【作者】 张尉；

【导师】 陈颙； 陈汉林； 杨树锋；

【作者基本信息】 浙江大学 ， 地质学， 2008， 博士

【摘要】 获取区域尺度地下结构的4D图像是探索地震物理预测、研究大陆动力学的重要途径。天然地震释放的能量大,但因为震源定位存在较大误差,故主要用于研究全球性大尺度问题,在研究区域性问题时分辨率常常显得不足。传统的深部结构探测,通常使用大当量炸药为震源,不环保且成本较高,难以在城市地区开展研究;又因采取流动地震测线的接收方式,覆盖区域有限,无法用来获取4D图像。对此不足,如果能做到小当量人工震源激发,区域地震台网(阵)远距离也能探测到信号,这将为地球科学建立研究地下的技术平台。本论文围绕2004年至2007年中国地震局联合众多单位在首都圈地区开展的多次人工地震实验,以采集到的首都圈固定地震台网和流动地震台数据为基础资料,从震源的激发、接收、数据处理和应用等方面较系统地研究了小当量人工震源探测区域性深部结构及其演化的可行性。炸药是陆地上使用时间最久、用途最广的震源。利用小药量的震源激发,能否获得较大范围的探测尺度是地球物理学家们十分关注的问题。论文利用10-25kg炸药量的陆地井下激发实验研究了小当量炸药的探测范围。结果表明25kg炸药的激发能量相当于M_L0.69天然地震,最远接收台的探测距离却超过200km,穿透深度约40km,振幅仅1.6nm。可识别Pg,Pm和Pc震相。此外,10-25kg炸药激发的波形具有相似性,可构成一定精度的重复地震。而利用三次大小炮联合实验计算出的振幅比和药量比关系,可用来提高小当量炸药的检测距离。综合分析得出:在良好的激发和接收条件下可以选择小当量炸药作为区域性震源,但炸药激发效果的可控性不高,重复性有限,应用潜力有限。气枪是海洋地震勘探使用最广的震源,将气枪移植到陆地,需对其激发特性和激发效果进行分析。论文研究了6000-8000inch~3的陆地水库气枪的探测距离、可重复性和影响因素等特征。结果表明气枪单次激发能量仅等同于1.4-1.6kg的炸药,但因为水中激发的地震波能量转换率比陆地炸药高10倍左右,激发产生的震级却相当于M_L0.5-0.6的天然地震。单次激发信号可传播120km,采用信号叠加技术后185km长的流动测线和近400km处的固定地震台可清晰记录到Pg,Pc,PmP,Pn,Sg,SmS,Sn等震相,穿透深度可至上地幔顶部。分析还表明:气枪具有环保、经济、较高可操作性和高度可重复性(相关系数平均值大于0.96)的特点,是较理想的区域性研究震源,而且激发效果容易控制,通过增加气枪容量和沉放深度的方式就可以有效提高气枪的探测范围。区域地震台网是人工震源主动探测地下结构、研究地下介质物性变化的重要组成部分,认识并充分利用固定地震台网对4D地震学研究意义重大。为了研究台网对短周期微弱信号(1-20Hz)的检测能力,论文通过分析首都圈固定地震台网2000多条噪声数据,得出基岩台的背景噪声平均水平低于沉积层台约13dB。台址的好坏可以根据对某一次地震记录的信噪比大小来比较,2006年朝鲜地下核试验提供了这次难得的机会,波形记录经1-5Hz滤波后,台网中的18个基岩台可以清晰辨认核爆破产生的P波或Lg波,研究表明背景噪音较小的台站通常也具有较高的信噪比;而通过台网的基岩台计算得到的朝鲜核试验震级和NEIC计算的震级结果相同,表明基岩台的记录是真实可靠的。因此一些较好的基岩台可在4D地震学研究中重点研究,发挥其重要作用。首都圈的大城市都建在沉积层上,研究沉积层区地震波激发、传播和衰减特征对开展城市区域的地下结构成像至关重要。研究发现2570inch~3的气枪在渤海激发,区域地震台网可在近100km处检测到波速为1.7-1.8km/s的强震相,但P波的传播距离仅为21km。8000inch~3水库气枪激发的P波在沉积层区传播距离也不超过30km。研究还表明沉积层会从波形、噪声、信噪比和衰减等各方面较严重地影响深部探测的结果。根据人工地震实验和天然地震的衰减规律,引入半衰距离的概念来表征衰减的快慢,得出地震波在浅部沉积层的半衰距离为几十米,远远低于基岩的几公里。地震波的衰减并非线性变化,因此人工地震波的检测距离存在一定的突变性。S波分裂可以为地震预测探索实践提供可靠的物理途径。论文对水库气枪实验数据进行S波分裂分析,发现气枪激发可以产生S波(Sg和SmS)。S波来自于气枪激发的P波在液-固界面的转化,S波能量较强,叠加后和M_L 1.6天然地震相当。对布置在燕山隆起带的流动地震台的气枪信号进行S波分裂参数分析,结果表明快剪切波偏振优势方向为NWW和NNE向,偏振方向和区域断裂的性质密切相关。气枪是高度可重复性人工震源,利用气枪定点激发和定点接收有可能精确获取S波分裂参数,参数随时间的变化规律可以反映应力场随时间的变化,可应用于地震的物理预测。更多还原

【Abstract】 The 4D map of regional structure of deep earth provides important information for earthquake prediction and understanding the continental geodynamics. Suitable seismic source and advanced recording and processing systems are necessary to produce the 4D map. Although natural earthquakes can release huge amounts of energy, the epicenters of natural earthquakes are not accurate enough to be used for producing the 4D map. Traditionally, explosives are used as the seismic sources for exploring depth structure. However, explosives are expensive and have a large negative effect on the environment, so it is difficult to use them. Alternative seismic sources are necessary. If we can use a regional seismic network or seismic array to detect the seismic signal generated from small artificial seismic sources far away, it will provide a practical way to explore the regional deep structure of the Earth.From 2004 to 2007, China Earthquake Administration with some other organizations held several artificial seismic experiments. In this paper, I will discuss some topics of using small artificial seismic sources to explore regional structure of the deep earth, including features of the artificial seismic sources, signal detection, data processing technics and application. The data used in this paper is based on the seismic datasets recorded by Capital Area Seismograph Network(CASN) and portable seismic stations.Explosion is the most common source for seismic exploration in the land. However, is a small explosive a good source to investigate the deep structure? A borehole explosion experiment with 10-25kg chemical charges was conducted to answer this question. The energy of the explosive equals to a about M_L0.69 natural earthquake. The seismic wave generated by 25kg explosive can be detected by a permanent seismic station with offset up to 218km, and the seismic phases Pg, Pm and Pc can be identified. Pm waves can be detected with the peak-to-peak amplitude being about 1.6 nanometers and the depth of propagation is about 40km. The comparability of seismic waves generated by several 10-25kg explosions shows that the explosion is a fairly repeatable source. The relationship between amplitude of seismic wave and charge of the explosives was calculated. The result shows that the explosive can be used as the seismic source for deep structure exploration if the source and receiver conditions are good. However, it is not easy to control the conditions and the explosive has a limited application.The airgun is the most important seismic source in marine exploration. We conducted two experiments using airguns in a reservoir. The airgun-array (4 airguns) had a volume of 6000-8000 cubic inches and the energy released by one shot is equivalent to a 1.4-1.6 kg TNT explosion. Because more energy is transformed into seismic waves in the water than in the land, the energy of an airgun-array is equivalent to a M_L0.5-0.6 earthquake. In order to enhance the signal, we stack the recordings of about 272 shots for each receiver. The phases of Pg, Pc, PmP, Pn and Sg, SmS, Sn can be picked easily in the stacked seismic recordings. We found that once stacked, the signal can be detected by receivers with offset up to 185km and by a permanent station with offset upto 400km. The crosscorrelation coefficient (CC) of the signal is higher than 0.96. The results shows that airgun is also a green, cheap source with high controllability and repeatability, so it is the suitable source for exploring regional deep structure of the Earth.The regional seismic network plays an important role for studying deep crustal structure and monitoring the changes of rock properties via artificial seismic source. The artificial sources generally generate short-period seismic signals between l-20Hz. In order to study the ability of the seismic network to detect short -period weak signals, I analyse noise datasets of the CASN. The results show that the background noise level at seismic stations on bedrock is about 13dB lower than that at stations located on sedimentary layers. An underground nuclear explosion detonated by North Korea in 2006 is a good opportunity to examine the ability of CASN to detect weak signals. The records band-pass filtered between 1-5Hz show that the P or Lg waves generated by the nuclear explosion can clearly be recorded by 18 seismic stations located on bedrock. The average amplitude of the P waves is 16 nanometers. The magnitude calculated is mb4.3, the same as that given by NEIC. The result also shows the background noise level of a station is one of the most important factors for signal detectability. The good bedrock stations can play a significant role in 4D seismology.Nearly all the big cities in the Beijing area are located on sedimentary layers. The propagation and attenuation of seismic waves in sedimentary layers are different from that in the deep earth. It is important to know the features of the sedimentary layers when we want to obtain underground structure maps of the urban areas. A strong phase is found in the signal generated by airguns with a volume of 2570 cubic inches in Bo-Sea. Its velocity is about 1.7-1.8km/s and can be detected with offset up to nearly 100km. Note that the P wave can only be detected at offsets up to 21km. Even by increasing the the airgun volume to 8000 cubic inches, the P wave generated by airguns in a reservoir is only detected at offsets up to 30km. The result shows that the waveforms, signal-to-noise ratio and attenuation of seismic waves in the sedimentary layers are factors affecting the result of detection. The attenuation of seismic wave in sedimentary layers is much stronger than in rocks, and the variation is not linear, which means the detection range of the signal is not a linear relation with distance.Shear wave splitting is an possible method for earthquake stress-forecasting. I applied shear-wave splitting analysis to a seismic dataset generated by airguns in a reservoir. We found that the seismic data contains shear-waves (Sg and SmS). The shear-waves are converted at the water-solid interface from P-waves generated by the airgun source, and the energy of the converted shear-wave is equivalent to the energy released by a M_L 1.6 earthquake. We analyzed the data recorded by a seismic line deployed over the Yanshan uplift. The results shows that the predominant polarizations of the fast shear-wave are in the directions of NWW and NEE, which are affected by the characteristics of the local fault system. Using an airgun as a repeatable seismic source and recording the data at a fixed point, the variation of shear-wave splitting parameters can indicate the variation of local stress-strain fields, and hence provides a method for earthquake stress-forecasting.更多还原