自适应虚同相轴方法压制地震层间多次波

地震数据中发育的层间多次波是影响速度分析和偏移成像的精度和可靠性的关键.通常情况下,层间多次波的动校正量、叠加速度和频率与一次波并无明显差异,从而对识别、预测和压制多次波带来了极大挑战.传统虚同相轴方法基于物理图像和定性公式,其预测的层间多次波振幅和相位精度难以满足实际需求,造成了其对匹配算法的过度依赖.本文针对传统虚同相轴方法的理论缺陷和计算精度问题,通过理论推导得到了新的自适应虚同相轴方法.相比于传统方法,自适应虚同相轴方法能够显著提高压制多次波能力,同时减少对匹配算法的依赖.本文给出了自适应虚同相轴方法的推导过程,并运用一维和二维模型算例验证了方法相较于传统虚同相轴方法的多次波预测精度优势.通过在PLUTO模型和实际陆地地震数据上的应用实例,证明了本文新研究的自适应虚同相轴方法对去除层间多次波,恢复并突出目标储层同相轴,提高地震成像分辨率的显著作用.

     

Full‐wavefield migration: using surface and internal multiples in imaging

Multiple scattering is usually ignored in migration algorithms, although it is a genuine part of the physical reflection response. When properly included, multiples can add to the illumination of the subsurface, although their crosstalk effects are removed. Therefore, we introduce full‐wavefield migration. It includes all multiples and transmission effects in deriving an image via an inversion approach. Since it tries to minimize the misfit between modeled and observed data, it may be considered a full waveform inversion process. However, full‐wavefield migration involves a forward modelling process that uses the estimated seismic image (i.e., the reflectivities) to generate the modelled full wavefield response, whereas a smooth migration velocity model can be used to describe the propagation effects. This separation of modelling in terms of scattering and propagation is not easily achievable when finite‐difference or finite‐element modelling is used. By this separation, a more linear inversion problem is obtained. Moreover, during the forward modelling, the wavefields are computed separately in the incident and scattered directions, which allows the implementation of various imaging conditions, such as imaging reflectors from below, and avoids low‐frequency image artefacts, such as typically observed during reverse‐time migration. The full wavefield modelling process also has the flexibility to image directly the total data (i.e., primaries and multiples together) or the primaries and the multiples separately. Based on various numerical data examples for the 2D and 3D cases, the advantages of this methodology are demonstrated.     

利用地震信息节点技术平台建设虚拟测震台网的可行性分析

通过介绍宝鸡市地震局虚拟测震台网建设过程,及网络集成,技术应用、运行状态等情况,表明市、县一级地震工作机构,在现有环境和条件下,完全可以利用地震信息节点平台建设虚拟测震台网,实现地震数字观测资料共享、地震事件实时监测、快速获取震情信息、建立强震预警机制等功能,这无疑是逐步提高防震减灾队伍快速响应能力和全社会综合防灾能力的有效途径。     

Time-space variation of volcano-seismic events at La Fossa (Vulcano, Aeolian Islands, Italy): new insights into seismic sources in a hydrothermal system

We investigated the relationship between volcano-seismic events, recorded at La Fossa crater of Vulcano (Aeolian Islands, Italy) during 2004-2006, and the dynamics of the hydrothermal system. During the period of study, three episodes of increasing numbers of volcano-seismic events took place at the same time as geothermal and geochemical anomalies were observed. These geothermal and geochemical anomalies have been interpreted as resulting from an increasing deep magmatic component of the hydrothermal fluids. Three classes of seismic events (long period, high frequency and monochromatic events), characterised by different spectral content and various similarity of the waveforms, have been recognised. These events, clustered mainly below La Fossa crater area at depths of 0.5–1.1 km b.s.l., were space-distributed according to the classes. Based on their features, we can infer that such events at Vulcano are related to two different source mechanisms: (1) fracturing processes of rocks and (2) resonance of cracks (or conduits) filled with hydrothermal fluid. In the light of these source mechanisms, the increase in the number of events, at the same time as geochemical and geothermal anomalies were observed, was interpreted as the result of an increasing magmatic component of the hydrothermal fluids, implying an increase of their flux. Indeed, such variation caused an increase of both the pore pressure within the rocks of the volcanic system and the amount of ascending fluids. Increased pore pressures gave rise to fracturing processes, while the increased fluid flux favoured resonance and vibration processes in cracks and conduits. Finally, a gradual temporal variation of the waveform of the hybrid events (one of the subclasses of long period events) was observed, likely caused by heating and drying of the hydrothermal system.     

Voluminous lava flows at Oldoinyo Lengai in 2006: chronology of events and insights into the shallow magmatic system

The largest natrocarbonatite lava flow eruption ever documented at Oldoinyo Lengai, NW Tanzania, occurred from March 25 to April 5, 2006, in two main phases. It was associated with hornito collapse, rapid extrusion of lava covering a third of the crater and emplacement of a 3-km long compound rubbly pahoehoe to blocky aa-like flow on the W flank. The eruption was followed by rapid enlargement of a pit crater. The erupted natrocarbonatite lava has high silica content (3% SiO₂). The eruption chronology is reconstructed from eyewitness and news media reports and Moderate Resolution Imaging Spectroradiometer (MODIS) satellite data, which provide the most reliable evidence to constrain the eruption’s onset and variations in activity. The eruption products were mapped in the field and the total erupted lava volume estimated at 9.2 ± 3.0 × 10⁵ m³. The event chronology and field evidence are consistent with vent construct instability causing magma mixing and rapid extrusion from shallow reservoirs. It provides new insights into and highlights the evolution of the shallow magmatic system at this unique natrocarbonatite volcano.     

Scattering and dissipation of seismic waves in the presence of nonlinearity

The processes of seismic wave scattering and dissipation are examined in a medium that, in addition to being inhomogeneous and anelastic, is nonlinear and seismically active (seismic emission). In such a medium, there is a complex interrelation between the effects of nonlinearity, scattering and dissipation. Thus, nonlinear interactions between the various components (primary, scattered and induced) of a developing wavefield cause nonlinear (or wave-on-wave) scattering and, by transferring part of the wave energy to the high-frequency region, contribute to its scattering and dissipation. On the other hand, whereas dissipation opposes the accumulation of nonlinear effects by reducing the wave amplitudes, scattering assists it by increasing the propagation distance (and hence the interaction time).Estimates based on results of field experiments involving vibrators indicate that, as a rule, scattering on inhomogeneities is much stronger than nonlinear scattering, and that nonlinear effects may often dominate dissipative ones.The nonlinearity of the transmitting medium explains observedQ-value anomalies, and its seismic activity explains the existence ofP coda and the temporal changes in codaQ.     

浅水无定位拖缆观测系统定义及多次波压制效果分析

浅水无定位拖缆在近岸海洋工程及浅水多道地震勘探中应用广泛,但受无定位拖缆实际位置获取的限制,野外观测系统准确定义受影响.基于传统直线假设的观测系统定义简单易行,但计算的共反射点面元无法获得实际坐标,位置偏差较大,严重影响后续处理方法的有效应用.基于实际炮点轨迹坐标,本文提出了一种新的无定位拖缆实际坐标计算方法,新方法首先确定接收排列的相对空间位置,采用反距离比线性插值算法准确计算接收道实际坐标值,再根据实际坐标重新定义观测系统,获得了实际跑航弯曲测线观测系统图.文中对传统方法与新方法定义的不同观测系统数据,分别采用SRME（Surface-related Multiple Elimination）方法压制浅水海底鸣震类自由表面相关多次波,结果显示采用新方法定义的观测系统反射数据多次波衰减效果明显,有效反射连续,基底反射成像清晰,说明文中实际坐标计算方法符合海上弯曲测线观测系统的定义,提高了SRME方法压制多次波的效果.

     

Kirchhoff scattering series for sources and receivers located in an elastic medium: Another insight into the concept of virtual events

The convolution-type and correlation-type representation theorems are building blocks of wave-scattering theory whose usefulness expands in many seismological applications. For example, the Kirchhoff scattering series currently used for attenuating free-surface multiples has been derived from the convolution-type representation theorem. The recently introduced concept of virtual events, which allows us to put virtual sources and virtual receivers inside the subsurface based on the data collected at the sea surface, has been derived by a combined use of the convolution-type and correlation-type representation theorems. The formulation of inverse Kirchhoff scattering series and virtual events has been limited so far to the cases in which sources or receivers, or both, are located in the water. Unfortunately, this assumption is not valid, especially in the context of virtual events, in which both sources and receivers will often be located in a solid. We here redescribe the Kirchhoff scattering series and reformulate the concept of virtual events for the cases in which sources and receivers are in a solid. Moreover, we describe a new form of Kirchhoff series based on the correlation-type representation theorem and new formulae for computing virtual events which do not include the complex renormalization operation of the previous formulation.     

Stratigraphic filtering of seismic waves: The influence of mode conversion multiples onP-wave andS-wave phase and attenuation

We demonstrate how multiples, generated at the interfaces of plane parallel beds, modify the propagation characteristics of an originally coherent seismic wave. For waves propagating at an angle to the bedding plane we find that theSV andP-waves couple so that neither is a pure mode. TheSH-wave, while modified in its propagation characteristics by multiples, remains a pure mode. The coupling ofSV-multiples into the quasi-P-mode appears weaker than the coupling ofP-wave multiples into the quasi-SV mode; at least this is so for the two simple cases of (a) density fluctuations only and (b) correlatedV _p andV _s fluctuations which conserve Poisson's ratio.We also find that the coupling is sensitive to both the angle of propagation and frequency. In addition there is a cut-off angle forP-wave multiples influencing the quasi-SV mode. Propagation angles larger than the cut-off permit theP-multiples to modify the phase of the quasi-SV mode, but not its effective attenuation. No such cut-off effect is found for SV-multiples influencing the quasi-P mode, whose angle-dependent and frequency-dependent phase distortion and effective attenuation are influenced both byP-wave multiples andSV-multiples.In view of the mathematical complexity of the expressions describing the phase, and effective attenuation of modes when allowance is made forP-andS-wave multiples, we strongly advocate numerical coding of the major mathematical formulae. By so doing a systematic study can be undertaken of the frequency and offset dependence of seismic waves as a function of seismic source input and power spectral behavior of the fluctuations in density and elastic constants of beds. It is our opinion that the full mathematical expressions are too involved to permit an analytic, systematic investigation to be given of the phase and attenuation of seismic waves with any degree of sophistication or generality.     

Scattering and anelastic attenuation of seismic energy in northern Greece

The relative contribution of scattering (Q _s ^–1 ) and intrinsic (Q _i ^–1 ) attenuation to the totalS-wave attenuation for the frequencies of 1.5, 3.0, 6.0 and 12.0 Hz has been studied by applying the radiative energy transfer theory, Data of local earthquakes which occurred in northern Greece and were recorded by the permanent telementered network of the Geophysical Laboratory of the University of Thessaloniki have been used. The results show that in this area the scattering attenuation is dominant over all frequencies while intrinsic attenuation is significantly lower. The estimatedQ _s ^–1 andQ _i ^–1 values have frequency dependences off ^–0.72 andf ^–0.45, respectively. The frequency dependence ofQ _s ^–1 is the same as that of the codaQ _c ^–1 , obtained by applying the single scattering model, which probably implies that the frequency dependence of the coda wave attenuation is attributed to the frequency dependence of the scattering attenuation.Q _c ^–1 values are very close to scattering attenuation for short lapse times, (10–20 sec), and intermediate between scattering and intrinsic attenuation for the longer lapse times, (50–100 sec). This difference is explained as the result of the depth-dependent attenuation properties and the multiple scattering effects.     

New insights from legacy seismic data: reprocessing of legacy 2D seismic data for imaging of iron-oxide mineralization near Sishen Mine,South Africa

Two overlapping legacy seismic profiles, 130 km long end to end, were shot in the 1990s over the Kuruman Hills on the western margin of the Kaapvaal Craton in southern Africa. The 6-s profiles were aimed at investigating the crustal structure of the western Kaapvaal Craton as well as to locate potential continuation of the Witwatersrand gold-bearing horizons beneath the cover rocks, the latter of which was unsuccessful. In this study, the legacy seismic data are reprocessed and used to image the iron-oxide (mainly haematite) mineralization found in the Kuruman Formation of the Griqualand-West Supergroup, which outcrops along the two seismic profiles. The seismic profiles are located close to the Sishen open pit iron mine, where one of the world's largest iron ore concentrations (986 Mt) is mined. The reprocessed and merged seismic data are combined with magnetic, magnetotelluric, borehole and outcrop data to constrain the interpretation, and all indicate the mineralization host rocks to have ∼500 m thickness and 950 m depth. The seismic data further reveal seismic reflections associated with multiple iron ore horizons, which are affected by a first-order scale syncline and numerous near-vertically dipping (∼65–80°) normal and reverse faults of various orientations and throws, thus providing insight into the structurally controlled iron ore mineralization in the area. Seismic tomography and magnetotellurics characterize the sediments to have a velocity ranging between 5000 and 6000 m/s and a resistivity of <10 Ωm. The seismic imaging of the syncline and associated structural disruptions is important for future mining purposes and plans in the area as these structures might have preserved iron-oxide mineralization from erosion. The reprocessed data thus provide information that could be incorporated in potential future underground mine planning in the area, improving the resource evaluation of the iron-oxide deposit. Legacy seismic data are thus shown to hold intrinsic quality and possible untapped potential that can be realized via data reprocessing.     

单震相微地震事件识别与反演

为了对单震相微地震事件进行识别,同时将识别出来的微地震事件进行定位.根据单一震相任意两道到时差与微地震事件、检波器空间位置及震相速度关系的特征规律,研究了单震相微地震事件识别方法.首先分析到时差与以上各变量的内在变化规律,建立起到时差与各变量之间的定量计算关系,然后就相邻道到时差和检波器排列的首尾两道到时差,研究了具体的定量计算关系表达式.利用以上关系式能够计算出上述两种到时差的分布区间.把计算的实际微地震事件的两种到时差与设定的事件的到时差的分布区间对比,以落入区间判断识别单一震相.综合搜索法和遗传算法的特点,提出基于正演模型迭代的解域约束下微地震事件联合反演方法.从迭代误差解的期望概率密度分布、不同解方向的变化特征分析,研究了解域约束的界定条件.在确定的解域范围内,利用遗传算法进行微地震事件的反演.将研究的单震相识别方法,通过对实际资料的应用验证分析,极大地减小了震相的误识率.根据识别资料,利用研究的反演方法对实际资料进行反演.经实际情况对比分析,反演结果和实际的裂缝分布带是一致的,并且反演结果聚敛性效果较好.     

Utilization of multiple scattering: the next big step forward in seismic imaging

Surface removal and internal multiple removal are explained by recursively separating the primary and multiple responses at each depth level with the aid of wavefield prediction error filtering. This causal removal process is referred to as “data linearization.” The linearized output (primaries only) is suitable for linear migration algorithms. Next, a summary is given on the migration of full wavefields (primaries + multiples) by using the concept of secondary sources in each subsurface gridpoint. These secondary sources are two‐way and contain the gridpoint reflection and the gridpoint transmission properties. In full wavefield migration, a local inversion process replaces the traditional linear imaging conditions. Finally, Marchenko redatuming is explained by iteratively separating the full wavefield response from above a new datum and the full wavefield response from below a new datum. The redatuming output is available for linear migration (Marchenko imaging) or, even better, for full wavefield migration. Linear migration, full wavefield migration, and Marchenko imaging are compared with each other. The principal conclusion of this essay is that multiples should not be removed, but they should be utilized, yielding two major advantages: (i) illumination is enhanced, particularly in the situation of low signal‐to‐noise primaries; and (ii) both the upper side and the lower side of reflectors are imaged. It is also concluded that multiple scattering algorithms are more transparent if they are formulated in a recursive depth manner. In addition to transparency, a recursive depth algorithm has the flexibility to enrich the imaging process by inserting prior geological knowledge or by removing numerical artefacts at each depth level. Finally, it is concluded that nonlinear migration algorithms must have a closed‐loop architecture to allow successful imaging of incomplete seismic data volumes (reality of field data).     

各向同性介质长偏移距地震同相轴动校正

传统二阶动校正方法基于较小最大偏移距与目标层深度比和地震波沿直线传播假设,进行长偏移距地震资料处理时,这些假设不再成立.高阶项动校正公式能提高长偏移动校正精度,文中对几种典型的高阶项动校正方法进行了比较,并提出了优化四阶、优化六阶动校正方法.模型计算表明,高阶项动校正方法能取得较常规动校正方法好的动校正结果,但并非阶数越高动校正精度就越高;在纵向速度变化剧烈时,高阶动校正或优化高阶动校正方法一般不能适用于最大偏移距与目标层深度大于3.5的地震反射同相轴,优化四阶和优化六阶动校正公式由于考虑了无穷大偏移距的影响,具有更稳定、更加精确动校正效果,适合于实际的各向同性长偏移距地震资料处理.     

Scattering of seismic waves by cracks in multi-layered geological regions: I. Mechanical model

In this work, a hybrid boundary integral equation method (BIEM) is developed, based on both displacement and hypersingular traction formulations, for the analysis of time-harmonic seismic waves propagating through cracked, multi-layered geological regions with surface topography and under plane strain conditions. Specifically, the displacement-based BIEM is used for a multi-layered deposit with interface cracks, while the regularized, traction-based BIEM is used when internal cracks are present within the layers. The standard uni-dimensional boundary element with parabolic shape functions is employed for discretizing the free surface and the layer interfaces, while special discontinuous boundary elements are placed near the crack tips to model the asymptotic behaviour of both displacements and tractions. This formulation yields displacement amplitudes and phase angles on the free surface of a geological deposit, as well as stress intensity factors near the tips of the cracks. Finally, in the companion paper, numerical results are presented which show that both scattered wave and stress concentration fields are sensitive to the incidence seismic wave parameters and to specific site conditions such as surface topography, layering, the presence of cracks and crack interaction.     

Scattering of seismic waves by cracks in multi-layered geological regions: II. Numerical results

The mechanical model for plane strain, time-harmonic seismic wave propagation problems in cracked, multi-layered geological regions with surface topography and non-parallel interfaces was described in the first part of this work. Here, this model is used to investigate the response of such a region to the presence of traveling elastic waves generated by a seismic source. The computational methodology that was developed in the first part is based on a combination of both the regular (displacement-based) and the hypersingular (traction-based) Boundary Integral Equation Method (BIEM). First, the accuracy and convergence characteristics of this hybrid BIEM are studied. Then, a series of problems involving four different configurations of a reference geological deposit with both interface and internal cracks are solved, for a loading that is due to a seismically-induced pressure wave propagating upwards from the underlying rigid half-plane. The purpose of the numerical study is to investigate the influence of various key parameters of the problem, such as frequency and incidence angle of the incoming wave, size of the surface relief, location and size of the buried cracks, interaction effects between cracks and finally the presence of layers, on both the scattered displacement field and the stress concentration field.     

Study of crustal seismic velocity in the Weihe fault depression basin by seismic tomographic imaging

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Geometry and growth of sill complexes: insights using 3D seismic from the North Rockall Trough

Doleritic sill complexes, which are an important component of volcanic continental margins, can be imaged using 3D seismic reflection data. This allows unprecedented access to the complete 3D geometry of the bodies and an opportunity to test classic sill emplacement models. The doleritic sills associated with basaltic volcanism in the North Rockall Trough occur in two forms. Radially symmetrical sill complexes consist of a saucer-like inner sill at the base with an arcuate inclined sheet connecting it to a gently inclined, commonly ragged, outer rim. Bilaterally symmetrical sill complexes are sourced by magma diverted from a magma conduit feeding an overlying volcano. With an elongate, concave upwards, trough-like geometry bilaterally symmetrical sills climb away from the magma source from which they originate. Both sill complex types can appear as isolated bodies but commonly occur in close proximity and consequently merge, producing hybrid sill complexes. Radial sill complexes consist of a series of radiating primary flow units. With dimensions up to 3 km, each primary flow unit rises from the inner saucer and is fed by primary magma tube. Primary flow units contain secondary flow units with dimensions up to 2 km, each being fed by a secondary magma tube branching from the primary magma tube. Secondary flow units in turn are composed of 100-m scale tertiary flow units. A similar branching hierarchy of flow units can also be seen in bilaterally symmetrical sill complexes, with their internal architecture resembling an enlarged version of a primary flow unit from a radial sill complex. This branching flow pattern, as well as the interaction between flow units of varying orders, provides new insights into the origin of the structures commonly seen within sill complexes and the hybrid sill bodies produced by their merger. The data demonstrate that each radially symmetrical sill complex is independently fed from a source located beneath the centre of the inner saucer, grows by climbing from the centre outwards and that peripheral dyking from the upper surface is a common feature. These features suggest a laccolith emplacement style involving peripheral fracturing and dyking during inner saucer growth and thickening. The branching hierarchy of flow units within bilaterally symmetrical sill complexes is broadly similar to that of primary flow units within a radially symmetrical sill complex, suggesting that the general features of the laccolith emplacement model also apply.Editorial responsibility: J. Stix     

Analysis of the resolution function in seismic prestack depth imaging

We consider the problem of estimating subsurface quantities such as velocity or reflectivity from seismic measurements. Because of a limited aperture and band-limited signals, the output from a seismic prestack reconstruction method is a distorted or blurred image. This distortion can be computed using the concept of resolution function, which is a quantity readily accessible in the Fourier space of the model. The key parameter is the scattering wavenumber, which at a particular image point is defined by the incident and scattered ray directions in a given background model. Any location in any background model can be considered. In general, the resolution function will depend on the following four quantities: the background velocity model, the frequency bandwidth, the wavefield type and the acquisition geometry.
We first establish the resolution function for a general scattering model assuming local reaction. We then adapt this result for two well-known scattering models: Born and Kirchhoff. For each of these approximations the corresponding resolution function is derived and discussed. Finally, by employing a simple synthetic data example we demonstrate the ability of the resolution function to predict the image distortions.