Bistatic Reverberation Research Articles

Modeling and interpretation of bistatic bottom reverberation in deep water near seamounts.

The received reverberation signal can be beamformed by utilizing a vertical array, generating a vertical-angle time record (VATR) that enables analysis of spatiotemporal distribution characteristics. Due to the influence of multipath propagation effects, deep-sea reverberation exhibits highly complex characteristics, especially in a seabed with significant depth variation. In a recent bistatic reverberation experiment with a vertical array receiver, peculiar bright stripes were observed in the VATR. These stripes are the result of scattering caused by large-scale bottom structures and are closely associated with seamounts. To accurately model and interpret these stripes, a bistatic reverberation model is initially established to reproduce the VATR. This model enables us to numerically simulate the spatiotemporal distribution of reverberation in the VATR, offering a qualitative explanation for these stripes. However, the model alone is incapable of predicting the specific stripe structure associated with a particular seamount. To address this limitation, an equation system is introduced to calculate the stripe parameters based on the seamount parameters. By analyzing and deducing the dependency of the stripes on the seamount, conclusions were drawn using the equation system. Ultimately, the presented model and equation system successfully reproduce and comprehensively explain the observed abnormal stripes from the experiment.

A look at reverberation received on a vertical array during the TREX13 experiment

The TREX13 Target and Reverberation Experiment was conducted in shallow water off Panama City, Florida. Previously published work focused on reverberation from a fixed horizontal array, which resolves scattering in the azimuthal direction, but has very limited information about the vertical dependence of the scattering. Here, we look at the TREX13 datai—LFM pulses of various bandwidths in the 1.8–3.6 kHz frequency range—received on the vertical line arrays (VLAs) to see what information can be gleaned. The depth and time dependence of the reverberation and echo from a target are investigated. Beams are formed at various vertical angles, which should provide information about the grazing angle dependence of the scattering. A complication arises in determining the scattering strength in non-uniform environments, since the reverberation comes from all azimuths, and since the source and vertical array receivers were not co-located. In such bistatic geometry, the energy arriving at any instant comes from different ranges, azimuths, and vertical angles. However, if the source and receiver are close together, at longer times the geometry approximates the simpler monostatic situation. To help interpretation, results are compared with predictions from a range-dependent bistatic reverberation model. [Work supported by ONR, Ocean Acoustics.]

Reverberation due to a moving, narrowband source in an ocean waveguide.

In this paper, a model for the bistatic reverberation associated with seafloor scattering of sound from a moving, narrowband source in an ocean waveguide is developed. Studies of the Doppler effect for moving sources in waveguides have typically focused on the forward propagating field where the Doppler shift leads to a splitting or broadening of the received spectrum. In contrast, the contributions to the scattered field come from all directions and as a consequence the spectrum of the received energy is spread across the entire range of Doppler-shifted frequencies possible for the speed of the source. The model developed here uses rays for the incident field, ray-mode analogies for the scattering, and normal modes to propagate the scattered field to the receiver. Results from this model are compared with data collected using a towed source during the Target and Reverberation Experiment 2013. The possible applications of this Doppler reverberation for seafloor characterization are also considered.

Study on Doppler characteristics of underwater bistatic reverberation

Active sonar has two typical types of geometry configuration respectively named monostatic and bistatic like a radar system. In the monostatic scenario, co-located underwater transducers transmit and receive sound energy, while in a bistatic situation the transducers are physically separated. Both object detection and identification can be significantly enhanced through utilization of the additional dimension provided by a bistatic geometry. So, more people care about bistatic scattering characteristics of underwater objects as well as bistatic reverberation in recent years. In this paper, Doppler characteristics of bistatic reverberation generated by moving transmitter and receiver are studied. Theoretical formulism for receiving frequency of bistatic reverberation is derived in case of a tone signal being transmitted. Further analysis shows bistatic reverberation is more complicated than monostatic reverberation when Doppler is concerned. In monostatic case, the Doppler frequency shift of reverberation relates to centre frequency of the tone signal, velocity of the transmitter as well as the arriving direction. While in bistatic situation, it varies not only with the above factors, but also the locations, moving directions, velocities of both transmitter and receiver, and also the arriving time of the reverberation, which makes extraction and utilization of Doppler information more difficult.

Bottom reverberation for large receiving depth in deep water

Ocean reverberation is an important issue in underwater acoustics, which usually influences the working performance of the active sonars significantly. The deep-water reverberation data are collected from the South China Sea experiment including the reverberation signals at large receiving depths near the bottom, showing that the wave intensity increases obviously at some moments with time increasing. To analyze in depth the data, a uniform bottom-reverberation model is proposed based on the ray theory, which can calculate monostatic and bistatic reverberation intensity and explain the generation process of deep-water reverberation. The mesh method is first used in this model by dividing bottom scatterers into a number of grids. Then reverberation is calculated based on the exact time of generating the scattering signal from each grid. Due to the exact arrival time, the presented model can provide more accurate result than classical models, in which scatterers are usually treated as circular rings or elliptical rings. Numerical results are compared with experimental reverberations at different receiving distances and depths. The simulated and experimental results agree well overall for large receiving depths, whereas agreement extent decreases for the case of receiving depth close to the sea surface. The analytical results indicate that the applied scattering coefficient is suitable for this experimental sea area, and meanwhile verify that this scattering model is more accurate for low-angle bottom backscatters corresponding to the reverberation at large receiving depths.

A three-dimensional bistatic reverberation model of underwater explosion by interface scattering at variable sound speeds

Underwater reverberation is a main limitation of the sonar performance and thereby the reverberation level estimation becomes crucial. In this study, based on the Lambert law and ray theory, a model for predicting 3D bistatic reverberation performance by interface scattering at linear sound speed profile is established and then verified through the underwater explosion experiments. The Influences of source and receiver positions, and relative sound speed gradient on reverberation performance are further investigated. The results indicate that: (1) the proposed model can predict the short-range mean reverberation level effectively, with the deviation 2–4 dB and describe the whole reverberation level distribution in some details; (2) at the early reverberation phase, the interference effects between the interface scattering sounds are considerable and a dominating interface exists; the counterbalance between the losses by scattering, spreading and medium absorption results in the local high-intensity zones close to corresponding interfaces, respectively; (3) as the sound source moves towards some interface, associated local high-intensity zone gradually expands, while the other one shrinks; if the sound speed approaches are constant, an extra local high-intensity zone will appear between the previous two but with a lower magnitude.

A Deep-Water Bottom Reverberation Model Based on Ray Theory

Ocean reverberation is an important issue in underwater acoustics due to the significant influence on working performance of the active sonars. In this paper, a uniform bottom-reverberation model is proposed based on ray theory, which can calculate monostatic and bistatic reverberation intensity and explain the generation process of deep-water reverberation. The mesh meth-od is firstly used in this model by dividing bottom scatterers into a number of grids. Then reverberation is calculated based on the exact time of scattering signal generated on each grid. Due to exact arrival time, the presented model can provide more accurate result than classical models, in which scatterers are usually treated as circular rings or elliptical rings. Numerical results are compared with reverberation data collected from the South China Sea deep-water experiment with different receiving distances and depths. The simulated and experimental results agree well overall.

Relative Importance of Bottom Attenuation and Scattering in the Shallow-Water TREX13 Environment

In this paper, the results of applying normal mode models to explain measured data collected during the 2013 Target and Reverberation Experiment (TREX13) are presented. An adiabatic, and one-way coupled, Born approximation normal mode model (Nautreverb) and an energy conserving two-way coupled mode model (Aeneid) are described. Both monostatic and bistatic reverberation time series solutions are produced with the models for a rough bottom spectrum or a realization from it. Next, monostatic reverberation time series are produced, for both the Nautreverb and Aeneid models, with the measured bottom roughness parameter values obtained from measurements made during TREX13. In addition, loss mechanisms for depth averaged transmission loss are assessed. The relative effects of bottom attenuation and loss due to scattering, for several variants of the model parameters for the TREX13 environment, are also discussed. Based on the small roughness measured at the TREX13 site, bottom attenuation appears to be the dominant loss mechanism. Adjustments of the frequency dependence of the bottom attenuation were constructed to reproduce salient features of the measured transmission loss.

음원과 수신기 사이에 교차 산란단면적을 이용한 양상태 잔향음 모의

소나 운용에서 잔향음은 수중 표적 탐지의 제한요소이기 때문에 정확한 예측이 중요하다. 최근에는 단상태소나 연구에서 공간적으로 송수신기의 위치가 다른 양상태 소나에 대한 연구로 확장되고 있는 추세이다. 양상태 잔향음을 모의하기 위해서는 양상태 음파전달, 양상태 산란강도 및 산란단면적 등에서 단상태와 다른 복잡한 계산이 요구된다. 전 세계적으로 양상태 잔향음에 대한 연구가 활발하게 진행되고 있지만 잔향음을 예측하는데 중요한 요소인 양상태 산란단면적을 정확하게 계산하는 방법에 대한 연구는 미비하다. 본 논문에서는 거리 독립 환경의 양상태 잔향음모의에서 두 원의 교차되는 면적을 응용하는 새로운 산란단면적 계산 방법을 제안한다. 최종적으로 본 논문의 양상태 산란단면적 계산 방법으로 모의된 잔향음 준위는 기존에 제안되었던 방법들의 예측값과 비교되며 2013년 5월에 수행된 해상 실험의 측정값과 비교를 수행하였다. It is important to predict accurately reverberation level, which is a limiting factor in underwater target detection. Recently, the studies have been expanded from monostatic sonar to bistatic sonar in which source and receivers are separated. To simulate the bistatic reverberation level, the computation processes for propagation, scattering strength, and scattering cross section are different from those in monostatic case and more complex computation processes are required. Although there have been many researches for bistatic reverberation, few studies have assessed the bistatic scattering cross section which is a key factor in simulate reverberation level. In this paper, a new method to estimate the bistatic scattering cross section is suggested, which uses the area of intersection between two circles. Finally, the reverberation levels simulated with the scattering cross section estimated using the method suggested in this paper are compared with those estimated using the methods previously suggested and those measured from an acoustic measurements conducted in May 2013.

Peculiarities of prereverberation formation during long-range propagation of explosion signals

In experiments carried out in different regions of the World Ocean on long range propagation of explosion signals, researchers repeatedly observed bistatic surface reverberation that not only accompanied the probing (specularly reflected from the sea surface) signal but also preceded it. The part of the reverberation that preceded the probing signal was called the prereverberation. A theoretical explanation for this phenomenon in the context of an idealized model of the sea surface for the conditions of a near-surface sound channel was presented in 1978, and a technique for calculating the prereverberation time structure with allowance for the real conditions of the sound propagation in the ocean was developed only in the beginning of the 2000s. A series of calculations carried out according to this technique for the conditions of performing two experiments (for a near-surface sound channel and an underwater sound channel with the speed of sound near the bottom exceeding its value near the surface) resulted in the refinement of our knowledge about the mechanism forming the structure of a prereverberation signal. This paper, in addition to a brief description of other authors’ publications on this topic, presents a description of a technique for calculating the time structure of a prereverberation signal, a brief description of the conditions of two experiments in which prereverberation was observed, and results of analyzing the main prereverberation characteristics.

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.

浅海混响互相关函数与格林函数的关系

Using the bistatic coherent reverberation model based on normal mode theory the effects of the reverberation time and the correlation integral time interval on the cross-correlation of the reverberations which received by two range-departed receivers in shallow water are analyzed in frequency domain. The possibility to retrieval Green’s function is also discussed in this paper. The result shows that the arrival-time structure of Green’s function can be recovered from the cross-correlation if the following conditions are satisfied: (1) the reverberation time is enough to make an approximate diffused field; (2) the correlation integral time interval is double at least of the time scale, the ratio of the maximum of inter-mode interference wavelength to the minimum modal group velocity; (3) the cross-term in the cross-correlation can be cancelled by a vertical source array.

The modeling and analysis of the reverberation on the continental shelf off of New Jersey.

As interest of operating sonar system in the littoral area increases, study of reverberation on a continental shelf is needed. In this study, an incoherent reverberation on a continental shelf was simulated using a 2-D propagation model based on ray theory with scattering strength formula based on various theories. Each scattering cross section was modified for application to von Karman roughness spectrum, which was observed during an experiment on the continental shelf off of New Jersey. Various reverberation patterns were shown according to each scattering cross section. By comparing the results from the model and experimental data, an adequate scattering strength was found for each source frequency, and the dominant mechanism of the reverberation on the continental shelf was identified for each time step. In case of the monostatic reverberation measurement, the source and receiver were located on the border between slopes. So both slopes affected the reverberation. In case of the bistatic reverberation, scattered signal from each slope affected the reverberation at different times. The characteristic of bistatic reverberation was different from that of monostatic reverberation. The effect of source-receiver separation on the reverberation of a continental shelf was studied through simulating monostatic and bistatic reverberations.

Surface prereverberation in long-range propagation of explosion-generated signals in underwater sound channe

Experimental data on long-range propagation of explosion-generated signals in a well-developed underwater sound channel are analyzed. In the experiments, the wind speed reached 10–11 m/s and the sea state was Beaufort 4–6. At distances of 80–120 km from the source and at two different reception depths, a prereverberation is observed, that is, the advancing of a part of the bistatic surface reverberation with respect to the direct signal. The conditions for the prereverberation to arise are discussed for different distances from the source. On the basis of data processing, the increase rate of the prereverberation signal, its level relative to the direct signal, and frequency dependence are estimated.

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.

The theoretical and simulation prediction of bistatic reverberation average strength

In researching long-distance interface bistatic reverberation, the scattering area is taken as an elliptical ring. In researching short-distance bistatic reverberation, the model should be a three-dimensional ellipsoid. The ellipse and ellipsoid models can work without wireless communication, only using GPS clock to make sure the systems work synchronously so as to realize ranging and orienting. It is convenient for the integration of bistatic sonar system. The two-dimensional and three-dimensional prediction formulas of bistatic pressure reverberation are deduced, the vector-sensor three-dimensional reverberation is simulated and the anti-reverberation performance of vector-sensor directivity composite is discussed.

Reverberation modeling with BiRASP—The bistatic range-dependent active system prediction model

The calculation of bistatic reverberation is a computationally intensive exercise. Efficient computation of timely results typically involves numerous physical and algorithmic assumptions or approximations. Consequently, it is not unusual for predicted time series from different models to differ in level or structure. At a recent Reverberation Modeling Workshop [9–13 November 2006, Austin, TX], a set of test cases were proposed with the intent of establishing benchmark reverberation problems and solutions. This talk will focus on the test case performance of BiRASP, the Naval Research Laboratory’s bistatic reverberation model. BiRASP is a ray-based code, originally developed for deep-water, long-range, low-frequency reverberation predictions and data analysis. Results for the various test case problems will be shown. The consequences of some of the typical algorithm and physical assumptions/approximations will be demonstrated and discussed. [Work supported by the Office of Naval Research.]

Time structure of bistatic reverberation in the long-range propagation of explosion-generated signals

In experiments on the long-range propagation of explosion-generated signals, a noise background accompanying the classical signal quartets are repeatedly observed. The background smoothly increases before the arrival of the first signal of the quartet, then decreases, and completely vanishes after the arrival of the last signal of the same quartet. The data of an experiment performed in winter in a deep-water region of the northwestern Pacific are considered. These data are used to demonstrate the phenomenon at hand and to show that the noise background of the quartets is the manifestation of the bistatic reverberation caused by sound scattering by the rough ocean surface towards the receiver located a long distance from the source. An easy-to-use technique is proposed for calculating the time relations between the direct signal and the surface-reverberation one. Calculations are performed for the time structure of the bistatic surface reverberation to explain the observed features of the phenomenon.

Closed Form Bistatic Reverberation and Target Echoes With Variable Bathymetry and Sound Speed

Some existing closed-form expressions for target echo and reverberation, assuming classical mode-stripping and Lambert's law in isovelocity water, are extended to the case of uniform slope bathymetry combined with range-independent linear sound speed and bistatic geometry. The analytical, or extremely fast numerical, calculation of signal-to-reverberation-ratio provides insight useful for sonar design, and operational research. There are three types of path, each of which may be important at some range: Those that only interact with one boundary, those that always interact with both boundaries and those that sometimes interact with both. Each provides a separate closed-form contribution. The earlier finding, that with Lambert's law the reverberation tends to follow the same range trend as the target echo, is slightly modified by the addition of refraction. The stronger reverberation from the low sound speed side of the duct tends to fall off slightly less slowly than the target echo according to the strength of the refraction. Some graphical examples show that the very large parameter space can be handled very easily

Bistatic clutter statistics in underwater acoustics

For the case of a spatially separated (i.e., bistatic) source-receiver pair, the joint distribution of the time delay, bearing, and Doppler shift of acoustic clutter is derived. The clutter is assumed to be spatially uniform with a fluctuating velocity that is Gaussian distributed with zero mean and components that are uncorrelated and of equal variance. For the Doppler distribution, the source and receiver are assumed, in general, to be moving relative to one another. Unlike the monostatic case, the time delay and bearing are found to be statistically dependent. Conditioned on the time delay, the distribution in bearing exhibits two peaks appearing to either side of the source direction for times less than three direct-blast times. Later times show a single peak in the direction of the source which diminishes in relative amplitude with time. The Doppler shift distribution, conditioned on time delay and bearing, is derived under the assumption that the clutter velocity dispersion is much smaller than the speed of sound. The statistical distribution is then related to bistatic reverberation, and a qualitative comparison is made between measured bistatic data and the derived statistical distributions.