| Surface wave exploration is an important tool to detect the shallow velocity structure of S waves non-destructively, which consists of three main steps:acquisition of surface wave in the field, extraction of dispersion data and, and inversion. To invert S waves velocity is the ultimate objective of surface wave exploration, which is the most important step among the three main steps. The inversion process includes two main algorithms. One is forward modeling, in which the model profile is assumed, and the theoretical dispersion data are derived. The other is optimization algorithm. The second algorithm is an iterative numerical process to search for the theoretical model profile that produces dispersion data that most closely match the field data. The process precision of each step will affect the final inversion accuracy. On the other hand, only the fundamental mode dispersion data are used in most surface wave inversions at present. Even some old methods such as halfwavelength approximation method, inflection point method, asymptote approximation method and so on, are also used now. These simplely and roughly methods include certain subjectivities to be used. They only use the approximate relation between the S-wave velocity and the Rayleigh velocity in inversion, which are not ture inversion indeed and leade to large errors in the inversion results. However, there are several modes in the surface dispersion data. Higher mode dispersion data often possess more information about the media. Especially, when a low velocity interlayer exists in the media, higher mode dispersion data occupy the dominant energies of high frequency segments. So, fundamental mode and higher mode dispersion data should be considered in the inversion. In this dissertation, three aspects contents are studied deeply. The first is the forward modeling of multiple-mode dispersion curves and their characteristics as well as the Rayleigh wave field modeling by staggered-grid finite-difference method. The second is the inversion of multiple-mode dispersion data and some measures to improve the uniqueness of inversion. The third is the methods to extract multiple-mode dispersion curves from field data. This study was supported by the National 863 Foundation Project of China (2005AA615010), and was the follow-up study of the project of'Surface Wave Extraction of Reflection Seismics and Shallow Structure Detecting Technique'. This PhD dissertation obtains such achievements and conclusions as follows:1. The conventional Thomson-Haskell fast compution method is achieved successfully. It is also revised to be suitable for ocean models. The multiple-mode dispersion curves of ocean models are calculated and analyzed. And simultaneously, high-order staggered-grid finite-difference scheme is used to calculate the Rayleigh wave field. The dispersion characteristics of Rayleigh waves and how their multiple-mode can be excitated in practice are studied by comparing the theoretical dispersion curves from dispersion function with those extracted from synthetical record.2. Dispersion curves are calculated numberically respectively for tow-layers, three-layers and low velocity interlayer models, which are covered with a liquid layer. And their corresponding wave field are simulated at the same time. The characteristics of multiple-mode dispersion curves for models with different thickness of the overlying liquid layer are analysed thoroughly. The results got from dispersion function are also verificated by wave field simulating. The geometric characteristics of the multi-mode dispersion curves are compared to those of that without a liquid surface. And then, some problems which should be taken into consideration when seismic exploration is applied in off-shore areas were pointed out.3. In the inversion of multiple-mode dispersion curves, a subdividing layering method to invert the dispersion curves of surface wave is put forward. After subdivision, there remains just only the shear velocity to be inverted in the inversion process. The inversion results obtained from subdividing layering method approach closely to the actual model, which shows that the subdividing layering method is feasible and effective in dispersion curves inversion. Comparing with the conventional inversion method in which the shear velocity and the layer thickness are inverted simultaneously, the subdividing layering method avoids changing the layer numbers and the thickness of each layer, which simplifies the inversion process greatly. This method can not only satisfy the requirement of inversion resolution, but also make the inversion result close to the actual situation.4. Because the inversion result of Levenberg-Marquardt method depend on initial model greatly, it is very important to select a rational in the inversion. The S-wave velocity Vs in the depth of 1/3 (or 1/2) times of the Rayleigh wavelength is considered to be equal to the phase velocity VR. An initial model p0 is constructed using the Rayleigh wave velocity (VR) and frequency (f) data pairs from fundamental mode by the formulae of z=LR/3 and LR=vR/f. Cubic spline interpolation is used to calculate S-wave velocity Vs at specified depth. An initial model p0 calculated by this method is close to the true model ptrue, which avoid selecting initial model blindly, save iteration time greatly, and ensure the inversion process running fastly and steadily.5. Levenberg-Marquardt (LM) method and Genetic Algorithm (GA) are used in fundamental and multiple-mode inversion. Theoretical model testing proved that GA obtained a better result than LM. But GA is much expensively in time consumption. Considering this disadvantage of GA, this dissertation put forward an updated scheme of Levenberg-Marquardt method (ULM), whose accuracy is comparatively with GA. But the time consumption of ULM is far bellow GA. In practical application, ULM can not only ensure high accuracy for inversion, but also cost small time. So, ULM is an ideal inversion method.6. In the aspect of extraction of multiple-mode dispersion curves from field data, f-k method,f-p methods and phase-shift method are used. The extraction result of these three methods are studied and analyzed. This PhD dissertation update the present extracting method of dispersion curves by combining the advantage of f-p method with that of phase-shift method together, and then get a f-p & phase shift-stacking method. This method possesses both the advantage of f-p method and phase shift method, which not only renew the information of fundamental mode, but also remain higher modes to have a good resolution.7. The field actual data are processed to obtain dispersion data which are used to invert 2D S-wave profile. And the results are compared with those obtained by half-wavelenth approximation methods in which just one mode of dispersion data is used in the inversion. At the same time, half-wavelength approximation is also used in the inversion. These will certainly affect the accuracy of inversion. On the other hand, the procedure of layering and inversion request abundant experience for the processer. That is to say, the inversion results of half-wavelenth approximation methods are easy to be affected by personal factors. However, this study adopts the subdividing layering method, which need not user to control the inversion. At the same tme, the program can also invert multiple-mode dispersion data. So, it improves the accuracy of inversion in certain degree.In a word, this PhD dissertation study the dispersion characteristics of Rayleigh waves and how their multiple-mode can be excitated in practice by comparing the theoretical dispersion curves from dispersion function with those extracted from synthetical record. The study results deepen people's understanding on dispersion characteristics of surface waves. They also have important theoretical and practical value in the aspects of surface wave exploration and non-destructive testing. At the same time, this study may provide some thought and theoretical bases for surface wave exploration in liquid-covered areas.The dissertation accomplished the extraction and inversion of multiple-mode dispersion curves, and simplified the inversion parameters into one variable by subdividing layering method. All this studies not only enrich the inversion study of surface wave, but also make it possible to use multiple-mode dispersion data to invert the S-wave velocity in practice.it make up the deficiency of present inversion which only use fundamental dispersion curves, and improve the accuracy of inversion. Corresponding processing programs have been written in this study. The actual data processing results shows that the programs are in possession of great economic benefits and useful value. This study is one part of the National 863 Foundation project of'Surface Wave Extraction of Reflection Seismics and Shallow Structure Detecting Technique'. One production of the project is the software of'Reflection Seismics Multi Channel Analysis of Surface Waves and Forward Modeling and Inversion(SW863 V1.0)', where the forward modeling and inversion parts are the main work of this dissertation. At present, this software has obtain the software copyright registration of China(registration number:2008SR09155).
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