Seismic scattering near the earth's surface
Seismic scattering near the earth's surface
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DOI：
10.1007/bf00874356
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发表时间：
1990-03
影响因子：
2
通讯作者：
A. Levander
中科院分区：
文献类型：
--
作者：
A. Levander
地震尾波通常被认为是由地球中的不均匀性散射的波组成的。三类散射机制已被确认：重复镜面反射的一次波在一个均匀的分层结构，体积散射的一次波从局部体积不均匀性，和散射的一次波从不规则性，否则一般分层结构。当散射障碍物靠近或位于地表时，地表的存在使所有这些散射现象的描述变得复杂。在本文中，我回顾工作，证明了散射的影响，在地球表面附近，强调三个一般领域的调查：散射体波从一个不规则的自由表面，散射体波在不规则的层，和传播的表面波不规则地形或不规则波导。大多数重要的影响已经在理想几何形状的模型研究中得到了承认。支持模型研究的观察证据是存在的，但往往是推断出来的。测量散射的受控实验很少，地形可以聚焦或散焦入射体波，并可以将体波转换为表面波，反之亦然。不规则的表面层可以放大入射体波，耦合体波和表面波，并在空间有限的低速谷结构和高度不规则的层中产生共振。勒夫波传播对波导中的不规则性高度敏感。在不规则层状介质上测量的乐甫波色散与接收器阵列下方的平均平面层状结构的色散有很大不同。瑞利波色散对波导中的光滑不规则性不太敏感，通常代表接收阵列下方的平均结构。
The seismic coda is usually thought to be composed of waves scattered from the heterogeneities in the earth. Three classes of scattering mechanisms have been recognized: repeated specular reflection of primary waves in a uniformly layered structure, volume scattering of primary waves from localized volume heterogeneities, and scattering of primary waves from irregularities in an otherwise generally layered structure. The presence of the earth's surface complicates the description of all these scattering phenomena whenever the scattering obstacles are near or at the surface. In this paper I review work which demonstrates the effects of scattering near the earth's surface, emphasizing three general areas of investigation: scattering of body waves from an irregular free surface, scattering of body waves in irregular layers, and propagation of surface waves across irregular topography or in irregular wave guides. Most of the effects of importance have been recognized in model studies of idealized geometries. Observational evidence in support of the model studies exists, but is often inferred. Few controlled experiments to measure scattering have been performed.Topography can focus or defocus incident body waves and can convert body waves to surface waves and vice versa. Irregular surface layers can amplify incident body waves, couple body and surface waves, and produce resonances in spatially limited low velocity valley structures and highly irregular layers. Love wave propagation is highly sensitive to irregularities in a wave guide. Love wave dispersion measured over irregularly layered media can be quite different from the dispersion of the mean plane layered structure beneath the receiver array. Rayleigh wave dispersion is far less sensitive to smooth irregularities in a wave guide and is usually representative of the mean structure beneath the receiving array.