PROCESSING SHEAR-WAVE VSP DATA*

Shear-wave arrivals in vertical seismic profiling recordings made with Vibroseis can be more accurately determined by wavelet processing and spectrum whitening. This method aids discrimination of shear-wave arrivals against P-wave disturbances. The data, thus improved, can be used to determine the main axis of shear-wave polarization for each sounding depth in order to maximize shear-wave amplitude by properly adding the two horizontal component signals vectorially.     

ANISOTROPY DETECTED IN SHALLOW CLAYS USING SHEAR-WAVE SPLITTING IN A VSP SURVEY1

Shear waves can provide valuable information about seismic anisotropy. On entering an anisotropic medium, a shear wave generally splits (shear-wave splitting) into a fast and a slow quasi-shear wave with polarizations fixed by the elastic properties of the medium and direction of travel. If the medium contains planar discontinuities with common normals, the fast shear wave will be suitably propagated if its polarization lies in the plane of the discontinuities. Measuring this polarization, using a VSP geometry with oriented three-component geophones in the borehole, offers the possibility of monitoring the orientation and density of the discontinuities as a function of depth. Such a shear-wave VSP was carried out in an uncased 0.3 m diameter borehole drilled to a depth of 120 m in the north of The Netherlands. The upper 80 m of the sequence, consisting of a glacial till and sands and clays of Pleistocene age, was studied. The clays in this sequence have been subjected to glacial deformation and as a result are overconsolidated and locally fissured. In our shallow VSP experiment, shear-wave splitting and therefore anisotropy was identified at various geophone depths for one source offset. Hodograms showed a consistent polarization of the fast shear-wave component over a large depth interval. Under the assumption that the anisotropy was caused by planar discontinuities with common normals, this polarization direction gives the strike of the fissures in this interval. The polarization direction of the fast S-wave did not correspond exactly with the strike which was obtained from geological information on the fissures. The geological information was from undisturbed oriented 70 mm core samples taken at 3 m intervals in the borehole. The discrepancy, however, could be explained in terms of dipping fissures, and such a dip was confirmed by the geological and geotechnical information. The orientation of fissures is an important factor in the directional deformation and strength characteristics of clays as far as geotechnical behaviour is concerned. This study thus illustrates a practical application of shear-wave splitting observed in shallow shear-wave VSP for geology and geotechnical engineering.     

COMPARISON OF SIGNAL PROCESSING TECHNIQUES FOR ESTIMATING THE EFFECTS OF ANISOTROPY1

Three-component recordings of shear-waves in exploration surveys provide an opportunity to measure crustal anisotropy, which may be important in estimating the geometrical and physical parameters of reservoir rocks. VSPs are particularly important for this purpose as they are less subject to the complex interactions of the shear wavefield with the free surface. The first stage in characterizing the subsurface anisotropy requires that the distinctive phenomenon of shear-wave splitting must be examined for every arrival at each geo-phone. This effect may be defined by two parameters: the polarization of the leading shear-wave and the time-delay between corresponding split shear-waves. A variety of techniques have been designed to estimate these parameters of shear-wave splitting. Here, we classify the published techniques into four main categories and review their properties. Representative procedures from each group are applied to a common synthetic data set contaminated with signal-generated noise. The results allow some general statements to be made about the utility of these methods for processing shear-waves in VSP data.     

APPLICATION OF ELASTIC MODELLING IN PROCESSING AND INTERPRETATION OF VSP DATA: A CASE HISTORY1

Data from offshore Norway is used to study applications of elastic VSP modelling in detecting shear waves and observing the effects of successive mode conversion in field-recorded VSP data. The shear-wave velocities and densities from log data are used in conjunction with compressional wave velocities determined from surface seismic and log data in the VSP modelling. The time domain non-normal incidence elastic VSP modelling technique of Aminzadeh and Mendel is used as the modelling algorithm. Two surface seismograms are computed first. One is the vertical component and the other is the horizontal component for plane waves that have specified incident angles. A downward continuation method is then applied to generate seismograms at different depth points. The collection of these seismograms constitutes non-normal incidence VSPs. Both vertical and horizontal components of VSP data can be obtained by this procedure. In this paper non-normal incidence VSPs are generated for a 12.5° incident plane wave. The modelling results of layered earth systems of thin layers and thick layers are both compared with field data, and the effect of mode conversions in thin layers is observed. Several events in the field data can be explained by this elastic VSP modelling. Comparison of the model data and field data enabled a probable tube wave or out-of-plane event to be identified, the removal of which significantly improved the final VSP section. This study also shows how the VSP data helped the interpretation of the surface 3D data.     

THE SHEAR-WAVE SPLITTING OF GAIZHOU EARTHQUAKE SWARM IN LIAONING

An M_L4.7 earthquake occurred on February 2,2012 in Liaoning Gaizhou (40.56°N,122.36°E),since then,small earthquakes are frequent in this area,and until now the seismic activity does not stop,several earthquakes with magnitude larger than 4.0 have occurred.As of October 30,2014,1223 earthquakes have happened in the Gaizhou area,including 934 earthquakes with the magnitude M_L1.0~1.9,247 with the magnitude M_L2.0~2.9 and 45 with the magnitude M_L3.0~3.9.Meanwhile,earthquakes are continuously active in Haicheng area where the M_S7.3 earthquake happened in 1975,and there are over 1100 earthquakes (M_L ≥ 1.0) having occurred since the Gaizhou earthquake swarm activity.Because the polarization direction of the fast shear wave is very sensitive to the variation of the principal stress environment,the shear wave splitting parameter can reflect the regional stress state and the local structural features,especially effective for the analysis of small-scale stress environment characteristics.So based on the seismic activities of the two earthquake clusters,this study analyzes the characteristics of shear-wave splitting in Gaizhou-Haicheng area.Preliminary results show that predominant polarization direction of fast shear-waves in the old earthquake region of Haicheng is stable,consistent with the direction of regional stress field.Due to presence of active fault below the Gaixian station (GAX),the predominant polarization direction of fast shear-waves is more complicated.There are two predominant polarizations,consistent respectively with Jinzhou Fault strike which is below the station and the maximum principal stress direction in this area.In addition,Gaizhou earthquake swarm activity increased after December 22,2013,and after the time node,the predominant polarization direction of fast shear-waves in Gaixian station is SEE,which is close to the predominant polarization direction of fast shear-waves in Yingkou station,at the same time consistent with the maximum principal stress direction of this region.Thus it can be inferred,the enhanced activity of Gaizhou earthquake swarm since December 22,2014 may be related to local enhancement of regional stress.In addition,the average time-delays of slow waves in station YKO and GAX show that there are no obvious changes before and after the time point of December 22,2013,which is different greatly with the previous related researches on the variation of slow wave time-delays,and there is no possibility that the Gaizhou earthquake swarm evolved into foreshock sequences from current preliminary results.We should do more work to study the details of the time delay variation of shear wave splitting parameter.     

波形极相似的地震丛集中剪切波分裂的变化

在1991年大同MS5.8地震余震的记录中,一些小地震丛集中可见波形极为相似的地震对,利用各对地震记录的互相关函数可以确认其相似性．为了辨认剪切波波形中发生的微小变化,采用重采样技术提高采样率．可以直接从地震图中观察到丛集中成对地震的剪切波分裂的变化．      

2000年姚安地震余震序列的剪切波分裂研究

2000年1月15日姚安MＳ6.5地震后,云南省地震局在余震区布设的流动数字台网接收到了大量余震序列的数字波形,这为研究姚安地震的性质提供了丰富的资料. 由于剪切波经过上地壳传播时的分裂现象反映了上地壳介质的各向异性,因此在对余震事件进行精确定位的基础上,文中应用互相关系数法计算了余震序列剪切波的分裂参数,并对不同台站和不同区域得到的结果进行了分析讨论. 主要得到以下结论:余震区的平均快波偏振方向受区域应力场控制,与主震最大水平主压应力方向一致;不同台站与不同区域事件的平均快波偏振方向是有差异的,与区域内的构造变化和应力场调整有关;延时大小受多种因素影响,与剪切波传播时经过介质的性质关系密切.      

亚洲MAGSAT卫星磁测数据处理

本文简要介绍了使用的ＭＡＧＳＡＴ数据变量，磁测数据的筛选，卫星磁练习薄宾提取，以及卫星磁异常图的编绘。     

浅层地震勘探资料处理中的速度分析参数选取

浅层人工地震勘探是当前城市地震活断层探测的主要方法,勘探精度不仅受激发、接收等因素的影响,还与地震数据处理的精度密切相关。速度分析是地震勘探资料处理中的关键环节,它的准确与否对动校正、共深度点叠加、叠后偏移及时深转换等都将产生影响。针对浅层地震资料目的层较浅、叠加次数少和城市背景干扰大等特点,文中对速度分析的参数选取问题进行了研究。首先通过对相邻CDP道集内各接收道炮检距的分析,以及单个和多个CDP道集的对比,提出了用于速度分析的CDP大道集的抽取原则。并通过对不同的CDP大道集、速度扫描间隔、计算时窗长度、动校切除比例等所获得的速度谱的对比分析,对速度谱扫描过程中各参数的选取提出了建议,并对速度分析及时深转换过程中值得注意的问题进行了讨论     

天祝-古浪地震剪切波分裂及电性各向异性变化特征

采用与永登地震相同的方法,研究了1996年6月1日天祝-古浪5.4级地震前后地壳介质剪切波分裂、电性各向异性随时间的变化特征.结果表明临震前电性主轴、快S偏振方向都是从北偏西方向转为北偏东,而该次地震的P轴为北偏东50°,三者总体上是一致的.由于该次地震与松山电磁台同处一个断层构造上,因此视电阻率变化在各频段均较强,但ρyx比ρxy变化大,这一点与永登地震不同.另外,由于收集的小震资料较少,快慢S波时间延迟的分析结果没能反映出应力强度的积累,另一个原因可能是该次地震孕育与永登地震后应力调整纠缠在一起的结果(两者距离120多公里).     

云南2001年施甸地震的剪切波分裂参数变化特征

云南地区2001年4月10日至6月8日相继发生了4月10日MＳ5.2、4月12日MＳ5.9和6月8日MＳ5.3的施甸地震. 本文利用云南遥测数字地震台网的保山地震台记录的地震波形资料，使用SAM分析方法（即剪切波分裂系统分析方法），采用地震应力预测的分析手段，对这个强震系列进行剪切波分裂参数变化特征的研究. 初步结果表明，随着地震活动的增强，剪切波分裂时间延迟值t基本呈现增加的特征，但在强震即将发生之前的短时间内，发生t减小的现象，符合我们早先研究发现的震前应力释放现象. 研究认为，在地震活动强烈的构造地区和有强震发生危险的经济发达地区或重要城市，开展强烈地震的应力预测应用实践，可以减轻地震的危害程度.      

A COMPARATIVE STUDY OF THE SHORTENING OPERATORS USED IN VARIOUS DATA PROCESSING TECHNIQUES IN GRAVITY INTERPRETATION*

Discrete Fourier transform analysis provides an infinite number of weight coefficients for filters like upward and downward continuation. For practical applicability, the lengths of such filters have been reduced to a manageable number by various shortening operators, viz. those by Peters, Martin, Mufti, v. Hann, Hamming, and the truncation operator. A comparative study for choosing an operator which approximates the theoretical filter response best has indicated that Martin's shortening operator and the truncation operator are best, respectively, for normalized and non-normalized sets of weight coefficients.     

地震台站观测资料微机管理实用程序

本文简要地论述了台站观测资料微机管理实用程序的设计原则、结构及其特点，给出了操作流程示意图和简要说明。该程序使用效果较好，使观测资料处理及时，月报输出快速准确，减少了工作人员的工作量。     

地震资料处理中的聚束滤波方法

聚束滤波方法的基本原理,包括信号与噪音模型、设计准则和所获得的聚束滤波器．本文所用的自适应聚束滤波器是根据具有参数化的动校正量、振幅及相位随偏移距变化（ＭＶＯ、ＡＶＯ及ＰＶＯ）的一次反射和多次反射信号模型而设计的．但在实际应用中ＰＶＯ通常不能得到证实．结果分析能为进一步的从ＡＶＯ及ＰＶＯ获取岩石类型信息的专题研究提供资料．人工合成资料的例子给出了参数化的自适应聚束滤波的实施细节和所设计滤波器的响应特性．实际资料的例子表明数据自适应聚束滤波在叠前共中点道集地震多次波消除上比Ｒａｄｏｎ变换方法更灵活、有效．类似于其它叠前处理过程,自适应聚束滤波的优越性在信噪比较高的资料上体现得最为明显．     

地震资料处理中的聚束滤波方法

聚束滤波方法的基本原理,包括信号与噪音模型、设计准则和所获得的聚束滤波器．本文所用的自适应聚束滤波器是根据具有参数化的动校正量、振幅及相位随偏移距变化（ＭＶＯ、ＡＶＯ及ＰＶＯ）的一次反射和多次反射信号模型而设计的．但在实际应用中ＰＶＯ通常不能得到证实．结果分析能为进一步的从ＡＶＯ及ＰＶＯ获取岩石类型信息的专题研究提供资料．人工合成资料的例子给出了参数化的自适应聚束滤波的实施细节和所设计滤波器的响应特性．实际资料的例子表明数据自适应聚束滤波在叠前共中点道集地震多次波消除上比Ｒａｄｏｎ变换方法更灵活、有效．类似于其它叠前处理过程,自适应聚束滤波的优越性在信噪比较高的资料上体现得最为明显．     

REFLECTION AND POLARIZATION OF TUBE WAVES AS SEEN IN VSP DATA1

P-wave and S-wave data acquired with vertical seismic profiling (VSP) often include tube waves propagating in the borehole, although considerable efforts are generally made to ensure that these waves are not recorded. However, several theoretical studies have indicated that tube waves could provide important information about the rock formation and thus should not be considered as pure noise. In order to study some of these aspects experimentally, tube waves were acquired by VSP in a well in the Paris Basin both before and after casing. A sparker was used as source inside the borehole, which ensured that the data recorded contained high-amplitude tube waves. It is shown that the casing is an obstacle which prevents the study of formation parameters, and thus further tube-wave acquisitions should be carried out in open holes only. The before-casing tube-wave reflection log is compared to a synthetic log computed from the sonic log. The high resolution of the tube waves is of particular interest, revealing layers that are too thin to be detected in body-wave surveys. It has recently been suggested that the projection of the tube-wave polarization in the horizontal plane can be used to determine directions of stress-induced anisotropy in the rock formation. Strong polarization anomalies are observed in the data sets but are attributed to tool problems rather than any rock-formation feature.     

剪切波在双层方位各向异性介质传播中分裂参数的变化特性

以质元运动方程和应力应变关系为基础建立了各向异性介质的波动方程．从数学上对体波在各向异性介质中的传播进行了严格的数学推导,给出了各向异性介质中三种体波波速与介质弹性常数及波传播方向的数学关系．由此对剪切波在单层和双层弱方位各向异性介质中的分裂行为进行了系统讨论,给出了合成分裂参数、每一层分裂参数与入射剪切波偏振方向的数学表达式．假定每一层分裂参数之后,得到了合成分裂参数随入射方位角变化的合成地震图．研究表明,剪切波在双层各向异性介质中传播时产生的两对分裂剪切波可用一合成分裂因子来描述．由合成分裂因子确定的分裂参数随入射剪切波偏振方向的变化呈现出π／2的变化周期．其结果可用于双层各向异性介质反演研究,对进一步解释板块运移、地幔对流规模、深源地震成因、浅部地震构造及深部力源问题等有参考意义．     

人工爆炸地震野外数据处理装置

1988年底在华南地区连县—港口近400km的测线上,首次使用了中国科学院地球物理研究所研制的人工爆炸地震野外数据处理装置.该装置由AT型微机及必要的接口板和外围装置组成,能在野外条件下把模拟或数字记录仪记录结果以数字数据形式输入计算机,并进行显示、打印、存储等操作.由于成功地解决了诸如磁带信号直接屏幕扫描搜寻,时间信号自动识别、解码,钟差和零时信号屏幕光点比测,采样率随磁带速度自动调节,多通道长时间信号连续存储,数字滤波和再采样,用普通打印机快速打印高分辨率图形等技术问题,我们在野外记录后2—3天内即能绘出全测线120道记录的剖面图,达到了前所未有的高速度.     

地震勘探数据处理的一种算法——分布参数系统最优理论的应用

本文将根据地震数据识别地层结构问题归结为分布参数系统的最优化问题,利用脉冲变分原理给出了一个新的算法,此算法与文献[4]中算法相比较,计算效率要高。     

岩组资料的计算机处理

岩组分析在地质学研究中是一种很有用的手段。但这种方法是建立在大量数据统计分析基础之上的,因此资料的处理及作图需化费很多时间和精力。在国外的研究中,大都实现了电子计算机处理数据和作图。但在我国还都是人工作图。为了引进先进方法,我们查阅了有关资料,但没发现可利用的现成程序。根据A.B.Spencer等1967年一篇文章所讲的原理,我们提出适用于我们工作的用计算机处理一般岩组资料的方法。关于碳酸盐矿物的动力学分析的资料处理方法,因无参考文献,是我们自己推导出来的。该方法经实际应用证明是行之有效的,它不仅节约了大量时间,而且提高了精度