Image-domain wavefield tomography for tilted transversely isotropic media

Transversely isotropic models with a tilted symmetry axis have become standard for imaging beneath dipping shale formations and in active tectonic areas. Here, we develop a methodology of wave-equation-based image-domain tomography for acoustic tilted transversely isotropic media. We obtain the gradients of the objective function using an integral wave-equation operator based on a separable dispersion relation that takes the symmetry-axis tilt into account. In contrast to the more conventional differential solutions, the integral operator produces only the P-wavefield without shear-wave artefacts, which facilitates both imaging and velocity analysis. The model is parameterized by the P-wave zero-dip normal-moveout velocity, the Thomsen parameter δ, anellipticity coefficient η and the symmetry-axis tilt θ. Assuming that the symmetry axis is orthogonal to reflectors, we study the influence of parameter errors on energy focusing in extended (space-lag) common-image gathers. Distortions in the anellipticity coefficient η introduce weak linear defocusing regardless of reflector dip, whereas δ influences both the energy focusing and depth scale of the migrated section. These results, which are consistent with the properties of the P-wave time-domain reflection moveout in tilted transversely isotropic media, provide important insights for implementation of velocity model-building in the image-domain. Then the algorithm is tested on a modified anticline section of the BP 2007 benchmark model.     

Combined inversion of first-arrival travel times and reflection travel times

In this work, we propose a method for determining reflection travel times based on the acquisition of first-arrival travel times via the fast sweeping method. The accuracy of this scheme was proven by conducting model experiments to establish a foundation for first-arrival tomography, reflection tomography and combined tomography. Reflection tomography was subsequently achieved using the adjoint-state method; on this basis, we propose a combined tomography method involving both first-arrival and reflection tomography. In the model experiments, excellent results were obtained via first-arrival tomography, reflection tomography and our combined tomography method. Finally, full-waveform inversion was performed, with the inversion produced by combined tomography used as the initial model. Excellent results were obtained using this approach. However, combined tomography reproduced and characterized the model much more realistically.     

河南及邻区瑞利波群速度噪声层析成像

本文选取河南及邻区39个宽频带和甚宽频带地震台站记录的2014年1月～2015年12月垂直分量的连续波形资料,通过波形互相关和叠加运算得到台站间的经验格林函数,并用时频分析方法提取了498条瑞利波群速度频散曲线,进而采用噪声层析成像方法反演获得了河南及邻区8～25s的瑞利波群速度分布图像。结果表明,8s周期的群速度分布与地表构造特征密切相关,平原地区表现为低速异常,而基岩出露的山区呈现为高速异常;14s、20s周期的群速度在山区表现的高速异常与8s相似,说明中上地壳与地壳浅部结构有一定的相关性和继承性;8～20s周期的群速度在华北平原地区存在低速异常范围随周期增加而减小的现象。华北平原地区在下地壳表现出的低速异常,可能与大陆地壳减薄有关。大致以太行-武陵重力梯度带为界,下地壳表现出的西部低速、东部至研究区中部高速异常差异,体现了地壳东薄西厚的结构特征。     

Colliding Blast Waves Driven by the Interaction of a Short-Pulse Laser with a Gas of Atomic Clusters

Collisions between shocks are commonly found in many astrophysical objects, however robust numerical models or laboratory analogues of these complex systems remain challenging to implement. We report on the development of scaled laboratory experiments which employ new techniques for launching and diagnosing colliding shocks and high Mach number blast waves, scalable to a limited subset of astrophysically-relevant regimes. Use of an extended medium of atomic clusters enables efficient (>80%) coupling of 700 fs, 1 J, 1054 nm laser pulses to a “cluster” gas with an average density of ≈10¹⁹ particles cm⁻³, producing an initial energy density >10⁵ J cm⁻³, equivalent to ≈5×10⁹ J/g. Multiple laser foci are used to tailor the spatial profile of energy deposition, or to launch pairs of counter-propagating cylindrical shocks which then collide. By probing the collision interferometrically at multiple view angles in 5^{^} increments and applying an inverse Radon transform to the resulting phase projections we have been able to tomographicall reconstruct the full three-dimensional, time-framed electron density profile of the system.     

OCT在珍珠研究中的应用

光学相干层析成像技术(OCT)是一种对物体进行层析成像的新技术。作为一种无损的高分辨率的检测手段,OCT可用来鉴别真假珍珠、区别海水有核珍珠与淡水无核珍珠、测定珍珠层厚度,并可用于探讨珍珠生长动力学规律,在珍珠研究中具有较广阔的应用前景。     

Shear zones in porous sand: Insights from ring-shear experiments and naturally deformed sandstones

In a high-resolution small-scale seismic experiment we investigated the shallow structure of the Wadi Araba fault (WAF), the principal fault strand of the Dead Sea Transform System between the Gulf of Aqaba/Eilat and the Dead Sea. The experiment consisted of 8 sub-parallel 1 km long seismic lines crossing the WAF. The recording station spacing was 5 m and the source point distance was 20 m. The first break tomography yields insight into the fault structure down to a depth of about 200 m. The velocity structure varies from one section to the other which were 1 to 2 km apart, but destinct velocity variations along the fault are visible between several profiles. The reflection seismic images show positive flower structures and indications for different sedimentary layers at the two sides of the main fault. Often the superficial sedimentary layers are bent upward close to the WAF. Our results indicate that this section of the fault (at shallow depths) is characterized by a transpressional regime. We detected a 100 to 300 m wide heterogeneous zone of deformed and displaced material which, however, is not characterized by low seismic velocities at a larger scale. At greater depth the geophysical images indicate a blocked cross-fault structure. The structure revealed, fault cores not wider than 10 m, are consistent with scaling from wear mechanics and with the low loading to healing ratio anticipated for the fault.     

一种改进的电离层层析成像算法

针对电离层层析成像算法中确定相位积分常数以及合理选择迭代初值等关键问题,本文提出一种联合使用差分多普勒相位和差分多普勒频率数据的新算法,将相位积分常数的计算以及迭代初值的确定与图像的重建过程统一起来．该算法克服了单独计算相位积分常数的困难,初值的选择也较为合理,数值模拟反演与对实测数据反演的结果证明,本文提出的算法对电离层ＣＴ重建更为合理有效．     

CT对隐性骨折的诊断价值

目的：探讨ＣＴ对隐性骨折的诊断价值。方法：回顾性分析８４例隐性骨折患者,其中男６４例,女２０例,年龄７￣７３岁,平均４２．６岁。所有患者均行轴位或冠状位ＣＴ扫描。结果：扫描发现脊柱骨折２８例,髋臼骨折８例,胫骨平台骨折１１例,眶骨骨折１２例,颞骨骨折１３例。肩胛骨骨折１２例。结论：ＣＴ对隐性骨折的诊断具有极大价值,对于高度怀疑骨折而Ｘ线片未显示骨折征象的患者应进一步给予ＣＴ扫描检查。     

FMTT方法研究华北及邻区上地幔P波速度结构

本文利用华北地震科学台阵190套宽频带地震仪在2006年10月至2009年3月记录的远震事件,采用波形互相关技术提取了52414条P波初动到时数据,进一步使用FMTT(Fast Marching Teleseismic Tomography)方法,获取了华北克拉通中、东部陆块下方上地幔分辨高达0.5°×0.5°的P波速...