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。     

Research on the acoustic scattering function and coherence properties from rough seafloor based on finite element model

Acoustic scattering from a rough sea bottom is recognized as a main source of reverberation. In this study, scattering properties from a layered bottom were exploited based on the finite element model. The scattering strength and loss from the layered rough seabed were investigated by ensembling the realizations of rough interface. They were found to be dependent on the thickness of sediment, and interference was significant in the case of thin sediment. Through verification of the finite element model, the scattering loss could be evaluated using the Eckart model with a proper sound speed in the thick sediment. The multiple scattering effect on the sound field was also exploited. It revealed that the effect depended strongly on the bottom type.     

浅海近程混响的振荡现象*

研究浅海近程混响特性对于评估和提高主动声纳性能具有重要意义。多次浅海混响实验显示,近程混响强度存在稳定的振荡现象,脉宽基本对振荡的幅度和周期没有影响。为解释这一现象,本文基于射线理论和小斜率近似给出了浅海近程混响模型,仿真与实测数据结果基本吻合。数值仿真结果表明：海底反射声场对单站声纳接收到回声信号的贡献远小于海底近垂向大掠射角散射声场的作用;混响强度振荡现象是海底近程散射声场的多途现象造成的,并由此给出了振荡周期与海深及收发深度的关系。     

浅海小掠射角的海底界面声反向散射模型的简化

在浅海,尤其是负梯度声速剖面和海面较为平静的浅海波导,海底界面反向散射是浅海混响的主要来源.经验散射模型只适用于分析浅海混响平均强度衰减特性,而基于物理机理建立的反向散射模型克服了这一缺陷,但同时也引入了其受地声模型约束的问题.本文结合了海底反射系数的三参数模型,对浅海远场海底反向散射模型进行了简化,以减少地声模型的输入参数.理论分析了海底反射系数的相移参数可以描述海底对声场的散射作用,无需任何海底地声参数的先验知识.通过对海底反向散射模型近似简化,结果表明在临界角附近和甚小掠射角范围内的海底粗糙界面反向散射模型的角度特性和强度特性受海底沉积层的影响不同:在临界角附近,海底反向散射的角度特性受海底反射系数的相移参数加权,而其散射系数则近似与相移参数无关;对于甚小掠射角,海底反向散射的角度特性近似与海底反射系数的相移参数无关,其散射系数则近似与相移参数的4次方成正比.     

界面混响的耦合简正波理论研究

利用耦合简正波理论分析了粗糙界面散射引起的混响。引入了粗糙海底以及海面引起的各个模态间的耦合系数表述粗糙界面对声场的散射过程。结合风浪谱Pierson模型,仿真了不同海况下的海面混响,重点在于分析不同海况下海面起伏对海底混响的影响。利用耦合系数的求解从粗糙海面引起模态间能量耦合的角度对这种影响进行了机理性的探究。结果表明,尽管在浅海中海底混响占支配性地位,但随着海况等级的增加,海面散射对海底混响是有影响的,而这种影响可以从粗糙界面对声波模态间的耦合进行机理分析。      

双基地冰-水界面混响强度的理论预报

针对北极冰层冰水界面具有小尺度粗糙界面的情况,给出由冰层造成的混响平均强度的理论预报公式。首先将北极冰层等效为具有粗糙界面的弹性介质,并且采用小粗糙度微扰理论在精细拟合粗糙度谱的基础上,建立粗糙冰层的三维散射强度模型,然后建立三维双基地声呐几何模型确定有效散射区域,最后给出双基地冰下混响平均强度的估算公式,并在所拟合的粗糙度谱的情况下,利用该预报公式计算双基地声呐配置参数和海冰物理及声学特性对冰下混响强度的影响算例。数值仿真表明该公式能够估算出北极冰下双基地声呐产生的混响平均强度,并且分析出了时延、基线长度等声呐配置参数和冰层声速比、密度比等海冰物理参数会对混响强度造成的影响。声呐的配置参数主要影响散射的有效面积,海冰的物理参数则影响着冰层的散射强度。其中,冰层声速比不仅影响混响强度的大小,还会影响混响强度随时间的衰减速度。      

An oscillation phenomenon of low frequency reverberation in the shallow water and its physical explanation

~~An oscillation phenomenon of low frequency reverberation in the shallow water and its physical explanation1 Zhang, R.. Jin. G. Normal-mode theory of average reverberation intensity in shallow water, Journal of Sound and Vibration, 1987 19(2):215-223 2. Ellis, D. D., A shallow-water normal-mode reverberation model, J. Acoust. Soc, Am., 1995, 97(5): 2804-2814. 3. McDaniel, S. T., Seafloor reverberation fluctuations, J. Acoust. Soc. Am., 1990, 88(3): 1530-1535. 4. Abraham, …     

Application of small-roughness perturbation theory to reverberation in range-dependent waveguides

A rough-interface reverberation model is developed for range-dependent environments. First-order perturbation theory is employed, and the unperturbed background medium can be layered and heterogeneous with arbitrary range dependence. To calculate the reverberation field, two-way forward scatter due to the slowly changing unperturbed environment is handled by fast numerical methods. Backscatter due to small roughness superimposed on any of the slowly varying interfaces is handled efficiently using a Monte Carlo approach. Numerical examples are presented to demonstrate the application of the model. The primary purpose of the model is to incorporate relevant physics while improving computational speed.     

Time reversed reverberation focusing in a waveguide

Time reversal mirrors have been applied to focus energy at probe source locations and point scatterers in inhomogeneous media. In this paper, we investigate the application of a time reversal mirror to rough interface reverberation processing in a waveguide. The method is based on the decomposition of the time reversal operator which is computed from the transfer matrix measured on a source-receiver array [Prada et al., J. Acoust. Soc. Am. 99, 2067-2076 (1996)]. In a similar manner, reverberation data collected on a source-receiver array can be filtered through an appropriate temporal window to form a time reversal operator. The most energetic eigenvector of the time reversal operator focuses along the interface at the range corresponding to the filter delay. It is also shown that improved signal-to-noise ratio measurement of the time reversal operator can be obtained by ensonifying the water column with a set of orthogonal array beams. Since these methods do not depend upon a priori environmental information, they are applicable to complex shallow water environments. Numerical simulations with a Pekeris waveguide demonstrate this method.     

Modeling interface roughness scattering in a layered seabed for normal-incident chirp sonar signals

Downward looking sonar, such as the chirp sonar, is widely used as a sediment survey tool in shallow water environments. Inversion of geo-acoustic parameters from such sonar data precedes the availability of forward models. An exact numerical model is developed to initiate the simulation of the acoustic field produced by such a sonar in the presence of multiple rough interfaces. The sediment layers are assumed to be fluid layers with non-intercepting rough interfaces.     

Effects of rough interface on impedance of solid LiPON in MIM cells

The influences of the rough interface on the impedance of solid lithium phosphorus oxynitride (LiPON) in its metal/insulator/metal (MIM) cells over a wide frequency are studied. Using magnetron sputtering method, Al/LiPON/Al cells with different rough interfaces are fabricated. With increasing the Al films electrode roughness, the deposited LiPON films on these electrodes are correspondingly roughened, which result the interface in MIM cell roughened. Rough LiPON films make their ionic resistance decreasing and the real contact area and the bulk capacitance of MIM cells increasing. These lead to the ionic conductivity of LiPON films improved from 1.09 to 2.70 μS/cm. Meanwhile, the bulk capacitance and the electrolyte resistance in MIM cells could be separated by changing the interface roughness. Rough interfaces in MIM cells contribute to the differences in the morphology of LiPON thin films, which result in the decrease of the activation energy of ionic conductivity E _g from 0.45 to 0.38 eV. Without changing LiPON components ratio and sputtering conditions for the thin films deposition, or using any heat treatment, we demonstrate an effective way to improve the ionic conductivity by increasing the roughness of the contact electrode in MIM cells, corresponding roughening the LiPON films and decreasing its ionic impedance.     

Reverberation at the Mid-Atlantic Ridge during the 1993 ARSRP experiment seen by R/V Alliance from 200-1400 Hz and some modeling inferences

In July 1993 SACLANTCEN participated in an experiment for the Acoustic Reverberation Special Research Program (ARSRP). The primary objective was to take high resolution measurements to shed light on the detailed physical processes dominating the low frequency scattering from rough topographic features and from deep sediment pond areas. A very detailed set of monostatic and bistatic scattering experiments were conducted just west of the Mid-Atlantic Ridge near 26 degrees N, 47 degrees W. Results from SACLANTCEN's R/V ALLIANCE are presented which show monostatic and bistatic scattering from selected bathymetric features using pulsed sources from 200-1400 Hz and SUS charges. Area water depths ranged from 3300 to greater than 5200 m. The receivers were a horizontal array of 128 elements spaced at 0.5, 1, and 2 m. Source depths varied from 120 to 1200 m and receiver depths ranged from 400 to 500 m. Received reverberation levels are presented as functions of the estimated scatterer position on area bathymetry maps. Results show the same high scattering areas from one seamount were observed in three distinct frequency bands centered at 227, 350, and 700 Hz. Even at 1350 Hz the correlation of scattering highlights with nearby bathymetric features is quite good. Comparisons of measured reverberation and modeled reverberation are presented which suggest that the dominant longer range scattering from the large features is driven by the steep slopes present above the critical depth but that small scale roughness is also essential for strong backscatter.     

The surface cracking behavior in air plasma sprayed thermal barrier coating system incorporating interface roughness effect

The objective of this work is to understand the effect of interface roughness on the strain energy release rate and surface cracking behavior in air plasma sprayed thermal barrier coating system. This is achieved by a parameter investigation of the interfacial shapes, in which the extended finite element method (XFEM) and periodic boundary condition are used. Predictions for the stress field and driving force of multiple surface cracks in the film/substrate system are presented. It is seen that the interface roughness has significant effects on the strain energy release rate, the interfacial stress distribution, and the crack propagation patterns. One can see the completely different distributions of stress and strain energy release rate in the regions of convex and concave asperities of the substrate. Variation of the interface asperity is responsible for the oscillatory characteristics of strain energy release rate, which can cause the local arrest of surface cracks. It is concluded that artificially created rough interface can enhance the durability of film/substrate system with multiple cracks.     

Mechanisms for subcritical penetration into a sandy bottom: experimental and modeling results

This paper presents preliminary results of a recent study whose overall objectives are to determine the mechanisms contributing significantly to subcritical acoustic penetration into ocean sediments, and to quantify the results for use in sonar performance prediction for the detection of buried objects. In situ acoustic measurements were performed on a sandy bottom whose geoacoustical and geomorphological properties were also measured. A parametric array mounted on a tower moving on a rail was used to insonify hydrophones located above and below the sediment interface. Data covering grazing angles both above and below the nominal critical angle and in the frequency range 2-15 kHz were acquired and processed. The results are compared to two models that account for scattering of sound at the rough water-sediment interface into the sediment. Although all possible mechanisms for subcritical penetration are not modeled, the levels predicted by both models are consistent with the levels observed in the experimental data. For the specific seafloor and experimental conditions examined, the analysis suggests that for frequencies below 5-7 kHz sound penetration into the sediment at subcritical insonification is dominated by the evanescent field, while scattering due to surface roughness is the dominant mechanism at higher frequencies.     

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 monoand the bi-static cases.     

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.     

粗糙接触界面超声非线性效应的概率模型

提出描述粗糙接触界面超声非线性效应的概率模型,利用分段均匀概率函数描述粗糙接触界面劲度系数变化,结合表面粗糙峰的几何分布特征,得到界面粗糙度和两侧表面相对运动对声波的非线性调制作用。实验观测了铝合金材料的粗糙接触界面的高次谐波现象和阈值现象,进一步分析了归一化非线性参数与界面加载压力、粗糙度之间的关系。实验测量结果与概率模型的理论预测一致,证明了该模型的正确性,为利用超声非线性效应评价粗糙接触界面提供了理论依据。      

Low frequency seabed scattering at low grazing angles

Low-frequency (LF) seabed scattering at low grazing angles (LGA) is almost impossible to directly measure in shallow water (SW), except through inversion from reverberation. The energy flux method for SW reverberation is briefly introduced in this paper. The closed-form expressions of reverberation in an isovelocity waveguide, derived from this method, indicate that in the three-halves law range interval multimode/ray sea bottom scattering with different incident and scattering angles in forming the reverberation may equivalently be represented by the bottom backscattering at a single range-dependent angle. This equivalent relationship is used to derive the bottom backscattering strength (BBS) as a function of angle and frequency. The LF&LGA BBS is derived in a frequency band of 200-2500 Hz and in a grazing angle range of 1.1°-14.0° from reverberation measurements at three sites with sandy bottoms. This is based on three previous works: (1) The closed-form expressions of SW reverberation [Zhou, (Chinese) Acta Acustica 5, 86-99 (1980)]; (2) the effective geo-acoustic model of sandy bottoms that follows the Biot model [Zhou et al., J. Acoust. Soc. Am. 125, 2847-2866 (2009)] and (3) A quality database of wideband reverberation level normalized to source level [Zhou and Zhang, IEEE J. Oceanic Eng. 30, 832-842 (2005)].     

浅海海底反射系数幅值参数的反演

理论分析了一种通过混响强度衰减特性获取海底反射系数的幅值参数的方法.将海底反射系数的幅值参数和相位参数引入到全波动混响模型中,为海底反射系数的反演提供理论基础。理论分析和数值仿真表明,在小掠射角条件下,利用混响强度衰减特性反演海底反射系数幅值参数的可行性和准确性。该反演方法只需要输入4个变量:本地混响强度的衰减特性,反射系数的相位参数,海深以及海深处的声速,同时要求混响数据具有一定的混响噪声比(大于6 dB)才能够使反演结果准确可信。根据本地静态混响实验数据成功反演得到海底反射系数的幅值参数.      

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。