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1.
控制照明与面向目标成像的观测系统设计 总被引：1，自引：0，他引：1
Liu Shouwei Geng Jianhua Feng Wei 《应用地球物理》2005,2(4):230-234
常规地震观测系统设计方法基于地下水平层 状介质的假设,通常不能适应复杂构造情况。我们 从控制照明的思想出发,提出了一种面向目标成像 的地震观测系统设计方法,该方法需要一个由初步 地震解释得到的速度模型。利用单程傅立叶有限差 分波场传播算子将目标层的平面源延拓到地表,通 过分析从目标层延拓到地表的波场能量的分布,可 以确定目标层成像所需要的炮点或者检波点的位置。 利用SEG-EAGE盐丘模型数值试算结果表明,该方法 用于设计面向目标成像的特定地震采集系统。 相似文献
2.
In order to perform resistivity imaging, seismic waveform tomography or sensitivity analysis of geophysical data, the Fréchet derivatives, and even the second derivatives of the data with respect to the model parameters, may be required. We develop a practical method to compute the relevant derivatives for 2.5D resistivity and 2.5D frequency-domain acoustic velocity inversion. Both geophysical inversions entail the solution of a 2.5D Helmholtz equation. First, using differential calculus and the Green's functions of the 2.5D Helmholtz equation, we strictly formulate the explicit expressions for the Fréchet and second derivatives, then apply the finite-element method to approximate the Green's functions of an arbitrary medium. Finally, we calculate the derivatives using the expressions and the numerical solutions of the Green's functions. Two model parametrization approaches, constant-point and constant-block, are suggested and the computational efficiencies are compared. Numerical examples of the derivatives for various electrode arrays in cross-hole resistivity imaging and for cross-hole seismic surveying are demonstrated. Two synthetic experiments of resistivity and acoustic velocity imaging are used to illustrate the method. 相似文献
3.
Seismic ambient noise of surface wave tomography was applied to estimate Rayleigh wave empirical Green's functions(EGFs) and then to study crust and uppermost mantle structure beneath the Makran region in south-east Iran.12 months of continuous data from January 2009 through January 2010,recorded at broadband seismic stations,were analyzed.Group velocities of the fundamental mode Rayleigh wave dispersion curves were obtained from the empirical Green's functions.Multiplefilter analysis was used to plot group velocity variations at periods from 10 to 50 s.Using group velocity dispersion curves,1-D vs velocity models were calculated between several station pairs.The final results demonstrate significant agreement to known geological and tectonic features.Our tomography maps display low-velocity anomaly with SW-NE trend,comparable with volcanic arc settings of the Makran region which may be attributable to the geometry of Arabian Plate subducting beneath the overriding the Lut block.The northward subducting Arabian Plate is determined by high-velocity anomaly along the Straits of Hormuz.At short periods(20 s),there is a sharp transition boundary between low- and high-velocity transition zone with the NW trending at the western edge of Makran which is attributable to the Minab fault system. 相似文献
4.
P. B. DILLON 《Geophysical Prospecting》1990,38(7):757-777
A modern approach to migration is to perform wavefield extrapolation, subject to an imaging condition. Correct wavefield extrapolation requires that the boundary conditions at the array of geophones satisfy the wave equation. A sufficient condition is to perform the survey with a single stationary source. Contrary to this condition, many VSPs are conducted in deviated wells, where the source is maintained vertically above the down-hole geophone at each well station. Such a survey fails to provide the boundary conditions theoretically necessary for wave-equation migration. A recently published inversion scheme, referred to as acoustic generalized Radon transform migration (GRT migration), was developed to handle any configuration of sources and geophones, including moving-source deviated-well VSP surveys. GRT migration may be viewed as a weighted version of the generalized Kirchhoff migration, derived in this paper from the exploding-reflector model. When a VSP-survey geometry has been specified, GRT migration can be expressed in terms of array parameters, and compared with the equivalent expression for Kirchhoff (wave-equation) migration. The differences between the two integrals are significant and their effect is demonstrated on VSP data. 相似文献
5.
Three‐dimensional seismic survey design should provide an acquisition geometry that enables imaging and amplitude‐versus‐offset applications of target reflectors with sufficient data quality under given economical and operational constraints. However, in land or shallow‐water environments, surface waves are often dominant in the seismic data. The effectiveness of surface‐wave separation or attenuation significantly affects the quality of the final result. Therefore, the need for surface‐wave attenuation imposes additional constraints on the acquisition geometry. Recently, we have proposed a method for surface‐wave attenuation that can better deal with aliased seismic data than classic methods such as slowness/velocity‐based filtering. Here, we investigate how surface‐wave attenuation affects the selection of survey parameters and the resulting data quality. To quantify the latter, we introduce a measure that represents the estimated signal‐to‐noise ratio between the desired subsurface signal and the surface waves that are deemed to be noise. In a case study, we applied surface‐wave attenuation and signal‐to‐noise ratio estimation to several data sets with different survey parameters. The spatial sampling intervals of the basic subset are the survey parameters that affect the performance of surface‐wave attenuation methods the most. Finer spatial sampling will reduce aliasing and make surface‐wave attenuation easier, resulting in better data quality until no further improvement is obtained. We observed this behaviour as a main trend that levels off at increasingly denser sampling. With our method, this trend curve lies at a considerably higher signal‐to‐noise ratio than with a classic filtering method. This means that we can obtain a much better data quality for given survey effort or the same data quality as with a conventional method at a lower cost. 相似文献
6.
A matched-filter approach to wave migration 总被引：1，自引：0，他引：1
Wave migration is a technique in which the reflectivity of the Earth is interpreted by extrapolating the fields measured on the surface into the ground. The motivation of this paper is to develop a generalized imaging algorithm based on a matched-filter that shows a mathematical connection between currently used migration techniques. The filter is determined by estimating the received signal when a specific test target exists in the ground. To keep the method general, a point scatterer is used as this target, while distributed objects are modeled without changing the filter characteristics by a collection of independent point scatterers. Also, the specific forms of the Green's functions, which describe wave propagation in the ground, are not included in the formation of this approach leaving more freedom in the implementation. When the filter is applied to measured data of a monostatic survey, the resulting method becomes a forward scattering problem in which these data become time-reversed current sources. Next, specific forward scattering techniques are applied to this matched-filter approach and the resulting methods are compared to traditional migration techniques. In doing so, we find that the general form of most migration techniques can be shown using a matched-filter, while the major differences lie in the actual interpretation of the wave propagation that is used to implement the filter. The similarities of the matched-filter-based approaches to traditional techniques are used to show a connection and general overview of wave migration. Finally, these methods are applied to data collected over pipes buried in sand. 相似文献
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8.
The effect of viscosity, non linearities, incident wave period and realistic eastern coastline geometry on energy fluxes are investigated using a shallow water model with a spatial resolution of 1/4 degree in both meridional and zonal directions. Equatorial and mid-latitude responses are considered. It is found that (1) the influence of the coastline geometry and the incident wave period is more important for the westward energy flux than for the poleward flux, and (2) the effect of the inclination of the eastern ocean boundary on the poleward energy flux, for the Pacific and Atlantic Oceans, decline as the period of the incident wave increases. Furthermore, the model simulations suggest that the poleward energy fluxes from meridional boundaries give plausible results for motions of seasonal and annual periods. For comparatively shorter periods, a realistic coastline geometry has to be included for more accurate results. It is recommended that any numerical model involving the reflection of baroclinic Rossby waves (of intraseasonal, seasonal or annual periods) on the eastern Pacific or Atlantic Oceans, should consider the effect of the coastline geometry in order to improve the accuracy of the results. 相似文献
9.
Decoupled elastic prestack depth migration 总被引：1，自引：0，他引：1
This paper presents a new decoupled form of the formula for common-shot or common-receiver amplitude-preserving elastic prestack depth migration (PreSDM), which can be used for estimating angle-dependent elastic reflection coefficients in laterally inhomogeneous anisotropic media. The multi-shot or multi-receiver extension of this formula is suitable for automated prestack amplitude-versus-angle (AVA) elastic inversion of ocean-bottom cable (OBC), walkaway VSP (WVSP) or standard towed-cable data at any subsurface location. The essence of the theory is a systematic application of the stationary-phase principle and high-frequency approximations to the basic elastic Green's theorem. This leads to nonheuristic explicit wave mode decoupling and scalarization of vector PreSDM. Used in combination, ray-trace and finite-difference (FD) eikonal solvers create a useful tool to calculate accurate Green's function travel time and amplitude maps. Examples of synthetic OBC data and applications to field WVSP data show that the new imaging technique can produce a clear multi-mode elastic image. 相似文献
10.
与其他偏移方法相比,逆时偏移基于精确的波动方程而不是对其近似,用时间外推来代替深度外推.因此,它具有良好的精度,不受地下构造倾角和介质横向速度变化的限制.激发时间成像条件的求取是叠前逆时偏移的难点之一,本文采用求解程函方程的方法得到地下各点的初至波走时,以此作为叠前逆时偏移的成像条件.基于任意矩形网格和局部平面波前近似的有限差分初至波走时计算方法精度较高并适用于强纵横向变速的复杂介质.试算结果表明,在复杂介质模型中利用叠前逆时深度偏移收到了很好的成像效果. 相似文献
11.
Steven D. London 《地球物理与天体物理流体动力学》2013,107(6):616-628
We consider an electrically conducting fluid confined to a thin rotating spherical shell in which the Elsasser and magnetic Reynolds numbers are assumed to be large while the Rossby number is assumed to vanish in an appropriate limit. This may be taken as a simple model for a possible stable layer at the top of the Earth's outer core. It may also be a model for the thin shells which are thought to be a source of the magnetic fields of some planets such as Mercury or Uranus. Linear hydromagnetic waves are studied using a multiple scale asymptotic scheme in which boundary layers and the associated boundary conditions determine the structure of the waves. These waves are assumed to be of the form of an asymptotic series expanded about an ambient magnetic field which vanishes on the equatorial plane and velocity and pressure fields which do not. They take the form of short wave, slowly varying wave trains. The results are compared to the author's previous work on such waves in cylindrical geometry in which the boundary conditions play no role. The approximation obtained is significantly different from that obtained in the previous work in that an essential singularity appears at the equator and nonequatorial wave regions appear. 相似文献
12.
Zero-offset-source VSP surveys provide information about the subsurface only within the Fresnel zone centered at the well. Offsetting the source location moves the reflection zones away from the well thus providing lateral cover. Conventional processing of this type of data gives rise to a distorted image of the subsurface. Using a simple ray-tracing scheme, this image may be reconstructed into the more familiar coordinate system of the surface seismic section. This simple data-independent mapping is based on the assumption of horizontal layering and requires a vertical velocity profile. The technique of placing the source away from the borehole was first applied to the single-offset-source VSP survey. However, data from any survey geometry (such as deviated well with rig source, walkaway VSP, etc.) can be mapped to the coordinate system defined by the appropriate seismic section. To obtain the best results from this type of survey the target area must be defined and simple modeling techniques used to optimize the source location(s). These pre-survey modeling methods may also be used to anticipate—and hence avoid a number of problem areas which experience has highlighted. The data from any VSP survey is the result of a realizable experiment and as such obeys the wave equation. This implies that the wave equation may be used to migrate the data to its true subsurface location. Theoretically, such a process is more secure than ray-tracing techniques, although its practice presents many difficulties. 相似文献
13.
A. H. BALCH H. CHANG G. S. HOFLAND K. A. RANZINGER C. ERDEMIR 《Geophysical Prospecting》1991,39(7):887-913
The principles of imaging, for example that of prestack migration, can be applied to cross-borehole seismic geometry just as they can to surface seismic configurations. However, when using actual cross-borehole data, a number of difficulties arise that are rarely or never encountered in imaging surface seismic data: discontinuities may reflect or diffract incident seismic waves in any direction. If a discontinuity lies between the lines of sources and receivers, forward-scattered, or interwell, events may be recorded. If a discontinuity lies outside the interwell region, back-scattered, or extra-well, events may be recorded. Many angles of incidence are possible, and all possible reflected modes (P–P, P–S, S–P and S–S) are present, frequently in nearly equal proportions. The planes of the reflectors dip from 0 to ±90°. In order to deal with these complexities we first separate propagation modes at the receiver borehole using both polarization and velocity. Next we compensate for phase distortion due to dispersion. Finally, and most importantly, we migrate or image the data in cross-borehole common-source gathers. To do this, a finite-difference solution to the 2D scalar wave equation, using reverse time, for an arbitrary distribution of velocities, is used to project the separated, reflected-diffracted wavefield back into the medium. There are four reflection modes (P–P, P–S, S–P and S–S), so we can apply four different imaging conditions. The zones outside the boreholes as well as inside the boreholes can be imaged with these conditions. These operations are repeated for each common-source gather: each common-source gather generates four partial images in each image space. This multiplicity of partial images can be stacked in various combinations to yield a final image of the subsurface. Our experiments using solid (not fluid) physical models indicate that when these procedures are correctly applied, high quality cross-borehole images can be obtained. These images appear with great clarity even though some of the weak diffractions causing diffraction images may be almost totally obscured by other high-amplitude events on the raw data. 相似文献
14.
Atahebson B. Santos Milton J. Porsani Manuelle S. Góis 《Geophysical Prospecting》2019,67(7):1720-1731
The determination of three-dimensional geometry and acquisition parameters, the seismic acquisition survey design, is constantly subject of studies in obtaining data with the highest seismic quality, operational efficiency and cost minimization. In this paper, we propose a methodology for inverting geometry parameters of three-dimensional orthogonal land seismic surveys based on a direct search method using a mixed-radix based algorithm. In this algorithm, the search space is discretized on a mixed-radix base, which depends on the extreme values and the search resolution of each parameter. We will show how to reparametrize the orthogonal acquisition geometry elements in order to obtain the independents and integers parameters that are necessary to construct the mixed-radix base. For the optimization purpose, we define an objective function to contemplate target parameters associated with the elements of the acquisition geometry directly related to the geophysical and operational constraints. Taking in account that the mathematical functions and the objective function we define for the problem have no significant computational cost, all model space parameters are fast and efficiently tested. We applied the algorithm, using as input data, provided by a one-line roll orthogonal reference geometry, assuming a pair of geological objectives as shallow and deep targets. All selected models that meet both the proposed objectives and the constraints are organized by decreasing order of fitness so that with the mixed-radix inversion algorithm we found not only the best model, but also a set of suitable models. Likewise, with the best set of geometries, it is possible to establish a direct comparison between them, analysing their adherence to the technical and operational requirements according to the availability and degree of detail of each one. We show the top 10 best results as a table, allowing a direct comparison between all aspects of these geometries, and we summarize the results showing graphically the fitness of all selected geometries and the inverted geometry elements for the 1000 best geometries. These graphical displays provide a direct way to understand how each model behaves as the fitness decreases. The algorithm is very flexible and its application can be extended to any environment and type of acquisition geometry, and in any phase study of an area be it regional, exploratory or development. 相似文献
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Helmholtz's equation with a variable wavenumber is solved for a point force through use of a first-order differential equation system approach. Since the system matrix in this formulation is non-constant, an eigensolution is no longer valid and recourse has to be made to approximate techniques such as series expansions and Picard iterations. These techniques can accommodate in principle any variation of the wavenumber with position and are applicable to scalar wave propagation in one, two and three dimensions, with the latter two cases requiring radial symmetry. As shown in the examples, good solution accuracy can be achieved in the near field region, irrespective of frequency, for the particular case examined, namely a wavenumber which increases (or decreases) as the square root of the radial distance from source to receiver. Finally, the resulting Green's functions can be used as kernels within the context of boundary element type solutions to study scalar wave scattering in inhomogeneous media. 相似文献
17.
本文给出了单位点质量负荷作用在球状成层地球模型上的解。对较新的G-D1066A地球模型求出了n直到10000阶的负荷勒夫数,并利用这组负荷勒夫数计算了格林函数的值。简述了如何利用格林函数计算地球对表面任何负荷的响应。讨论了负荷潮研究在地球物理和海洋学方面的一些应用。 相似文献
18.
天山造山带作为世界上陆内最大的造山带之一,现今地震活动频繁,造山运动强烈,是开展陆内造山和内陆地震活动研究的天然试验场.本文利用整个天山造山带地区国内及国际台网的108个地震台站连续三年的背景噪声资料,提取了8~50 s周期的瑞利面波相速度频散曲线,并构建了整个天山造山带地区的二维瑞利面波相速度与方位各向异性分布图像.结果表明：浅部结构与地表的地质构造单元具有较大的相关性.低波速异常主要分布于沉积层厚度较大的盆地地区,而高波速异常主要分布于构造活动比较活跃的山脉地区.东天山地区中下地壳存在比较弱的低波速异常,而塔里木盆地和准噶尔盆地汇聚边缘的上地幔区域则表现为明显的高波速异常,各向异性快波方向呈现近NS向的特征,暗示着塔里木盆地和准噶尔盆地的岩石圈已经俯冲至东天山的下方.中天山地区的中下地壳至上地幔区域均呈现为明显的低波速异常,且各向异性快波方向变化比较复杂,表明中天山地区的整个岩石圈结构已经弱化,热物质上涌可能对介质的方位各向异性有一定的影响.西天山及帕米尔高原的上地幔区域存在低波速异常,各向异性表现为NW-SE方向,可能与欧亚板块的大陆岩石圈南向俯冲有关.塔里木盆地内部存在相对弱的低波速异常,推测塔里木盆地可能已经受到上涌的地幔热物质的侵蚀和破坏. 相似文献
19.
面波多道分析方法(MASW)通过分析高频瑞雷波确定浅地表剪切波速度.在过去的20年中,由于该方法具有非侵入性、无损、高效及价格低的特点,越来越受到浅地表地球物理和地质工程学界的重视,视为未来最有希望的技术之一.这篇综述论文将介绍中国地质大学(武汉)浅地表地球物理团队近年来在研究高频面波的传播理论和应用中取得的部分成果.非几何波是一种仅存在于浅地表介质,尤其是未固结的沉积物中的独特的地震波.它的存在对快速而准确地获得表层S波速度有一定价值.我们的研究表明非几何波是一种具有频散特性的泄漏波.泄漏波的存在可能导致将其误认为瑞雷波的基阶或高阶能量,从而造成模式误判.这种模式误判会导致错误的反演结果.我们通过求取高基阶分离后的瑞雷波格林函数证明虚震源法瑞雷波勘探的可行性.这个结果将极大地降低野外瑞雷波勘探成本.勒夫波多道分析方法(MALW)中未知参数比瑞雷波的少,这使得勒夫波的频散曲线比瑞雷波的简单.因此,勒夫波反演更稳定,非唯一性更低.勒夫波数据生成的能量图像通常比瑞雷波的清晰,并具有更高的分辨率,从而可以更容易地拾取精确的勒夫波的相速度.利用雅克比矩阵分析波长与探测深度的关系表明对相同波长的基阶模式而言,瑞雷波的探测深度是勒夫波的1.3~1.4倍;而两种波的相同波长的高阶模式波的探测深度相同.我们也尝试了时间域勒夫波反演.按照勒夫波分辨率将地球模型剖分成了不同尺寸的块体,利用反卷积消除了地震子波对勒夫波波形的影响,通过更新每个块体的S波速度来拟合勒夫波波形,从而获得地下S波速度模型.该方法不基于水平层状模型假设,适用于任意二维介质模型. 相似文献
20.
George D. Manolis Tsviatko V. Rangelov 《Soil Dynamics and Earthquake Engineering》2006,26(10):952-959
Geological media are invariably non-homogeneous, which complicates considerably the analysis of seismically induced wave propagation phenomena. Thus, closed-form solutions in the form of Green's functions are difficult to construct, but are quite valuable in their own right and often play the role of kernels in boundary integral equation formulations that are used for the solution of complex boundary-value problems of engineering importance. In this work, we examine in some detail the types of wave-like equations that result from vector decomposition of the equations of motion for the infinitely extending non-homogeneous continuum, which would be a first step for evaluating Green's functions. Specifically, an eigenvalue analysis is first performed, followed by computations using the finite difference method for a specific example involving a soil layer with quadratically varying material parameters. The aforementioned wave-like equations, defined in terms of dilatational and rotational strains, are originally coupled. Their uncoupling involves use of algebraic transformations, which are in turn valid for certain restricted categories of non-homogeneous materials. Numerical solution of these equations clearly shows attenuation patterns and phase changes that are manifested as the incoming wave disturbance is continuously scattered by non-constant material stiffness values encountered along the propagation path. 相似文献