The Common-Reflection-surface Stack Theory And Application In The Biyang Sag

Common-Reflection-Surface (CRS) stack is the best way at present to simulate zero-offset section. Common Reflection Surface is a circle segment around an underground reflector. Its travel time response in the time domain, which is CRS stack surface, can be regraded as a combination of all Common Reflection Point (CRP) trajectores in the segment. CRS stack can focus more energy in the vicinity of the reflector, therefore can obtain a better zero offset section by stacking events with same phase.The notable advantage of CRS stack is that it can calcalate the stack area using Projected Fresnel zone, and select valid seismic data to stack. This process of selection is like usual DMO method, and it is the key of CRS stack. All of methods that calculate the stack area are approximative and their results is different. After a detailed introduction about the theoretical foundation of CRS stack in the beginning of this thesis, a detailed description of the selection of CRS stack area will be presented.At first, based on wave equation, high-frequency ray solution and its character are given to clarify theoretical foundation of the method. The hyperbolic and parabolic travel time of the reflection in layer media are presented in expression of matrix with paraxial ray theory. Because the reflectiive wave field mainly comes from the Fresnel zone, thereby the conception of Fresnel zone is explained. The matrix expression of Fresnel zone and projected Fresnel zone are given in sequence. With geometrical optics, the relationship between object point in model and image point in image space is built for the complex subsurface. The travel time formula of reflective point in the monuniform media is deduced. Also the formula of reflective segment of zero-offset and nonzero offset section is provided.The CRS stacking surface depends on three search parameters. These are the emergence angle α of ZO ray, as well as two wavefront curvatures R_N and R_NIP These attributes are associated with the normal wave and the normal-incidence-point wave, and it can confirm the position and shape of reflective interface. This enabled me to compute the associated projected Fresnel zones and expressed their equations in terms of CRS stack attributes. But the projected Fresnel zone only can be estimated quite well with the hyperbolic ZO traveltime approximation for small offsets and a simple subsurface. Thus, I would propose three changes of projected Fresnel zone using the CRS stack traveltime: First, use the NMO-velocity instead of the velocity of the first layer. Second, assume flat horizons, R_N=∞. Third, divide the resulting projected Fresnel zone by cosα . The...