Use of model substitution in the inversion of dispersion curves in weak subsurface layers

Abstract

The primary concern for advanced geological prediction in tunnels is the weak layer, which can be explained by its properties based on the shear wave velocity. Ordinarily, the conventional technique of estimating longitudinal and transverse wave velocities using the reflection-wave theory has low precision and strong ambiguities, resulting in inaccurate predictions. To overcome this problem, surface waves can accurately predict multiparameter constraints and achieve high precision. However, the phenomenon of “mode kissing” is common during the surface wave dispersion mode of weak layers, leading to misinterpretation and escalating dangers during tunnel construction.Therefore, this study conducted numerical simulations of Rayleigh and Love waves using various weak-layer models and observation systems for low-speed weak layers. In addition, we analyzed the relationship between the dispersion curves obtained using the model substitution method and numerical models. We suggest a novel approach by employing the simplex method simulated annealing algorithm (ASSA) and the model-substitution method for inverting actual weak layer models and measurement data.Our findings demonstrate that the model substitution method can effectively delineate the “mode kissing” in the fundamental and higher modes of Rayleigh and Love waves. However, inconsistencies exist when characterizing the “mode kissing” frequency dispersion of Rayleigh waves when the low-speed layer velocity is equal to the surface velocity. Additionally, the model substitution method can accurately invert the velocity structure of various types of weak layer models. However, for low-sensitivity strata parameters, the accuracy improvement is limited. Therefore, we used coordinate rotation and model substitution to analyze the development of weak layers in front of the palm surface using spatial measurement data obtained from the tunnels. Our findings showed good agreement with the results of advanced drilling, verifying the effectiveness of using the model substitution method to invert the velocity structures of weak layers and providing a new approach for predicting geological features using surface- and reflection-wave methods.