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1、2017 年 地球物理学基础第 3 次作业 第 1 页 共 5 页 1. The techniquesof refractionandreflection seismology, using artificial, controlled explosions as sources, were developed in the search for petroleum. Since the 1960s these methods have been applied with notable success to the resolution of detailed crustal structu
2、re under continents and oceans. The development of powerful computer technology enabled refinements in earthquake location and in the determination of travel-times of seismic body waves. These advances led to the modern field of seismic tomography, a powerful and spectacular technique for revealing
3、regions of the Earths interior that have anomalous seismic velocities. In the field of earthquake seismology, the need to protect populations and man-made structures has resulted in the investment of considerable effort in the study of earthquake prediction and the development of construction codes
4、to reduce earthquake damage. To appreciate how seismologists have unraveled the structure of the Earths interior it is necessary to understand what types of seismic waves can be generated by an earthquake or man-made source (such as a controlled explosion). The propagation of a seismic disturbance t
5、hrough the Earth is governed by physical properties such as density, and by the way in which the material of the Earths interior reacts to the disturbance. Material within the seismic source suffers permanent deformation, but outside the source the passage of a seismic disturbance takes place predom
6、inantly by elastic displacement of the medium; that is, the medium suffers no permanent deformation. Before analyzing the different kinds of seismic waves, it is important to have a good grasp of elementary elasticity theory. This requires understanding the concepts of stress and strain, and the var
7、ious elastic constants that relate them. 利用人工的、 控制爆炸作为震源的折射地震和反射地震技术在寻找石油领域 得到了发展。19 世纪 60 年代以来，这些方法已经令人瞩目的成功应用于分辨位 于大陆和大洋下的精细地壳构造。 强大的计算机技术的发展使地震位置的改进和 地震体波走时的测定成为可能。这些进展导致了现代地震层析成像领域的发展， 这是一项强大而引人注目的技术，它揭示了地球内部具有异常地震速度的区域。 在天然地震地震学领域， 对人口和人造建筑的保护需求导致了在地震预测研究和 制定减少地震损害的建筑规范方面投入了大量精力。 想要领会地震学家是如何揭开地
8、球内部结构的， 就有必要理解什么类型的地 震波可以被天然地震震源或人工地震震源（如控制爆破）产生。地震扰动在地球 的传播受密度等物理特性的控制，以及地球内部物质对扰动的反应方式的控制。 震源中的物质承受永久变形， 但震源外的地震扰动主要通过介质的弹性位移而发 生，即介质不发生永久变形。在分析不同类型的地震波之前，掌握基本弹性理论 是十分重要的。 这需要理解应力和应变的概念， 以及与它们有关的各种弹性常数。 2. 波前- Wavefront；反射波- Reflected Wave； 高速带-High Velocity Zone；球面层- Spherical Layers； 积分法- Integr
9、al method；平面波-Plane Wave； 射线路径-Ray path；斯奈耳定律-Snells Law； Head wave-首波；Ray parameter-射线参数； 2017 年 地球物理学基础第 3 次作业 第 2 页 共 5 页 travel time curve-走时曲线；Direct wave-直达波； apparent velocity-视速度；Low velocity discontinuity-低速间断面； Seismic phase-震相；Curvature of spherical ray-球面射线曲率； 3. On a cross-section of th
10、e Earth, sketch the paths of the following seismic rays: (i) PKP, (ii) SKS, (iii) PcP, (iv) PPP.（10 points） 4.Astrong earthquake off the coast of Japan sets off a tsunami that propagates across the Pacific Ocean (average depth d=5 km).（20 points） (a) Calculate the velocity of the wave in km/hr and t
11、he corresponding wavelength, when the wave has a dominant period of 30 min. (b) How long does the wave take to reach Hawaii, which is at an angular distance of 54from the epicenter ? （a）hrkmsmgdc/797/4 .221100058 . 9 kmcT5 .3985 . 0797 （b）hr c L t53. 7 797 63712 360 54 5. The following table gives t
12、wo-way travel-times of seismic waves reflected from different reflecting interfaces in a horizontally layered medium.（20 points） 2017 年 地球物理学基础第 3 次作业 第 3 页 共 5 页 (a) Draw a plot of (travel-time)2against (distance)2. (b) Determine the vertical two-way travel-time (“echo-time”) and average velocity t
13、o each reflecting interface. (c) Compute the depth of each reflector and the thickness of each layer. (d) Compute the true velocity (interval velocity) of each layer. (e) Verify your results by computing the total vertical travel-time for a wave reflected from the deepest interface. （a） 01234567 x 1
14、0 6 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 (travel-time) 2 (distance)2 First reflector Second reflector Third reflector （b） According to the plot of (travel-time)2against (distance)2of each reflector above，the slope of each straight line is equal to 1/(Va)2， where Va is the average velocity， and the
15、its intercept with the t2-axis is equal to the square of echo time t0，then the average velocity and echo-time can be computed，the results are list in following table 2017 年 地球物理学基础第 3 次作业 第 4 页 共 5 页 That is， to the 1st layer,average velocity is about 1800 m/s; its echo time is about 0.111 s; to the
16、 2nd layer,average velocity is about 2360 m/s; its echo time is about 0.296 s; to the 3rd layer,average velocity is about 3640 m/s; its echo time is about 0.576 s; （c） For the first reflector，the average velocity is its true velocity and its depth（D1） is equal to its thickness（d1），that is Va1=V1，D1=d1，so mtVatVd1002/02/0D 111111 For the second reflectors， mtVadd3502/0D 22212 mdDd250 112 For the third reflectors， mtVadD10492/0D 33323 mDDd
