共查询到20条相似文献，搜索用时 31 毫秒
1.
Chao-ying Bai Tao Wang Shang-bei Yang Xing-wang Li Guo-jiao Huang 《Journal of Seismology》2016,20(2):475-494
Traditional traveltime inversion for anisotropic medium is, in general, based on a “weak” assumption in the anisotropic property, which simplifies both the forward part (ray tracing is performed once only) and the inversion part (a linear inversion solver is possible). But for some real applications, a general (both “weak” and “strong”) anisotropic medium should be considered. In such cases, one has to develop a ray tracing algorithm to handle with the general (including “strong”) anisotropic medium and also to design a non-linear inversion solver for later tomography. Meanwhile, it is constructive to investigate how much the tomographic resolution can be improved by introducing the later arrivals. For this motivation, we incorporated our newly developed ray tracing algorithm (multistage irregular shortest-path method) for general anisotropic media with a non-linear inversion solver (a damped minimum norm, constrained least squares problem with a conjugate gradient approach) to formulate a non-linear inversion solver for anisotropic medium. This anisotropic traveltime inversion procedure is able to combine the later (reflected) arrival times. Both 2-D/3-D synthetic inversion experiments and comparison tests show that (1) the proposed anisotropic traveltime inversion scheme is able to recover the high contrast anomalies and (2) it is possible to improve the tomographic resolution by introducing the later (reflected) arrivals, but not as expected in the isotropic medium, because the different velocity (qP, qSV and qSH) sensitivities (or derivatives) respective to the different elastic parameters are not the same but are also dependent on the inclination angle. 相似文献
2.
In downhole microseismic monitoring, accurate event location relies on the accuracy of the velocity model. The model can be estimated along with event locations. Anisotropic models are important to get accurate event locations. Taking anisotropy into account makes it possible to use additional data – two S-wave arrivals generated due to shear-wave splitting. However, anisotropic ray tracing requires iterative procedures for computing group velocities, which may become unstable around caustics. As a result, anisotropic kinematic inversion may become time consuming. In this paper, we explore the idea of using simplified ray tracing to locate events and estimate medium parameters. In the simplified ray-tracing algorithm, the group velocity is assumed to be equal to phase velocity in both magnitude and direction. This assumption makes the ray-tracing algorithm five times faster compared to ray tracing based on exact equations. We present a set of tests showing that given perforation-shot data, one can use inversion based on simplified ray-tracing even for moderate-to-strong anisotropic models. When there are no perforation shots, event-location errors may become too large for moderately anisotropic media. 相似文献
3.
In geological materials, anisotropy may arise due to different mechanisms and can be found at different scales. Neglecting anisotropy in traveltime tomographic reconstruction leads to artefacts that can obscure important subsurface features. In this paper, a geostatistical tomography algorithm to invert cross‐hole traveltime data in elliptically anisotropic media is presented. The advantages of geostatistical tomography are that the solution is regularized by the covariance of the model parameters, that known model parameters can be used as constraints and fitted exactly or within a prescribed variance and that stochastic simulations can be performed to appraise the variability of the solution space. The benefits of the algorithm to image anisotropic media are illustrated by two examples using synthetic georadar data and real seismic data. The first example confirms suspected electromagnetic anisotropy in the vadose zone caused by relatively rapid water content variations with respect to wavelength at georadar frequencies. The second presents how sonic log data can be used to constrain the inversion of cross‐well seismic data and how geostatistical simulations can be used to infer parameter uncertainty. Results of both examples show that considering anisotropy yields a better fit to the data at high ray angles and reduces reconstruction artefacts. 相似文献
4.
We estimate velocity anisotropy factors from seismic traveltime tomographic data and apply a correction for anisotropy in the inversion procedure to test possible improvements on the traveltime fit and the quality of the resulting tomographic images. We applied the anisotropy correction on a traveltime data set obtained from the investigation of the foundation structure of a monumental building: a Byzantine church from the 11th century AD, in Athens, Greece. Vertical transverse isotropy is represented by one axis of symmetry and one anisotropy magnitude for the entire tomographic inversion grid. We choose the vertical direction for the symmetry axis by analysing the available data set and taking into account information on the character of the foundations of the church from the literature and past excavations. The anisotropy magnitude is determined by testing a series of values of anisotropy and examining their effect on the tomographic inversion results. The best traveltime fit and image quality are obtained with an anisotropy value (Vmax/Vmin) of 1.6, restricted to the high velocity structures in the subsurface. We believe that this anisotropy value, which is significantly higher than the usual values reported for near‐surface geological material, is related to the fabric of the church foundations, due to the shape of the individual stone blocks and the layout of the stonework. Inversion results obtained with the correction for anisotropy indicate that both the traveltime fit and the image quality are improved, providing an enhanced reconstruction of the velocity field, especially for the high‐velocity features. Based on this enhanced and more reliable reconstruction of velocity distribution, an improved image of the subsurface material character was made possible. In particular, the pattern and state of the church foundations and possible weak ground material areas were revealed more clearly. This improved subsurface knowledge may assist in a better design of restoration measures for monumental buildings such as Byzantine churches. 相似文献
5.
6.
7.
正交各向异性介质P波走时分析及Thomsen参数反演 总被引：10，自引：3，他引：10
对于包含有垂向裂缝的横向各向同性地层或含有多组正交裂缝的各向同性地层，正交各向异性介质模型是最简单的与实际地层相符的方位各向异性模型.本文对单层水平反射界面正交各向异性模型采用射线追踪法计算了全方位角变化的P波走时，时距曲线表现出强方位各向异性.采用小生境遗传算法，对三条成一定角度的测线的走时信息进行速度和各向异性参数反演.模型算例表明，此方法可以得到高精度的裂缝方位角、P波垂直速度和较高精度的Thomsen各向异性参数. 相似文献
8.
3-D multi-parameter type traveltime tomography in a spherical coordinate frame: comparison of double and triple class simultaneous inversions 
下载免费PDF全文
下载免费PDF全文 It is now common practice to perform simultaneous traveltime inversion for the velocity field and the reflector geometry in reflection/refraction tomography, or the velocity field and the hypocenter locations in regional earthquake tomography, but seldom are all three classes of model parameters updated simultaneously. This is mainly due to the trade-off between the different types of model parameters and the lack of different seismic phases to constrain the model parameters. Using a spherical-coordinate ray tracing algorithm for first and later(primary reflected) arrival tracing algorithm in combination with a popular linearized inversion solver, it is possible to simultaneously recover the three classes of model parameters in regional or global tomographic studies. In this paper we incorporate the multistage irregular shortest-path ray tracing algorithm(in a spherical coordinate system) with a subspace inversion solver to formulate a simultaneous inversion algorithm for triple model parameters updating using direct and later arrival time information.Comparison tests for two sets of data(noise free and added noise) indicate that the new triple-class parameter inversion algorithm is capable of obtaining nearly the same results as the double-class parameter inversion scheme. Furthermore,the proposed multi-parameter type inversion method is not sensitive to a modest level of picking error in the traveltime data, and also performs well with a relatively large uncertainty in earthquake hypocentral locations. This shows it to be a feasible and promising approach in regional or global tomographic applications. 相似文献
9.
Constraining the anisotropy structure of the crust by joint inversion of seismic reflection travel times and wave polarizations 总被引：2，自引：0，他引：2
In the context of wide-angle seismic profiling, the determination of the physical properties of the Earth crust, such as the
elastic layer depth and seismic velocity, is often performed by inversion of P- and/or S-phases propagation data supplying
the geometry of the medium (reflector depths) or any other structural parameter (P- or S-wave velocity, density...). Moreover,
the inversion for velocity structure and interfaces is commonly performed using only seismic reflection travel times and/or
crustal phase amplitudes in isotropic media. But it is very important to utilize more available information to constrain the
non-uniqueness of the solution. In this paper, we present a simultaneous inversion method of seismic reflection travel times
and polarizations data of transient elastic waves in stratified media to reconstruct not only layer depth and vertical P-wave
velocity but also the anisotropy feature of the crust based on the estimation of the Thomsen’s parameters. We carry out
a checking with synthetic data, comparing the inversion results obtained by anisotropic travel-time inversion to the results
derived by joint inversion of seismic reflection travel times and polarizations data. The comparison proves that the first
procedure leads to biased anisotropic models, while the second one fits nearly the real model. This makes the joint inversion
method feasible. Finally, we investigate the geometry, P-wave velocity structure and anisotropy of the crust beneath Southeastern
China by applying the proposed inversion method to previously acquired wide-angle seismic data. In this case, the anisotropy
signature provides clear evidence that the Jiangshan-Shaoxing fault is the natural boundary between the Yangtze and Cathaysia
blocks. 相似文献
10.
在地震波正反演研究中,考虑起伏地表和地震各向异性具有非常重要的理论意义和实际应用价值.本文在前人研究的基础上,将最短路径追踪算法引入到起伏地表各向异性介质模型的地震波走时计算中.模型剖分时,整体模型划分成正方形单元,起伏边界附近以不规则网格逼近,进而采用非规则节点布置实现非规则网格处的最短路径计算.追踪计算中采用Sena群速度近似公式,得到各向异性地震波的走时,实现了复杂地表情况下各向异性介质模型中地震波的射线追踪.理论模型计算结果显示,本文方法能够可靠地应用于复杂各向异性介质模型,具有较高的计算精度. 相似文献
11.
Despite the complexity of wave propagation in anisotropic media, reflection moveout on conventional common-midpoint (CMP) spreads is usually well described by the normal-moveout (NMO) velocity defined in the zero-offset limit. In their recent work, Grechka and Tsvankin showed that the azimuthal variation of NMO velocity around a fixed CMP location generally has an elliptical form (i.e. plotting the NMO velocity in each azimuthal direction produces an ellipse) and is determined by the spatial derivatives of the slowness vector evaluated at the CMP location. This formalism is used here to develop exact solutions for the NMO velocity in anisotropic media of arbitrary symmetry. For the model of a single homogeneous layer above a dipping reflector, we obtain an explicit NMO expression valid for all pure modes and any orientation of the CMP line with respect to the reflector strike. The contribution of anisotropy to NMO velocity is contained in the slowness components of the zero-offset ray (along with the derivatives of the vertical slowness with respect to the horizontal slownesses) — quantities that can be found in a straightforward way from the Christoffel equation. If the medium above a dipping reflector is horizontally stratified, the effective NMO velocity is determined through a Dix-type average of the matrices responsible for the ‘interval’ NMO ellipses in the individual layers. This generalized Dix equation provides an analytic basis for moveout inversion in vertically inhomogeneous, arbitrarily anisotropic media. For models with a throughgoing vertical symmetry plane (i.e. if the dip plane of the reflector coincides with a symmetry plane of the overburden), the semi-axes of the NMO ellipse are found by the more conventional rms averaging of the interval NMO velocities in the dip and strike directions. Modelling of normal moveout in general heterogeneous anisotropic media requires dynamic ray tracing of only one (zero-offset) ray. Remarkably, the expressions for geometrical spreading along the zero-offset ray contain all the components necessary to build the NMO ellipse. This method is orders of magnitude faster than multi-azimuth, multi-offset ray tracing and, therefore, can be used efficiently in traveltime inversion and in devising fast dip-moveout (DMO) processing algorithms for anisotropic media. This technique becomes especially efficient if the model consists of homogeneous layers or blocks separated by smooth interfaces. The high accuracy of our NMO expressions is illustrated by comparison with ray-traced reflection traveltimes in piecewise-homogeneous, azimuthally anisotropic models. We also apply the generalized Dix equation to field data collected over a fractured reservoir and show that P-wave moveout can be used to find the depth-dependent fracture orientation and to evaluate the magnitude of azimuthal anisotropy. 相似文献
12.
13.
高频假设下的地震射线理论以及相应的地震成像理论表明,在射线稀疏条件下,不可能得到较高分辨率的构造成像;而有限频射线理论更符合实际地震的传播规律,即地震波的走时不仅与中心射线(传统的几何射线)上的速度分布有关,而且与中心射线附近一定范围(称其为第一菲涅耳体)内的速度异常分布有关.鉴于此,本文提出了计算多震相地震波菲涅耳体有限频射线的方法,并定义了走时敏感核函数,同时给出了利用多震相菲涅耳体有限频射线进行速度模型和反射界面同时反演成像的公式.利用多震相走时资料,使用传统射线层析成像方法与有限频射线层析成像方法进行了速度和界面的同时反演成像.结果表明,当射线密度较小时,无论是对速度模型的重建还是对反射界面几何形状的更新,有限频射线层析成像方法均优于传统射线层析成像方法, 而变频有限频射线层析成像则是实际地震层析成像的首选反演算法. 相似文献
14.
BRUCE J. VERWEST 《Geophysical Prospecting》1989,37(2):149-166
The study of wave propagation in media with elliptical velocity anisotropy shows that seismic energy is focused according to the horizontal component of the velocity field while the vertical component controls the time-to-depth relation. This implies that the vertical component cannot be determined from surface seismic velocity analysis but must be obtained using borehole or regional geological information. Both components of the velocity field are required to produce a correctly focused depth image. A paraxial wave equation is developed for elliptical anisotropic wave propagation which can be used for modelling or migration. This equation is then transformed by a change of variable to a second paraxial equation which only depends on one effective velocity field. A complete anisotropic depth migration using this transformed equation involves an imaging step followed by a depth stretching operation. This allows an approximate separation or splitting of the focusing and depth conversion steps of depth migration allowing a different velocity model to be used for each step. This split anisotropic depth migration produces a more accurate result than that obtained by a time migration using the horizontal velocity field followed by an image-ray depth conversion using the vertical velocity field. The results are also more accurate than isotropic depth migration and yield accurate imaging in depth as long as the lateral variations in the anisotropy are slow. 相似文献
15.
Seismological studies generally suggest that the Earth’s inner core is anisotropic and the anisotropic structure changes significantly both laterally and with depth. Previous body-wave studies of the inner core have relied on ray tracing or waveform modeling using one-dimensional (1D) models. Here we present non-linear tomographic inversions of the inner core anisotropy using three-dimensional (3D) ray tracing, spline parameterization, and a large collection of PKP differential travel times. We adapt a pseudo-bending ray tracing (PBR) method in spherical coordinates for seismic rays that traverse the inner core (PKP(DF) phase). The method iteratively perturbs each discontinuity point and continuous segment of the ray through 3D earth structure so that its travel time is minimum. The 3D anisotropic structure of the inner core is approximated to the first order as 3D heterogeneous (but isotropic) structure for a given ray. The data are corrected using a scaled mantle tomographic model. The inner core anisotropy model obtained has the following major features. (1) The model has strong hemispherical and depth variation. The isotropic velocity in the topmost inner core is greater in quasi-eastern hemisphere (QEH) (40–160°E) than in quasi-western hemisphere (QWH) (other longitudes). The anisotropy is weak in QEH to the depth of 600–700 km below the inner core boundary (ICB), while in QWH, the anisotropy increases at much shallower depth (about 100–200 km below the ICB) to about 3–4%, then remains at about 2–4% throughout the rest of the inner core. (2) The anisotropy form changes abruptly (over a depth range of about 150 km) at the radius of about 600 km, slightly less than half of the inner core radius, forming a distinct inner inner core (IIC). The velocity in the IIC has maximums at equatorial and polar directions and minimum at an angle of about 40° from the equatorial plane. The velocity in the outer inner core (OIC), however, changes little for ray directions 0–40° from the equatorial plane. (3) Despite large variation of the anisotropy, the isotropic velocity (Voigt average) throughout the inner core is nearly uniform. The results suggest that the OIC is likely composed of the same type of iron crystals with uniform chemistry, but the IIC may be composed of a different type of crystal alignment, a different iron phase, or a different chemical composition. Our tests on model parameterization, mantle correction, and linear and non-linear inversion suggest the main features of our model are very robust. However, fine scale structures are likely to differ, particularly in the major transition zones, e.g., in the topmost QWH (isotropy to anisotropy), between OIC and IIC (change in the form of anisotropy), and between QEH and QWH in OIC (difference in anisotropy strength). Searches for possible waveform complications from these boundaries need to be aware of the directional dependence and geographical variation to be successful. 相似文献
16.
A transmission + reflection wave-equation traveltime and waveform inversion method is presented that inverts the seismic data for the anisotropic parameters in a vertical transverse isotropic medium. The simultaneous inversion of anisotropic parameters and ε is initially performed using transmission wave-equation traveltime inversion method. Transmission wave-equation traveltime only provides the low-intermediate wavenumbers for the shallow part of the anisotropic model; in contrast, reflection wave-equation traveltime estimates the anisotropic parameters in the deeper section of the model. By incorporating a layer-stripping method with reflection wave-equation traveltime, the ambiguity between the background-velocity model and the depths of reflectors can be greatly mitigated. In the final step, multi-scale full-waveform inversion is performed to recover the high-wavenumber component of the model. We use a synthetic model to illustrate the local minima problem of full-waveform inversion and how transmission and reflection wave-equation traveltime can mitigate this problem. We demonstrate the efficacy of our new method using field data from the Gulf of Mexico. 相似文献
17.
V.B. Piip 《Geophysical Prospecting》2001,49(4):461-482
A method using simple inversion of refraction traveltimes for the determination of 2D velocity and interface structure is presented. The method is applicable to data obtained from engineering seismics and from deep seismic investigations. The advantage of simple inversion, as opposed to ray‐tracing methods, is that it enables direct calculation of a 2D velocity distribution, including information about interfaces, thus eliminating the calculation of seismic rays at every step of the iteration process. The inversion method is based on a local approximation of the real velocity cross‐section by homogeneous functions of two coordinates. Homogeneous functions are very useful for the approximation of real geological media. Homogeneous velocity functions can include straight‐line seismic boundaries. The contour lines of homogeneous functions are arbitrary curves that are similar to one another. The traveltime curves recorded at the surface of media with homogeneous velocity functions are also similar to one another. This is true for both refraction and reflection traveltime curves. For two reverse traveltime curves, non‐linear transformations exist which continuously convert the direct traveltime curve to the reverse one and vice versa. This fact has enabled us to develop an automatic procedure for the identification of waves refracted at different seismic boundaries using reverse traveltime curves. Homogeneous functions of two coordinates can describe media where the velocity depends significantly on two coordinates. However, the rays and the traveltime fields corresponding to these velocity functions can be transformed to those for media where the velocity depends on one coordinate. The 2D inverse kinematic problem, i.e. the computation of an approximate homogeneous velocity function using the data from two reverse traveltime curves of the refracted first arrival, is thus resolved. Since the solution algorithm is stable, in the case of complex shooting geometry, the common‐velocity cross‐section can be constructed by applying a local approximation. This method enables the reconstruction of practically any arbitrary velocity function of two coordinates. The computer program, known as godograf , which is based on this theory, is a universal program for the interpretation of any system of refraction traveltime curves for any refraction method for both shallow and deep seismic studies of crust and mantle. Examples using synthetic data demonstrate the accuracy of the algorithm and its sensitivity to realistic noise levels. Inversions of the refraction traveltimes from the Salair ore deposit, the Moscow region and the Kamchatka volcano seismic profiles illustrate the methodology, practical considerations and capability of seismic imaging with the inversion method. 相似文献
18.
Reflection moveout inversion in azimuthally anisotropic media: accuracy, limitations and acquisition 总被引：1，自引：0，他引：1
AbdulFattah Al-Dajani Tariq Alkhalifah & Frank Dale Morgan 《Geophysical Prospecting》1999,47(5):735-756
Parameter estimation from the elliptical variations in the normal-moveout (NMO) velocity in azimuthally anisotropic media is sensitive to the angular separation between the survey lines in 2D, or equivalently, the source-to-receiver azimuth in 3D, and to the set of azimuths used in the inversion procedure. The accuracy in estimating the orientation of an NMO ellipse, in particular the parameter α, is also sensitive to the magnitude of anisotropy. On the other hand, the accuracy in estimating the semi-axes of the NMO-velocity ellipse is about the same for any magnitude of anisotropy. To invert for the NMO ellipse parameters at least three NMO-velocity measurements along distinct azimuth directions are needed. In order to maximize the accuracy and stability in parameter estimation, it is best to have the azimuths for the three source-to-receiver directions 60° apart. Having more than three distinct source-to-receiver azimuths (e.g. full azimuthal coverage) provides a useful data redundancy that enhances the quality of the estimates. In order to maximize quality in the inversion process, it is recommended to design the seismic data acquisition such that it contains small sectors (≤10°) with adequate fold and offset distribution. Using three NMO-velocity measurements, 60° apart, an azimuthally anisotropic layer overlain by an azimuthally isotropic overburden (as might occur for fractured reservoirs) should have a relative thickness (in time) with respect to the total thickness at least equal to the ratio of the error in the NMO (stacking) velocity to the interval anisotropy of the fractured layer. Coverage along more than three azimuths, however, improves this limitation, which is imposed by Dix differentiation, by at most 50%, depending on the number of observations (NMO velocities) that enter the inversion procedure. 相似文献
19.
横向各向同性介质是地球内部广泛分布的一种各向异性介质.针对这种介质,我们对各向同性介质的最小走时树走时模拟方法进行了推广,推广后的方法可适用于非均匀、对称轴任意倾斜的横向各向同性介质模型.为保证计算效率,最小走时树的构建采用了一种子波传播区域随地震波传播动态变化的改进算法.对于弱各向异性介质,我们使用了一种新的地震波群速度近似表示方法,该方法基于用射线角近似表示相角的思想,对3种地震波（qP, qSV和qSH）均有较好的精度.应用本文地震波走时模拟方法对均匀介质、横向非均匀介质模型进行了计算,并将后者结果与弹性波方程有限元方法的模拟结果进行了对比,结果表明两者符合得很好.本文方法可用于横向各向同性介质的深度偏移及地震层析成像的深入研究. 相似文献
20.
本文提出-种利用有偏VSP资料反射波旅行时信息重建椭圆各向异性介质中水平向与垂直向速度的方法。其中,地下介质假定为层状椭圆各向异性介质。反射波旅行时间采用射线追踪理论及几何关系计算得到,反演中的线性方程组采用奇异值分解(SVD)技术进行求解。 方法检测时,我们对各向同性介质及椭圆各向异胜介质情况下有限差分法正演模拟的深井有偏移距VSP地震资料分别进行各向同性和各向异性方法反演成像。结果表明,本文所述方法较之各向同性介质模型反演方法对介质类型有很好的适用性,同时也说明了本方法的司行性。最后,我们分别介绍了对实际有偏VSP资料反演得到的地下介质的速度结构图像。 相似文献