水下掩埋目标的散射声场计算与实验

水下掩埋目标声散射问题是识别和探测掩埋目标的理论基础, 是声散射研究领域的热点问题. 本文基于射线声学推导了掩埋情况下目标声散射计算的格林函数近似式, 并在此基础上进一步给出了相应的远场积分公式. 在有限元方法的基础上, 将推导得到的公式写入有限元仿真软件, 对软件功能进行拓展, 构建二维轴对称目标的声散射模型, 并计算掩埋情况下弹性实心球在不同条件下的目标强度, 获得了其散射声场随频率、掩埋深度、沙层吸收系数等参数的变化规律. 开展实心球的自由空间和浅掩埋条件下水池声散射实验, 利用共振隔离技术处理实验数据, 提取目标声散射的纯弹性共振特征进行分析, 结果表明可将其用于掩埋目标识别和探测. 最后利用总散射声场与理论计算结果进行对比, 验证了理论仿真的正确性.     

Acceleration target detection based on LFM radar

In radar systems, the echo signal caused by an accelerated target can be similarly considered as linear frequency modulation (LFM) signal. In high signal-to-noise ratio (SNR), discrete polynomial-phase transform (DPT) algorithm can be used to detect the echo signal, as it has low computation complexity and high real-time performance. However, in low SNR, the DPT algorithm has a large mean square error of the rate of frequency modulation and a low detection probability. In order to detect LFM signal in low SNR, this paper proposes a detection method, segment discrete polynomial-phase transform (SDPT), which means, at first, dividing the whole echo pulses into several segments with same duration in time domain, and then, using coherent accumulation method of DFT to segments, at last, processing this signal with DPT in intra-segment. In the case of a large number of segments, the SDPT can improve the output SNR. In addition, in a certain SNR, to the target signal with big sampling interval, large acceleration and less segments, this paper proposes an algorithm to detect the LFM signal generated from the combination of an improved DPT (IDPT) and fractional Fourier transform (FRFT). The output SNR of this algorithm is connected with the length of time delay. In the simulation, when the length of the time delay is 0.2 N, the output SNR is 2.5 dB more than that which results from directly using DPT. Finally, the detection performance and algorithm complexity of the proposed algorithm were analyzed, and the simulated and measured data verify the effectiveness of the algorithm.     

正交多相码连续波主动声呐回波检测算法设计

与常规脉冲式主动声呐相比,连续波主动声呐能够提高目标回波的时间处理增益和目标信息更新速率。该文提出一种由正交多相码合成的连续发射波形,分析了发射信号和目标回波模型,设计了多通道匹配滤波器组完成回波检测。为了进一步提高接收机检测性能,提出了多通道非相干积累的处理方法。通过数值仿真,分析提出的连续波形的多普勒分辨性能和目标信息更新率。通过蒙特卡罗法获取接收机工作特性曲线,比较了脉冲式主动声呐和连续波主动声呐在均匀混响背景下对单目标检测性能的差异。仿真结果表明,该文设计的连续波波形具有较好的多普勒分辨性能,在均匀混响背景下,回波检测算法能够明显提高单个目标的探测能力。     

Measurement of the echo reduction for underwater acoustic passive materials by using the time reversal technique

A measuring method of the echo reduction of passive materials by using the time reversal(TR) technique is presented. To measure the echo reduction of a sample with this approach, the received signals are firstly focused according to the TR theory. Then, the sample is removed and the TR processing is again employed to realize the focus of the received signal.Finally, the echo reduction of the sample is evaluated with these focusing signals. Besides, to calibrate the measured echo reduction via the TR technique, a standard sample is employed to measure a constant coefficient that only depends on the measurement environment. An aluminum plate sample and a steel plate sample with the same size of 1.1 mxl.O m x0.005 m axe tested in a wave guide tank. The experimental results show that the calibrated values are well consistent with theoretical results under the free field at the measured frequency range of0.5-20 kHz. The relative errors of all the measured values are less than 10% and the values of the expanded uncertainty are less than 1.5 dB. The TR processing focuses the energy in spatial domain and temporal domain, so it can be used to measure the echo reduction of passive materials in the environments with reflections induced by boundaries and low frequency sources.     

水下目标回波与混响的时频形态特征域盲分离

水下沉底或掩埋目标识别受海底混响干扰严重,并且目标回波与混响在时-频域上均存在混叠,增大了对目标回波与混响的分离难度。根据目标回波与混响的产生机理差异,将图像形态学与时频分析相结合的盲分离算法用于目标回波与混响的分离。推导了目标几何声散射成分与混响在Wigner-Ville时频面的形态特征表达式,利用图像形态学滤波去除Wigner-Ville时频面中的混响及刚性亮点之间交叉项,在时频形态特征域获取解混矩阵,实现了目标回波和混响的分离。仿真与实验数据处理结果表明,结合图像形态学的时频域盲分离算法提高了目标回波信号的信混比。      

水中分层弹性球壳高频时域回波的声学编码研究

针对水下集群目标及敌我目标识别的难题,该文提出了一种基于水中分层弹性球壳高频时域回波的声学编码原理及方法。推导了水中4层弹性球壳目标散射声压的简正级数解,并与有限元结果进行了对比验证。通过构造高频主动声呐的探测脉冲信号,与4层弹性球壳声传递函数的简正级数解做卷积运算,获得了目标的时域回波脉冲序列。研究了分层弹性球壳的厚度、各层材料属性、排布顺序等对时域回波特征的影响规律,提出了基于时域回波特征的声学编码方法。研究表明：利用水中分层弹性球壳目标高频时域回波特征能够实现声学编码,回波结构稳定,且不受限于探测方向。通过携带或安装这种分层弹性球壳结构,有望识别水下航行体/悬浮体等目标。该文的研究对水下目标的主动探测身份识别及导航等具有一定的参考价值。     

水声无源材料回声降低测量时反聚焦方法

提出了利用时间反转(时反)聚焦技术的水声无源材料回声降低测量方法。首先实现有界水声环境下有、无试样接收信号的时反聚焦,然后利用聚焦信号测量试样反射系数,最后通过标准试样对反射系数进行修正,从而得到试样的回声降低测量值。在小型水池中进行了铝板试样和钢板试样回声降低的测量,铝板试样的尺寸为1.1m×1.0×0.005m,测量频率范围为3~20kHz,钢板试样尺寸与铝板试样相同,测量频率范围为0.5~20kHz,测量结果经修正后与平面波理论计算值基本一致,和理论计算值的相对误差小于10%,扩展不确定度小于1.5dB。本方法采用时反原理实现了接收信号的空时聚焦,提高了测量信混比,因此适用于非自由场环境下无源材料回声降低的测量试验,尤其适用于低频的回声降低测量。      

Robust minimum-variance time-reversal beamforming for target detection and localization in uncertain environment

The target detection and localization in uncertain environment with robust time reversal （TR） technique was investigated. TR is a physical process as well as a method of signal processing. Therefore, it is natural to perform beamforming via TR, i.e., TR beamforming （TRBF）. To reduce the effects of environmental uncertainty on TR spatio-temporal focusing, transmitting TRBF with modeled instead of physical probe source was studied and robust minimum-variance receiving TRBF with diagonal loading was put forward. Both of them were applied to the detection and distance estimation of the target, which was dealt with in a waveguide experiment. The experimental results show the validity of the methods in uncertain environment.     

时间反转和逆时偏移混合法用于层状介质中目标的声学检测和定位

一般超声回波检测、时间反转和逆时偏移技术,都不能将界面附近的目标(缺陷)检测出来。这是因为目标反射波和界面(或底面)反射波相重叠的缘故。虽然改进的时间反转法(即换元的时间反转法)能够区别目标和界面(或底面),但还是不能定位。本文提出一种时间反转和逆时偏移混合方法。分析和仿真结果表明该方法不但能够鉴别出层状介质中的目标,而且可以定位。对于多个目标,也能给出其位置分布图。特别地,本文方法对于分层介质中的缺陷和浅海海底下的目标检测和定位具有重要的潜在价值。      

Acoustic field analysis of detection and location of targets in layered media by time reversal-reverse time migration mixed method

Using the time reversal(TR)-reverse time migration(RTM) mixed method, the numerical analysis towards the acoustic field for detection and location of targets in layered media is performed. First the approaches and steps of the numerical analysis and simulation for acoustic field are proposed, then, the acoustic field distribution of single target and multiple targets are further computed by using the approaches of single emission-multiple receiving and multiple emission-multiple receiving. The numerical simulation results indicate that target detection and location can be well realized. By comparison, good results are obtained from the both two approaches, but large time-consuming is observed in way of multiple emission-multiple receiving, and so would be unacceptable. Additionally, the improved way is also proposed for multiple target detection.     

多基地空时码探测信号设计及时反相关检测技术

为实现浅海复杂环境下的多基地声纳多源目标回波分辨,本文设计了一种适用于多入多出垂直阵信道环境下的空时码探测信号,并针对倾斜垂直阵的多途子信道差异问题,提出了信号的时反相关检测技术.空时码探测信号采用伪随机信号调制,具有良好的正交性,能在抗子信道严重衰落的同时,分辨多源目标回波.垂直阵受水流冲击,呈倾斜状态时,其多途子信道不一致性会导致各子信道传递信号无法在接收端聚焦,使阵列增益受损,同时导致时延测量能力下降和信号判决错误率上升,为此本文设计了信道训练信号用以估计多途子信道环境,通过虚拟时间反转镜获得子信道不一致条件下的最佳匹配检测信号,实现对接收信号的时反相关检测.仿真结果表明,本文所设计的探测信号和检测方法,能够克服复杂的信道条件和多途子信道不一致性引起的检测问题,满足多基地声纳探测需求,实现多源目标回波分辨.     

水下连续波有源探测的回波检测算法

研究线性调频连续波在水下探测中的信号模型、回波信号的检测和目标参数估计方法,通过分析回波信号与发射信号间的差拍信号,提出差拍-分数阶傅里叶变换结构的处理算法。为了减少分数阶傅里叶变换二维搜索时的计算量,首先使用Radon-Ambiguity变换估计差拍信号的调频斜率,再做相应阶次的分数阶傅里叶变换。数值仿真说明提出的算法能够有效消除线性调频连续波的距离-速度耦合现象,准确估计目标参数。通过湖上试验比较线性调频脉冲信号和线性调频连续波的处理增益,提出的算法处理的连续波比匹配滤波方法处理的脉冲信号处理增益大13 dB。研究结果表明该算法在连续波水下探测中可行有效,不仅能够准确估计目标参数,还具有良好的检测性能。      

Increasing the speed of relaxometry-based compartmental analysis experiments in STEAM spectroscopy

In this work we present a method for improving the speed of spin-spin relaxation time (T2) measurements for compartmental analysis in stimulated echo localized magnetic resonance spectroscopy without reducing the sampling density. The technique uses a progressive repetition time (TR) to compensate for echo time (TE) dependent variations in saturation effects that would otherwise modulate the received signal at short TRs. The method was validated in T2 studies on 10 young healthy subjects in spectroscopic voxels localized along either the right or left Sylvian fissure (2 x 2 x 1.5 cm3, 10 ms mixing time (TM), 2048 data points, 819.2 ms acquisition time). The TR was automatically adjusted so that TR-TM-TE/2 was kept constant as the TE was incremented. Compared to long TR T2 experiments, the progressive TR technique consistently replicated the T2 relaxation times and reference signals of the tissue water compartment while reducing the data acquisition time by more than 50%. The percent error was on average less than 2% for estimates of T2 and S(0) for the tissue water, an indication that the progressive TR technique is a useful method for determining the tissue water signal for internal referencing.     

Dual-echo breathhold T(2)-weighted fast spin echo MR imaging of liver lesions

The purpose of this study was to develop a multi-shot dual-echo breathhold fast spin echo technique (DFSE) and compare it with conventional spin echo (T2SE) for T(2)-weighted MR imaging of liver lesions. The DFSE acquisition (EffTE1/EffTE2/TR = 66/143/2100 ms) imaged 5 sections per 17 s breathhold. T2SE imaging (TE1/TE2/TR = 60/120/2500 ms) required 16:55 (min:s) for 14 sections. Both techniques used a receive-only phased-array abdominal multicoil and provided 192 x 256 effective resolution. The results showed first and second echo relative DFSE/T2SE contrast values for 27 representative lesions (15 consecutive patients) were 1.08 +/- 0.05 and 1.16 +/- 0.09 (mean +/- STD mean), respectively. Corresponding CNR values were 1.12 +/- 0.09 and 0.97 +/- 0.12. Overall DFSE was comparable-to-superior to T2SE for lesion sizing and image artifact. DFSE lesion detection was inferior to T2SE's in several patient studies because of decreased conspicuity of lesions located near multicoil edges and because of poor breathhold-to-breathhold reproducibility and lack of breathholding. However both DFSE (and T2SE) provided lesion detection rated to be of diagnostic quality for all patient studies. In conclusion, we found that DFSE provides diagnostically useful dual-echo T(2)-weighted MR liver images in a greatly decreased acquisition time.     

浅海负跃层条件下的双基地有源探测实验及定位声速修正

为了研究浅海条件下海底固定水平阵和机动声源的双基地有源探测性能,建立了浅海双基地有源探测仿真模型,分析了实验海区负跃层条件下的传播损失和多途能量扩展损失,实现了双基地有源探测的性能预估。以模型仿真为基础,在南海北部海域开展了一次浅海双基地有源探测实验.针对定位中的声速与实验中目标回波的脉冲传播速度的偏差导致定位精度下降的问题,提出了一种目标回波脉冲传播速度近似估计方法。实验结果表明,双基地有源探测可在浅海负跃层条件下实现对水中目标的有效探测,多个实验站位的回波信噪比实测值与仿真预测值符合较好,定位声速近似估计方法可进一步提高定位精度。      

目标粗糙对合成孔径激光雷达回波的退相干效应

本文研究了目标表面粗糙对回波信号探测的影响,通过蒙特卡罗方法建立一维和二维目标高斯随机粗糙面模型,并对目标回波信号的探测过程进行仿真,研究目标表面粗糙对中频信号的影响.还进行了粗糙面和光滑面两组光外差探测实验,验证目标粗糙对合成孔径激光雷达回波信号严重的“退相干”效应.同时,采用数字波前分析仪对探测器处本振光和信号光波前分别进行检测,检测结果与仿真一致,证明粗糙面回波相位畸变严重.文中研究结果对设计光源参数、接收系统参数以及评估系统作用距离奠定基础,为合成孔径激光雷达系统设计提供定量参考.     

掩埋目标声散射特性及其实验

基于变分原理建立2D-FE弱形式解模型,针对轴对称掩埋目标进行散射远场的快速、高精度数值计算。入射波垂直于对称轴,给出:掩埋深度不变,以1°为间隔,改变掠射角所得到目标散射声压级相对于垂直海底照射获得结果的差值随频率、掠射角变化的表达式;掠射角不变,以0.1m为间隔,改变掩埋深度得到的散射声压级相对于浅掩埋条件下获得结果的差值随频率、深度变化的表达式。实验结果表明:在回波最强的正横方向,掩埋弹性目标表现出的共振散射特性与在自由场空间中具有相似性,表明了掩埋条件下目标内填充的透射和表面环绕波理论的适用性,验证了大掠射角入射得到散射声压级差值的变化规律。研究成果对宽带、高频入射声波探测更深掩埋目标提供思路。      

Target detection by an echolocating harbor porpoise (Phocoena phocoena)

Two echolocation experiments are described. They were conducted on the same harbor porpoise housed in a sea pen, one year apart at Neeltje Jans, The Netherlands. The aims were to determine the target detection ability of an echolocating harbor porpoise, with the ultimate goal to predict the distance at which harbor porpoises can detect fishing nets. In experiment 1, the maximum distance at which the 3-year-old porpoise could detect a 7.62-cm diameter water-filled stainless-steel sphere by echolocation was determined psychophysically. The 50%-current detection threshold was reached when the sphere was at a distance of 26 m from the porpoise's rostrum. In experiment 2, conducted a year later, the maximum detection distance for a 5.08-cm water-filled stainless-steel sphere was 15.9 m. The target strengths of both targets were measured using simulated harbor porpoise echolocation signals and the results, coupled with transmission-loss calculations, indicated that the echo levels received by the porpoise with the targets at the threshold ranges in the two experiments were only 1.3 dB apart. Together with information on the target strengths of various fishing nets, the results of the present study can be used to predict the distance at which the nets can be detected by harbor porpoises.     

基于双向反射函数的水下激光引信回波仿真方法

针对鱼雷激光近炸引信探测水下近场目标的需求,开展了水下激光引信回波蒙特卡洛仿真方法研究.结合水下激光引信探测特点建立水下目标回波的蒙特卡洛仿真模型.为了提高水中非朗伯目标表面回波仿真的准确度,推导了基于双向反射函数的光子反射方向概率分布,根据概率分布随机抽样光子反射方向.仿真了不同距离和入射角度条件下的水中目标回波信号.仿真结果表明:目标回波幅度随目标距离和入射角度的增大迅速下降,目标距离在6~12m内变化时,信号峰值动态范围为11.5dB;目标距离为8m,激光入射角在0~45°内变化时,信号峰值动态范围为9.2dB.为验证仿真方法的正确性,在水池中进行水中目标蓝绿激光探测实验,实验结果和仿真结果一致.研究成果可为解决传统蒙特卡洛方法在水中非朗伯面目标回波仿真中的适用性问题及水下激光引信优化设计提供参考.     

Introduction of fast MR imaging in the assessment of hepatic steatosis

We determined the utility of fast gradient echo techniques (modified Dixon method) in the assessment of hepatic fat content. Fast spoiled gradient echo was performed on bovine liver/corn oil homogenates with known fat fractions (FF_E) to assess the accuracy of fat quantitation (FF_MRI). The pulse sequence was manipulated via alterations in TE (echo time), TR (repetition time), and α (flip angle). In vivo studies were then performed using breath-holding maneuvers on normal adult volunteers and subjects at risk to develop hepatic steatosis, with cystic fibrosis or morbid obesity. At out-of-phase, TE, TR, and α were 2.1 ms, 7.3 ms, and 30–50° and in-phase TE, TR, and α were 4.2 ms, 9.3 ms, and 30–50°; FF_MRI correlated well with FF_E. An elevated fat fraction was observed in a high percentage of subjects with cystic fibrosis and morbid obesity. Fast gradient echo techniques were used successfully in the assessment of hepatic steatosis. The reduced acquisition times permitted in vivo analysis on adults and children using breath hold maneuvers.