【作者】 杨微；

【导师】 葛洪魁；

【作者基本信息】 中国地震局地球物理研究所 ， 固体地球物理学， 2013， 博士

【摘要】 地震波在地球介质中的传播,携带了丰富的地下介质物性的信息,是我们获取地球内部结构、物质组成及状态的最重要的研究手段。通过对天然地震激发地震波的研究,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已取得了巨大的进步,较为完整地勾画出了地球整体结构的三维图像。但在面对严重的地震灾害和强烈的火山喷发等问题时仍显得不能满足要求,我们不仅需要地球内部清晰的静态图像,还要了解和掌握地球内部的运动变化过程。到目前为止,国内外学者已分别尝试利用重复天然地震、自然噪声和人工震源等研究地下介质的动态变化。由于天然地震发生的频率低,地震定位特别是深度确定的精度较低,重复地震发生的时间和地点不可控制,而噪声源能量弱,需要长时间的叠加才能获得可靠的测量,这些因素限制了利用天然源测量地下介质变化的时间和空间精度及分辨率。而人工震源具有精准的激发时间、地点和震源特性可测,并有可能实现高度可重复,观测系统分布灵活方便、可以进行密集观测等优点,从而实现很高的走时变化探测精度,有望弥补天然源的不足。随着人工主动震源探测技术的发展,利用重复人工震源进行地下介质波速变化测量已经成为一个重要的发展趋势。本文结合区域尺度地下介质动态变化监测的需要,围绕主动源探测技术的关键问题：1)信噪比；2)震源可重复性；3)波速变化精确测量等开展相关研究。在全面分析和总结现有的理论与实验研究成果的基础上,利用现有的观测天然地震信号的固定台站,与观测位置灵活的流动地震测线相结合作为信号接收方式,对几种大功率人工震源特征进行对比分析和筛选,分别以精密控制机械震源(Accurately Controlled Routinely Operated Seismic Source,以下简称ACROSS)和大容量气枪震源为基础,构建相应的高性能主动源探测技术系统,并进行了现场试验研究。通过对现场试验数据的分析,获取了地下介质波速变化的一些信息,得到了一些有意义的结果,并对区域尺度人工震源波速变化主动测量精度的影响因素、引起波速变化的机理、需要进一步完善的问题、介质波速变化探测的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四个方面。第一,区域尺度主动源探测原理。介绍了主动源探测技术的原理和方法,针对传统方法波速变化测量精度的不足,发展了波速变化精确测量方法,并采用实际观测的大气压或理论计算得到的固体潮连续变化进行标定和检验。在探测系统构建上,从地震信号的激发、传播、接收以及信号分析等方面入手,给出了开展区域尺度主动源探测系统的技术要求,对震源激发和信号接收之间进行匹配分析。观测方式上结合现有天然地震的观测模式,采用固定台与流动台相结合,台网与测线相结合的观测方式。在震源方面,对现有大功率(激发能量能超过106J)人工震源特征进行对比和筛选,分别从连续源和脉冲源中选用具有代表性的高度重复性震源,即ACROSS和大容量气枪震源。第二,ACROSS震源特征及其信号检测方法。根据港震公司生产的10吨ACROSS震源,对其发展历程、工作原理及震源特征进行了全面的介绍,搭建了相应的主动探测技术系统。ACROSS震源激发的扫描信号频带范围为2-10Hz,具有重复性好、频率低的优势,激发能量随频率的上升呈二次方增加,通过多次叠加,在～10km的流动台站检测到了有效信号。并对实际观测数据进行了信号检测方法对比研究,得出对于具有较高信噪比的ACROSS信号,相干和反褶积法的处理结果优于互相关和短时相关法,且能有效的去除震源因素的影响。构建的ACROSS主动源系统探测精度高,观测到的波速变化与大气压呈明显的正相关性。第三,汶川地震余震同震波速变化监测。汶川地震后,结合汶川断裂带科学钻探项目,利用ACROSS震源,跨龙门山断裂带—前山断裂带进行了动态监测试验。在观测期间,监测区域内发生了一次Ms5.6级地震,观测到了短期的同震／震后波速变化,在地震前后,跨过断裂带的流动台站的P波走时发生了5-9ms的延迟。综合现有的人工地震剖面和汶川地震科学钻探在前山断裂带附近的钻井资料,在观测区域内跨断裂带建立了二维速度模型,通过相对走时变化模拟分析。研究结果表明：Ms5.6级地震引起的P波走时延迟,主要是由于地震诱发应力调整导致～120m的断裂带内介质同震／震后波速变化,变化幅度约为2.0%。第四,大容量气枪区域尺度探测试验。以大容量气枪震源为基础,设计、加工并配备了相关设备,构建了一套大容量气枪震源主动探测技术系统,并与多家单位合作,分别在河北、云南的内陆水库和新疆的人工水体等不同环境下开展了探索性试验研究,从探测尺度和探测精度两方面对利用气枪震源进行地下介质应力变化主动监测的可行性和有效性进行了分析和探讨。研究表明：大容量气枪震源具有激发能量强,优势频率低,可重复性好,可激发较强的S波震相等优点；构建的重复探测技术系统探测距离远,在380km的偏移距上还能检测到有效信号,且探测精度高,观测到了由固体潮引起的地下介质连续变化,为用于区域尺度地下介质变化监测和研究地震孕育过程的动力学过程提供了可靠的技术保障。更多还原

【Abstract】 The seismic wave carries a wealth of information about the physical properties of the subsurface when propagates in the earth medium. It is the most important research tool for us to get knowledge of the Earth’s interior structure, material composition and state, and so on. Through studied on the seismic waves excited by 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 static image of the overall Earth’s structure. However, it is still not meeting the requirements, when faced with serious earthquake disasters, strong volcanic eruptions and other issues, what we needs is not only the static images of the Earth’s interior, also to understand and master the process of change in the Earth’s interior.Now many scholars used the repeated earthquake, noise and artificial source to study the dynamics monitoring of the subsurface in domestic and foreign. As the low frequency of earthquake occurrence, especially the low accuracy in the depth of the earthquake location, and the occurrence time and location of the repeat earthquake can not be controlled, so it is not suitable for carry out monitoring in long-term. The energy of the noise source is very weak, needed a long time to stack for get reliable measurements. The resolutions and precisions of passive source monitoring are limited by the spatial and temporal distributions of source, but the artificial sources with precise timing, location and characteristics can be measured, and likely to achieve highly repeatable. In addition, the observation system can achieve high accuracy detection of changes in travel time with flexible and intensive, expected to compensate the lack of the natural source in accuracy with the precise time and location. With the development of the artificial source detection technology, monitoring subsurface changes with the seismic wave generated by repeatable artificial source has become another goal to pursue.In this dissertation, combined with the need of dynamic monitoring subsurface on regional scale, we carried out research focus on the key issues of the dynamic monitoring technology with active source, which are the signal to noise ratio (SNR), repeatability of the seismic source and accuracy in measure velocity change. Based on comprehensive analysis and summary of the existing theoretical research achievements and experiment results, we adopted fixed stations monitored for earthquakes and portable seismic station with flexible and intensive to receive signal, conducted comparative analysis on several high-power artificial source and select two sources representative for continuous and pulse sources, respectively. We constructed the corresponding high-performance dynamic monitoring system with Accurately Controlled Routinely Operated Seismic Source (ACROSS) and large volume airgun, and studied on field experiments. We observed some information of velocity change, 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 four aspects.First, the detecting principle used active source on regional scale. We introduced the principle and method of dynamic monitoring technology with active source, developed the precise measurement method in velocity changes contrary to the low precision used the traditional methods, and took the observed atmospheric or tidal change by theoretical calculation as a continuous calibrated and test signal. On the aspect of building up the active monitoring system, we take the hand from the signal excitation, propagation, reception, signal analysis and other aspects, given out the technical requirements for carry out dynamic monitoring with artificial source on regional scale, and conducted matching analysis between the active source excitation and signal reception. Considering the existing observation mode for earthquake, we used fixed station and portable seismic station, combined network with mobile survey line mode. With the seismic source, we carry out comparative analysis the existing high-power (excitation energy can exceed106J) artificial source characteristics, and select two sources representative for continuous source and pulse source, respectively, namely ACROSS and large volume airgun sources.Second, the characteristics and signal detection method of ACROSS. According to10tons ACROSS produced by Beijing Gangzhen Mechanical and Electronic Technology Company, we give a comprehensive introduce its development process, working principle, source characteristics, and build up a high-performance dynamic monitoring system, which was widely used to detect and monitor the velocity change in subsurface, with its simple structure, easy operation and maintenance. The ACROSS signal has the characteristic of high repeatability and low frequency rang from2Hz to10Hz, and there is a relationship of square times between the energy and scan frequency. On signal detection, the cross-coherence and deconvolution method are more suitable than the cross-correlation and short-window correlation with comprehensive analysis from repeatability, travel time profile, spectral characteristics, SNR, and can remove the effect of the source characteristic. The dynamic monitoring system has high detection precision with ACROSS, and observed velocity changes have a relationship of positive correlation with atmospheric pressure.Third, the monitoring and analysis on coseismic velocity change in Wenchuan aftershock. After the Wenchuan earthquake, combined with Wenchuan Fault Scientific Drilling (WFSD) Project, we carried out continuous monitoring across the range-front fault zone of Longmenshan fault using10tons ACROSS, and adjacent to WFSD. We observed travel time delay of～5-9ms associated with a Ms5.6earthquake from the seismic stations on hanging wall of the Jiangyou-Guanxian fault, while from stations on the footwall, they did not observe any travel time delay larger than measuring error. We in this study carry out forward modeling by using two-dimensional Spectral Element Method to further investigate the amplitude and spatial distribution of observed velocity change. The model is well constrained by results from seismic reflection and WFSD coring. Our model strongly suggests that the observed coseismic velocity change is localized within the fault zone with width～120m rather by dynamic strong ground shaking, and a velocity decrease of～2.0%within the fault zone is required to fit the observed travel time delay distribution, which coincides with rock mechanical experiment and theoretical modeling.Fourth, the active monitoring experiment on regional scale used large volume airgun source. Based on the large volume airgun source, we designed, processed and allocated relative equipments, and constructed a high-performance active monitoring system with large volume airgun source. Cooperation with a number of units, carried out experiment with inland reservoirs and artificial water body in Hebei, Yunnan and Xinjiang, and conducted a feasibility and effectiveness analysis with dynamic monitoring the stress state and its change used airgun from detection distance and precision. The results showed that:The large volume airgun source has power of energy, the characteristic of low frequency, high-repeatability, can stimulate strong S wave, and so on. The active monitoring system has long detection distance at380km also can detect signal by stack, high detection precision, and have observed the continuous change caused by the Tide. It can be applied to carry out the active monitoring of subsurface and4D seismology on regional scale, which are provide a new technology to study on the medium properties, crustal stress distribution and dynamic variety.更多还原