On the detection of objects buried at a shallow depth using seismic wave reflections

This paper concerns the detection of shallow (of the order 1 m) buried objects using seismic excitation. Time-extended signals are used to generate a compressional wave using a shaker attached to the ground. The wave propagates through the ground, reflects off a buried object and is captured by an array of geophones on the surface. The envelopes of the cross-correlation functions between the measured ground velocities and the excitation signal are calculated and summed to generate a cross-sectional image of the ground. The wide cross-correlation peaks caused by high ground attenuation are partially compensated for by using the generalized cross-correlation function called the phase transform. Simple simulations are conducted to demonstrate the method, and some field experiments have been carried out aimed at the detection of a buried concrete pipe. In the experiments the pipe could be detected using the method proposed, with experimental and simulated data producing good agreement.     

自来水铸铁管道泄漏声信号频率特征研究

针对基于声发射技术的自来水管网泄漏检测定位方法，研究因泄漏而形成的声信号频率分布及不同泄漏量对频率分布的影响。实际泄漏检测时，通常采用互相关法估计泄漏信号到达不同传感器间的时间延迟实现漏点定位，因此，借助互相关分析法研究了管道的不同口径及泄漏信号传播距离对泄漏信号频率分布的影响。同时，泄漏声信号的传播不可避免要经过管道间的接口，因此分析了两种管道接口对信号频率成分的影响。进而为设计合理的管道泄漏检测过程提供依据，并为泄漏声信号形成及多种因素对泄漏声信号特征产生影响的机制研究奠定基础。     

浅海多径时延估计的功率谱加权自相关方法

针对浅海近距离时倒谱多径时延峰变弱难以提取的问题,提出了一种功率谱加权自相关多径时延估计方法。该方法采用FFT估计信号功率谱,对功率谱取1-α次幂(0<α<1),再进行IFFT变换,对其实部进行峰值检测获得多径时延估计。根据目标能量通过特性在正横前递增正横后递减的特点,在已检测时延前后一定区间内搜索下一个时延值,实现多径时延的自动提取.理论分析表明,多径时延估计的倒谱法与自相关法都可以看作功率谱加权自相关法的特例.海试数据处理与基于Bellhop射线模型的仿真计算表明,该方法在近距离时多径时延峰比倒谱法更清晰,输出信噪比提高了3~4 dB.采用该方法自动提取的海底反射多径时延用于单个水听器对目标距离的估计,与GPS测量距离基本相符,验证了方法的有效性.     

Attenuation characteristics of the fundamental modes that propagate in buried iron water pipes

The attenuation of the fundamental non-torsional modes that propagate down buried iron water pipes has been studied. The mode shapes, mode attenuation due to leakage into the surrounding medium and the scattering of the modes as they interact with pipe joints and fittings have been investigated. In the low frequency region the mode predicted to dominate over significant propagation distances approximates a plane wave in the water within the pipe. The established acoustic technique used to locate leaks in buried iron water pipes assumes that leak noise propagates as a single non-dispersive mode at a velocity related to the low frequency asymptote of this water borne mode. Experiments have been conducted on buried water mains at test sites in the UK to verify the attenuation and velocity dispersion predictions.     

深海声影区稀疏时延估计与声源测距

研究了深海声影区中经一次海底反射的多途声线到达垂直双水听器的时延差与声源位置的关系,提出了一种稀疏时延估计与声源测距方法。首先利用近海面布放的短间距垂直双水听器接收一定频带的声信号,然后计算接收信号的广义互相关函数,并利用频谱搬移和稀疏解卷积技术提取时延差,最后通过时延差匹配,估计水下声源的距离。仿真实验表明,在4300 m深海中,所提方法能够正确提取多途到达时延差,估计声影区内的声源距离。海试结果表明,当垂直接收孔径分别为21 m和30 m时,声源测距误差分别小于13.6%和8.1%。上述结果表明,所提出的时延估计方法可适应带宽较窄的接收信号,多途到达时延估计参数可用于实现声影区中的水下声源测距。     

用于时延估计的一种相关峰内插算法

本文介绍一种基于余弦函数的相关峰内插算法，在时延估计系统中，为了减少运算量和增加观察时间，采用内插算法是必要的，因为在频率估算时应用了迭代算法，所以无论对于ｃｏｓｘ，ｓｉｘ／ｘ或实际的相关峰，都有相当高的内插精度。     

Locating the position of objects in non-line-of-sight based on time delay estimation

Non-line-of-sight imaging detection is to detect hidden objects by indirect light and intermediary surface(diffuser).It has very important significance in indirect access to an object or dangerous object detection, such as medical treatment and rescue. An approach to locating the positions of hidden objects is proposed based on time delay estimation. The time delays between the received signals and the source signal can be obtained by correlation analysis, and then the positions of hidden objects will be located. Compared with earlier systems and methods, the proposed approach has some modifications and provides significant improvements, such as quick data acquisition, simple system structure and low cost, and can locate the positions of hidden objects as well: this technology lays a good foundation for developing a practical system that can be used in real applications.     

互谱域MVDR自适应时延估计方法研究

针对窄带信号,通过构造互谱时间序列,在互谱域建立了平稳时间序列时延估计的最小方差无畸变响应(MVDR)滤波器模型;利用分段近似处理,类比空间MVDR自适应算法,给出了其具体算法(Algorithm of MVDR in cross spectral domain,CSMVDR);进行了数值仿真实验研究和海上实验数据处理。数值仿真与实验数据处理结果初步验证了CSMVDR时延估计对于舰船辐射噪声的适用性,CSMVDR时延估计有比相关检测更好的时延估计性能,能够提高信噪比增益和时延估计精度。     

基于小波变换的广义相关时延估计算法

为了估计两个在空间上分离的传感器所接收信号之间的时间延迟,本文结合小波变换的特点,提出了一个基于小波变换的广义相关时延估计算法并给出了它的时域和频域公式,此算法与直接互相关法相比,能对信号与噪声的假设条件放宽且在低信噪比下能有效估计时延。为了提高时延估计精度,在广义相关时延估计的基础上本文又提出了基于基小波的二次加权法。经仿真证明了这两种算法的有效性。     

The effect of dispersive and non-dispersive channels on time delay estimation

The cepstrum and complex demodulation for both wrapped and unwrapped phase outputs are examined as to their performance in the presence of amplitude distortion and dispersion in a channel. With amplitude distortion, complex demodulation is a more powerful processor than cepstrum unless zeros occur in the amplitude spectrum of the distortion. Such singularities disrupt the unwrapped phase output completely and, unless removed, limit the useful region of the wrapped phase output. With phase distortion or dispersion, both the cepstrum and complex demodulation (wrapped phase output) yield biased time delay estimates. A successful phase unwrapping process in complex demodulation enables the correct estimate of the time delay in the presence of dispersion. In addition, the dispersion itself is extracted.     

基于二次相关的语音信号时延估计改进算法

目前语音信号的时延估计研究,大部分采用的是广义互相关算法。然而,广义互相关时延估计算法易受噪声和混响环境影响。为此,本文提出了一种基于二次相关的语音信号时延估计改进算法,该算法对语音信号进行二次互相关运算,并结合Hilbert变换,对二次互相关峰值进行进一步的锐化处理,使得反映时延的峰值点检测更为准确。实验结果表明,改进的时延估计方法在非平稳的语音信号中能够有效地抑制噪声干扰,且在不同混响条件下时延估计具有更好的性能。     

An adaptive time delay estimati on algorithm based onquadratic weighting of the frequency domain

Time delay estimation (TDE) plays an important role in many engineering appli-cations. A new time delay estimation configuration, the quadratic weighting of the frequency domain adaptive TDE model, is put forward. The quadratic weighting of the frequency domainSCOT (Smoothed Coherence Transform) and ML (Maximum Likelihood) adaptive TDE algo-rithms are presented, respectively. The variance of the quadratic weighting of the frequency domain SCOT algorithm is derived. Then the proposed algorithms are applied in the TDE of helicopter passive acoustic location. The simulation results are presented which verify that the proposed algorithm has better performance in the low signal to noise ratio.     

Dynamic discrimination of convergence of the LMS time delay estimation in complicated noisy environments

For nonstationary signals, time constant can no longer represent the actual convergence performance of the adaptive delay estimation. A scheme is thus proposed for dynamically discriminating the convergence of the LMS time delay estimation to get an optimal estimate even under complicated noisy environments. Based on the fact that the filter output gets orthogonal to the estimation error only when an adaptive filter operates in its optimal condition, an approximate orthogonality constraint is derived for discriminating the practical adaptation’s convergence. This discrimination condition, which is obtained through the estimated mean-square values of related available variables at each iteration, can be updated in the adaptation process according to the varying statistic characteristics of observed signals. Thus, it can dynamically evaluate the quality of the adaptive solution and end the iteration opportunely. Simulation results are also included to demonstrate its effectiveness to give exact convergence results in both stationary and nonstationary cases.     

一种维纳加权频域自适应时延估计算法

提出了一种基于频域自适应滤波的维纳加权时延估计算法。该算法首先实现了对输入信号的维纳自适应滤波预处理,然后在频域实现了自适应时延估计。该算法在将相位数据拟合时,做了一次加权处理,有效地改善了时延估计的精度和自适应收敛速度。最后用实测的直升机噪声数据进行了仿真,验证了新算法的有效性。     

地源热泵耦合地埋管换热计算若干问题的研究

针对地质构造特点,进行地下一定深度的温度场测试;对耦合地埋管的换热设计计算中的若干问题进行了研究;在简化计算换热模型的基础上,在Excel上用VBA编写宏功能,得到实用的地埋管换热的工程设计计算方法,是一种工程易用的计算软件。同时将这种计算方法应用到了一个实际工程中,并对计算结果进行了验证。     

Axisymmetric fluid-dominated wave in fluid-filled plastic pipes: Loading effects of surrounding elastic medium

Axisymmetric (n = 0) waves that propagate at low frequencies are of practical interest in the application of acoustic techniques for the detection of leaks in fluid-filled pipelines. A general expression for the fluid-dominated (s = 1) wavenumber is presented in a thin-walled fluid-filled pipe surrounded by an elastic medium. In this paper the analysis is extended to investigate the loading effects of surrounding medium on the low-frequency propagation characteristics of the s = 1 wave. The analytical model is subsequently applied to MDPE water pipes surrounding by three media, namely an air, water and soil. It is used to demonstrate explicitly the loading effects of surrounding medium, acting as a combination of mass, stiffness and radiation damping on the s = 1 wavenumber. Good agreement is achieved between the measurements and predictions. The theory with experimental validations provides the basis for improving acoustic leak detection methods in fluid-filled pipe systems.     

Leak detection in water-filled plastic pipes through the application of tuned wavelet transforms to Acoustic Emission signals

A new method to detect leakage in a water-filled plastic pipe is proposed. In this method, a leakage signal-signature in time domain is first captured by monitoring the Short Time Fourier Transforms (STFT) of AE (Acoustic Emission) signals over a relatively long time-interval. The captured signal is then used to find a mother wavelet (tuned wavelet) for the best signal localization in time and frequency domains. The technique for AE signal detection using tuned wavelet is then described. Practical application of the method proposed herein is then presented using a water-filled plastic pipe as a case study. Signals generated from this experimental setup are collected to identify leakage signal-signatures from other interfering signals (background, pipe natural frequency, splash and environmental noise). The results of the experiment prove that using tuned wavelet, AE events can be detected and identified precisely in time. In addition, sources of signals due to leakage and their respective energy levels can also be recognized.     

On the estimation of persistence in geophysical time series

The estimation of persistence (or: self-correlation) is necessary to evaluate the effective degree of freedom (or: the number of statistically independent samples) of a geophysical time series. The application of textbook definitions may result in problems when the time series contains periodic signals. This is demonstrated with analytical solutions for a given auto-correlation function. On physical grounds, in this paper we estimate the persistence time as the integral over the absolute value of the auto-correlation function. This procedure has been proposed by Stratonovich and is shown to work for slow and fast oscillations involved. In the practical part of the paper, a 42-year long time series of phase height measurements at Kühlungsborn (54 ^∘N, 12 ^∘E) is studied. In the period band between 2.7 and 27 years, it shows a persistence time of about 12 years and an effective degree of freedom of 3.5. Hence, the negative trend of 310 meters per decade was found to be significant at a level of 86 %. This level could be raised if the time series were longer.     

二次加权频域自适应时延估计算法与应用

时延估计问题在许多工程应用中具有重要的意义。本文提出了一种二次加权频域自适应时延估计模型,给出了二次加权频域SCOT和ML自适应时延估计算法,推导了二次加权频域SCOT自适应时延估计的方差。把该算法应用于对武装直升机的声测无源定位中,用实测直升机噪声数据进行了计算机仿真。结果表明,在低信噪比情况下,二次加权频域自适应时延估计具有更好的时延估计性能。