Exploration And Imaging Of 3D Subsurface Velocity Structure By Microtremors

Site condition governed distribution of strong ground motion and earthquake damage significantly. The studies on this issue have been important in the areas of engineering seismology and seismology for decades. Many problems on ground motion are associated with in the lack of site condition information of the observation stations. To detect 3D subsurface Vs structure of engineering site by observing microtremors, must be a powerful tool for development in this subject of Earth Science. In this thesis, the exploration and imaging of the 3D subsurface Vs structure by means of microtremor approaches are studied. The main achievements are summarized as follow.1. An immune-heuristic genetic inverse strategy for deducing Vs structure from the dispersion curves of Rayleigh wave is developed, which combines float coding with roulette selection, arithmetical uniform crossover and mutation into a basic genetic inversion frame, adds the immune heuristic method as local maximum searching scheme and adopts no-repeating forward calculation as speedup scheme, and determines the final structure from the best one of the local optimum by conducting Simulation Annealing algorithm to the result of each individual run. Various combinations of genetic operator and codings are examined by the virtual inversion, as the result it is pointed out that the offline capability is best in dispersion curves inversion by combining float coding with roulette selection. It's suggested to integrate the above two with arithmetical uniform crossover and mutation into a basic genetic inversion frame.In this inversion strategy, the immune heuristic method and the no-repeating forward calculation method are added to the above basic frame. The immune heuristic method speeds up the evolution by adopting the best individual's information, enhances the local search in its neighboring space by adding one random number with normal distribution, while giving attention to the search in other areas else by rectifying the distribution deviation. The local search and global search are linked effectively, the disturbance to evolution of the genetic algorithm is minimized. The times of forward caculation are reduced greatly, the calculating cost is saved and the evolution efficiency is increased, from no-repeating forward calculation.According to simulation results for complex soil layered condition, the deficiency in assumption of equal small thickness layers in inversion is pointed out, and the scheme to assume the number of layers and take the Vs and thickness of each layer as parameters to be inversed is proposed. The final result is determined from the best of the local optimum by conducting Simulation Annealing algorithm to the result of each individual run, is proposed. The shortage of averaging the results of all individual runs is avoided. The stability and precision of inversion is increased and the non-uniqueness is reduced.2. An exploration approach for regional Vs structure is developed, by jointly inversion of multiple sites and of microtremors array method with H/V method. A pseudo-Parallel Genetic inversion strategy is designed, in which sub-populations with different running parameters are evolved simultaneously and results in evolution are exchanged timely. A jointly microtremors exploration approach for regional Vs structure is developed. The joint inversion includes the joint of multiple sites and the joint of the microtremors array method and H/V method. The Vs of each soil layer keeps the same at the all sites, to consider the commonness of the soil structure and the depositing connection in the region, and the influence of the errors in observation and in dispersion curves extraction and inversion in individual site can be trailed off. The Vs of each layer in the region and the thicknesses of soil layers in each site are inversed jointly by combination of the microtremor array method and H/V method. By this way, the excellent correlation between the fundamental frequency of microtremors H/V and the thickness of sediment is fully exerted to make up the shortage in array method which is not sensitive with the depth of the buried rock surface, and the speciality of Vs structure at each site is well described. In the inversion procedure, H/V fundamental frequency, which is commonly recognized as an index of soil depth, rather than the ellipticity of Rayleigh wave which remains in dispute, is adopted as the target reasonably.A pseudo-Parallel Genetic algorithm is designed, against the number of parameters to be determined increasing greatly and the inversion efficiency reducing and the possibility of non-unique increasing, in the inversion. In the pseudo-Parallel Genetic algorithm, sub-populations are evolved simultaneously with different running parameters to emphasizing respectively on'exploration'or'exploitation'. The diversity of the population is holding while the premature is avoided. The inversion efficiency is improved, by exchanging results in evolution timely.3. A two–step approach for 3D Vs structure imaging from sparsely and non-uniformly distributed data by means of integrating the interpolation method of biharmonic Green function as spline in tension with Kriging method, is established. 3D subsurface Vs structure in Tangshan and surrounding region is imaged by microtremors survey data.A two-step space prediction approach is developed in the dissertation. Firstly the average Vs of each layer are fixed, the interfaces between each two soils layers are predicted by the interpolation method of biharmonic Green function as spline in tension, to keep soil interfaces varying smoothly in space. Furthermore, a method to determine tension coefficient is proposed based on microtremor H/V method, which makes most use of H/V information and avoids the determination of the tension coefficient at will.Secondly, the variance essence of the Vs structure as a regional variable is deeply excavated by Kriging method and its structural characteristic is taken into account. The spatial successive distribution of the Vs structure between soil interfaces is simulated by high precision space extrapolation and prediction, and finally the detailed 2D or 3D model is constructed.The two-step space prediction approach describes the characteristic of Vs structure as a regional variable from the sparsely and non-uniformly distributed microtremors data. Obviously, it also can easily make good use of the other exploration method.4. The microtremor method proposed in this dissertation is validated by comparing with the international advanced microtremor methods and SASW methods via ESG2006 Noise Blind Test and results of microtremors observations at 26 sites in Tangshan region. Some understandings and conclusions crucial for guiding microtremors array observation are also drawn.As an exploration method developed in practice, it is necessary to be validated in test. The method presented in the paper is tested with results from the international advanced approach of SASW at 26 sites in Tangshan region and ESG2006 Noise Blind Test. The results show that the method obtains reliable results either from artificial records or in-situ observation records in acceleration or velocity . Either of them can be chosen in observation contingent on the capability of instruments. Although high mode of dispersion curves are observed in 2 cases in artificial records analysis, but in microtremors records observed in-situ it is not notable. Based on analysis of the former researchers'conclusion and the above results, the reliable wave length range of SPAC is proposed as Rm in <λ< 4Rmaxto instruct microtremor array design. The Vs structure inverted may reach deeper with the same precision as the international advanced approach of SASW, well applied in geotechnical exploration. In conclusions, an integrated microtremors method to explore and image 3D subsurface Vs structure is developed in this dissertation. The dispersion curve extraction, inversion strategy and 3D structure imaging are studied separately, then integrated and improved.