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Research Of Multi-components Seismic Wave Reverse-time Depth Migration

Posted on:2016-05-01Degree:DoctorType:Dissertation
Country:ChinaCandidate:J J YangFull Text:PDF
GTID:1220330473956354Subject:Marine geophysics
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Even through the scalar wave theory-based acoustic prestack reverse-time depth migration played an important role in the conventional energy development process, but imaging of complex reservoirs is difficult to obtain satisfactory results because of defects such as theoretical limitations and unitary wavefield information; and also due to the lack of fidelity amplitude, it is very difficult to apply to the later prestack migration work. In recent years, the vectorial wave theory-based elastic wave prestack reverse-time depth migration attracted widespread attention to the industry. Compared to the acoustic reverse-time migration, elastic wave prestack reverse-time depth migration can simultaneously process multi-component seismic records, so that the results obtained are more similar to the actual situation. Meanwhile, with multi-component simultaneous tomography, the ability to solve practical problems is more powerful, as well as high precision seismic exploration for some special target. This paper studies the basic technique of elastic reverse-time migration firstly, and studies for its three key technologies primarily. Including:(1) The extraction of angle domain common image gathers of elastic reverse-time migration. (2) The study of correction of P-wave and S-wave’s wavelet stretch of elastic reverse-time migration. (3) The study of correction of P-wave and S-wave’s amplitudes after reverse-time migration.First, from the first-order velocity-stress elastic wave equation, high-order staggered-grid finite difference scheme for forward wavefields extrapolation and reverse-time extrapolation have been derived, and the technique of using mixed absorbing boundary condition to suppressing’ boundary reflection has been studied. Decoupling of longitudinal wave and shear wave during the forward extrapolation of source wavefields and reverse-time extrapolation of receiver wavefileds, and achieve three-component S-vector standardization, then independent imaging for P-wave and S-wave components by choosing the right imaging conditions. Then, the technique of using Laplace filter method to suppress the section scattered noise of reverse-time migration has been studied, and the imaging precision has been improved. Moreover, aimed at the problems of huge storage and the need of I/O in conventional reverse-time migration, the method of using effective boundary storage strategy to reconstruction source wavefields has been developed. Reverse-time migration storage and I/O time both greatly reduced.The angle domain common imaging gathers (ADCIGs) which transformed by shot domain common-offset gathers can provide input data for prestack inversion. Because of ADCIGs is the theoretically non-illusion restock gathers, so it is widely recognized as the most accurate prestack gathers. In this paper, the extraction method based on elastic wave ADCIGs of reverse-time migration has been studied. First, provide the geometric relationship for the source P-wave field incidence angle on the basis of the difference of the source P-wave field propagation angle and the reflector normal direction. Second, obtain the propagation angle of the source P-wave field by calculating the decoupled down-going P-wave’s amplitude gradient or phase gradient at the maximum amplitude time of the down-going wavefields. Third, obtained reflector normal direction by using complex wave number of migrated stack section, thus, each grid point’s incidence angle of each shot can be calculated. At last, the ADCIGs of P-wave and S-wave can be obtained by rearrange the common shot migration gathers by incident angle. In the paper, the usage of horizontally stratified medium model, the tilted stratified medium model, part of Marmousi-II elastic model and measured data of two areas have proved the effectiveness of the algorithm.Wavelets stretch effects of reverse-time migration is the wavelets’length and frequency in vertical (depth) range of migrated P-wave and S-wave vary with the velocity of seismic wave, stratigraphic dip and reflection angle. This paper mainly studies the wavelets stretch effects of P-wave and S-wave and calibration method in the elastic reverse-time migration theory. By extracting each image point’s source wavefields and receiver wavefields’ seismic propagation wavelets in time domain which is central of it’s image time during the elastic wave extrapolation, without change the amplitude and phase of imaging result, compress the wavelets for P-wave and S-wave after the wavefields extrapolation and before application of imaging conditions, vertical resolution and amplitude fidelity of the reverse-time migrated P-wave and S-wave increased.The purpose of the amplitude correction of the elastic reverse-time migration is to output the results which Zoeppritz equation assumed, so that it can be used in subsequent prestack inversion work. This paper analyzes the energy losses during the elastic wave reverse-time migration first. Under the assumption of perfectly elastic medium, reverse-time migration generates geometry diffusion effects and transmission energy losses. When using full two-way wave equation to extrapolate the source wavefields and receiver wavefields with sufficiently wide recording aperture and using deconvolution-type imaging condition, geometrical spreading effect and energy compensation are automatically included in reverse-time migration algorithm. Transmission loss compensation is based on ray tracing to get the imaging point propagation path, and determine the position and the incident angle which the transmission loss is generated, then calculate the transmission coefficient combined with the background velocity, amplitude correction on the imaging results directly. Model trail compute shows the effectiveness of the algorithm.
Keywords/Search Tags:reverse-time migration, vector wavefield, angle domain common imaging gathers, migration stretch correction, preserved-amplitude migration
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