REFLECTION-WINDOW MAPPING OF SHALLOW BEDROCK*

A field study was undertaken to evaluate the effectiveness of the high frequency seismic reflection technique for mapping of shallow and irregular bedrock. Bedrock reflections were obtained using a hammer source with both in-line and common offset field layouts. The recording equipment included 12-channel enhancement seismographs, 28 Hz vertical geo-phones and a microcomputer. The latter increased the overall versatility of the seismic system. Field sites for this study are typical of the geological settings of the tin mining areas of Malaysia. The topographical ‘lows’ of the irregular bedrock control the localization of tin ore. The subsurface geology consists of a thin low velocity layer (± 300 m/s) overlying the compact overburden (± 1700 m/s) which in turn lies on bedrock. This paper discusses various criteria for designing an optimum window for obtaining usable reflections between the first arrival and the leading edge of the ground roll cone. Detailed mapping of the overburden and the bedrock interface by the reflection method can be useful in delineating areas for exploratory drilling and for optimum planning of mining operations.     

柴达木盆地东部地震地面运动放大效应

柴达木盆地是青藏高原东北部大型断陷山间盆地,该地区的流动观测记录了2008年11月10日发生于大柴旦附近的M_W6.3地震。和附近的基岩上的记录相比,盆地内部的记录显示出非常显著的地面运动放大效应,表现为峰值速度的增大、持续时间的延长,其呈现出长持续时间的后续震相。傅里叶频谱分析表明盆地内部显著的后续震相的频率和直达波相比较低,地面质点运动轨迹图显示后续震相为面波运动特征。为了解释地面运动的差异,构建二维模型,通过交错网格高阶有限差分方法计算了地震波在盆地内部的传播过程,结果显示盆地内部低速层的存在造成直达波的放大以及多次反射与转换,盆地边缘结构造成的波的相干叠加产生了强烈的次生面波,其低频、大振幅、长持续时间的特征是盆地内部地面运动放大的主要原因。     

Broadband ground-motion simulation of the 24 May 2014 Gokceada (North Aegean Sea) earthquake (Mw 6.9) in NW Turkey considering local soil effects

On 24 May 2014, a Mw 6.9 earthquake occurred in the west of Gokceada Island, northern Aegean Sea. The earthquake was close to Canakkale, Enez, Tekirdag cities, and damaged 300 buildings in the Marmara Region, NW Turkey. We simulated its broadband (0.1–10 Hz) ground motions including 1D deep and shallow structures soil amplification effects at the 12 strong ground motion stations in the western Marmara Region. The 1D deep velocity structures from the focal layer to the engineering bedrock with an S-wave velocity of 0.78 km/s in different azimuthal directions were tuned by comparing the observed group-velocity dispersion curves of Rayleigh and Love waves from the mainshock with theoretical ones. We also added the shallow parts from previous surveys into the 1D models. Synthetic seismograms on the engineering bedrock were generated using the discrete wave number method with a source model and the 1D deep velocity structures. Then the surface motion was generated considering shallow soil amplification. The synthetic seismograms are generally in good agreement with the observed low and high-frequency parts at most of the stations indicating an appropriateness of the source model and the 1D structural model.     

较复杂介质波场计算中的一种透射边界

传统的人工边界是在声波或弹性波分解为内行和外行分量的基础上设计的.对于较复杂的介质模型（如粘弹介质、弹塑介质、塑性介质模型等）,地震波不能简单地分解为各自满足其运动方程的纵波、横波等成分,且地震波在传播过程中还有衰减现象,因此传统的人工边界已不能有效地用于非弹性的较复杂介质的波场模拟.本文提出了一种适合于更一般的地震模型的波场计算的透射边界条件,并以粘弹介质的一个较简单的二维几何模型为例,通过有限元数值求解这种透射边界和刚性边界下粘弹波的激发和传播,得到地面合成地震记录,证明了这种透射边界用于较复杂介质波场计算是可行的.     

DOWNHOLE SYNTHETIC SEISMIC PROFILES IN ELASTIC MEDIA1

A multichannel lattice filter structure is utilized to represent seismic waves propagating in adjacent layers in an elastic medium. Using this model, an explicit time-domain solution for arbitrary source and receiver locations is obtained as an ARMA (AutoRegressive and Moving-Average) process. The lattice and ARMA structures have given rise to an effective algorithm for the calculation of offset/downhole synthetic seismograms. A large range of recently developed offset/downhole seismic survey geometries, such as the ‘Yo-Yo’ arrangement, can thus be simulated. In addition, the explicit solutions for upgoing and downgoing waves provide new insight into the properties of general downhole seismic signals, including wave-mode conversion effects and multiple reflections. Furthermore, offset/downhole seismograms generated by a line source (i.e. 2D point source) can also be constructed by superposition of plane waves with different incidence angles. Synthetic seismograms generated using a different source-receiver arrangement indicate that the properties especially associated with offset/downhole seismic signals can be predicted by this modelling method. These properties include arrival times, amplitude attenuation and wave-mode conversion effects. Finally, utilizing this numerical modelling method to a real downhole survey with Yo-Yo geometry may lead to a proper data acquisition and processing procedure, and improves the interpretation confidence of the field section.     

SYNTHETIC SEISMOGRAMS AT NON-VERTICAL INCIDENCE*

Synthetic seismograms are usually computed for reflections from vertical incidence of P waves for a horizontally layered medium. In actual practice the angle of incidence departs from the vertical, as receivers are usually located at some distance from the source. At angles other than the vertical, the conversion of P- to S-wave energy and changes in the reflection coefficient affect the shape of the synthetic seismograms. The effect of non-vertical incidence on synthetic seismograms is examined in this paper. Seismograms at non-vertical incidence have been computed using the plane-wave approach of Haskell (1953) for a layered medium. The use of plane waves is an approximation to the actual case of spherical wavefronts from a surface source. Using plane-wave theory, the expected wave forms as a function of angle of incidence were computed numerically for several simple models. The results indicate that the synthetic seismograms do not change significantly for angles of incidence between o and 25 degrees. For larger angles the changes in the wave forms may be severe. The effect is more pronounced for high-velocity layers than for low-velocity layers.     

Some SH-wave seismic reflections from depths of less than three metres

Shallow SH-wave reflections are far from routine, although their study can provide insights into important properties of near-surface materials that cannot be inferred from P-wave data alone. Difficulties in separating SH-wave reflections from Love waves are generally considered the major obstacle to progress in shallow SH-wave seismic reflection. This may be the case in surveys undertaken at great depths, but it is not necessarily true for reflection data gathered at shallow and ultra-shallow depths. This paper shows that when SH-wave data possess wavelengths greater than the thickness of the superficial layer, Love waves are not greatly dispersed. In this case, misinterpretation between parts of reflection hyperbolae and waveguide arrivals is sufficiently limited. In a one-layer model earth, which well approximates typical situations of the near-surface underground, the most energetic modes (the lowermost modes) of the dispersed surface waves have a dominant frequency band that falls below the wavelet spectrum of the shallow reflections; therefore, they can be filtered out in the frequency domain. Higher modes, although their spectral content overlaps that of the reflections, exhibit small amplitudes on seismograms and leave strong reflections unaffected.We present field examples from three different sites where we were able to obtain ultra-shallow reflections (< 3 m) in unconsolidated sediments. The high level of resolution (vertical resolution up to 15 cm) suggests that SH-wave reflection imaging has the potential to complement other high-resolution techniques, such as P-wave reflection and ground-penetrating radar (GPR) imaging, allowing a better and more complete characterization of the near-surface environments.     

区域地震范围内地壳介质的低频响应

本文对于计算P_nl波格林函数的广义地震射线方法进行了改进。包括:1.简化了地震射线的描述方法;2.将射线按其运动学相似性进行分组计算,从而较大地提高了计算速度;3.进一步推广用于计算SH波部分的响应。对于1983年山东菏泽地震和1985年云南禄劝地震,用综合地震图方法模拟了763型长周期地震仪的P_nl波记录,求出两个地震的断层面解答,其结果与有关研究所得该地区构造应力场的图象基本一致。     

Preliminary study of the straticulate structure of crust-mantle beneath east Guangdong and south Fujian Provinces of China

Using the 12 deep focus teleseismic P waveforms recorded by 6 short-period seismographs in east Guangdong and Fujian region of China as the observed data, synthetic P waveforms comparable to the observed ones are obtained by gradually adjusting the crust-mantle model and calculating the corresponding synthetic seismograms. The results suggest that the crust-mantle structure in this region is a vertically straticulate media structure consisting of 3 pairs of thin layers with a high- and low-velocity alternation. The crustal thickness tends to increase gradually from south to north, being 31.5 km in the south and 32.4 km in the north. Finally, the suggested model is tested using the explosive data of Yunfu, Guangdong Province. The theoretical travel-time of P waves agrees fairly well with the observed travel-time. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,14, 172–179, 1992.     

天然地震频率范围内首都圈地区近地表S波速度结构c

近地表沉积层的S速度结构是强地面震动模拟和地震灾害估计的重要参数,尤其是浅部的S波速度结构在工程上具有重要的应用意义．目前大部分资料来源于工程钻孔或工程地震探测,很少有地震波频率范围内的S波速度结构,或者深度达数百米的S波速度结构．通过对天然地震的井下摆波形记录的分析,提供了一种测量地震波频率范围深达数百米的S波速度的有效方法．收集了首都圈地区44个井下摆的近震记录,利用广义射线方法确认了直达S波及其在地表的反射波震相,并通过测量不同台站上两个震相的到时差,获得了首都圈地区浅层100——500m 深度范围的S波速度结构．研究发现,浅部100m 的平均S波速度低于300m/s．当深度增加到500m 时S波速增加到800m/s,平均速度梯度为0.8 （m/s）/m．研究结果表明,井下摆地震记录波形是研究沉积盆地浅层S波结构的重要资料,将为沉积盆地的强地面震动模拟提供重要基础参数．      

How deep was the Dudley (UK) earthquake of 22 September 2002?

On 22 September 2002, the largest UK earthquake (m_b4.3) of the last 10 years occurred near the town of Dudley in the West Midlands. Here we determine the earthquake focal mechanism and depth using data from stations at regional and teleseismic distances. Short-period teleseismic seismograms are interpreted in terms of P and surface reflections pP and sP. This analysis suggests that the source depth is deeper than the 9.7 km initially determined by the British Geological Survey (BGS). The relative amplitude method is applied to four teleseismic seismograms to support our interpretation of the surface reflections, and constrain the focal mechanism. Our preferred focal mechanism, a near vertical strike-slip with _s = 94°, = 88° and = –179°, is in reasonable agreement with a moment tensor determined by the Swiss Seismological Service. Synthetic regional surface wave seismograms match the observed seismograms for a model focal depth of 19.5 (±3.0) km and scalar moment, M₀, of 3.2 × 10¹⁵ N m. Our results emphasize that due to the well-known trade-off between depth and M₀ from inversions of long period (0.02–0.1 Hz) surface waves, it is preferable to combine long- and short-period data to constrain reliably the depth and hence estimate M₀. Our focal mechanism and depth are further validated by generating short-period synthetic seismograms that match the observations.     

Discussion of the Features of Synthetic Amplitude-Distance Curves of Teleseismic P Waves for Global Earth Models

The reason why the synthetic amplitude distance curves of P-waves for models IASP91 and PREM, observed for shallow sources up to epicentral distance of 28°, oscillate is illustrated by means of synthetic seismograms. Furthermore the position of the beginning of the diffraction of the P-wave at the CMB, depending on the prevailing signal period, is discussed in connection with the extension of Fresnel volumes.     

Grid dispersion in generating finite-differences synthetic seismograms

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PARAXIAL RAY TRACING IN 3D INHOMOGENEOUS,ANISOTROPIC MEDIA1

Many investigations of the propagation elastic waves within the earth require a technique for producing synthetic seismograms which is capable of modelling 3D propagation effects. Ray methods are an excellent option for these problems, because they can be made fully 3D and allow a relatively quick and flexible computation of synthetic seismograms. However, the two point problem of finding the ray which connects exactly a specific source and receiver, may still be difficult and time consuming. Therefore, application of the paraxial method, which allows extrapolation of the information on a given ray to nearby receiver locations, is very valuable. With this approach, great savings in computation time and significant simplification of computer codes are possible. We investigate the application of the paraxial ray method to two problems in which the effects of 3D seismic wave propagation are important. The first is a model of a reef structure. In this case, we consider synthetic seismograms for a VSP experimental configuration. When the SV source and well are located along the axis of the reef, only 2D propagation effects are observed. If the source-receiver plane is located to the side of the reef, however, the 3D shape of the reef causes significant amplitudes to be predicted for shear-wave arrivals on the transverse component of the synthetic seismograms. The second example is a ID, layered earth model, but it includes two layers which are azimuthally anisotropic due to the presence of aligned, vertical fractures. This anisotropy leads to 3D raypaths. Synthetic seismographs are presented for a cross-hole geometry both for an equivalent isotropic model and for the direction parallel to the fractures and at an angle of 45° to the fractures in the anisotropic case. These synthetics show that the differences between the isotropic case and the case for source and receivers aligned with the plane of the fractures are small and subtle. On the other hand, the predictions for the direction at 45° to the cracks show shear-wave splitting and significant transverse component signal. These results have important implications for both modelling and for applications such as tomography. It is clear that for some of the cases considered, a 2D algorithm will lead to errors in interpretation of data. In addition, ray-based tomographic techniques will have great difficulty in obtaining a well-defined 2D planar image when the signals are propagating in regions outside the image plane.     

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.

     

弹性介质中瑞雷面波有限差分法正演模拟

为研究瑞雷面波的形成机理及传播规律,促进瑞雷面波资料处理方法的发展,本文根据弹性波方程,采用交错网格有限差分数值求解算法,对浅层各向同性弹性介质进行了包括瑞雷面波和体波在内的全波场模拟. 提出了变系数吸收边界条件并将之应用于正演模拟,使边界条件的处理简单而高效,同时给出了角点的处理方法. 对工程勘察中常见的连续和层状介质模型进行了模拟,获得了更加接近实际情况的地震记录. 结合模拟记录,探讨了瑞雷面波的形成条件,同时讨论了震源埋深对面波能量的影响.     

各向同性薄层反射理论地震图

本文在各向同性介质假设下,计算了薄层顶、底反射P波和PS波的理论地震图.理论模拟发现薄层的反射P波与PS波是一复合波,包括层内的多次透射和反射转换波型,且具有类似单一界面反射的、脉冲式的波形特征.两类薄层反射整体振幅随着薄层厚度的降低而缩小;在地震子波主频40 Hz条件下,2 m左右厚度的薄层反射相比单一界面反射具有等同的振幅水平.1 m以下极薄层仅有弱反射甚至无反射;薄层反射复合波振幅与炮检距的关系依然成立,但不惟一;单界面反射AVO原理、方法不适用于薄层反射解释与反演.     

Application of the edge wave superposition method

Numerical examples of high-frequency synthetic seismograms of body waves in a 2-D layered medium with complex interfaces (faults, wedges, curvilinear, corrugated) are presented. The wave field modeling algorithm combines the possibilities of the ray method and the edge wave superposition method. This approach preserves all advantages of the ray method and eliminates restrictions related to diffraction by boundary edges and to caustic effects in singular regions. The method does not require two-point ray tracing (source-to-receiver), and the position of the source, as well as the type of source, and the position of receivers can be chosen arbitrarily. The memory and the time required for synthetic seismogram computation are similar to ray synthetic seismograms. The computation of the volume of the medium (the Fresnel volume or Fresnel zones), which gives the essential contribution to the wave field, is included in the modeling program package. In the case of complicated irregular interface (or a layered medium with a regular ray field at the last interface), the method displays a high accuracy of wave field computation. Otherwise, the method can be considered a modification of the ray method with regularization by the superposition of edge waves.     

SsPmp震相地壳探测方法

SsPmp波是远震S波经地表反射转换的P波在莫霍面发生反射后被地表台站接收得到的震相.震中距在30°~50°之间的远震S波震相经地表反射转换的P波射线参数较大,在莫霍面发生全反射,使得台站接收的SsPmp波具有较强的能量,能够从地震记录中清楚地识别出来,为探测台站附近的莫霍面形态提供新的途径.本文通过合成理论地震图分析了SsPmp震相与地壳厚度、射线参数和Pn波速度之间的关系.结果表明:对于水平界面,地壳厚度只影响SsPmp与Ss波之间的相对到时差;Pn波速度只影响SsPmp的相位;射线参数既对SsPmp波的相对到时有影响,也会引起SsPmp波的相位变化.对于复杂的界面,SsPmp反映的深度与速度梯度最大的深度接近,而反映的Pn波速度与实际的Pn波速度一致.     

Seismic migration and absorbing boundaries with a one-way wave system for heterogeneous media1

A first-order one-way wave system has been created based on characteristic analysis of the acoustic wave system and optimization of the dispersion relation. We demonstrate that this system is equivalent to a third-order scalar partial-differential equation which, for a homogeneous medium, reduces to a form similar to the 45° paraxial wave equation. This system describes accurately waves propagating in a 2D heterogeneous medium at angles up to 75°. The one-way wave system representing downgoing waves is used for a modified reverse time migration method. As a wavefield extrapolator in migration, the downgoing wave system propagates the reflection events backwards to their reflectors without scattering at the discontinuities in the velocity model. Hence, images with amplitudes proportional to reflectivity can be obtained from this migration technique. We present examples of the application of the new migration method to synthetic seismic data where P-P reflections P-SV converted waves are present. Absorbing boundaries, useful in the generation of synthetic seismograms, have been constructed by using the one-way wave system. These boundaries absorb effectively waves impinging over a wide range of angles of incidence.