水平层状介质中Rayleigh波多模式速度频散和椭圆极化频散特征物理实验模拟研究

Multi-mode velocity and elliptical polarization dispersion characteristics of Rayleigh waves in horizontal layered media physical simulation research

Rayleigh波勘探作为地震勘探的主要方法之一,一直受到地球物理勘探学界的广泛关注,而Rayleigh波研究领域的焦点就是对频散特性的研究.本文基于超声虚拟仪器实验平台,从超声波和超声换能器特性出发,对均匀介质、低速覆盖和高速覆盖等模型中的Rayleigh波波场进行观测,运用时域分析、f-v域波场变换和H/V谱比等方法,研究层状介质中Rayleigh波多模式速度频散和椭圆极化的频散特征.在实验过程中,虚拟仪器技术的运用为Rayleigh波频散特性的多模态展现和识别提供了良好的平台,是本研究有力的技术保障.实验研究发现,在低速覆盖的水平层状介质中,Rayleigh波存在多模式频散,高阶模式对浅层速度变化更敏感,即便基阶模式面波占优,高阶成分仍易对地层信息识别造成误判;然而,当上覆层为高速介质时,此类模型中未观测到明显的Rayleigh波多模式现象.研究还发现Rayleigh波椭圆极化率与介质泊松比是密切相关的,该结论与前期理论研究结果非常吻合,从而进一步验证了Rayleigh波椭圆极化频散用于评价地层速度结构的可行性.

Rayleigh wave exploration,as one of the main methods of seismic exploration,has been widely concerned by the field of geophysical exploration,and the focus of Rayleigh wave research is the study of dispersion characteristics.Based on the ultrasonic virtual instrument experimental platform,starting from the characteristics of ultrasonic waves and ultrasonic transducers,this paper observes the Rayleigh wave field in the models of homogeneous medium,low-velocity coverage and high-speed coverage.Meanwhile,the dispersion characteristics of Rayleigh wave multi-mode velocity dispersion and elliptical polarization in laminar media are investigated by using time domain analysis,frequency-velocity domain wavefield transformation and H/V spectral ratio.In the experimental process,the use of virtual instrumentation provides a good platform for the multimodal presentation and identification of Rayleigh wave dispersion characteristics,which is a strong technical guarantee for this study.Experimental studies have found that in horizontally layered media covered by low velocities,Rayleigh waves have multi-mode dispersion,with higher-order modes being more sensitive to shallow velocity variations,and even if the fundamental mode surface waves are dominant,the higher-order components are still prone to misinterpretation of stratigraphic information identification;However,no significant Rayleigh wave multi-mode phenomena are observed in such models when the overlying layer is a high-speed medium.In addition,it is found that the Rayleigh wave elliptical polarization rate is closely related to the medium Poisson’s ratio,a finding that is in good agreement with the results of previous theoretical studies,thus further validating the feasibility of Rayleigh surface wave elliptical polarization dispersion for evaluating the velocity structure of the formation.