Experimental detection and focusing in shallow water by decomposition of the time reversal operator

A rigid 24-element source-receiver array in the 10-15 kHz frequency band, connected to a programmable electronic system, was deployed in the Bay of Brest during spring 2005. In this 10- to 18-m-deep environment, backscattered data from submerged targets were recorded. Successful detection and focusing experiments in very shallow water using the decomposition of the time reversal operator (DORT method) are shown. The ability of the DORT method to separate the echo of a target from reverberation as well as the echo from two different targets at 250 m is shown. An example of active focusing within the waveguide using the first invariant of the time reversal operator is presented, showing the enhanced focusing capability. Furthermore, the localization of the scatterers in the water column is obtained using a range-dependent acoustic model.     

Echo-to-reverberation enhancement based on decomposition of time reversal operator with forward and backward reverberation nulling constrains

Combined the decomposition of time reversal operator and the time reversal reverberation nulling, a new time reversal processing approach for echo-to-reverberation ratio enhancement is proposed. In this method, a 2-dimensional signal subspace for the range of the target and two bottom focusing weight vectors for the ranges near the target are obtained by the decomposition of time reversal operator. From the signal subspace and focusing weight vectors, a constrained optimal excitation weight vector of source receiver array can be deduced to null the acoustic energy on the corresponding bottom and maximize the energy at the tar- get. This method remedies the shortages of conventional time reversal processing, time reversal reverberation nulling and time reversal selective focusing method. It focuses sound energy at the target and nulls the energy at the bottom near the target range simultaneously, therefore enhancing the echo-to-reverberation ratio without probe source and prior-knowledge of the relative scattering intensity of target and bottom. Numerical simulations in typical shallow water environments showed the effectiveness of the proposed method and its improved performance for echo-reverberation enhancement than conventional time reversal processing.     

Passive reverberation nulling for target enhancement

Echo-to-reverberation enhancement previously has been demonstrated using time reversal focusing when knowledge of the channel response between a target and the source array elements is available. In the absence of this knowledge, direct focusing is not possible. However, active reverberation nulling still is feasible given observations of reverberation from conventional source array transmissions. For a given range of interest, the response between the source array elements and the dominant sources of boundary reverberation is provided by the corresponding reverberation from this range. Thus, an active transmission can be projected from the source array which minimizes the energy interacting with the boundaries at a given range while still ensonifying the waveguide between the boundaries. As an alternative, here a passive reverberation nulling concept is proposed. In a similar fashion, the observed reverberation defines the response between the source array elements and the dominant sources of boundary reverberation at each range and this is used to drive a range-dependent sequence of projection operators. When these projection operators subsequently are applied to the received data vectors, reverberation can be diminished. The improvement in target detectability is demonstrated using experimental data with an echo repeater simulating the presence of a target.     

多输入多输出与时反联合的波导声呐有源目标定位

为了提高对浅海静止小目标的探测能力,提出了将多输入多输出和时反处理相结合的波导声呐处理框架。利用收发合置垂直阵,假设目标为点目标,每一个阵元轮流时反发射正交信号照射目标,整个阵接收对应的目标回波。当所有的阵元发射正交信号结束以后,对所有接收的目标回波数据进行分集处理,然后与时反发射驾驶向量做匹配滤波完成对目标的定位。分时发射策略克服了水声信道时延扩展和多普勒扩展导致常规多输入多输出处理接收端不同回波信号分离的困难。采用物理时反发射和数值时反接收聚焦,有助于抑制混响、提高回混比。数值仿真和波导水池实验结果表明多输入多输出时反处理较常规的有源匹配场处理以更高的精度对波导中的目标进行定位。      

Reverberation of Wideband Signals in Shallow Water when Using Sound Focusing

Within the framework of the normal mode approximation, expressions are obtained for calculating bottom reverberation signals recorded by a horizontal array in an inhomogeneous shallow-water waveguide in a wide frequency band. These expressions can be used to simulate bottom-scattered signals both for a monostatic and bistatic geometry, as well as in the case when sound focusing is applied. The constructed model is used to numerically study the structure of bottom reverberation in a waveguide with different parameters and characteristics of the receiver and source systems. The considered bottom inhomogeneities are the slope of the bottom, change in thermocline depth, and wind waves. The study demonstrates the promise of using sound focusing as time reversal using a single receiver–transmitter element to enhance the reverberation signal arriving from a given bottom area.     

浅海低频本地海底混响的波导不变性结构及抑制方法研究

根据浅海混响的射线简正波相干混响理论和可分离的海底散射模型,从理论上推导了浅海低频本地海底混响信号中存在的稳定的距离-频率干涉结构(波导不变性结构),并在此基础上基于谱相减的思想提出了一种基于波导不变性的本地海底混响预测对消方法。针对某个检测距离,其对应的中心频率上的海底混响强度可以根据波导不变性通过频移的方法在另一个与之相邻的距离上估计得到,利用该估计的混响强度可以对消检测距离对应的回波信号中的海底混响强度分量,增强回波信混比,提高目标检测能力。针对典型水平分层浅海波导环境,通过计算机仿真实验分析了低频海底混响存在的稳定波导不变性结构,验证了本文提出的基于波导不变性的混响预测对消方法的有效性。      

Finite element modeling of reverberation and transmission loss in shallow water waveguides with rough boundaries

A finite element model for the reverberation and propagation in a shallow water waveguide with a sandy bottom was calculated for five different environments at a center frequency of 250 Hz. The various environments included a rough water/sediment interface, a rough air/water interface, roughness at both interfaces and downward and upward refracting sound speed profiles with roughness at both interfaces. When compared to other models of reverberation such as ray theory, coupled modes, and parabolic equations, finite elements predicted higher levels of reverberation. At early times, this is due to the "fathometer" return, energy that is normally incident on the boundaries at zero range. At later times, the increased reverberation was due to high angle scattering paths between the two interfaces. Differences in reverberation levels among the environments indicated that scattered energy from the air/water interface is transmitted into the bottom at steep angles. This led to a large decrease in reverberation for a rough air/water interface relative to a rough water/sediment interface. Sound speed profile effects on reverberation were minimal at this frequency range. Calculations of the scintillation index of the different environments indicated that most of the reverberation was relatively Rayleigh-like with heavier tailed distributions at longer ranges.     

Adaptive instant record signals applied to detection with time reversal operator decomposition

Time reversal arrays are becoming common tools whether for detection or tomography. These applications require the measurement of the response from the array to one or several receivers. The most natural way to record the impulse responses for several sources is to generate pulses successively from each emitting point and record simultaneously the signals from the receivers. However, this method is very time consuming or inefficient in terms of signal-to-noise ratio. To overcome this limitation quasi-orthogonal pseudonoise signals like Kasami sequences can be used. For guided wave propagation, a very high degree of orthogonality between the signal is necessary to allow an accurate measure of the whole multipath structure of the transfer function. Hence, in this work, we propose a new family of pseudo-orthogonal signals that is adapted to the environment and more specifically, to highly dispersive media. These adaptive instant records signals are used experimentally to detect targets using the time reversal operator decomposition method. The accuracy of the 15 x 15 transfer functions acquired simultaneously, and therefore the detection capability, are demonstrated in an experimental ultrasonic waveguide as a small-scale model of shallow water propagation including bottom absorption and reverberation.     

Travel-time tomography in shallow water: experimental demonstration at an ultrasonic scale

Acoustic tomography in a shallow ultrasonic waveguide is demonstrated at the laboratory scale between two source-receiver arrays. At a 1/1,000 scale, the waveguide represents a 1.1-km-long, 52-m-deep ocean acoustic channel in the kilohertz frequency range. Two coplanar arrays record the transfer matrix in the time domain of the waveguide between each pair of source-receiver transducers. A time-domain, double-beamforming algorithm is simultaneously performed on the source and receiver arrays that projects the multi-reflected acoustic echoes into an equivalent set of eigenrays, which are characterized by their travel times and their launch and arrival angles. Travel-time differences are measured for each eigenray every 0.1 s when a thermal plume is generated at a given location in the waveguide. Travel-time tomography inversion is then performed using two forward models based either on ray theory or on the diffraction-based sensitivity kernel. The spatially resolved range and depth inversion data confirm the feasibility of acoustic tomography in shallow water. Comparisons are made between inversion results at 1 and 3 MHz with the inversion procedure using ray theory or the finite-frequency approach. The influence of surface fluctuations at the air-water interface is shown and discussed in the framework of shallow-water ocean tomography.     

Waveguide invariance structure for low frequency monostatic bottom reverberation mitigation in shallow water

The stable range-frequency interference structure(waveguide invariance) of the low frequency monostatic bottom reverberation in shallow water is derived theoretically in this paper by using ray-normal mode coherent reverberation model and separable bottom scattering model.And combined the concept of spectral subtraction,a reverberation forecasting and mitigating method based on the waveguide invariance is presented.According to the waveguide invariance,the reverberation intensity for a detecting range can be estimated from the reverberation returns scattered from some other range cells nearby the detecting range with frequency shift.Then the reverberation component for the detecting range in the echo can be canceled mostly by using the estimated reverberation intensity,so as to enhance the echo-to-reverberation ratio and improve the detection capability of the active sonar.Simulations in typical horizontal layered shallow water environment show the stability of waveguide invariance structure in the low frequency monostatic bottom reverberation and the efficiency of the reverberation forecasting and mitigating method based on the waveguide invariance.     

浅海波导中水平接收阵被动时反混响抑制方法研究

使用基于射线简正波异地混响理论建立的水平接收阵接收海底混响模型,提出了水平阵被动时反混响抑制方法。以水平接收阵接收回波信号的某一时刻作为开始,分别对其前一时刻和后一时刻的回波信号做时反算子分解获得两个混响子空间,将这两个混响子空间的平均作为该时刻的混响子空间的一个估计,然后利用投影处理来抑制该时刻接收信号中的混响成分。该方法可以在一定程度上克服由于海底粗糙引起的局部时间(距离)段内接收回波信号的不平稳性,使得估计得到的混响子空间更加接近真实混响子空间。因此,利用其进行被动时反混响抑制可以增强回波信混比,提高对目标的检测和方位估计能力。针对典型浅海波导环境开展的仿真验证了本文方法的有效性。      

浅海海底界面混响平均强度特性仿真

海底粗糙界面是产生混响的主要因素之一。本文通过理论分析和数值仿真的方法,根据浅海全波动混响模型对不同海底粗糙界面所引起浅海混响平均强度特性进行研究,主要考虑Goff-Jordan谱、Gauss谱和指数谱三种不同粗糙界面条件下的海底反向散射强度和混响平均强度特性。计算结果表明:海底粗糙界面会引起海底反向散射强度的频率特性的差异,进而导致海底平均混响强度的频率特性的差异,但随入射角度的变化不大。即使界面起伏的方差和相关长度相同,不同的粗糙度谱也会引起平均混响强度的差异。     

The relation between the waveguide invariant, multipath impulse response, and ray cycles

The waveguide invariant, β, that manifests itself as interference fringes or "striations" in a plot of frequency vs source-receiver separation, is usually thought of as a modal phenomenon. This paper shows that striations can be explained simply through the variation of the eigenray arrival times with range, in short, the variation of the multipath impulse response. It is possible to calculate β for a number of sound speed profiles analytically and to find what β depends on, why it switches from one value to another, how it depends on source-receiver depth, how it depends on variable bathymetry, and how smooth the sound speed profile needs to be for clear fringes. The analytical findings are confirmed by calculating striation patterns numerically starting from eigenray travel times in various stratified environments. Most importantly the approach throws some light on what can be deduced from β alone and the likelihood and utility of striations in reverberation.     

Horizontal array beamforming in an azimuthally anisotropic internal wave field

A numerical study of beamforming on a horizontal array is performed in a shallow water waveguide where a summer thermocline is perturbed by a time evolving realization of an internal wave field. The components of the internal wave field consist of a horizontally (azimuthally) isotropic, spatially homogeneous contribution, and a horizontally anisotropic, spatially inhomogeneous component. These terms represent a diffuse ("background") internal wave field and a localized solitary wave packet, respectively. Conventional beamforming is performed as a function of time while the internal wave field evolves throughout a computational volume containing the source-receiver paths. Source-receiver orientation with respect to the azimuthally anisotropic component has a significant effect on the beamformed output. When the source-receiver configuration is oriented approximately parallel to the solitary wave crests, beam wander, fading, beam splitting and coherence length degradation occurs in a time-dependent manner as the solitary wave packet passes through the environment. Both horizontal refraction of energy and a time-dependent modal source excitation distribution are responsible for these beamforming effects. In cases where source-receiver orientation is not approximately parallel to the wave crests, these effects are substantially reduced or eliminated, indicating that an azimuthally selective perturbation of the acoustic field can be attributed to the wave packet. Modal decomposition of the acoustic field and single mode starting fields are used to infer that, for the source-receiver orientation along the wave crests and troughs, acoustic propagation is predominantly adiabatic. A modal phase speed analysis explains several features associated with the beamformed power.     

基于时反算子分解的时反高分辨率定位技术研究

基于时反算子分解技术,提出了主动时间反转选择性定位方法。为提高此定位方法的分辨率,根据时反算子和协方差矩阵的相似性,提出了时反MUSIC和对角线加载时反MVDR定位技术。传统的获取时反算子的方法是通过单个阵元依次发射,该方法的缺点是每次只有单个元发射,导致输入信噪比较小,本文提出通过多个波束正交发射获取时反算子,能得到较大的输入信噪比,从而改善三种定位方法的定位性能。波导实验分别对三种定位方法和正交发射技术进行了验证,结果显示三种定位算法中,子空间基于的时反MUSIC方法具有较好的定位精度,时反MVDR技术具有较高的分辨率。      

Time reversal imaging of suspended target in strong reverberation

The weighted wideband time-reversal imaging approach of full signal subspaces was proposed.The extended target is modeled as an infinite number of independent point-like scatterers,and the imaging is constructed by accumulating all time-reversal images over all signal subspaces and the entire bandwidth in the region of suspected targets.On the bottom of a laboratory water waveguide,a cylindrical shell was used to produce reverberation.Two experiments were carried out.The first experiment is that an extended target was suspended near the cylindrical shell,whose echo and the reverberation reflected by the cylindrical shell were in different time windows.The second experiment is that a point-like target was suspended above the cylindrical shell,thus the echo and the reverberation were in the same time window.The experimental results show that the imaging quality of the proposed method is better than that of the conventional time reversal imaging methods for weak suspended targets in a strong reverberation background.     

强混响背景下悬浮目标的时反成像

提出了一种加权宽带全信号子空间时反成像方法,将扩展性目标建模为有限数量个、相互独立的点散射体构成,实现了时反成像在可疑目标区域的全信号子空间和整个宽带的累加。在实验室波导水池中,底部放置圆柱筒制造混响背景,共进行了两个实验,一是扩展性的悬浮目标位于圆柱筒附近,其回波与圆柱筒反射的混响不在同一个时间窗内,二是弱点目标回波与圆柱筒反射的混响重叠于同一个时间窗内,对所提出的方法进行强混响背景下弱悬浮目标的成像能力验证。结果表明,此方法对强混响背景下的弱悬浮目标的成像质量明显优于传统的时反成像方法。      

Modal time reversal of waves for shallow-water sea

A method is proposed for calculating the time-reversed wave field generated by a point source in a waveguide by using signals received by a vertical antenna array. The procedure of time reversal is based on representing the wave field in the form of the decomposition into modes of the ideal waveguide. In contrast to the earlier proposed simplified numerical method of time reversal of waves, the method presented here allows one to obtain the reversed field for the entire thickness of the waveguide. The method is successfully applied to a shallow-water sea with a depth of 120 m, at distances of 7, 10.5, and 12 km. It is shown that an opportunity arises to increase the gain of the array; to determine the parameters of the medium, including its stability in the presence of currents; and to match the point of transmission of an arbitrary received signal and the point of transmission of the reversed signal.     

Combination of time reversal and synthetic aperture beamforming for active detection of small bottom objects in waveguide environments

To enhance detection of small targets, the combination of time reversal processing (TRP) and synthetic aperture beamforming (SABF) is investigated. With the spatial–temporal focusing, the potential application of TRP for active detection has been demonstrated [Kim S, Kuperman WA, Hodgkiss WS. Echo-to reverberation enhancement using a time reversal mirror. J Acoust Soc Am 2004;115(4):1525–31]. When a physical probe source (PS) replaced by a modeled source (MS), the “potential” is turned into being more practical. Similar to matched field processing, the robustness of TRP with MS needs to be considered. Meanwhile by the improvement of the extended towed array measurement (ETAM) algorithm of passive SABF, a segmented ETAM algorithm is discussed for its use in active sonar. With the echo-signal enhancement by time reversal transmission, the echo-to-reverberation ratio is further improved by SABF. Finally a matched filter is used to detect the target and the range of the target is estimated by the time delay referenced to the transmission time. The results of the waveguide tank experiment demonstrate that the TRP–SABF method can effectively detect and locate a bottom cylinder shell of 0.51 m long and 0.21 m in diameter.     

Comparison of a subrank to a full-rank time-reversal operator in a dynamic ocean

This paper investigates the application of time-reversal techniques to the detection and ensonification of a target of interest. The focusing method is based on a generalization of time-reversal operator techniques. A subrank time-reversal operator is derived and implemented using a discrete set of transmission beams to ensonify a region of interest. In a dynamic ocean simulation, target focusing using a subrank matrix is shown to be superior to using a full-rank matrix, specifically when the subrank matrix is captured in a period shorter than the coherence time of the modeled environment. Backscatter from the point target was propagated to a vertical 64-element source-receiver array and processed to form the sub-rank time-reversal operator matrix. The eigenvector corresponding to the strongest eigenvalue of the time-reversal operator was shown to focus energy on the target in simulation. Modeled results will be augmented by a limited at-sea experiment conducted on the New Jersey shelf in April-May 2004 measured low-frequency backscattered signal from an artificial target (echo repeater).