【作者】 王彬；

【导师】 陈颙；

【作者基本信息】 中国科学技术大学 ， 固体地球物理， 2009， 博士

【摘要】 地震波作为“照亮地球内部的一盏明灯”,是我们获取地球内部结构、物质组成及状态等知识的最重要的研究手段。过去一个多世纪,由于观测技术的不断进步,通过对天然地震激发的地震波的研究,人们对全球地震分布、地球内部圈层结构及动力学过程、物质组成和横向不均匀性等方面的认识取得了巨大的进步,为我们较为完整地勾画出了地球整体结构的三维图像。也是基于对地震波传播特性的研究,人工地震勘探也已成为了解地球浅部能源分布、勘查构造环境的重要手段。然而,面对严重的地震灾害、强烈的火山喷发、地下水污染的日趋严重和资源的深度开发,都要求我们必须了解和掌握地球内部的运动变化过程,这些变化过程与地球内部介质的应力场和物性变化关系密切。由于地震波是目前所知唯一能够穿透整个地球的振动,因此研究地球介质应力场和物性变化最有效的手段依然是利用地震波,但这种对变化过程的研究与传统地震学研究的区别在于,要了解地球介质这种运动变化过程,我们必须研究不同时间地震波穿透变化了的地球介质所产生的细微变化,以获取地球介质的短期变化信息为前提。进入新世纪以来,由于观测技术和计算技术的发展,使测量和研究地震波速度的这种短期变化成为可能,并已在火山地震监测、介质物性变化检测等方面获得了应用,为我们研究地球的短期变化提供了很多新的信息。当前,通过精确测量地震波速度的变化探测地球介质的短期变化过程已成为地球物理学新的研究热点和未来重要的发展方向。本文围绕着地震波速度变化精确测量这一方向,在全面总结已有的理论研究成果、实验和数据处理方法的基础上,利用人工震源探测实验、天然重复地震波形分析和背景噪声提取Green函数方法等不同震源产生的地震波对地震波速变化这一问题进行了详细地分析与实验研究,通过对地震波速变化的精确测量,获取地球介质短期变化和地震孕育过程中的一些信息,得到了一些有意义的结果,并对引起波速变化的原因、实验的测量精度及有关方法中需要进一步完善和研究的问题、介质波速变化探测的实际应用等方面进行了探讨。本论文的主要工作大致可以分为以三个方面。第一,人工震源由于其时间的精确可控和激发波形的高度相似性,是当前地震波速变化精确测量最重要的手段。我们研究小组进行了为期30天的野外主动源探测实验,采用了两套地震观测系统,观测基线分别长250米和1千米。实验结果表明,利用相关检测方法,直达波的波速相对变化测量精度达到了10^-4至10^-5的量级,脉冲序列的相关检测可有效地提高信号的信噪比,增加信号的探测距离。利用尾波干涉方法测量得到波速相对日变化量达10^-2至10^-3,测量精度是10^-4,波速在具有长趋势变化的背景上表现出明显的日变化,这种日变化与实验场地附近深井观测的地下水位的日变化显示出同步的关系,并与大气压的变化成负相关关系。实验结果表明,由于应力变化引起的波速变化的敏感性可达10^-6/pa。第二,2003年7月和10月云南大姚地区先后发生了6.2、6.1级两次地震,震后我们在震区立即布设了较为密集的流动数字化地震观测台网,获得了大量的余震观测资料。利用这些余震的数字化波形资料,我们开发了一套基于距离的聚类分析法重复地震波形识别系统,以不同的波形相关系数为阈值识别出余震序列中的重复地震事件。对相关系数为0.9的两对重复地震事件在固定台站上的记录波形,采用尾波干涉方法对地震波速度变化进行了测量,但我们用天然重复地震并没有得到明显的波速扰动规律性的结果,这显示了由于天然重复构造地震在时间和空间分布上的不可控性,使得这种方法在速度变化监测的应用方面受到很大的限制,对此,尚需进行深入的研究。第三,用背景噪声提取Green函数进行地下介质结构的反演和波速变化的监测是近年来地震学研究中的一个新方向。我们利用云南数字地震台网,选取2008年3月21日云南盈江5.0级地震前后七个月的震区附近三个地震台的连续观测资料,提取了Green函数,用干涉的方法对Green函数的变化进行了测量,结果表明,地震发生前,孕震区出现波速的异常变化,不同的路径（台站对之间）波速相对变化有的在震前上升,有的在震前表现为下降,相对变化幅度可达10^-3至10^-2的量级。用同样的方法,对2009年7月3日云南姚安6.0级地震前后孕震区周围三个台站连续22个月的观测资料进行了分析,不同台站对之间介质的波速变化也显示出类似的现象,波速相对变化最大可达1%。这个结果表明,利用背景噪声提取Green函数通过干涉测量获取地球介质的波速变化的方法可成为对地震孕育过程监测的一种新手段。对地球介质波速变化的精确测量,是为了获取地球介质的短期变化信息,使我们对地球的认识从静态的3-D图像发展到与时间过程相关的、动态的4-D图像。本项研究工作是对4-D地震学研究的初步探索,是地震学研究中的一个新的发展方向,这种探索在获取更多的地球运动变化信息深化地球科学研究的同时,必将进一步拓展地震科学的应用领域。本文研究所涉及的三个方面,特别是人工主动源探测、背景噪声的应用将在今后4-D地震学的研究中发挥重要作用。更多还原

【Abstract】 The seismic-wave, as a light illuminating the Earth’s interior, is the most important research tool for us to get knowledge of the Earth’s interior structure, material composition and state, and so on. Over the past time period more than a century, due to the continuous progresses in observation technology, through studies of seismic-waves excited by natural earthquakes, the human being has made tremendous progress in understanding the global distribution of earthquakes, the spherical structure, geodynamic process, material composition, lateral heterogeneity, etc, about the Earth’s interior, which sketches out for us a relatively complete three-dimensional image of the overall Earth’s structure. It is also based on the studies of the characteristics of seismic-wave propagation that the artificial seismic exploration has become an important means for understanding the energy distribution in the superficial part of the Earth and exploring the tectonic environment.However, we are now facing with the serious earthquake disasters, strong volcanic eruptions, seriously increased groundwater pollution, and excessively exploiting of the natural resources. All these require us to understand and grasp the processes of the moving and changing of the Earth’s interior. These changing processes are closely related to the changes of the stress field and physical properties of the medium in the Earth’s interior. The most effective means to study the stress field and the physical property changes of the Earth’s medium is still the seismic waves that can penetrate through the deep interior of the Earth. Yet, the present study on the changing processes is different from the traditional seismological researches in that we have to study the subtle changes produced when the seismic-waves penetrate through the changed Earth’s medium at different times to obtain information on short-term changes in the Earth’s medium as a precondition to understand the Earth’s medium change process. Based on the development of observation and computation techniques since the new century, it becomes possible to measure and study the short-term changes of seismic-wave velocity, which has been applied for monitoring the volcanic earthquakes, and test for changes in the physical property of the medium, and provided a lot of new information for us to study the short-term changes of the Earth. At present, it has become a new hot research issue in geophysics and an important future direction of development to explore the short-term change processes of the Earth’s medium through the precise measurement of the seismic-wave velocity changes.In this dissertation, based on comprehensive summary of the existing theoretical research achievements, experiments and data processing methods, we focused on the precise measurement of seismic-wave velocity changes, conducted detailed analyses and experimental researches using exploration experiments by artificial sources, seismic waveform analyses by repeat natural earthquakes, and Green function methods from ambient noises. Through the precise measurement of seismic-wave velocity changes, we obtained some meaningful results, and discussed the reasons for seismic-wave velocity changes, the experimental measurement accuracy, the problems in the method that need further refinement and study, and practical applications of the exploration for seismic-wave velocity changes in the medium. The main researches of this dissertation can be divided into the following three aspects:First, an artificial seismic source is an import means at present to accurately measure the seismic-wave velocity changes, because its time can be precisely controlled, and the waveforms excited have a high degree of similarity. Our research group conducted a 30-day field exploration experiment using active sources and two sets of observation systems with baseline lengths of 250 meters and 1 km respectively. Experimental results show that the correlation test of the first arrivals made the measurement accuracy of seismic-wave velocity up to10^-4 to 10^-5. The impulse correlation test can effectively improve the signal to noise ratio, and increase the signal detection range. The relative diurnal variation of seismic-wave velocity changes measured by coda wave interference method was 10^-2 to 10^-3 with a measurement accuracy of 10^-4, and the seismic-wave velocity displayed a clear diurnal variation on the background with a long-term trend change. The diurnal variation showed a synchronous relationship with the diurnal variation of groundwater level in a deep well near the earthquake prediction test site, and a negative correlation relationship with the atmospheric pressure. The experimental results showed that the sensitivity of seismic-wave velocity changes caused by the stress changes was 10^-6/Pa.Second, we deployed a relatively dense mobile network of digital instruments after the 2 seismic events with M6.2 and M6.1 occurred in Dayao, Yunnan province, in July and October 2003 respectively, and obtained a large number of observation data from the aftershocks. Taking advantage of these digital waveform data from the aftershocks, we developed a system for identification of repeat seismic waveforms based on distance-based clustering analysis method to identify the repeat seismic events in an aftershock sequence with different correlation coefficients as the thresholds for the aftershock sequences. For two pairs of waveform data with correlation coefficient of 0.9 from the repeat events at the fixed stations, we measured the seismic-wave velocity changes in medium using the coda-wave interference method, and did not get a result with apparent regularity of seismic-wave perturbation. This indicated that the method was much limited in application for monitoring the velocity changes due to the uncontrollable nature of the repeat earthquakes in the tempo-spatial distribution, which needs further studies.Third, it is a new direction developed in recent years in seismological researches to monitor the seismic-wave velocity changes and invert for the underground medium structure by Green functions extracted from the ambient noises. We selected data recorded continuously at three stations near the earthquake area 7 months before and after the March 21, 2008 Yingjiang M5.0 earthquake, Yunnan province, extracted Green functions from the ambient noises, and measured the Green function changes using the interference method. Results showed that anomalous changes of seismic-wave velocity occurred in the seismogenic area: some of the relative velocity changes for different paths （station to station） increased before the event, while some other decreased with the relative change amplitude up to a degree of 10^-3-10^-2. Using the same method, we analyzed the data continuously observed 22 months before and after the July 3, 2009 Yao’an M6.0 Earthquake in Yunnan province at three stations near the seismogenic area. The seismic-wave velocity changes in medium between different stations also displayed the similar phenomena, and the maximum amplitude of the relative changes of seismic-wave velocity may get to 1%. This result indicated that the measurement of seismic-wave velocity changes by interference of Green functions extracted from the ambient noises can be used as a new means to monitor the seismicgenic process.It is for us to obtain the short-term change information in the Earth’s medium by the precise measurement of seismic-wave velocity changes in the Earth’s medium, so as to understand the Earth from the static 3-D images to the time-related and dynamic 4-D images. This research work is an initial exploration for the 4-D seismological studies, and a new direction of development for the seismological studies. This kind of exploration would obtain more information on changes in the Earth’s movement, deepen the Earth science researches, and meanwhile would certainly further expand the application areas of seismology. The three aspects involved in this dissertation, especially the exploration by an artificial active source, and applications of the ambient noises will play an important role in the future studies of the 4-D seismology.更多还原