地下浅部异质体对表面波场谱扰动分析

均匀半无限体中表面源激发近表面波场由瑞利波主导，当浅部存在异质体，前行瑞利波发生散射，异质体周围表面波场响应谱发生变化。当异质体长度相对波长较大时，由异质体上方表面质点响应谱提取绕射波频散曲线可分析异质体与周围介质刚度差异对绕射波传播特性的影响。与含软层或硬层半无限体中瑞利波特性比较，可以发现绕射波具有与瑞利波类似的传播特性。异质体上方谱变化特征与异质体和周围介质间刚度差异有关，本研究基于异质体绕射波与入射瑞利波位移结构差异解释谱变化现象。结果表明：异质体上方谱与周围谱明显不同，在偏移距−波长域，相对于前方波场，软质体上方谱密度整体呈减弱趋势，硬质体上方谱密度整体呈增强趋势。根据谱密度变化对应偏移距、波长及谱密度强弱，可以预估异质体在传播方向的位置、埋深，并识别异质体类型。

Analysis of spectral changes of the surface wave-field caused by underground shallow heterogeneities

In homogenous half spaces, the wave-fields in the shallow depth are dominated by Rayleigh waves elicited by surface sources. When the forward Rayleigh waves encounter a shallow heterogeneity, the surface response spectrum over the heterogeneity is changed. When the length of the heterogeneity is significantly long compared to the wavelength of the forward Rayleigh waves, the effects of the stiffness contrast between the heterogeneity and the surrounding soil on the behavior of the diffracted waves are analyzed from the dispersion curves extracted from the spectrum performed on the surface response over the heterogeneity. Compared to Rayleigh waves in the half space containing the soft or hard soil layer, it can be found that the behavior of the diffracted waves is similar to that of Rayleigh waves. The spectral changes over the heterogeneity are related to the stiffness contrast. The phenomena of the spectral changes are explained using the displacement structure differences between the diffracted waves and the incident Rayleigh waves. It is shown from the results that the spectrum over the heterogeneity is obviously different from those in front of and behind the heterogeneity. In the offset-wavelength domain, compared to the spectrum in front of the heterogeneity, the spectral density decreases as a whole over a soft heterogeneity, while the spectral density increases over a hard heterogeneity. The location and the burial depth of a suspected heterogeneity can be estimated from the offsets and the wavelengths corresponding to the spectral changes, respectively. The type of the heterogeneity can be identified from the relative decrease or increase in the spectral density.