声波方法在城市活断层探查与评价中的应用--人工地震、雷达和声波方法的比较与组合

断层上断点深度是确定平原隐伏断层活动年代的关键数据。对同一断层用不同物探方法获得的数据相差可达几十到几百米，这一差值可能导致对断层活动性的判定结论完全相反，由于缺乏宽频带、中心频率高的人工震源，只能采用多种方法联合探测。人工地震波、电磁波和声波的频率越高，分辨率越高，但衰减快，可探测深度也降低。相反，频率低，分辨率下降，但探测深度增大。人工地震波频率为数十赫兹，探测深度大，分辨能力低。雷达电磁波为10^6Hz量级，分辨能力强，但探测深度很小。声波为数千赫兹，分辨能力优于人工地震，探测深度大于雷达，性状介于二者之间，适合厚层第四纪沉积层的超浅层探查。声波高分辨率超浅层探测的初步结果表明．以往华北平原东部某些断层活动性的论断可能要做很大的修正。     

地震雷达

利用绿色、环保的人工震源,主动向地下发射地震波,构建地震雷达,实现大范围的地下探测,是地球科学的一个前沿课题.最早,探测地球内部利用的是天然地震产生的地震波,这是因为天然地震释放的能量大,一次天然地震释放的能量相当于万吨级炸药.几千公里远处仍可以接收到信噪比大于1的地震波信号,但天然地震发生频度低和震源位置定位精度低限制了利用天然地震进行地下探测的精度;后来,利用人工源的地震勘探得到迅速的发展,探测精度明显提高,但受到人工源能量的限制,探测的空间尺度有限;今天,以小当量激发实现大尺度探测是发展地震雷达的一个关键问题,是将地震波理论和现代信息科学相结合的一个新的领域.发展人工震源的编码和接收信号的解码等理论和技术,可以从电磁波雷达在过去半个世纪所经过的道路得到许多借鉴和启发.可以预期,地震雷达的发展将会对观测地震学带来全面的影响.     

井间电磁法综述

地面电磁法在实际工作中在很大程度上受到了探测深度和分辨率的限制.为了克服这些缺点,出现了井间电磁法.井间电磁法指的是在两个(或多个)钻孔中分别发射或接收电磁波信号,利用电磁波信号进行成像并探测井间物理性质的地球物理方法.由于发射机和接收机可以分别放置在很深钻孔中,其具有大透距、大探测深度的特点,因而广泛应用于工程环境物探、矿产勘查、石油勘探等中.针对不同的应用,产生了各具特点的一些特殊方法,包括井间无线电波成像、跨孔雷达、井间电磁成像.井间无线电波成像仪目前只测量电场强度数据,工作频率低,一般是单频的电磁波,频率范围通常在1 kHz至10 MHz.由于缺少走时数据修正射线路径,井间无线电波成像主要是进行基于直射线追踪的衰减层析成像.井间无线电波成像既可用于工程与环境地球物理也可用于找矿.跨孔雷达是钻孔雷达的一种探测方式,用高频电磁脉冲探测两个井孔间介电常数和电导率的变化.跨孔雷达层析成像也叫地质雷达CT,既可进行走时成像,还可进行衰减成像.一般来说,地质雷达CT的电磁波工作频率较高,中心频率通常在10 MHz和1 GHz之间,因此在分辨率指标上占有优势,跨孔雷达主要用于工程与环境地球物理.井间电磁成像采用更低的频率,测量复电磁信号,适合油气储集层监测,是一种地球物理前沿技术.经过在多个试验区初步试验表明,井间电磁成像是油藏研究的有效手段,可用于分析剩余油分布,寻找油气富集区,进而达到提高钻探高效井成功率和提高采收率的目的.本文详细介绍对比了这三种方法在理论和实践中的一些特点,并对未来的发展进行了展望.     

汶川地震断裂带东北端浅部结构的人工地震探测

结合汶川地震断裂带动态监测,利用快速响应探测系统,开展了断层带浅部结构人工地震探测.针对地震断裂带动态监测条件下的复杂波场和低信噪比的情况,在f-k波场分离的基础上,分别利用了折射波共中心点成像、面波速度反演、反射波叠加成像方法,进行了浅层断层和构造成像处理,并对处理结果进行了综合解释,给出了断裂带浅部断层分布和速度特征.为汶川地震龙门山断裂带东北端动态监测提供了基础结构信息,所发展的断裂带快速响应探测技术对于地震应急动态监测具有重要意义.     

2015年4月25日尼泊尔地震波场传播及烈度初步模拟分析

2015年4月25日尼泊尔发生Mw7.9地震.本文采用三维曲线坐标网格有限差分方法,依据USGS给出的震源运动学反演结果和震源区域地形数据,模拟了尼泊尔地震波场传播过程,得到震源区域烈度分布模拟结果,与实际观测的地表峰值速度(PGV)大体吻合.结果表明:地震烈度的空间分布整体上受控于震源的单边破裂特征,高烈度区域主要分布在破裂传播方向上,即震源东半部.震源南侧到东南侧近场,由于受到震源和地形的双重影响,形成最大烈度分布区域,最大烈度约为IX.南侧平原受低速沉积层影响形成高烈度区域.震源西侧及盆地内烈度相对较低.     

Temporal Variations of Seismic Wave Velocity Associated with 1998 M6.1 Shizukuishi Earthquake

—?We attempt to detect temporal variations of seismic wave velocity before and after 1998 M6.1 Shizukuishi, northeastern Japan, earthquake by using waveform data from explosions and earthquake doublets spanning the period immediately before and after the earthquake. Direct P waves of the second explosion are delayed by ～20 ms at observation stations with epicentral distances less than 15 km. This tendency does not change if the analysis frequency band is changed. Our result suggests that the P-wave velocity decreased by at least 1% in the extremely shallow region of the hanging wall of the M6.1 thrust event after its occurrence. On the other hand, there was the frequency dependence of the coda wave delays for both artificial sources and for natural events. At 5–10 Hz, immediate sharp increases by more than 20 ms in time delays and lower coherency were observed at several stations. We estimated the region in which P-wave velocity might have decreased after the M6.1 earthquake. Maximum depth of the region is 13 km. The region includes the aftershock area of the M6.1 earthquake, but is eccentric to the earthquake in the west. Considering the frequency band analyzed (5–10 Hz), the scale of the spatial inhomogeneity which led to the coda wave delay is several hundreds meters. We investigated candidates for the cause of the direct P-wave and coda wave delay. Observed direct P-wave delay can be partly explained by the stress changes caused by coseismic fault slip. However, the coda wave delay cannot be explained by the stress changes that are limited to the superficial area. Crustal heterogeneity should have changed at coseismic time in the deeper area where aftershocks of the M6.1 earthquake occurred.     

基于密集线性台阵的背景噪声成像在明光市城市活断层调查中的应用

近年来,短周期密集台阵被动源探测技术逐渐成为地壳精细结构探测研究中的重要手段.为了探讨其应用于城市活断层调查在勘探尺度的有效性,在横跨安徽省明光市郯庐断裂带东界两条主干断裂位置,沿同期的主动源S波反射剖面,安徽省地震局和中国科学技术大学联合布设了一条由133个短周期地震仪组成的被动源密集测线.我们通过该被动源测线获得了约28天的连续波形记录,并采用一种新的线性台阵背景噪声成像方法——拓距相移法提取了10 Hz~2.0 s宽频的基阶Rayleigh波相速度频散曲线.利用得到的宽频带频散曲线,我们反演了近地表 1.3 km以浅的高分辨率S波速度结构,并与基本重合的主动源S波反射剖面以及地质资料等进行对比,验证了反演结果的可靠性,同时进一步分析了郯庐断裂带东界两条主干断裂的断层特征.结果表明：（1）被动源与主动源反演结果在近地表100 m以浅高度一致,且被动源的反演深度更深;（2）池河—太湖断裂是隐伏于深部的走滑断裂和浅部伸展断裂的综合反映,浅部为倾向北西的正断层,向上穿透第四系地层,可能为活动断裂;（3）拓距相移法能有效拓宽频散曲线的提取频带,保证浅部结构具有高分辨率,同时大大加强对深部结构的约束.该方法能够获得可靠的地下结构反演结果,可以作为城市活断层调查的一种新的有效手段.

     

微动探测方法软件的实现

利用微动进行勘探无需振动源,方便快捷、对环境没有干扰,是一种很有发展前景的勘探方法。针对微动探测数据处理的需求,开发研制了一套可视化微动探测处理分析软件系统MSMSPAC。该软件采用C~#语言开发,主要功能包括台阵设计、台阵方位角分布分析、数据格式转换、微动有效数据段挑选、数据滤波、重采样、参数计算及频散曲线提取、编辑、合并、项目管理等功能。软件系统界面友好,操作简便。     

Chirp浅剖仪在水域地质调查中的应用

基于线性调频技术的Chirp声学浅剖仪系统,较好地解决了探测深度以及分辨率这两个最令人关心但又互为矛盾的问题。该浅剖仪系统主要由导航系统、信号控制系统、信号发射系统、信号接受系统以及测深系统等5部分组成,理论上,其纵向地层分辨率为十几厘米左右,最大探测深度在百米左右,具体视地层以及水质等条件而定。考虑到该系统的特殊性和复杂性,对其关键操作进行了较为详细的描述。最后介绍了该系统在活断层调查、管线走向调查以及水底抛石调查等不同类型水域岩土工程勘察中的成功应用,表明其具有良好的应用前景。     

基于短周期密集台阵的北川地区浅层地壳结构研究

北川地区位于龙门山中央断裂带的中北段,2008年M_S8.0汶川地震发震断裂(映秀—北川断裂)穿北川老县城而过,对当地造成了毁灭性的破坏.除北川老县城以外,北川县内多处为汶川地震极震区,汶川地震摧毁了县内多个镇落.本研究在北川地区布设了由242台短周期地震仪组成的一条一维长测线和一个小孔径的二维面状密集台阵,进行了22天的连续记录.利用这些数据分别开展了背景噪声层析成像和水平垂直向谱比法(HVSR)研究,最终得到了北川地区浅层地壳结构.首先我们从背景噪声连续数据中提取了0.1~1 Hz的瑞利面波信号,然后反演得到了研究区的三维浅层横波速度结构,结果显示研究区存在三个低速区,其中汶川地震的地表破裂带附近存在深约2 km的低速区.同时我们提取了各台站的HVSR曲线,结果显示位于地表破裂带附近的台站HVSR曲线具有双峰特征,而远离地表破裂带台站的HVSR曲线总体呈现单峰特征,研究区沉积层最厚可达200 m.

     

高频电磁测深在地下热水勘探中的应用

高频电磁测深法是一种利用天然高频电磁场作为场源的电磁勘探方法,具有高分辨率和相对直流电法较深的勘探深度的特点,特别适合中浅层地热和地下水勘探。本文介绍了一个应用高频电磁测深评价地下热水资源的一个成功实例。野外数据采集采用了高频电磁测深系统MT-U5A,采用远参考技术,以保证数据采集质量。高频电磁测深资料二维反演成像处理结果较好地描述了地下热水的空间分布范围,为查明该区的地下热水的来源提供了较有利的证据。     

Joint evaluation of fracture azimuth by electromagnetic wave and elastic wave

With the multi-wave, multi-component seismic wave exploration, one can apply the anisotropy of fracture media to analyze the attributes of the fracture media, including the fracture azimuth. In the meantime, the techniques of full-polarimetric electromagnetic wave, including full-polarimetric borehole radar, can also be used to analyze the attributes of the fracture. However, the analysis precision of both the multi-component elastic wave exploration and full-polarimetric electromagnetic wave exploration is prone to the influence of noise and other factors. So far, some researchers have conducted studies on the joint inversion of electromagnetic waves and seismic waves. This paper develops evaluation techniques of fracture azimuth by electromagnetic wave, elastic wave, and joint analysis of coincident elastic reflection and electromagnetic data. Firstly, based on the shear wave splitting of elastic waves, this paper develops a statistical analysis technique which applies Pearson correlation coefficient to count and analyze the azimuth angle of fracture. Secondly, based on the information of electromagnetic polarization rotated by fracture, this paper develops a statistical analysis method of full-polarimetric electromagnetic waves which applies the maximum amplitude ratio between the co-polarization and cross-polarization to analyze the azimuth angle of fracture. Furthermore, based on the analysis result of the elastic wave and full-polarimetric electromagnetic wave, this paper develops a joint analysis technique which adopts the standard deviation. At last, authors in this study conduct joint detection experiments on the coincident fracture medium by using the ultrasonic and full-polarimetric ground penetrating radar. The experimental result indicates that both single geophysical methods are capable of analyzing the fracture azimuth angle, but the joint analysis is more accurate.     

Island sheltering of surface gravity waves: model and experiment

A field experiment is used to evaluate a numerical model of the sheltering of gravity waves by islands offshore of the Southern California region. The sheltering model considered here includes only the effects of island blocking and wave refraction over the island bathymetry. Wave frequency and directional spectra measured in the deep ocean (unsheltered region west of the islands) were used as input to the sheltering model and compared with coastal observations. An airborne L-band synthetic aperture radar was used to image the directional properties of the waves in the deep ocean. In addition to the unsmoothed spectra, a unimodal directional spectrum model obtained from fits to the radar spectra was also employed to suppress the high noise level of this system. Coastal measurements were made in about 10 m depth at Torrey Pines Beach with a high resolution array of pressure sensors. The model predictions and data at Torrey Pines Beach agree well in a limited frequency range (0.082 to 0.114 Hz) where the unimodal deep ocean model is appropriate. The prediction that unimodal northern swell in the deep ocean results in a bimodal directional spectrum at Torrey Pines Beach is quantitatively verified. The northern peak of the bimodal spectra is due to waves coming through the window between San Clemente and San Miguel-Santa Rosa Islands. The southerly peak is due to wave refraction over Cortez and Tanner Banks. For lower frequency waves, the effects of strong refraction in the island vicinity are shown qualitatively. Refraction can theoretically supply up to approximately 10% of the deep ocean energy that is otherwise blocked at this site. The modifications of the island shadows due to wave refraction become theoretically negligible for wave frequencies 0.11Hz. Also, local wave generation effects, which are not included in this sheltering model, are shown to be occasionally important for waves with frequencies 0.12Hz.     

临漳-大名断裂活动特征

临漳-大名断裂位于覆盖层较厚的平原地区,处于隐伏状态,为获得断裂的浅部构造特征以及活动特征,在断裂上采用高分辨浅层地震探测方法,获得了两条高质量叠加时间剖面图以及深度剖面图,这些图像清晰地显示了断裂的浅部构造以及活动特征。探测结果表明,临漳-大名断裂活动时代具有分段性,临漳段断裂上断点埋深120m,最新活动时代为中更新世早期;大名段断裂上断点埋深105m,最新活动时代为晚更新世中期。     

Exploration and Study of Major Faults in Anshan City,Liaoning Province

In order to provide a basis for the earthquake resistance protection zoning of Anshan City, westudied the activity of faults. In the study, the synthetic geophysical prospecting techniqueswere used. These techniques include the shallow artificial earthquake method, electric method,geologic radar method, etc., with shallow artificial earthquake sounding as the main means.In the meantime, the data of geophysical prospecting and borehole record of this city werecollected and the methods of field investigation and sample age dating were also used incombination. The results show that there are 5 hidden or semi-hidden faults in Anshon City.Among these faults, Dashitou-Songsantai fault, Ningyuantun fault and Dayangqi fault trendNW, the middle Pleistotene Tanggangzi fault trends NE, while the early Pleistocene or Pre-Quaternary Ertaizi fault trend NW. According to the definition of active faults, none of thesefaults is active. This paper also discussed the cause for the formation of seriously damagedareas in Ans     

临震电磁波信息研究

临震电磁波辐射与电磁扰动的现象,早在1966年邢台地震时就引起了人们的注意。1975年海城地震和1976年唐山大震后,一些单位从不同的方面开展了观测研究工作。我省一些中强震震前也曾出现过不同表现形式的电磁波辐射与电磁扰动现象。如1970年10月29日金湖4.2级地震前约2小时,震中区一鱼场工人发现收音机受到强烈干扰,震后即恢复正常;1974     

盈江5.8级和缅甸7.2级地震前电磁异常

本文分析了2010年10月-2011年3月云南通海ELF地震台站观测的电磁场资料,研究发现该资料变化与2011年发生在中国大陆"3·10"盈江5.8级及"3·24"缅甸7.2级地震存在以下关系,(1)470～1 Hz的磁场自功率密度谱(PSD)在2011年盈江5.8和缅甸7.2级地震前出现了脉冲丛集异常,异常幅度分别比正常月份变化约1～3个数量级.异常的幅度与观测点的电极方位和信号的频率有密切的关系.(2)2011年3月10日盈江地震的异常变化是先从地壳的深部向浅部发展,在震前25天2.6 km处的电阻率开始发生变化,震前10天400 m处的电阻率变化,出现了勺形的变化形态,阻抗相位同步变化.3月24日的缅甸7.2级地震,震前10天深部、浅部的电阻率同步发生了大幅度变化,阻抗相位发生转折,这可能是强震的变化特性.     

龙门山断裂带中上地壳速度结构的短周期环境噪声成像

本文利用在龙门山断裂带周边布设的57个台站自2008年11月至2009年11月为期一年的垂直分量连续地震环境噪声数据,通过短周期地震环境噪声成像方法,获得了龙门山断裂带中北段地壳25 km深度范围的S波精细速度结构.结果表明: (1)龙门山断裂带周边区域10 km以上的速度结构与地表断裂的分布形态具有良好的一致性,速度结构控制了龙门山主要断层的深部延展特征;在15 km及以下深度,S波速度结构呈现沿龙门山和沿岷山隆起走向的交叉构造格局,由此造成的速度结构差异可能影响了汶川地震的破裂过程; (2)速度结构随深度的分布特征为龙门山断裂带主要断层的深部延伸形态给出了良好的约束,结果进一步确认了龙门山断裂中段的高角度铲型断裂构造特征; (3)研究区的南端发现了龙门山断裂下方20 km以下深度具有与松潘地块中地壳低速层相关的低速结构的迹象,这可能是汶川地震破裂带南段22 km左右深度存在脆韧转换带的一个证据.研究结果显示出密集台阵和短周期环境噪声成像方法在地壳浅部精细结构和断层探测研究中具有巨大潜力.     

城市活断层探测中的浅层地震勘探方法

对大量地震灾害的研究表明，地震发生时，位于地表活动断层上的房屋或构筑物的破坏最严重，尤其是20世纪90年代后期美国北岭地震，日本阪神地震和中国台湾集集大地震等的发生后，世界许多国家的政府和地震科学家都清楚地认识到城市活断层探测与研究的重要性和急迫性，城市活断层的探测对于城市规划，抗震设防，减轻地震对城市设施的破坏都具有重要的现实意义，浅层高分辨地震勘探是城市活断层探测手段中最有效，最可靠的方法之一，可以在地表探测到地下活断层的位置，埋深，产状和空间展布情况，但由于城市环境的强干扰背景和场地条件的复杂性，必须针对实际情况，在观测系统，震源，数据采集环境的强干扰背景和场地条件的复杂性，必须针对实际 情况，在观测系统，震源，数据采集和处理方法等环节中，采用一系列提高分辨率， 提高信噪比的有效方法，才能取得可靠的探测成果，本文对城市活断层探测中的浅层地震勘探方法的技术难点和相应的解决方法进行了讨论，并结合我们近几年来在城市开展浅层地震勘探的一些经验，介绍一些实用性的浅层地震勘探工作方法。     

The April 24, 2013 Changning M s4.8 earthquake: a felt earthquake that occurred in Paleozoic sediment

The dense broadband seismic network provides more high-quality waveform that is helpful to improve constraint focal depth of shallow earthquake. Many shallow earthquakes occurring in sediment were regarded as induced events. In Sichuan basin, gas industry and salt mining are dependent on fluid injection technique that triggers microseismicity. We adopted waveform inversion method with regional records to obtain focal mechanism of an M _s4.8 earthquake at Changning. The result suggested that the Changning earthquake occurred at a ESE thrust fault, and its focal depth was about 3 km. The depth phases including teleseismic pP phase and regional sPL phase shows that the focal depth is about 2 km. The strong, short-period surface wave suggests that this event is a very shallow earthquake. The amplitude ratio between Rayleigh wave and direct S wave was also used to estimate the source depth of the mainshock. The focal depth (2–4 km) is far less than the depth of the sedimentary layer thickness (6–8 km) in epicentral region. It is close to the depth of fluid injection of salt mining, which may imply that this event was triggered by the industrial activity.