Use of low frequencies for sub-basalt imaging

Many prospective passive ocean margins are covered by large areas of basalts. These basalts are often extremely heterogeneous and scatter the seismic energy of the conventional seismic reflection system so that it becomes difficult to obtain information on deeper reflectors. Since high frequencies are scattered more than low frequencies, we argue that the acquisition system for sub-basalt targets should be modified to emphasize the low frequencies, using much larger airguns, and towing the source and receivers at about 20 m depth. In the summer of 2001 we obtained seismic reflection data over basalt in the northeast Atlantic using a system modified to enhance the low-frequency energy. These new data show deep reflections that are not visible on lines shot in the same places with a conventional system.     

A case study of sub-basalt imaging in land region covered with basalt flows

In this study, a set of 20 2D seismic lines, acquired over the Golan Heights basaltic plateau, was processed and analysed. Although the data were acquired and processed by standard techniques, in some cases good-quality seismic images were obtained under several hundred metres of basalts. We describe how the seismic characteristics of the top basalt layer were defined and show the effect of the numerous widespread volcanic sources on the quality of the final images. The new data reveal the first images of the sedimentary sequence under the basalt flows, and indicate that strands of the Dead Sea Transform extend into this area. The entire region was found to be very deformed. Several attractive traps for hydrocarbon exploration were also identified on the output sections.     

利用大地电磁资料对玄武岩成像的可行性

The magnetotelluric （MT） method has been among the favorite supporting tools for seismic imaging of sub-salt and sub-basalt targets. In this paper we present an example from Kachchh, India （where basaltic rocks overlie Mesozoic sedimentary rocks）, and discuss the feasibility of using MT method as an exploration tool in this geological setting. Our results highlight the difference in magnetotelluric response caused by the thin intrabasalt layering. The key issue addressed in this paper is what MT can and cannot provide in such geological settings. First, we compute apparent resistivity and phase response curves using representative resistivity-depth models and borehole data from the study area. Later, we compare these results to assess the plausibility of using MT to image the sub-volcanic sediments at Kachchh. Finally, we substantiate our discussion through one-dimensional inversion of the field observed MT data from this region that exhibits poor sensitivity of MT for thin basalt layers.     

Method for correction of the prestack migration amplitude of converted waves

The paper presents a method for correction of amplitude of prestack migration using the reflectivity function. The solution of wave equations for heterogeneous media expressed in the form of Neumann series for converted waves was used to find the reflectivity function. The performance of the proposed method was verified using synthetic models of wavefields. The synthetic models of wavefields were also used to compare amplitude correction levels required by the presented method and the standard amplitude correction methods, i.e., the methods using the spherical divergence and extention factor.     

New constraints on lithospheric thickness of the Iranian plateau using converted waves

The study of mantle lithosphere plays a key role to reveal predominant tectonic setting process of a region. The current geological and tectonic setting of Iran is due to the ongoing continental–continental collision of the Arabian and Eurasian plates. We applied a combined P and S receiver function analysis to the teleseismic data of nine permanent broadband seismic stations of the International Institute of Earthquake Engineering and Seismology located in different tectonic zones of Iranian plateau. More than 4 years of data were used to estimate the thickness of the crust and mantle lithosphere. According to our results, the crust is 50 km thick beneath the Zagros fold and thrust belt (ZFTB). We found the maximum Moho depth of approximately 70 km under the Sanandaj-Sirjan zone (SSZ) indicating the overthrusting of the crust of Central Iran onto the Zagros crust along the main Zagros thrust (MZT). Below the northeasternmost part of the Urumieh–Dokhtar Magmatic Arc (UDMA) and Central Iran, the Moho becomes shallower and lies at 40 km depth. Towards northeast, beneath the Alborz zone, the crust is 55 km thick. Based on S receiver functions, we provided new insights into the thickness of the Arabian and Eurasian lithospheres. The location of the boundary between these plates was estimated to be beneath the SSZ, which is slightly shifted northeastward relative to the surficial expression of the MZT. Furthermore, the Arabian plate is characterized by the relatively thick lithosphere of about 130 km beneath the ZFTB reaching 150 km beneath the SSZ, where the thickest crust was also observed. This may imply that the shortening across the Zagros is accommodated by lithospheric thickening. In contrast, UDMA and Central Iran are recognized by the thin lithosphere of about 80–85 km. This thin lithosphere may be associated with the asthenospheric upwelling caused by either lithospheric delamination or Neo-Tethys slab detachment beneath the Zagros collision zone.     

Phase behaviour of P-SV converted waves recorded at the sea-bed

Shear waves recorded at or near the sea-bed, i.e. a water–sediment interface, may suffer from unwanted phase change, which is detrimental to velocity analysis and processing and degrades the quality of the final stacked or migrated image. In this study, this phenomenon is analysed for P-SV converted waves recorded at the sea-bed. Theoretical analysis shows that phase change does not occur if the converted shear waves always maintain raypaths that lie within the critical angle, provided the subsurface layering is horizontal. A phase change that is asymmetric with offset can readily be explained as being due to dipping layers at targets or the dipping sea-bed. This analysis is extended to multiple layers and anisotropic media and shows that the same conclusions hold. The analysis performed on two sets of ocean-bottom-cable seismic data shows that the majority of observations show little evidence of phase change, and occasionally display the asymmetric phase change with offset. This finding underlines the robustness of converted shear waves for imaging the horizontal subsurfaces and sea-bed as all of the offset information may be used.     

考虑水层鸣震的海底多分量数据转换波走时反演方法

海底多分量地震数据在完成上-下行P/S波分离之后,基本消除了检波器端的多次波信号,但仍受到一些源端多次波的干扰.在浅水(水深在500 m以内)、软海底环境下,经海底与海面各反射一次的水层鸣震因能量较强,对反射波偏移成像与速度分析造成极大的困扰.就横波速度分析而言,含源端水层鸣震的PPS波对一次转换PS波形成主导性干扰....     

VTI介质转换波动态聚焦束偏移成像方法研究

采用多分量地震数据进行处理,可以得到高质量的转换波成像结果,进而为流体识别和裂缝预测等提供有效支持.本文在前人研究的基础上,通过求解各向异性介质qPSV转换波运动学和动力学射线追踪方程,获得转换波中心射线走时、路径及动力学信息;同时,通过修改动态聚焦型传播算子,构建出各向异性介质动态聚焦束,提出了一种各向异性介质转换波...     

Detection of waves converted from P to SV in the mantle

A method is described for the detection of P to SV converted waves in the long-period P coda. The procedure involves axis rotation, transformation of records to a standard form and stacking of processed records from events of various epicentral distances. When applied to NORSAR records, the procedure detected converted phases corresponding to the boundaries in the 410–440- and 640–690-km depth ranges.     

The separation and parameter determination of PS converted phases in deep sounding by converted waves of earthquakes

The practice of deep sounding by converted waves of Earthquakes shows that the time duration of the PS converted phases is usually longer than the arrival time delay between the converted phases coming from the adjacent boundaries. So, the mutual interference of the converted phases from adjacent boundaries may form a complicated interference series of the waves. It is difficult to recognize and determine exactly the parameters of the converted waves in the interfering records. In this paper the method of omni-azimuthal polarization analysis and the method of separation of interfering waves are described. The theoretical examples show that the procedure given in the paper is rather effective to separate the interfering waves and to determine their parameters, and so, is important to improve the resolving power and the reliability of the method of deep sounding by converted waves of earthquakes. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,15, 39–45, 1993. The research reported here is a part of proposal sponsored by State Seismological Bureau and China Joint Seismological Science Foundation.     

Heterogeneities in the mantle transition zone from observations of P-to-SV converted waves

The method of detection of P-to-SV converted waves from distant earthquakes (Vinnik, 1977) was applied to sets of long-period records from a few seismograph stations in Europe and the west of North America. The results obtained suggest that the converted phases related to the major boundaries in the mantle can be reliably detected and the depths of conversion evaluated with an accuracy of a few kilometres. The depth of the olivine-spinel transition is close to 400 km and no difference between the estimates for the north of Europe and the west of North America is found. The depth of the boundary separating the upper and lower mantle is close to 640 km, which is 30 km less than in the recent Earth-reference models. Fine S velocity stratification of this transition changes laterally from a high-gradient layer 50 km thick, terminated at the bottom by a sharp discontinuity, to a gradient layer 100 km or more thick without the discontinuity. A striking anomaly of the mantle transition zone is found in the Rio Grande rift area where a well pronounced boundary is found at 510 km depth.     

基于近震转换波的沉积层地区震源深度测定方法

基于合成地震图,并与观测数据对比,对沉积层地区近震波形的频率成分、偏振和走时等特征进行了分析,确认了沉积-基底界面的Sp转换波.研究表明：在给定震中距时,Sp转换波与直达P波的到时差随震源深度的增加近似呈线性增加,可以用来较好地约束震源深度.以2015年4月19日河北文安M3.0地震和2006年7月4日河北文安M5.1地震为例,验证了使用近震Sp转换波测定沉积层地区震源深度的可行性.利用Sp转换波对2015年4月19日河北文安M3.0地震重新测定震源深度的结果为18 km左右,而不是地震目录中给出的29 km,说明该地震发生在中上地壳,而不是下地壳.本文给出的方法可应用于测定沉积层地区的震源深度.

     

Use of marine controlled-source electromagnetic sounding for sub-basalt exploration

The problem of imaging sedimentary structure obscured by high-velocity layers, such as carbonate, basalt or salt, using conventional seismic techniques is well known. When this problem is encountered in offshore areas, marine electromagnetic data can provide valuable, alternative and complementary constraints on the structure. We concentrate on the use of marine controlled-source electromagnetic (CSEM) sounding in the frequency domain. The feasibility of mapping sub-basalt sedimentary structure using this technique is explored by means of modelling studies which mimic a type of survey which could readily be employed in practice. As a starting point the problem is addressed in terms of 1D resistivity structure. We show that sub-basalt sediments can be detected and their depth of burial quantified to within 200 m in the examples shown. The presence of small-scale inhomogeneities in the basalt (which cause much of the scattering in seismic data) is shown to have no appreciable effect on the ability of the CSEM data to detect the sediments. The modelling is then extended to 2.5 dimensions. Again the presence of sub-basalt sediments can be readily detected and their properties and geometry quantified, especially if the electromagnetic data are combined with constraints from complementary geophysical techniques.     

直达Ps与接收函数Ps联合分析地壳各向异性

本文尝试直接利用地震图记录中的Ps转换波,而非传统的接收函数方法,检测地壳介质的各向异性.虽然远震记录中的P波对Ps转换波会有干扰,但P波与Ps转换波具有不同的偏振特性,最小能量法可以将Ps转换波的各向异性参数检测出来,对时间窗的选择也很强壮.本文利用IC台网（New China Digital Seismograph Network,中国新数字地震台网）中的KMI,ENH,LSA,BJT,XAN,WMQ六个台站接收的宽频远震记录进行了测试,并且对比了接收函数方法的结果.研究表明,利用最小能量法直接测量与利用旋转相关法测量接收函数中Ps转换波的结果基本一致.两种方法测量所得的快波偏振方向与地壳运动方向,最大水平主压应力方向,震源机制解压缩轴方向,最大主压应变率方向以及已有的各向异性研究结果基本一致.     

Analytical expressions for Fresnel volumes and interface fresnel zones of seismic body waves. Part 2: Transmitted and converted waves. Head waves

Summary Fresnel volumes and interface Fresnel zones of transmitted and head waves are studied. The relation derived for transmitted waves may also be used for converted reflected waves. Considerable attention is devoted to the penetration of Fresnel volumes across structural interfaces, particularly for head waves.     

水平界面上P-SV转换波转换点的精确解

转换波共转换点的叠加和道集选取都需要准确地计算转换点的位置.Tessmer和Behle、Taylor分别给出了水平单层介质中P-SV转换波转换点坐标的解析解,由于其表达式的复杂性,在应用中几乎不被采用.在本文中,运用Snell定律重新建立了在水平反射界面上反射的P-SV转换波的转换点坐标的四次方程,并严格地推导出与纵波速度、横波速度、炮检距和反射深度有关的转换点坐标的解析解,确定了惟一的解析表达式.将这一结果应用于P-SV转换波的速度分析和叠加处理中.简化的公式有较好的应用价值.     

SH波地形效应特征的研究

许多实际的地震中已经观测到地表地形对地表地震动产生影响(地形效应),这种影响背后的成因、影响因素、影响规律至今还不太清楚,因此地震学中、地震工程学中都围绕这一课题展开了相应的研究.本文以地震波模拟为手段,以点力源激发下的SH横波地震波传播为对象,研究地形形态、震源的物理特征对SH波地表地震动的影响,研究内容主要包括:地形陡度和地形复杂度对地形效应的作用;震源所处位置、震源频率与地形效应的关系,揭示出地形效应的成因受局部地表张角的控制,张角越小地形的放大作用越强.     

Normal moveout velocity for pure‐mode and converted waves in layered orthorhombic medium

We study the azimuthally dependent hyperbolic moveout approximation for small angles (or offsets) for quasi‐compressional, quasi‐shear, and converted waves in one‐dimensional multi‐layer orthorhombic media. The vertical orthorhombic axis is the same for all layers, but the azimuthal orientation of the horizontal orthorhombic axes at each layer may be different. By starting with the known equation for normal moveout velocity with respect to the surface‐offset azimuth and applying our derived relationship between the surface‐offset azimuth and phase‐velocity azimuth, we obtain the normal moveout velocity versus the phase‐velocity azimuth. As the surface offset/azimuth moveout dependence is required for analysing azimuthally dependent moveout parameters directly from time‐domain rich azimuth gathers, our phase angle/azimuth formulas are required for analysing azimuthally dependent residual moveout along the migrated local‐angle‐domain common image gathers. The angle and azimuth parameters of the local‐angle‐domain gathers represent the opening angle between the incidence and reflection slowness vectors and the azimuth of the phase velocity ψ_phs at the image points in the specular direction. Our derivation of the effective velocity parameters for a multi‐layer structure is based on the fact that, for a one‐dimensional model assumption, the horizontal slowness and the azimuth of the phase velocity ψ_phs remain constant along the entire ray (wave) path. We introduce a special set of auxiliary parameters that allow us to establish equivalent effective model parameters in a simple summation manner. We then transform this set of parameters into three widely used effective parameters: fast and slow normal moveout velocities and azimuth of the slow one. For completeness, we show that these three effective normal moveout velocity parameters can be equivalently obtained in both surface‐offset azimuth and phase‐velocity azimuth domains.     

Crustal thickness variation across the northeast Iran continental collision zone from teleseismic converted waves

For the first time, we present the variation of crust–mantle boundary beneath the northeast Iran continental collision zone which is genetically part of the Alpine–Himalayan orogeny and beneath Central Iran which is a less-deformed tectonic block. The boundary was imaged by stacking teleseismic P–S converted phases and shows a strong variation of Moho from 27.5 km under Central Iran to 55.5 km beneath the Binalud foreland basin. The thickest crust is not located beneath the high topography of the Kopeh Dagh and Binalud mountain ranges suggesting that these mountain ranges are not supported by a crustal root. The simple gravity modeling of the Bouguer anomaly supports this idea.