High Models Rayleigh Wave And Forward Modelling And Inversion Of Its Dispersion Curves

Rayleigh wave prospecting is a new method of engineering geophysical exploration. It has been widely used in the domains of engineering geological exploration and engineering quality testing, since it is easy and quick to operate with less limit of the operating site and directly get surface wave velocity parameters regarding the depth of the strata. Dispersion curve of Rayleigh wave is the basis of inverting the structure of strata, and to get its proper interpretation is an important aspect of prospecting method based on Rayleigh wave. In practice, the dispersion curve of Rayleigh surface wave often appears as a zigzag pattern, with which only a rough position of the interface between strata might be estimated yet by intuitions or experiences sometimes, and no parameters of surface wave velocity of the stratum could be acquired. The inversion for the strata structure and the velocity parameter with the dispersion curves of zigzag pattern has been a difficult problem challenging the geophysical researchers.Having reviewed the developments of Rayleigh surface wave, this paper has carried out the theoretical analysis and inversion of Rayleigh surface wave抯 transmission in horizontal bedding medium, numerically calculated the dispersion curves under the conditions of embed of low velocity and joints, studied the physical me~hanism of zigzag dispersion curve. The study result shows that multiple models of guiding wave, in addition to fundamental-model, may be induced by the source of same frequency in the strata with weak embed or joints. It is also discovered that the occurrence of these high-models of guiding wave is the essential cause of the zigzag dispersion curves of Rayleigh surface wave. In this paper, it is pointed out that the/zigzag dispersion curves of Rayleigh surface wave can be hardly avoided when carrying out the prospecting of Rayleigh surface wave in stratified medium.This paper has also studied the basic features of waves of high-models, and analyzed the effects of several factors. The result indicates the number of high-models has relations with the source frequency, overburden thickness, longitudinal and transverse velocities of the stratum and the density of stratum. Among them, the stratum density and longitudinal velocity have much slighter effects on the guiding waves of high-models than the transverse velocity.The two important findings about Rayleigh surface wave have been concluded: 1) the Rayleigh surface wave of high-models penetrates longer and is easier to obtain than the fundamental-model. When the frequency range is not wide much and the main frequency is high, the wave of fundamental-model may be weak while the other high-models strong, and no reliable signal of Rayleigh wave of fundamental-model can be certainly obtained; 2) the Rayleigh surface wave of high-models is more sensitive to the parameters variety of the stratum than the fundamental-model. The dispersion curves of Rayleigh wave of fundamental-model obtained from strata of different parameters may be very similar, while the corresponding dispersion curves of high-models distinguish from each other obviously. This two conclusions deMonstrate thoroughly the importance and functions of Rayleigh surface wave of high-model in the prospecting of strata structure and the inversion of parameters. Meanwhile, they prove that the incorrect results may be led out if the parameters of the stratum are inverted only from dispersion curves of Rayleigh wave of fundamental-model, so that the inversion must be carried out with dispersion curves of high-model waves.Finally, this paper has inverted the zigzag dispersion curves based on the outputs of forward modeling. The methods used in the inversion include damping least square method, simulated annealing algorithm and genetic algorithms, and the advantages and disadvantages of each have been also analyzed and compared. The damping least square method is depending firmly on the initial model, of which the condition is difficult to m...