamforming algorithm developed by researchers at the NATO Undersea Research Centre (NURC, La Spezia, Italy) but the normalization for this algorithm is derived by the author. It is shown that this normalized algorithm has an upper frequency limitation not previously noted. Reverberation data are taken from two recent experiments. One was the 2004 Boundary Characterization Experiment near the Malta Plateau. The area is rich in clutter objects like wrecks and mud-volcanoes and has some subbottom features that may be important. Sources were monostatic coherent pulses and impulsive sources. The receiver was the NURC cardioid array. The other experiment was the U.S. Office of Naval Research's 2003 Geoclutter effort to study shallow-water bottom reverberation and clutter in the STRATAFORM area off New Jersey. Sources were bistatic coherent pulses. The receiver was the five-octave research array (FORA). The STRATAFORM has benign surface morphology but strong clutter is observed. Some highlights of the reverberant returns from that area are discussed that include returns from probable fish schools. From these two data sets, it is shown that up to 15-20-dB left/right discrimination is attainable on broadside cardioid beams in the 1200-1750-Hz region for both linear frequency modulation (LFM) pulses and sound-underwater-source (SUS) broadband data. Also, a (previously developed) constrained simulated annealing (SA) inversion technique is applied to the directional reverberation data from the NURC array. A two-layer fluid model of the bottom is assumed. The algorithm is best at estimating compressional speeds, layer thickness, and attenuations in that order. Extracted bottom parameters at the site are compared with independent inversion results previously reported by Holland. Inverting reverberation from directional arrays is shown to give somewhat better quality results since, unlike standard line arrays, the reverberation data apply to only one bearing along the seafloor.

Bottom reverberation in shallow water: Coherent properties as a function of bandwidth, waveguide characteristics, and scatterer distributions

Shallow water presents a difficult, reverberation-limited environment for active SONAR operations. It is important to understand the predictable structure of shallow-water reverberation in order to aid the design of processors and detectors which work properly in these environments. In this paper, the temporal characteristics of monostatic reverberation are predicted as a function of source bandwidth, source–receiver depth, and the propagation characteristics of range-independent shallow water. Results show that at early time, reverberation can be highly coherent across a vertical line array, violating the homogeneous noise assumption, while at late time the reverberation becomes increasingly uncorrelated. This is shown to be due to the insonification of independent bottom patches at late time. It is also shown that this decorrelation of the reverberation is dependent both on the propagation characteristics of the particular shallow-water environment, the correlation length scale of the scatterers, and the bandwidth of the source, with high-bandwidth sources causing decorrelated reverberation sooner than low-bandwidth sources. The results also show that there are several identifying characteristics in reverberation time series which may be useful for identifying the types of scatterers which cause reverberation during particular experiments.

Bottom backscattering strengths derived from shallow‐water reverberation measurements

A normal‐mode model for calculating surface and bottom reverberation in shallow water has been developed, and found to give good agreement with single‐hydrophone data for a fairly flat‐bottomed area [D. D. Ellis, SACLANTCEN Report SR‐196 (September 1992)]. The model has been extended to include beam patterns and has been compared with broadband data received on a horizontal line array. The model‐data agreement is again generally quite good. When the model‐data differences as a function of beam angle and time are overlaid on a map of the area, the differences are seen to be primarily due to changes in the bottom bathymetry. This can be used to extract a measure of the bottom backscattering strength versus location.

Shallow-water bottom reverberation under downward refraction conditions

In 1965, some propagation loss and reverberation level measurements were performed under downward refraction conditions at a location south of Long Island. The reverberation levels measured during those trials are reduced to scattering strength values by considering the details of the propagation to and from the bottom, and the results are shown to be similar to those that Urick derived by ignoring the details of the propagation paths. The manner in which downward refraction conditions facilitate the measurement of bottom scattering strength values for shallow-water areas is also discussed.

Shallow Water Bottom Reverberation under Downward Refraction Conditions

In 1965, some propagation loss and reverberation level measurements were performed under downward refraction conditions in a location south of Long Island. The interpretation of the propagation loss results was described in a previous paper [J. Acoust.Soc. Am. 41 (1967)]. This paper presents the reverberation level measurements collected during those trials, and reduces these measurements to scattering strength values by considering the details of the propagation to and from the bottom. The results are shown to be similar to those which Urick [J. Acoust. Soc. Am. 48, 129(A) (1970)] derived by ignoring the details of the propagation paths to and from the scatterets. Finally, the manner in which the downward refraction conditions facilitate the general gathering of bottom scattering strength values for shallow water areas is discussed.