Study On 2-D Seismic Imaging By Wavefield Extrapolation Method

We can get correct subsurface image for strata-media using conventional NMO+Stack imaging method. But if there exists complex structure, such as dipping reflectors and faults, included in the subsurface, image that is imaged by conventional NMO+stack method would depart from true subsurface structure, and then post-stack migration is necessary to move dipping reflectors into their true subsurface positions. If geological structure is more complex, stacked section cannot be treated as zero-offset section, NMO+stack+post-stack-migration imaging method doesn't work, and then pre-stack migration is necessary.On the basis of one-way wave equation, wavefield extrapolation post- or pre-stack imaging method continues the surface wavefield downward to the subsurface, and then imaging condition is applied to image. Wave-equation-based imaging method is widely used in 2-D seismic imaging for its precision. According to the domain that algorithm implemented in, wave-equation-based imaging method could be classified into F-K method and finite difference method. The former has the advantage of lower computing cost, but cannot accommodate lateral velocity variation, the latter can accommodate arbitrary velocity variation, but only can image limited dip reflector and the computing cost is much higher.In this dissertation, phase-shift wavefiled extrapolation that can accommodate vertically varying velocity and finite-difference wavefield extrapolation that take into account large dip reflector was implemented. Integrated exploded reflector model, corresponding post-stack migration algorithm was implemented, and migration impulse response was presented. I introduced a trick originally presented by Claerbout to improve the precision of difference approximation, impulse response that adopts this trick proved the precision. Split-step Fourier wavefield extrapolation was implemented according to the Split-step Fourier migration method originally presented by Stoffa in 1990, and co-shot gather prestack migration algorithm was also implemented integrated cross-correlation imaging condition. Model tests indicated that, phase-shift migration method was ideal to the model that can be approximated by vertically varying velocity model for its high precision and low cost; finite difference migration could accommodate arbitrary velocity variation, but could Only image limited dip reflector and its computing cost is much higher, even the large angle equation has the risk of operator aliasing; 1/6 trick evidently improved the precision of finite difference approximation without any additional calculating cost; imaging result by pre-stack migration was evidently superior to that imaged by post-stack migration if geological structure and velocity model are both complex.