SEISMIC REFRACTION AND SCREENING BY THIN HIGH-VELOCITY LAYERS *

The screening effect of thin, relatively shallow high-velocity layers often presents considerable problems in seismic exploration. Such layers prevent the greater part of the seismic energy from travelling to greater depths and introduce additional refraction arrivals, confusing the seismogram still further. In order to investigate both the screening and refractive properties of high-velocity layers, scale-model experiments have been made over a wide range of layer-thickness/ wavelength ratios (0.05 < d/λ < 2) for suitably chosen material contrasts. The results may be summarised as follows. Refraction arrivals from thin layers in the field may be recognised by their relatively rapid amplitude decay. Furthermore, the “echeloning”-effect observed for refraction first arrivals may be due to the presence of a (thin) layered structure. Since the apparent refraction velocity varies with d/λ when d/λ < 1, differences between vertical well-log velocities and velocities observed along the surface may be expected, making time/depth conversion using surface velocity data inaccurate. Transmission of elastic energy may be expected, if anywhere, only near the shotpoint, at small geophone offset, and for relatively thin screens (d/λ < 0.1). The transmitted signal shape is then independent of the layer thickness. This transmitted energy may be registered either in a reflection set-up with geophones near the shotpoint, or in long-distance refraction work. Three possibilities are offered for overcoming the screening effect of thin high-velocity layers: Use longer-wavelength signals Apply short-spread reflection shooting Apply long-distance refraction shooting The experimental results obtained in scale-model arrangements of such set-ups confirm the potentialities of these methods.     

DETECTION AND RESOLUTION OF THIN LAYERS: A MODEL SEISMIC STUDY1

The detection and resolution of a thin layer closely situated above a high-impedance basement are predominantly determined by both the frequency content of the incident seismic wavelet and the existence of the nearby high-impedance bedrock. The separation of the thin layer and the basement arrivals is investigated depending on the low-frequency content of the wavelet. The high-frequency content of the wavelet is kept constant. The initial wavelet spectrum with low frequencies has a rectangular shape. All wavelets used have zero-phase characteristics. Numerical and analogue seismic modelling techniques are used. The study is based on the geology of the Pachangchi Sandstone in West Taiwan. Firstly the resolution of a thin layer between two half-spaces is examined by applying the Ricker and De Voogd-Den Rooijen criteria. The lack of low-frequency components of the incident seismic wavelet reduces the shortest true two-way traveltime by about 20%. In addition, low-frequency components of the wavelet diminish the deviation between true and apparent two-way traveltime by about 65% for layer thicknesses in the transition from a thick to a thin layer. The second step deals with the influence of a high-impedance basement just below a thin layer on the detection and resolution of that thin layer. Reflected signal energies and apparent two-way traveltimes are considered. The reflected signal energy depends on the low-frequency content of the incident wavelet, the layer's thickness and the distance between the basement and the layer. This applies only to layers with thicknesses less than or equal to one-third of the mean wavelength in the layer, and a distance to basement in the range of one to one-half of the mean wavelength in the rock material between layer and basement. The minimum thin-layer thickness resolvable decreases with increasing distance to the basement; i.e. for a layer thickness of one-third of the mean wavelength in the layer the relative error of the two-way traveltime increases from 5% to 30%, if the distance is reduced from one to one-half of the mean wavelength in the material between the basement and the thin layer. Finally, a combination of vertical seismic profiling and downward-continuation techniques is presented as a preprocessing procedure to prepare realistic data for the detection and resolution investigation.     

INVESTIGATION OF THE SEISMIC RESPONSE OF CYCLICALLY LAYERED CARBONIFEROUS ROCK BY MEANS OF SYNTHETIC SEISMOGRAMS *

Seismic exploration for coal as well as basic scientific research indicate the existence of unsolved problems. These problems arise partly because the requirements are different from those in exploration for gas and oil and are partly due to the geological situation. The medium to be investigated is composed of cyclically changing layers with extremely high velocity and density contrast. Furthermore, the structure of the carboniferous rock is highly fractured and folded. This leads to difficulties in interpreting the seismic response of carboniferous rock. To overcome these difficulties synthetic seismograms are a useful tool. Calculating synthetic seismograms the carboniferous rock has been modelled as a sequence of seams and rock. The following results have been obtained

• (i) A single seam gives rise to a distinct reflection signal even for a thickness of 1/50 of the wavelenght.
• (ii) Individual reflections are not visible from a sequence of layers containing a great number of seams and interfaces. Due to constructive interference only a few high amplitudes appear. These high amplitudes are labelled “interference reflections”.
• (iii) With increasing travel time the interference reflections are mainly composed of short lag multiples such that the primary reflections have no significant influence.
• (iv) The sequence of seams acts on the reflected seismic signal as a high pass filter and on the transmitted signal as a low pass filter. The cut-off frequencies are determined by the average seam thickness, and the steepness of the slopes increases with increasing number of seams.
• (v) The interference reflections can be used for determining the geological structure at least for the upper part of the sequence.

     

DIRECT RESISTIVITY INTERPRETATION BY ACCUMULATION OF LAYERS*

The asymptotic approximation of Pekeris is replaced by two new procedures referred to as the two-point method and the multilayer method, other steps in the direct interpretation remaining unmodified. The new methods are based on the assumption that there are at least one or two consecutive sample points of the kernel curve containing the information on a particular layer and containing no information on the deeper layers. In any step, the identified covering layers are accumulated and the interpretation progresses to the successive deeper layer. The multilayer method is oriented towards interpretation of data severly contaminated by noise. The elimination of noise with simultaneous averaging of layer parameters is performed in the domain of Dar Zarrouk parameters.     

DIRECT LOCATION OF OIL AND GAS BY THE SEISMIC REFLECTION METHOD*

The seismic velocity of sedimentary rocks is determined by the porosity, the velocity in the matrix and that in the fluid. The last two are known experimentally. The velocity can be measured from the surface. By applying the time average equation we can find the porosity. If the only difference at two points is the nature of the fluid in one layer, the ratio of the reflectivities of the layer at these points determines the nature of the fluid; this ratio is equal to the ratio of the amplitudes at these points. The size of the reservoir can then be found. The method can be applied to both oil exploration and production. It is more accurate in the latter case because more information is available than in the former case. The method is also more effective in developing discovered fields than the conventional method of relying only on the geological structural configurations.     

用地脉动研究银川沉积地层的地震波放大特性

根据银川盆地不同厚度覆盖层上的三分向( 东西、南北和垂直) 地脉动测量数据,绘制出水平向振幅谱与垂直向振幅谱的比值曲线.结果表明,谱比曲线上最大峰值处的频率大小显著地反映了覆盖层的薄厚.通过银川市部分房屋自振频率的对比分析认为,该部分房屋在未来地震中将遭受严重破坏.     

ON TRACING SEISMIC RAYS WITH SPECIFIED END POINTS IN LAYERS OF CONSTANT VELOCITY AND PLANE INTERFACES*

A polygonal ray path connects the seismic source and detector positions when the intervening medium consists solely of constant velocity layers with plane interfaces which may have arbitrary orientation. The coordinates of the ray vertices satisfy a system of coupled equations resulting from the requirement that Fermat's principle be satisfied along the ray path. Solving the system of equations is equivalent to tracing the ray numerically. A notable feature of this approach is that a ray which is critically refracted over a segment of its path requires no special handling.     

广义S变换与薄互层地震响应分析

Stockwell等人提出的S变换虽然与Fourier谱能保持直接联系，然而，由于S变换中的基本小波不适用于地震资料处理. 为此本文采用两个步骤对S变换加以推广，得到两种新变换（统称为广义S变换）. 首先， 用带有4个待定参数的调幅简谐波来代替S变换中的基本小波,定义广义S变换1，给出对应的逆变换；然后，以第一步中的基本小波的线性组合为新的基本小波，定义广义S变换2，并构造其逆变换公式. 最后，分别使用S变换及广义S变换对几种典型的薄互层模型进行分析计算. 结果表明，后者比前者有更强的探测能力， 后者可准确地确定厚度为1/8波长的薄互层中波阻抗界面位置，但前者却不能. 文中还用实际资料处理的结果，证明了广义S变换方法的有效性.     

华北近年水平形变与地震关系的研究

近年华北地区GPS水平形变反映出阴山－燕山南缘构造带、张家口－蓬莱隐伏断裂带、郯断裂带及建昌－邢台构造带具有较强的活动性,成为不同时间的压缩区段。近年的基线和位移量的表明：形变压缩区和位移量较大的地区是地应力积累较大地区,因此,成为中、强地震发生的场所,这两项可作为判定地震危险区的标志。     

地变率指标与华北地震区划

该文讨论地变率指标的概念。它是指在由发震构造引起的垂直运动为主的地表形变场中,水平方向上单位距离内垂向速率变化的物理量。笔者以华北地震区划为例,认为它可作为预测未来震情的一种有效的定量依     

阿斯南地震地质条件与地震形变带特征

本文介绍了阿斯南地区的地震和地质概况,在此基础上重点论述了构造的成生、发展与地震形变带特征,认为谢利夫盆地是晚新生代以来逐渐发展形成的,北东向阿斯南断层第四纪以来活动强烈,本次地震是该断裂继续活动的结果。     

SEISMIC MODEL STUDIES ON DIFFRACTION OF WAVES BY EDGES OF VARYING RADIUS OF CURVATURE AND DEPTH *

Results of studies carried out with the help of a three-dimensional seismic model on waves diffracted from edges of varying radius of curvature R and depth h with respect to wave length λ are described. The amplitude decay, travel time, and apparent velocity of the wave diffracted from a sub-surface edge of semi-infinite length are found to depend on the parameters R, h, and distance from the edge on the surface provided the ratio of the parameters to λ are less than some limiting values. The nature of the amplitude decay is independent of R when the depth exceeds 2λ, and independent of h when R exceeds 1.5λ. When these are below the limiting values (h= 2λ and R= 1.5λ), the nature of the decay depends appreciably on R and h. The apparent decay in amplitude on the surface due to geometrical spreading by the diffracting edge is less than that of a cylindrical secondary wave source and decreases with increase in depth of the edge. The nature of the travel time curves of the diffracted waves near the edge depend on R/λ when the depth is within about one λ. Apparent velocity of the wave depends largely on R/λ in the zone of diffraction up to a distance of about one λ from the edge on the surface. Beyond this distance the velocity is almost the same irrespective of R/λ and depend only on h/λ. The width of the zone of diffraction caused by an edge of finite length comparable to λ is more and more narrow as the ratio of the distance of the edge on the surface to its depth increases.     

斜俯冲板块边界变形分配的力学分析

本文研究斜俯冲板块边界由俯冲的逆掩断层和平行于火山弧的走滑断层所切割出的岩石层窄条的宏观力学状态．分析计算表明,当断层强度小而构造压应力大时,海沟处板块俯冲形成的逆掩断层地震滑动方向与斜收敛板块运动方向将不一致,震源机制反映的主压应力和断层错动方向变得近乎垂直于海沟;板块间的强耦合和大陆板块边缘软弱带的存在为窄条滑动提供了有利条件．给定俯冲倾角和震源机制滑动角的观测值,可以计算逆掩和走滑断层上的有效摩擦系数．计算结果表明,两种断层上的有效摩擦系数一般都不大于０．２,反映或存在着高孔隙水压,或存在断层泥之类弱物质,或两者兼而有之．     

斜俯冲板块边界变形分配的力学分析

本文研究斜俯冲板块边界由俯冲的逆掩断层和平行于火山弧的走滑断层所切割出的岩石层窄条的宏观力学状态．分析计算表明，当断层强度小而构造压应力大时，海沟处板块俯冲形成的逆掩断层地震滑动方向与斜收敛板块运动方向将不一致，震源机制反映的主压应力和断层错动方向变得近乎垂直于海沟；板块间的强耦合和大陆板块边缘软弱带的存在为窄条滑动提供了有利条件．给定俯冲倾角和震源机制滑动角的观测值，可以计算逆掩和走滑断层上的有效摩擦系数．计算结果表明，两种断层上的有效摩擦系数一般都不大于０．２，反映或存在着高孔隙水压，或存在断层泥之类弱物质，或两者兼而有之．     

含地下水的土层对地震波传播的影响

本文综合地介绍了含地下水的土层对地震波传播的研究成果,并对一些普遍性的原则问题进行了讨论.在不变形介质的前提下详细研究了二维地下水的波动问题,应用和开发了线单元-特征线法、交替方向隐式差分法和伽辽金有限单元法,求解了一些边值问题.还研究了介质的不均匀性、地层的成层性等对地下水波动的影响以及地下水波在界面上的透射、反射现象,发现了一些新的规律.从普遍方程出发,论证了地下水波在可变形介质和不变形介质中的差别,导出了两者的换算关系.开辟了由不变形介质的解求可变形介质的解的一个间接途径. 在可变形介质的前提下,直接从两相介质动力方程组出发,对含水土层中的波动问题进行了详尽研究,考虑了弹塑性、非线性本构关系、剪胀性、刚度退化等各种因素的影响.通过计算对各种土壤刚度、渗阻条件、边界透水条件、荷载形式、本构关系和不均匀性等情况做了比较研究,发现一些新规律.在垂直地震荷载作用下发现了两种不同形态的孔隙水压反应--阶变形和振荡形的机制,并用间断波理论解释了孔压阶跃的原因.最后对发现的新现象做了一些讨论和总结.     

流变介质中无限平面滑动产生的长周期地震波及形变波的传播特性和时空分布

本文在文献[1]的基础上,对流变介质中无限平面滑动产生的长周期地震波及形变波的传播特性和三种源位移模型在两个特殊频段的合成波时空分布进行了研究。另外还探讨了上述合成波形随其他参数的变化规律。所得结果可用于分析某些短期和临震地震前兆及慢地震等。     

RESOLVING ANISOTROPY IN LAYERED MEDIA BY JOINT INVERSION*

A finite number of layers must normally be used to represent the electrical properties of a horizontally-layered region. It is well known that these representative layers must include anisotropy. Applying the concept of joint inversion to DC resistivity and magnetotelluric (electromagnetic) data from a structure consisting of many fine layers, we show how the resistivities in the horizontal and vertical directions can be separately determined in each layer. The results are related to the contrasts existing in the interbeds which comprise the composite layers. The method has applications in both engineering and exploration.     

水库水位变化对地震活动和变形的影响

对二滩电站水库蓄水后水位变化与水库库区、水库外围地区地震活动性进行了对比研究,并利用攀枝花南山台形变观测资料与水库水位变化之间的相关性,研究了"水库-小震-变形-中强震"之间的关系。结果表明,在由水位变化引起的加卸载过程中,加载时段是外围地区中强地震的有利发震时段,而卸载达到年度最低水平时,库区内地震活动明显增强。水库首次蓄水引起变形最为剧烈,之后影响逐渐减弱。     

MODELING OF SH-WAVE PROPAGATION IN AN IRREGULARLY LAYERED MEDIUM-APPLICATION TO SEISMIC PROFILES NEAR A DOME*

An approach that relies on a discrete representation of seismic wavefields allows the computation of synthetic SH-seismograms in a laterally varying medium with plane and curved interfaces in the two dimensional (2-D) case. The diffracting interface is represented by an array of body forces located along the interface at equal spacing. The numerical treatment is limited to the irregular boundary while the propagation in flat layered zones is obtained by the reflection-transmission matrix method. As an example we have studied the case of a dome in a stratified medium. The solutions obtained verify the reciprocity theorem with good accuracy. The computation of vertical profiles and of surface reflection profiles illustrates the effects of diffraction and the importance of lateral propagation in such a structure.     

地震振动液化形变的研究

多种地质作用可造成年青的松散沉积物发生变形。为了判别沉积形变中的地震振动液化形变形迹,着重讨论了振动液化形成机制、形变几何形态特征以及它们与融冻变形和准同生变形构造的区别。认为振动液化形变形迹可以作为鉴别古地震标志之一。