浅部地下洞穴散射波对波场扰动分析

均匀介质体表面源激发的瑞利波能量在介质浅部占主导地位,当前行瑞利波遇体浅部洞穴时,质点位移形状及轨迹会发生改变,瑞利波在洞穴发生散射。散射波导致波场能量出现扰动,波场不同区域扰动特征不同。在洞穴前方,反射瑞利波与入射瑞利波相干,能量谱出现干涉条纹;在洞穴上方,透射波发生频散,相速度不同于瑞利波,在某些条件下,透射波传播特性类似于自由板中兰姆波,透射波在洞穴后边界面绕射会导致表面波场能量减小;在后方区域,受几何衰减影响,表面波场逐渐由透射瑞利波主导,其低频成份相对高频成份能量减少较多。本文基于散射波理论,分析洞穴埋深/波长比对波场扰动影响,由数值模拟计算验证理论分析,结果表明由测点距—波长域能量谱扰动对应的临界波长可预测洞穴埋深。

Disturbances of surface wave field due to wave scattering at shallow cavities

In homogenous half spaces, the wave fields at shallow depth are dominated by Rayleigh waves activated by the surface sources. When the forward Rayleigh waves encounter a shallow cavity, the displacement shapes and the particle orbit are changed. Rayleigh waves are scattered at the cavity. The wave field is disturbed by the scattering waves. The different features of the disturbances can be found in different regions of the surface wave field. In the region in front of the cavity, the reflected and the incident Rayleigh waves interfere constructively or destructively with each other. The interference fringes are formed. In the region over the cavity, the transmitted waves are dispersive. The phase velocities are different from those of Rayleigh waves. Under certain conditions, the behaviour of the transmitted waves is similar to that of Lamb waves in the free plates. The diffraction of the transmitted waves along the back boundary results in the energy reduction in the surface wave field over the boundary. Influenced by the geometrical attenuation, the transmitted Rayleigh waves gradually dominate the surface wave field in the back region. The components at low frequencies have greater energy reduction as compared to those at high frequencies. Based on the wave scattering theory, the effects of the ratio of the buried depth to the wavelength of Rayleigh waves on the disturbances of the surface wave field are analyzed. The numerical simulations are used to verify the analyses. It is shown from the results that the buried depth can be estimated from the critical wavelength corresponding to the energy reduction in the offset-wavelength domain.