The Key Imaging Technology In Complex Piedmont Structure Exploration

There is a lot of piedmont foreland basin in the western part of China, such as Kuqa depression which locates along the north margin of Tarim basin. The low S/N ratio and the low accuracy of the imaging are caused by the rugged topography and the extremely complex deformation. The indeterminacy of structure, high positions and trap often occurs. The major reason which leads to drilling failure is that the structure is uncertainty.This paper analyzes and discusses all technical factors which influence seismic imaging accuracy and puts forward technical idea and method that must change traditional seismic acquisition, data processing ideas based on the prestack migration imaging. We also develop and promote the key acquisition technology which fits the prestack depth migration. In addition, we focus on integration and match of each key technology. Finally we put forward programme sutiable for complex structure exploration in foreland basin which is based on TTI anisotropy prestack depth migration technique applicable to rugged topography, and thus improve the seismic imaging accuracy and success rate of drilling.Under the support of national key scientific and technological project “large oil and gas fields and coal seam gas development”, the subproject “prestack depth migration integrated technology in piedmont complicated structure exploration” was carried out. Its content and key direction mainly focus not on prestack depth migration method itself, but on how to make processed data meet the requirements of prestack migration imaging algorithm better. The main contents, innovations, and new understandings are in the following four aspects:① According to the relationship between seismic observation system and prestack depth migration imaging, the general direction of observation system design and optimization are high density and wide azimuth.We point out that the evaluation of an observation system depends on wheather we can get the reflection of tager layer, meet the prestack migration of imaging algorithm, and suppress the noise from rugged topography and the process of prestack migration. We advance many quantization parameters, which are different from those in traditional prestack migration times, to evaluate the observation system, such as track density, CRP coverage density of target layer(illumination), simulation migration amplitude, dispersion and so on. It is clear that the key point of observed system design and optimization of prestack depth migration is that, on the basis of enough track density, the illuminance of target layer and space geometry relation between shotpoints and detecting points should be optimized. The purpose of observed system design and optimization is economy and effectivity which includes two aspects. On one hand, we hope to suppress the noise from nonuniform surface scattering、migration itself and to promote the quality and S/N ratio through high density, uniform collection which can obtain wavefield without alias. On the other hand, through the wide azimuth design we can not only realize the integrity and save cost, but also avoid azimuth anisotropy problem in the middle and shallow layers which is hardly sovled by the current imaging technology(HTI media imaging problem).②Through the comparison and analysis on different migration algorithms and imaging conditions, we concluded that only equivalent velocity model could be build which can hardly describe the details of velocity variation in complex high-steep structure of foreland basin under the current contidition of seismic imaging velocity inversion based on travel time. Therfore, seismic imaging in exploration should focus on prestack depth migration based on Kirchhoff integral method rather than the reverse-time migration which isn’t cost-effective. We could improve the imaging accuracy through reverse-time migration only with the highly development of exploration and sufficient accurate velocity model.③ Faced with the characteristics of seismic data from complex relief surface, we studied the data preprocess method and processing key point before migration which fit the prestack migration. We put forward key technology and application method of premigration process which is different from the traditional postprestack process, primary including: building and correction of migration datum 、 prestack noise suppressing、data regularization processing technology and so on.④ Under the foundation of much analysis of theory model and practice, we point out that the key technology and mission of complex structure prestack depth migration is building the migration velocity field. The key technology and important workflow includes the below four aspects: First, the primary arrival chromatography. Applying the surface velocity inversion and surface migration velocity modeling restricted by field near surface survey data,we solved the problem of seismic migration imaging brought by the variation of complex surface,which is different from the traditional way to solve the suface problem by statics correction. Second,the technology of multi-information primary migration velocity modeling in mid-deep strata.With the understanding of seismic and geology law, we constructed the mid-deep initial velocity model with multiple information from drilling, log, seismic and non-seismic joint inversion velocity,in order to make sure the basic law of velocity variation is correct.Third, multi-method velocity modeling and iteration optimization technique including migration and velocity analysis iteration and grid chromatography,which could improve the accuracy of velocity model to achieve the goal of migration and improving imaging equality.The last is TTI anisotropy migration parameter modeling. We could construct the anisotropy model to achieve the goal of TTI anisotropy prestack depth migration with drilling,log and migration iteration,only if the isotropy velocity modeling is confirmed.The application of prestack depth migration in complex hig-deep structure exploration of foreland basin is a very complex project. From the design of seismic acquisition observation system, field implementation to the series of data processing and imaging technology, we always need to develop, promote or even change the traditional imaging technology and appliation workflow based on poststack time migration. According to the research and discussion in this paper, we almost figured out the key problem of seismic exploration in current complex structure area through the developed technology, rebuilding the workflow, innovation and integration. Taking the high-precision pre-stack depth migration techniques as the core, the integrated technology for complex mountain seismic exploration based on rugged terrain was applied in oil and gas exploration in west china, remarkable effects had been gained.