共查询到10条相似文献，搜索用时 93 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.
Seismic ray path variations in a 3D global velocity model 总被引：2，自引：0，他引：2
A three-dimensional (3D) ray tracing technique is used to investigate ray path variations of P, PcP, pP and PP phases in a global tomographic model with P wave velocity changing in three dimensions and with lateral depth variations of the Moho, 410 and 660 km discontinuities. The results show that ray paths in the 3D velocity model deviate considerably from those in the average 1D model. For a PcP wave in Western Pacific to East Asia where the high-velocity (1-2%) Pacific slab is subducting beneath the Eurasian continent, the ray path change amounts to 27 km. For a PcP ray in South Pacific where very slow (−2%) velocity anomalies (the Pacific superplume) exist in the whole mantle, the maximum ray path deviation amounts to 77 km. Ray paths of other phases (P, pP, PP) are also displaced by tens of kilometers. Changes in travel time are as large as 3.9 s. These results suggest that although the maximal velocity anomalies of the global tomographic model are only 1-2%, rays passing through regions with strong lateral heterogeneity (in velocity and/or discontinuity topography) can have significant deviations from those in a 1D model because rays have very long trajectories in the global case. If the blocks or grid nodes adopted for inversion are relatively large (3-5°) and only a low-resolution 3D model is estimated, 1D ray tracing may be feasible. But if fine blocks or grid nodes are used to determine a high-resolution model, 3D ray tracing becomes necessary and important for the global tomography. 相似文献
3.
使用阻尼最小二乘法进行震源参数和地壳三维速度结构的走时联合反演．所用资料为Ｓ波和Ｐ波到时差,并用人工地震资料的二维解释结果作为三维速度模型的特定约束条件．为建立初始模型,又利用天然地震构成了准二维剖面．在走时反演基础上,利用遗传算法进行了几个地震事件的波形反演尝试,并对走时反演获得的地壳速度结构模型的局部进行了修正．以３４°～４２°Ｎ,９４°～１１２°Ｅ作为研究区域,在该区域中收集了１９８６年以来大量地震的Ｓ波和Ｐ波到时差资料,７条人工地震二维速度剖面资料和２个数字化地震台的几个地震的三分向记录资料．对这些资料进行了处理,最后得出了０～２５ｋｍ深度不同截面的速度分布,并对所得结果进行了分析． 相似文献
4.
用慢度分块均匀正方形模型将介质参数化，仅在正方形单元的边界上设置计算结点，这些结点构成界面网．根据Ｈｕｖｓｅｎｓ和Ｆｅｒｍａｔ原理，由不断扩张、收缩的波前点扫描代替波前面搜索，在波前点附近点的局部最小走时计算中对波前点之间的走时使用双曲线近似，通过比较确定最小走时和相应的次级源位置，记录在以界面网点位置为指针的３个一维数组中．借助这些数组通过向源搜索可计算任意点（包括界面网以外的点）上的全局最小走时和射线路径．这一方法不受介质慢度差异大小限制，占内存少，计算速度较快，适于走时反演和以Ｍａｓｌｏｖ射线理论为基础的波场计算． 相似文献
5.
Roberto Scarpa 《Pure and Applied Geophysics》1982,120(3):583-606
Jeffreys-Bullen P and PKP travel-time residuals observed at more than 50 seismic stations distributed along Italy and surrounding areas in the time interval 1962–1979, indicate the complex velocity pattern of this region. Strong lateral velocity inhomogeneities and low velocity zones are required to explain the observed pattern of residuals. In particular, late arrivals of about 1 sec are observed in the Apenninic mountain range, requiring both greater crustal thickness and low velocity layers, coherent with seismic refraction data and surface wave dispersion measurements. The seismic stations located in the Western and Eastern Alps indicate the presence of high velocities. In the Western Alps the strong azimuthal variation of residuals and the high values of early arrivals have a close relationship to the Ivrea body, an intrusive crustal complex characterized by a velocity as high as 7–7.2 km/sec.A travel-time inversion performed with theAki
et al. (1977) block model, confirms the peculiar characteristics and the sharp variations in the lithosphere of the whole Italian region, with values of velocity perturbations between many adjacent blocks, ranging in size from 50 to 100 km, and independent from the earth parametrization chosen, reaching values up to 10% in the lithospheric part and 5% in the asthenosphere. 3-D inversion requires also high velocity along the Tyrrhenian coastal margin, equivalent to an uprise of major crustal and lithospheric discontinuities along this part of the Italian peninsula. Moreover low velocity material must be present in the northern part of the Adriatic foreland, in the lithosphere-asthenosphere system, closely related to the stress and seismicity pattern, and the lateral bending of the lithosphere in the same region. 相似文献
6.
Forough Keshvari Z. Hossein Shomali Mohammad Tatar Ayoub Kaviani 《Journal of Seismology》2011,15(2):329-339
Non-linear teleseismic S-phase tomography across the Zagros collision zone in southwestern Iran is used to determine a high-resolution
image of the upper-mantle structure. The inversion was done using 41 high-quality earthquakes recorded by 19 broad-band and
medium-band stations along a 620 km long profile across the collision zone. Smearing from strong crustal velocity anomalies
into the upper-mantle is suppressed by travel-time corrections calculated based on a 3-D crustal model for the study area.
Our results show that the relatively old and cold Arabian shield has a higher velocity (up to 6% faster, at depths between
70 and 300 km) than the younger lithosphere farther north in Central Iran. These two upper-mantle domains are separated by
a sharp near-vertical transition whose surface expression coinciding with the Main Zagros Thrust. 相似文献
7.
基于单程波深度延拓方法,发展了一种地震波入射角度计算方法.入射角度的计算仅利用简谐波场,可得到整个成像区域内所有点的入射波波前面方向.该方法具有较高的计算效率,可服务于合成角道集等深度偏移方法;与偏移算法相比,其计算量几乎可以忽略.与射线法或基于走时梯度的入射角度计算方法相比,本文方法更稳健,避免了速度场的微小变化导致的入射角较大变化,因此更适用于实际偏移速度模型,也与波动方程深度偏移方法更匹配.数值算例表明,本文方法既有较高的计算效率又有很好的精度,且有很好的稳定性. 相似文献
8.
地震走时层析成像是地球物理反演中成熟的方法之一,已在许多领域得到广泛应用,并取得了良好的效果。本文介绍的地震层析成像方法包括模型参数化、射线追踪和理论走时计算、非线性方程组的线性化、线性化方程组的求解以及解的评价。观测数据使用了中国地震局地球物理勘探中心在岫岩陨石坑取得的浅层地震折射资料,反演得到两条近垂直交叉剖面的P波速度结构。结果表明,该坑为一简单坑,直接撞击形成的区域为直径约1．8km、深度约700m的坑体;坑中心深度约700m周围以及向下出现的7．0km／s以上的高波速可能是陨石撞击时所产生的高温高压使表层岩石达到熔融状态,改变了围岩的性质,致使围岩速度升高。 相似文献
9.
V. Pereyra 《Pure and Applied Geophysics》1996,148(3-4):345-386
We describe an integrated forward and inverse three-dimensional modeling system that can deal with complex geological structures. The system has been designed to handle large-scale problems by using a distributed approach. It uses seismic ray tracing for forward simulation, time-to-depth mapping, and nonlinear travel-time inversion.A novel decomposition method is our tool to attack large-scale problems in a parallel approach. The system is fully implemented and we demonstrate its performance with synthetic examples.This research was partially supported by the National Science Foundation under SBIR Grant III-9300992. 相似文献
10.
Fresnel zone inversion for lateral heterogeneities in the earth 总被引：2，自引：0，他引：2
Kiyoshi Yomogida 《Pure and Applied Geophysics》1992,138(3):391-406
We propose a different kind of seismic inversion from travel-time or waveform inversion for lateral heterogeneities in the earth: Fresnel zone inversion. Amplitude and phase delay of data in several frequency ranges are inverted for model space around ray paths with a width corresponding to the considered frequency so that primary effect of finiteness of wavelength be included. For vertically heterogeneous media, Fréchet derivatives for inversion are obtained very efficiently using the paraxial ray approximation, with nearly similar amounts of computation compared to travel-time inversion. As an example, Fréchet derivatives are computed for a teleseismic observation system for a three-dimensional structure in the lithosphere beneath an array of seismic stations. Even if the used frequency is around 2 Hz, the width of Fréchet derivatives cannot be neglected, particularly near the bottom of the lithosphere. Sensitivity of model parameters to observations is, moreover, different in our approach from conventional travel-time inversion: it is zero along ray paths but large slightly away from them. Some model calculations show that travel-time inversion, particularly with models divided into very fine meshes or blocks, might give misleading results. An example of inversion for a simple Camembert model, in the event that travel-time inversion gives no reliable results, shows how this technique works with much smaller data sets and computation than waveform inversions. 相似文献