摘要
地震波是照亮地球内部的一盏明灯,迄今为止,关于地球内部的结构、组成、过程和状态等知识几乎全部都来自地震波。地震学发展到现在,人们在利用天然地震研究全球尺度的结构(如地球的分层结构,板块俯冲等)上取得了巨大的成功。但是天然地震空间分布有限、发震时间无法预知且天然地震目前定位精度不高,这些因素大大限制了利用天然地震进行区域(几十到数百公里)尺度介质的精细结构及其变化的研究。人工震源主动探测由于震源位置已知、激发容易控制、观测系统分布完整合理、可以进行密集观测等优点,而成为研究区域结构的主要手段。但人工震源激发能量相对于天然地震太小,射线覆盖范围有限,深度探测能力较低。同时社会的发展也进一步限制大当量破坏性爆破的使用,因此寻找一种绿色环保的有效震源并发展相应的数据处理方法,成为用地震波探测地球深部亟待解决的关键问题。本文主要研究了人工震源的特性,并且根据震源特性在激发和数据处理方面探讨了提高地震信号检测和探测能力的方法。
首先,为研究水库大容量气枪震源应用于长炮检距深穿透的深地壳结构探测的可行性与有效性,在野外试验的基础上,本文研究了大容量气枪的震源特性、激发子波的特征和规律及其探测效果。结果显示气枪震源具有如下优点:(1)高效震源:6000 in~3的气枪源在水库激发所释放能量仅相当于1.6kgTNT炸药释放的能量,利用短周期流动地震仪可以在近185km处获得气枪激发的清晰信号;(2)低频震源:富含10Hz以下低频能量,优势频率范围为4-6Hz;(3)高度可重复震源:激发波形互相关系数大于0.94的占99.7%;(4)绿色环保震源:气枪震源对试验现场及环境没有产生破坏及噪声污染。
气枪震源可以用于区域精细结构的探测以及动态变化的监测。我们分析了气枪激发信号在长185km测线上的记录,拾取包含的丰富震相信息(如Pg,Pn,PmP等),并在此基础上反演了该地区的P波速度模型,模型结果显示该地区存在明显的低速层,这与在该地区进行的其他地球物理探测结果一致。鉴于气枪震源的高度可重复性,我们将重复地震方法应用到人工震源探测中,提出主动构建精确的人工重复地震,有效减小源位置的不确定性,提高重复地震方法的可靠性与可行性,为进一步利用人工重复地震进行地下物质状态动态监测与大尺度地震探测奠定了基础。
其次,为了探索在地震信号激发和处理中借鉴并利用雷达探测中编码原理的可行性,我们提出编码地震探测技术,通过理论分析和数值模拟对该方法提高人工震源地震探测能力的有效性进行了研究,并在此基础上进行了小尺度的野外试验。编码地震探测技术可简要描述为,利用人工震源向地下发送具有独特特征的编码脉冲序列作为一次等效激发,利用准确记录的编码震源源函数与远台地震记录之间的互相关进行“解码”分析获取地下信息。试验数据的处理结果表明该方法能够大幅度提高地震探测能力,利用小能量震源获得远距离、高分辨率的探测效果,是一种具有重要发展前景的人工探测技术。此外,为了尽可能地保持编码地震探测技术中的记录信号与源信号之间的相关性,引入偏振滤波器。我们通过三分量合成地震记录和野外探测实验的实际三分量记录进行验证,该滤波器不但具有很好的去噪功能,而且具有很好的零相位特征。
传统人工地震探测受到探测深度、分辨率和环保要求等限制,探索利用小能量震源进行深部地震探测具有重要意义。本文的研究为新型人工震源理论和方法的发展提供了新的尝试和基础。
Seismic waves give us the light to illuminate the Earth's interior. So far, almost all the information about Earth's interior such as its structure, composition and status has been gained by seismic wave. With the development of Seismology, people have achieved great success in studying the Earth's interior in global scale by earthquake, such as Earth's layered structure and plate subduction. However, the limited spatial distribution and the unpredictable occurrence of earthquakes, as well as the errors in the location of their epicenters, all these unfavorable factors heavily restrict the application of earthquakes in understanding the subsurface structure and its temporal variation in regional scale. In contrast with earthquake, seismic exploration with active sources has gradually become the dominant means in studying such regional scale structure for its well-known source location and well-controlled excitation thus the possibility of reasonably distributed observation systems. However, the energy released by active sources is relatively weak compared with that from earthquakes, thus reaches shallower in depth and smaller in lateral distance while propagating in the earth. In order to improve the capability of exploration with active sources, one way people used to take is to use large volume dynamite, the usage of which is restricted more and more or even forbidden for the requirement of environmental protection nowadays. It is therefore urgent to find an alternative and effective way as well as related data process methods. This thesis mainly focuses on the characteristics of active sources, and probes into the methods in improving the capability of seismic signal detection and exploration in both the excitation of seismic source and the process of seismic data.
Firstly, in order to study the feasibility and effectiveness of deep crustal structure exploration by reservoir based air gun source, we analyzed the characteristics of air gun excited in reservoir through field experiment. The results showed that the air gun source has advantages as follows: (1) High Efficiency. The energy released by air gun array of 6000 cubic inches in reservoir is equal to that generated by 1.6Kg dynamite, but the signal could be detected clearly even with offset bigger than 185Km. (2) Low dominant frequency. It is full of low frequency energy, and its dominant frequency is around 4 - 6Hz; (3) High repeatability. Seismic waves (99.7%) from air gun have big cross-correlation coefficients (> 0.94) with each other; (4) Green source. It has no damage to the reservoir dam and aquatic lives.
Air gun source could be used for exploring the regional fine structure and monitoring the temporal variation. We analyzed the air gun signals in the 185Km-long seismic line. And the records showed that the air gun signal is full of seismic phase information (Pg, Pn, PmP, etc). Based on these phases, we developed the P-wave velocity model for this area and find an obvious low-velocity layer, which is coherent to what people obtained from other geophysical methods in this area. With consideration of air gun source's high repeatability, we proposed an idea to construct active and accurate doublet by combination of doublet method and active sources. This method could effectively minimize the uncertainty of the source locations, thus enhance the feasibility and reliability of the doublet method. It laid the foundation for continuous monitoring on the status of the underground substances through repeated source.
Secondly, in order to explore the feasibility and potential of applying Code Principles to seismic signal excitation, which can be learned from radar detection, we developed Time Coded Impulsive Seismic Technique (TCIST). We studied the efficiency of this method through theoretical analysis, numerical simulation and field experiment. The principle of this method could be loosely described as follows, instead of using a high impact this method applies a series of moderate energy according to a time coding theme, and obtains the subterranean information by the correlation between the accurately recorded source function and long offset seismic record. Field experiment showed that this method could greatly improve seismic detection by moderate-energy source. In addition, we applied polarization filter to improve SNR of seismic record to contribute to its correlation coefficients with source function in TCIST. We tested the filter with three-component synthetic seismic data and actual record from field experiment. It is not only good for denoising, but also has good zero phase characteristics.
Traditional seismic exploration with active sources is restricted by source energy, resolution, environmental requirement and etc., thereby it is of great significance to develop methods to conduct seismic exploration in large scale with small energy source. This thesis provides a new attempt and research for the development of the theories and methods with active source.
引文
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