Study On Shallow Fine Structure In Central Jizhong Depression By Surface Wave Exploration Technology

Shear wave velocity structure and near surface engineering mechanics parameters are important parameters for underground space planning and development.It is the mainstream way to obtain the near surface dynamic parameters directly through limited engineering drilling holes and related test experiments.It is always the focus and frontier in the field of shallow surface geophysics to obtain the near surface engineering mechanical parameters of the whole area by using limited engineering holes as constraints.In this paper,the shallow structure of Jizhong depression is studied based on multiple near surface active source data.The shallow S-wave velocity structure is obtained by the surface wave fundamental order inversion,and the shallow surface layer is divided;the high-order surface wave inversion experiment is carried out to determine the inversion error of base high-order in different layers,so as to provide reference for the rational use of base high-order inversion;and then the engineering mechanical parameters are calculated according to the S-wave velocity to classify the engineering site.The research content of this paper is divided into three parts: the first part is the S-wave velocity structure inversion of surface wave base order.The surface wave data of three sections are processed by data conversion and filtering,and the fundamental dispersion curve is extracted,and the S-wave velocity structure is obtained by inversion.The second part is the experimental study of high order inversion.For the measured data,the high-order dispersion curve is extracted and the S-wave velocity structure is obtained by inversion.The third part is project site evaluation.Based on the P-wave and S-wave velocities obtained by surface wave data inversion,the mechanical parameters(shear modulus,dynamic elastic modulus and dynamic Poisson's ratio)are calculated by engineering mechanics formula to evaluate the engineering site in the study area.According to the above research,the following conclusions are drawn:(1)The inversion results of basic surface wave show that the stratification effect is obvious: the high-speed layer is 8-14 m,the low-speed layer is 14-20 m,and there is discontinuous low-speed layer in the range of 20-30 m.By comparing logging data,14-20 m is water bearing sand layer,and 20-30 m range is silty clay and silt.(2)The results show that the base order inversion is more accurate for the deep structure,and the higher-order inversion is more accurate for the low-speed and high-speed layers.(3)According to the P-wave velocity(first break wave tomography)and shear wave velocity structure(surface wave inversion)obtained from active source data,the study area is divided into three types of site soil.It is mainly composed of clay and silty clay with good construction conditions.