Study On Joint Velocity Modeling Of Near-surface And Deep Layers Under Complex Topography

Large amount of oil and gas energy is highly demanded because of the rapid economic development in our country. As a result, it is of great significance to explore those areas with both complex toporgraphy and underground geological structures. Poor data quality after migration, however, cannot satisfy the demand of exploration when conventional processing workflow is applied on acquired materials in these areas. So migration methods must be studied to obtain better imaging results. And the building of initial velocity models is one of the most important parts of migration from complex topography. There are two raw models for the building of initial velocity model. One of them is near-surface velocity structures obtained by tomography and the other one is deep layer model by velocity analysis. Since different seismic wave information is used to establish these two raw models, in a certain range of depth, the velocity information both the two models provided will be overlapped and differently changed. And it is also possible that neither of the two models could provide us with velocity information during a certain range of depth. Consequently, how to integrate these two into an entire one model by joint modeling technique of near-surface and deep layer velocity structures must be paid highly attention.As for integrating velocity information in the overlapped area, this thesis, preceeded from velocity trend varied with depth and the relationship between the two velocity models, discussed the selection of datum, initial and terminal surface for velocity combination. And the weighted value and the practical workflow for velocity combination are also elaborated in Chapter two. Then based on proposed criteria to judge the quality of the entire velocity after combination, the method for raw models modification is continuelly advised and improved to enhance the processing effect. And also the combination methods mentioned were applied on theoretical models and practical data to analyze the difference among the updated results.As for the velocity reconstruction in areas where no velocity information is provided by these two raw models, two kinds of methods, one is based on inverse distance weighting and the other one on velocity field continuation, are discussed and tested on models with gap areas. According to the two models having different boundaries(horizontally and undulantly, respectively), this thesis analyzed and compared these two methods by discussing difference values, similarity coefficient and the influence of gap area interfaces on the reconstructed velocity field. Finally, advantages, disadvantages and feasibility of these two methods are compared and discussed in detail.This thesis in Chapter four, based on imaging smoothing techniques, applied Gaussian smoothing and median value filtering methods on updated velocity models after combination or reconstruction. Firstly, velocity model extension was explained before smoothing and then the influence of some related parameters on smooth effect and the change of velocity models after smoothing were also analyzed and discussed. These parameters including Gaussian factor, the size of Gaussian model, and also the type and size of median filter model. Besides, the efficiency of both Gaussian and median filtering methods were also analyzed, especially from the aspect of sort algorithm in conventional median filtering method.By the study of velocity combination and reconstruction methods to intergrate the two raw models, this thesis elaborated the workflow of joint modeling technique of Near-surface and Deep layer velocity structures. And with the application on practical data, the methods proposed in this thesis could establish a good initial velocity model for the next processing work, such as pre-stack migration velocity analysis and pre-stack depth migration. This is a good endenvour for ensuring the good quality of the processing result by pre-stack depth migration in areas with rugged topography and complex underground geological structures.