Study On CRS Stack Of Complicated Surface And Application In Lu-Zong Basin

To obtain the Zero-offset(ZO)section is an important intermediate setp inseismic data processing. Common-midpoint(CMP)stack, Normal-moveout(NMO)correction stack, Dip-moveout(DMO)correction stack, as well as Pre-stackmigration are the main methods to obtain Zero-offset section in conventional seismicprocessing. These conventional methods require macro-velocity model ofsubsurface .and the image quality are depended on the accuracy of the velocity model.In the eighties of last century,new stacking techniques have been established whichyield better stacking results than the conventional methods mentioned above. Theyused several kinematic wavefield attributes instead of only one inNMO/DMO/stack.One of the most representative methods is Common ReflectSurface(CRS),which developed by Prof.Hubral of Karlsurhe university inGermany,has been widely accepted by geophysicists.Common Reflection Surface (CRS) stack is the best way at present to simulatezero-offset section.It can adapt to weak laterally inhomogeneous media. CommonReflection Surface is a circle segment around an underground reflector. Its traveltimeresponse in the time-space domain,this CRS stack surface,can be regarded as acombination of all Common Reflection Point (CRP) trajectories in the segment.Based on a general CRP trajectory expression,with two kinds of eigen wave-Normalwave and Normal incidence wave being introduced,the traveltime response of CRScan be given by extending CRP expression via considering that all rays are reflectedon the segment around the reflector according to paraxial approximation.The ray theory is a theoretical base of CRS stack. Based on the similarity of common reflection point(CRP)trace gathers in one coherent zone(Fresnel zone).CRS stack effectively improves signal/noise ratio than to use more CMP tracegathers to stack. Compared with conventional CMP stack and DMO stack,CRS stackcan focus more energy in the vicinity of the reflector. It not only improves the qualityof simulated zero-offset section and S/N ratio in deeper layers but also providesimportant seismic three-parameters section which can be used for inversion of amacro-velocity model and for depth image. It is regarded as an important method ofseismic data processing.At first, based on wave equation, high-frequency ray solution and its characterare given to clarify theoretical foundation of the method. The hyperbolic andparabolic travel time of the reflection in layer media are presented in expression ofmatrix with paraxial ray theory. With geometrical optics, the relationship betweenobject point in model and image point in image space is built for the complexsubsurface. The travel time formula of reflective point in the monunifonn media isdeduced. Also the formula of reflective segment of zero-offset and nonzero offsetsection is provided.The notable advantage of CRS stack is that it can calculate the stack area usingProjected Fresnel zone,and select valid seismic data to stack.This process of selectionis like usual DMO methed,and it is the key of CRS stack.All of methods that calculatethe stack area are approximative and their results is different. After a detailedintroduction about the theoretical foundation of CRS stack in the beginning of thisthesis,a detailed description of the selection of CRS stack area will be presented.For 2-D seismic data, the CRS stack can generate a stacking surface whichdepends on three search parameters. The optimum stacking surface needs to bedetermined for each point of the simulated zero-offset section. For a given primaryreflection, these are the emergence angle"of the Zero-offset ray, as well as tworadii of wavefront curvatures RN and RNTp .They all are associated with twohypothetical wavesahe so-called normal wave and the normal-incidence-point wave.We also addressthe problem of determining an optimal parameter triplet(a, RNIP, RN)inorder to construct the sample value(i.e.,the CRS stack value)for each pointin the desired simulated zero-offset section.This optimal triplet is expecteddetermine for each point the best stacking surface that can be fitted tomulticoverage primary reflectionevents.This paper was useful to test the performance of the extended CRS stack methodand to prove its validity, since the proper results are easy to estimate.