Study On Response Characteristics Of Shallow Surface Wave Dispersion Curve And Its Application

The shear wave velocity of shallow surface is an important parameter for seismic safety evaluation,engineering geological investigation and foundation treatment effect evaluation.Understanding the shear wave velocity structure of underground media is of great significance for engineering construction and geological hazard prediction and prevention.The direct method to obtain shear wave velocity on shallow surface can be obtained by geophysical logging,but this method is time-consuming and labor-consuming.Surface wave exploration is simple,efficient and non-destructive.Therefore,surface wave exploration is widely used in engineering geological exploration and engineering quality detection.The core of surface wave exploration lies in the research and analysis of the dispersion curve.Therefore,deepening the research of the dispersion curve of surface wave can more accurately obtain the underground S-wave velocity structure,which plays a guiding role in practical engineering practice.In this paper,the characteristics of dispersion curve of Rayleigh surface wave on shallow surface are studied from two aspects: numerical simulation of surface wave dispersion curve and Simulation of surface wave field.The numerical simulation of dispersion curve can study the characteristics of dispersion curve of layered medium model,and also can comprehensively analyze the characteristics of dispersion curve of different order guided wave modes.The numerical simulation of high-order finite difference of wave equation can simulate non-layered wave.The dispersion curves can be extracted from the surface wave model and the characteristics of surface wave field in topographic fluctuations.Combining the two methods can better analyze the response characteristics of surface wave dispersion curve on shallow surface.Firstly,this paper introduces the forward algorithm of Rayleigh wave dispersion curve,and carries out the forward simulation of the dispersion curve of the layered medium model with the increase of shear wave velocity and the layered medium model with soft and hard interlayers.It is concluded that for different layered media models,the phase velocities of the first-order guided wave modes all have zero-frequency limit and high-frequency limit;the dispersion curves of the higher-order guided wave modes of different orders have their respective cut-off frequencies,and the higher the order,the higher the cut-off frequency.At the cut-off frequencies,the phase velocities of the higher-order guided wave modes have the limit values.Then,wave field forward modeling is carried out based on the first-order stress-velocity equations.The wave field records of topographic change model,weak stratum model and weak interlayer model under complex surface conditions are obtained respectively.The dispersion spectrum is obtained by processing,and then the characteristics of the dispersion curve are analyzed.It is concluded that in surface wave exploration,in order to obtain the true phase velocity,in data processing,the channel spacing should take the true distance between geophones rather than the horizontal distance between them;the dispersion curve obtained under undulating terrain reflects the normal thickness of underground medium rather than its vertical thickness;the dispersion curves of layered strata with interbeds and non-layered strata with caves are both.There may be zigzag inflection points,which can be distinguished by wave field records when they are difficult to distinguish on dispersion curves.Under complex surface conditions,Rayleigh wave propagation is more complex.When using Rayleigh surface wave for exploration,the influence of topographic changes on Rayleigh wave field and dispersion curves should be considered.Finally,the practical application of surface wave exploration is discussed.The application of surface wave exploration in loess area is introduced.Then,based on the current situation of surface wave exploration and the actual application requirements,the advantages of active and passive source surface wave combined exploration are introduced.Combined with engineering examples,it is proved that surface wave combined exploration can guarantee both the accuracy and depth of surface wave exploration.