浅部地下洞穴散射瑞利波对表观相速度影响

均匀半无限体中表面源激发的瑞利波能量在介质浅部占主导地位。当前行瑞利波遇浅部洞穴时,瑞利波在洞穴发生散射。以洞穴作参照位置,波场沿前行波方向可依次分为前方、上方、后方三个区域。前方远场散射波以反射瑞利波为主,传播特性与入射波相同;当洞穴几何参数满足一定条件,上方散射波可形成与兰姆波传播特性类似的导波;后方远场由透射瑞利波主导。受散射波影响,表面波场表观相速度出现扰动,不同位置扰动特征不同,若测线下方有浅部洞穴存在,由不同位置相速度扰动特征差异,可预测洞穴方位,由前方相速度特征扰动对应波长可预测洞穴埋深。

Effects of Rayleigh Waves Scattered at Shallow Cavitieson Apparent Phase Velocity

In a homogenous half space acted by surface sources, the shallow wave-fields are dominated by Rayleigh waves. When the forward Rayleigh waves encounter a shallow cavity, the waves are scattered at the cavity. Taking the cavity as the reference position, the wave-field can be divided into the front, above and rear regions in sequence along the forward direction. The scattered waves in the far field of the front region are dominated by the reflected Rayleigh waves, which travel at a velocity the same as that of the forward Rayleigh waves. The guided waves with a propagation behavior similar to Lamb waves may be formed at certain geometrical sizes of the cavity. The far field of the rear region is controlled by the transmitted Rayleigh waves. Influenced by the scattered waves, the apparent phase velocity curves of surface waves are disturbed. The characteristics of the disturbances differ at different positions. If the shallow cavity is present under the measuring line, the cavity can be localized from the disturbance differences along the line. The buried depth of the cavity can be estimated from the wavelength corresponding to the characteristic disturbance of the phase velocity in the front region.