The Study On Delay-shot Plane-wave Prestack Depth Migration

Until recently, Prestack depth-migration based on the extrapolation of wavefields has been used for complex geological body imaging as the most effective method. However, it has to suffer from the large amount of computational cost. Conventional shot profile wave-equation prestack depth migration is usually implemented by recursively source and receiver wavefields independently followed by applying an appropriate imaging condition. The final image is constructed by stacking the images from each individual shot profile migration. Due to the limited aperture of a single shot migration,typically a large number of migrations are required to produce a good image, which raises the computation cost of shot profile migration so intensively. Additional, conventional wave-equation prestack depth migration not only lacks the direction that owns by ray method in wave field extrapolation process, but also do not have goal-oriented characteristics that owns by Kirchhoff pre-stack depth migration method. Plane-wave migration is a different migration method from conventional method which is proposed aiming at improving the computational efficiency and overcomeing the probloms in conventional method.Based on the several decades development of conventional shot profile imaging principle and imaging conditions, this article systematically studied the potential and problems of slant-stacking plane-wave decomposition and delay shot plane-wave migration. This article explors the advantages of plane-wave migration and provide solutions for some problems in the implement process, which has important theoretical and practical significance to the development of this method.Using symmetric nonstationary phase-shift method to extrapolation of wavefields, this paper implement the delay-shot plane-wave migration.1. This article introduces the concept, the physical meaning and the implement processing of slant-stacking. Wave-field plane-wave decomposition can be completed by linear moveout correction and stacking through out the offset axis. The processing of producing slant stack gathers by slant-stacking individual p is called linear Radon transform in mathematics. As the linear stacking of a number of common shot gathers, the plane wave source records of one incident angle is a common p profile, this is equivalent to the field records of delay shot. Therefore, a common p profile can also be called as a delay shot record, which covers the entire imaging area.This article analyzed the end-point effect caused by the cut-off of gather border, which produced pseudo-energy in slant stack processing. In this paper, three possible methods were purposed to reduce this end-point effect. The scissors-like pseudo-energy caused by this end-point effect is widespread in Radon transform, it would be great significance to Radon transform and plane-wave migration if we can avoided or reduced it.There are moveouts in the shot records caused by plane wave incident at the same point of the different angles. This article proved that for a invent generated by the underground interface, zero offset receive time is the biggest in plane wave records caused by different angles incident wave. The invent produced by an underground interface was similar to elliptical invent in the slant-stack gather, opening up.In the slant-stack gather, This zero offset receive time corresponds to the bottom of the invent, which shows the dip of the underground interface that produced this invent. According to this characteristic, this paper presents a new method that determines the ray parameter p corresponding to the target interface dip. Combine with plane-wave migration, a method used to strengthen aim interface imaging energy is proposed. This method can be carried out simply, which has a greater advantage of conventional shot profile.We need to re-setting the absolute offset on the raw data before slant stack transform.( different from the conventional meaning of offset). In this paper, delay shot plane wave decomposition was carried out separately according to plus or minus p ,which means the first shot point on the left was set to be base point for absolute offset in the range of minus p ,while in the range of plus p the first one on the right was the base point. It would be very helpful to the implement of plane wave migration and introduces no extra computation. Then we process the slant stack transform on the data, and sort the result to common p profiles, complete the conversion from point-source to plane-wave records. This paper also proposed another two plane-wave decomposition style besides delay shot plane-wave decomposition:a. Local plane wave decomposition, whose physical meaning is that line source shot at the same time incident along the angle corresponding to p .2.Slant stack plane wave decomposition on CMP data. Synthetic data is used to demonstrate the differences of the three styles.2. The phase shift plus interpolation (PSPI) method and the recently introduced nonstationary phase-shift (NSPS) method are combined into a single symmetric operator (SNPS) with improved accuracy and stability and with similar computational effort. The three operator are all x ? k double domain operator. Unlike the PSPI and NSPS, the SNPS operator is symmetric, by other words ,it is equal to its transpose. Migrations of synthetic data by the three operator are provided in the paper, which showed that the SNPS migration image is better than the other ones.3. There are two stages to migrate one shot gather using a typical shot-profile migration algorithm. First ,source and receiver wavefields are extrapolated from the surface to all depths in the subsurface.Second, the image is constructed by proper image condition(for example: by crosscorrelating the source and receiver wavefields). As with shot-profile migration,there are two steps to migrate a plane-wave source gather by typical plane-wave migrations.First,the source wavefield S p and receiver wavefield R pare extrapolated independently to all depths in the subsurface using one-way wave equations,respectively. Second,the image of a plane-wave source with a ray parameterPx sis constructed by crosscorrelation the source and receiver wavefields weighted with the circular frequencyω.A single common p profile produce a single image associating with the ray parameterPx s. The final image is constructed by stacking a range of common p profile migration.Current acquisition geometries do not honor the optimal applications of plane wave migration. This paper discusses the impact of one-side acquisition pattern and show that optimal application of the plane wave migration algorithm requires split spread common receiver gathers. Based on the reciprocity principle, this paper advances two methods to convert one-side acquisition data to split spread acquisition data, which have no extra computational cost in the process of migration and not only have deep significance to plane wave migration but also to conventional shot profile migration. Using the synthetic 2D Marmousi data, this paper shows the plane wave migration image equality of one-side common receiver, one-side common shot and split spread from one-side data common shot gathers, which demonstrate that the image from split spread data migration is better than image from one-sice data.Prestack Kirchhoff depth migration, conventional shot profile prestack depth migration and delay shot plane wave migration are compared in this pape.The result shows that migration based on the wave equation migration methods have the better image than kirchhoff migration method wich is based on the ray theory in the case of complex media. Compared to conditional shot profile migration methods, the biggest advantage of plane wave migration is increasing the calculate efficiency. Plane wave migration can produce equale or high quality images that are comparable to those of shot migration with much less computation cost. Plane wave migration is able to enhance the image on the target interface energy. It do not has the restriction from aperture, and has potential advantages over traditional shot profile method when dealing with steep geological body imaging. Plane-wave migration transforms the data from offset domain to ray parameter P domain.The migration would not be limitied by irregular field observation system.