鄂西地区宽频地震台阵背景噪声特征

利用鄂西地区长时间段宽频地震台站的三分量背景噪声记录,采用波形互相关方法得到台站对间的互相关函数,并通过聚束分析获得瑞雷波和勒夫波的慢度谱,研究鄂西地区背景噪声源的时空分布特征。结果表明,5~10 s周期范围,背景噪声来源于南太平洋且没有季节变化;10~20 s周期范围,慢度谱上显示明显的能量环,表明噪声源来源于多个方向,且表现出强烈和急剧的季节变化;20~40 s周期范围,慢度谱上也存在明显的能量环,其产生机制可能与此周期下提出的次重力波机制相似。在不同的周期范围内,噪声源分布方位有所不同,但在周期10~40 s范围噪声源在各方向均有分布。因此,利用长时间段连续噪声数据计算的互相关函数在周期10~40 s范围内满足背景噪声面波层析成像的理论前提。     

Passive Fathometry in Shallow Waters of the Indian Continental Shelf

The present study demonstrates the use of ambient noise for estimating the ocean depth in shallow waters of the Indian continental shelf. Ocean depth is estimated using a technique known as passive fathometer processing, which involves the correlation of surface-generated ambient noise with its reflection from the seabed. Ambient noise data collected using a vertical array from four locations (off Cochin, off Cuddalore, off Kakinada, and off Goa) along the Indian continental shelf were used for the study. The noise data recorded during windy conditions within the frequency band of 200–5000 Hz were used for analysis. Both conventional and adaptive beamforming techniques were applied for the passive estimation of the ocean depth. The estimated water column depth using the ambient noise measurement shows good agreement with the known depth from all the four locations. The advantages and limitations of the adaptive processing technique have also been discussed. The study clearly demonstrates the application of the surface-generated ambient noise in seabed image processing.     

用广义消除旁瓣聚束滤波方法消除三维地震资料中的相关噪音

本文提出了一个新的方法以消除三维地震资料中的相关噪音。广义消除旁瓣聚束滤波方法(GSC)作为自适应线性约束聚束滤波方法(LCMV)的一个广义形式,包括一个固定的自适应聚束滤波器和一个在无约束聚束滤波器前的控制矩阵。考虑到三维观测系统的复杂性、在三维聚束滤波方法设计中有两个关键点:(1)采用将地震资料按方位角分区,把三维时距关系简化为二维;(2)引入动态平衡面元,避免有的方位角分区内地震道数可能不够。合成资料和实际资料结果表明,新研究的广义消除旁瓣聚束滤波方法能够得到高效、稳定和高分辨率的解。     

地震资料去噪方法技术综合评述

地震资料去噪,无论是叠前还是叠后,都是处理中非常重要的内容.随着勘探技术的进步,地球物理界积累和开发的去噪软件已越来越多.对各种去噪方法进行分门别类,阐述其基本原理、物理意义、适用条件、发展前景,既有理论价值又有实际指导意义.本文从噪声的特征出发,首先对地震资料噪声进行了分类;然后综合评述了目前实际生产中常用的几种去噪方法,包括频率域滤波、频率波数域滤波、频率空间域滤波、Radon变换、聚束滤波、基于小波分解和重建的去噪方法等;最后还简述了去噪技术的应用及发展情况.     

Effect of uneven noise source and/or station distribution on estimating the azimuth anisotropy of surface waves

With the development of the dense array, the surface wave velocity and azimuthal anisotropy under the array can be directly obtained by beamforming the noise cross-correlation functions (NCFs). However, the retrieval of the Green's function by cross-correlating the seismic noise requires that the noise source has a uniform distribution. For the case with uneven noise source, the azimuthal dependence on the sources in the expression for the spatial coherence function, which corresponds to the NCF in the time domain, has the same form as the azimuthal dependence of the surface wave velocity in weakly anisotropic media. Therefore, the uneven noise source will affect the surface wave anisotropy extraction. In this study, three passive seismic methods, i.e., beamforming, SPAC (spatial autocorrelation), and NCF, are compared to demonstrate that an uneven source distribution and uneven station distribution have equivalent effects on the outcome from each method. A beamforming method is proposed to directly extract the velocity and azimuthal anisotropy of surface waves. The effect of uneven noise source and/or station distribution on estimating the azimuth anisotropy of surface waves was investigated using data from the ChinArray Phase II. A method for correcting the apparent anisotropy in beamforming results caused by an uneven station distribution is suggested.     

Review of research on sonar imaging technology in China

Over the past 20 years, sonar imaging technology particularly for the high-technology sector has been a focus of research, in which many developed countries, especially those with coast lines, have been competing with each other. It has seen a rapid development with increasing widespread applications that has played an important and irreplaceable role in underwater exploration with great prospects for social, economic, scientific, and military benefits. The fundamental techniques underlying sonar imaging, including multi-beamforming, synthetic-aperture and inverse synthetic-aperture sonar, acoustic lensing, and acoustical holography, are described in this paper. This is followed by a comprehensive and systematic review on the advantages and disadvantages of these imaging techniques, applicability conditions, development trends, new ideas, new methods, and improvements in old methods over recent years with an emphasis on the situation in China, along with a bold and constructive prediction to some development characteristics of sonar imaging technology in the near future in China. The perspectives presented in this paper are offered with the idea of providing some degree of guidance and promotion of research on sonar imaging technology.     

密集台阵背景噪声双聚束成像化龙断裂精细结构

双聚束成像法(Double beamforming, 简称DBF)是利用地震背景噪声进行地下速度结构成像的新方法, 该方法通过对环境噪声场中能量相干部分的叠加达到提高信号信噪比的目的, 识别和提取不同类型的地震波信号及其入射方位角(微弱体波、反射面波等), 从而进行地下速度结构成像研究等. 特别是进行背景噪声面波成像时, DBF能通过局部慢度(方位角)搜索直接提取面波信号的相速度而无需进行层析反演. 本文对广州番禺化龙断裂浅地表结构进行一维线性阵列背景噪声成像研究, 使用DBF方法提取了0.6~1.2 s周期的基阶瑞利波相速度, 并反演获得上地壳0~1200 m深度范围的二维S波速度结构剖面. 研究结果显示: 阵列东北侧靠近珠江位置出现的低速异常一直延伸至地下600 m深处, 反映了断裂两侧不同地块的沉积层厚度、压实率及岩性成分差异; 而900 m及以下深度处的相对高速异常则与第三纪岩浆活动形成的基岩层相对应. 化龙断裂在近地表的结构特征有效地提高了我们对珠江三角洲内断层系统的认识, 为进一步研究珠江三角洲的区域构造特征、发震机制及地震风险性评估提供了参考.

     

基于线性约束最小方差对角加载的稳健频率不变波束形成算法

针对实际环境中因麦克风阵列失配误差导致宽带波束形成器性能下降的问题,提出基于线性约束最小方差对角加载的稳健频率不变波束形成算法.该算法首先在线性约束最小方差准则的基础上,结合空间响应偏差函数,并对波束形成器权矢量的二范数施加不等式约束,再通过拉格朗日乘子法和凸优化工具箱CVX(ConVeX)分别在不等约束条件下求得权矢量的解析解和全局最佳解,以有效解决麦克风的增益、相位和位置等的不确定性造成的失配误差问题,提高频率不变波束器的稳健性.仿真结果表明:采用拉格朗日乘子法求解该算法的最优权矢量所得到的波束形成器对失配误差最不敏感,性能最稳健.     

Estimation of Seabed Properties Using Ambient Noise from Shallow Water Sites of the Indian Continental Shelf

In this study an attempt has been made to extract sediment geoacoustic properties using ambient noise measured from a vertical hydrophone array. Time series noise data recorded from three shallow water sites (Chennai, Cuddalore and Cochin) along the Indian continental shelf were used for the analysis. The compressional sound speed of sediment for all the sites was estimated from the vertical directionality of ambient noise. Using the value of the compressional sound speed remaining wave properties and material properties were deduced from the Grain-Shearing (G-S) theory of wave propagation in saturated granular media. The type of sediment extracted from the G-S theory correlates well with the results obtained from sieve and particle size analysis of grab samples, collected from all the sites. The study clearly shows the application of ambient noise in extracting environmental information in shallow water, and further applying it to improve sonar performance modeling.