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.     

CT在诊断小儿颅脑内出血中的应用

本文报告经CT 诊断的84例小儿颅脑内出血。CT 主要表现是颅脑内出血的高密度影。CT 扫描可确定出血的部位、数量和大小等,足一项十分有价值的检查方法。     

青藏高原东南缘地壳速度结构及云南漾濞MS6.4地震深部孕震环境

青藏高原东南缘岩石圈变形强烈、地震活动频繁,对其深部结构进行研究有助于提高对其演化及强震发震机理等问题的认识.本研究利用青藏东南缘固定和流动观测台站记录的地震P波走时数据,采用体波走时层析成像方法得到研究区地壳、上地幔顶部三维P波速度结构.结果显示,峨眉山大火成岩省内带范围呈明显的高速异常,推测为二叠纪时期地幔柱活动残留在地壳内的基性和超基性幔源物质.川西北次级块体和大火成岩省内带东西两侧存在低速带,可能是壳内部分熔融存在和中下地壳流动的证据:高原物质向南运移时受大火成岩省壳内高速体阻挡后分为两支,东支沿安宁河断裂—则木河断裂—小江断裂延伸,西支沿红河断裂向上地壳运移并逐渐穿过红河断裂.云南漾濞M_S6.4地震序列全部位于西支低速通道之上,推测构造块体SE向运动和地壳流作用使得应力在上地壳震源区进一步集中,共同驱动断裂活动导致漾濞地震发生.     

Integrated geophysical methods for the characterisation of an archaeological site (Massenzio Basilica — Roman forum, Rome, Italy)

A geophysical study that involved different techniques was carried out with the aim to improve the knowledge of the archaeological site where the Basilica of Maxentius was founded and to discern individual covered structures (foundations).Vertical Electrical Sounding (VES), seismic refraction and Electrical Resistivity Tomography (ERT) studies were performed in the archaeological site. VES and seismic refraction allowed to characterise the main geological formations of the hill where the Basilica was built and to distinguish the concrete floor and backfilling. Electrical data were processed using different algorithms; their results were compared to appraise the inverted models' robustness.ERT inversion algorithms were used to delineate shape and size of a much more complex structure, that were originally expected from archaeological excavation plan. The results of the commercial program were used as a posteriori information to include them in the algorithm proposed by the authors; the sequential use of the programs defined a processing procedure.The integrated use of different geophysical techniques reduced a great deal the intrinsic ambiguities of each method. Direct explorations (boreholes and archaeological excavations) confirmed the geophysical results.     

利用尾波包络线反演方法研究伽师强震群区地壳的非均匀结构

以单次散射模型、球形辐射源、一维线性速度模型为基础计算理论包络线,通过尾波归一化方法把理论与观测包络线结合起来形成非线性方程组,用迭代最小二乘方法反演伽师强震群区的散射系数空间分布.结果表明,伽师强震群区下方中上地壳中存在明显的非均匀结构.在10~25 km各深度图像上,研究区中部都存在一低散射系数区,这个低散射系数区在10 km、15 km深度被高散射系数区包围,在20 km深度仅其东侧存在高散射系数区.1997年的7次M_s≥6.0级地震发生在研究区中部的低散射系数区内或低散射系数区向高散射系数区过渡带的低散射系数一侧.     

华北东北部的面波相速度层析成像研究

本文用华北临时地震台阵197个宽频带台站2006年12月～2007年6月记录的面波资料,用双台法测定了435条路径上周期8～75 s的基阶瑞利波相速度频散曲线.通过Ditmar & Yanovskaya 方法反演得到33个周期分辨率为30～50 km的相速度分布图像.分析研究了4个具有代表性周期的相速度分布图像,这些图像揭示了华北东北部地壳上地幔速度结构的横向非均匀性质.10 s周期的相速度分布图像清晰地勾绘出了华北东北部盆地与山区及其内部次级构造单元的边界,平原与山区的相速度存在明显的差异;华北东北部6级以上的强震大都分布在16 s周期高相速度与低相速度急剧变化的过渡带上,规模较大的断裂也大部分位于过渡带的边界附近;在研究区域中部,26 s周期的相速度出现一个明显的呈北东向展布的相速度梯度带,该梯度带与重力梯度带位置相近,说明该重力梯度带两侧的地壳速度结构差异较大.     

圆弧状凹陷地形在平面SH波入射下的动力响应：高频解答

利用波函数的Fourier-Bessel级数展开法和外域型Graf加法公式,推导了圆弧状凹陷地形对平面SH波二维散射问题的解析解。与现有其他解析解不同的是,该解析解的获得无须求解线性方程组,从而避免了高频波入射下方程组系数矩阵可能出现的病态问题。因此,既使在入射波频率很高的情况下,该解析解也能够收敛。利用该解,在一个非常宽的频带范围内讨论了凹陷地形的几何形状对地面运动的影响。     

弹性介质中密度ρ与拉梅常数λ、μ的衍射层析成像方法研究

利用接收到的纵波信息，通过解纵波方程得到介质的密度和拉梅常数分布函数．首先，在频率波数域建立了描述入射纵波信号、接收纵波信号和介质三参数扰动量之间关系的积分方程．然后，建立了关于接收信号的各频率成份的方程组．经过正反傅氏变换求解此方程组即可得到介质三参数的分布函数．最后，就跨孔观测系统进行了数值模拟成像试验。结果表明方法有效．     

岩石非均匀性对超声衰减的影响及其修正

在充分考虑了岩石的非均匀性引起的Rayleigh散射对衰减测量值的影响后，通过有关理论和计算，将岩石的本征衰减和散射加以区分，对原先以均匀介质为基本假定的频谱比法进行了修正，改进后的频谱比法可在较宽的频率范围内进行Q值计算，避免了传统频谱比法在不同频段上得出不同的Q值的不确定性问题，同时，我们还引进并改进了频率移动法，我们将传统和改进的频谱比法，以及频率移动法应用于同一套实验数据，计算结果符合较好。     

新疆及邻区Pn速度层析成像

利用新疆及邻区2万多条Pn射线的走时资料,采用层析成像的方法反演该地区上地幔顶部的Pn速度结构.主要结果是:①新疆地区平均h速度为.2km/s,高于中国大陆地区的平均值(.0km/s),速度变化量从-0.3km/s到0.2km/s.②准噶尔盆地是明显的高Pn速度区,高速区与地貌形状吻合较好,最大值达.4km/s;塔里木盆地整体上呈现高速异常二者都可能与盆地是稳定地块、构造活动和变形较弱有关.③在盆地与山脉的交接带即主要的构造活动带,存在较低的Pn速度,可达7.9km/s.④塔里木南缘中部存在一显著的低速区,可能与岩浆活动和构造活动较强有关.⑤上地幔顶部波速各向异性整体上盆地中心较弱,边缘地带较强.新疆南部波速的各向异性可能是因印度板块对欧亚大陆的碰撞使地幔物质向北蠕动的结果.