Method Research On Two-way Wave-equation Forward Modeling And Illumination Analysis

In the process of petroleum seismic exploration, wave-equation forward modeling and illumination analysis both have great significance to help people understand and analyze the seismic wave propagation law, provide guidance for seismic data acquisition and seismic survey design, improve seismic data processing quality, reduce the uncertainty of subsurface velocity modeling and determine interpretation scheme of oil and gas reservoir.Based on previous researches and following the order from simple to complex, I conduct a further study on the seismic wave-equation forward modeling schemes and illumination analysis methods based on the one-way acoustic wave equation, two-way acoustic wave equation and two-way elastic wave equation, and then program to realize them respectively. The computation results of different velocity models indicate that the methods are correct and satisfied.At first, I make a research on the one-way acoustic wave equation seismic numerical simulation and illumination analysis. The one-way acoustic wave equation modeling is an simulation method which extrapolate wave field in the frequency wave-number domain and correct the perturbation due to transversely variable velocity in the frequency-space domain. By using SSF operator and FFD operator, Zero-offset synthetic records are gained. On the basis of positioning principle, I develop a faster method to get nonzero- offset common shot records that receive the extrapolated wave field on the surface of the model directly at each geophone, without extra lateral stacking. According to this kind of method, P-P common shot records and P-SV common shot records are simulated effectively. Based on one-way seismic simulation, seismic illumination analysis is conducted through which the results of Zero-offset seismic illumination, source illumination, source-receiver illumination and plane wave source illumination are achieved. The conclusions are that zero-offset seismic illumination is an ideal vertical incidence illumination, the results of source illumination and two-side receiving illumination are similar,while the one-side receiving case is different from them,in the case of small incident angles, SSF operator and FFD operator have no major difference, while not the case when the incident angle is greater than 40 degrees.The same as one-way wave simulation, the one-way wave seismic illumination has high efficiency.Then I make a research on the seismic numerical simulation and illumi- nation analysis based on two-way acoustic wave equation and two-way elastic wave equation. The numerical simulation methods of these two kinds of wave equations are similar. Adopting their own one order velocity-stress wave equation, I deduce their staggered-grid finite-difference algorithm in detail and program them correctly.Considering the intrinsic problems of finite-difference technique such as numerical dispersion, stability problem and boundary reflection, I give out the grid-dispersion relations for the P and S waves, stability condition and the discrete formula of perfectly matched layers of arbitrary even order accuracy, analyze detailedly the influence of different stability parameter, different passion ratio and different spatial sampling ratio on the grid-dispersion.According to the theoretical analysis, grid discrete slows the wave propagation, and along the axis, the numerical dispersion is most severe and decreases as the incident angle increasing,in the diagonal direction the dispertion is lowest.The smaller is the stability parameter,the more severe is the dispertion.Then I adopt the perfectly matched absorbing layer to reduce the boundary reflection.Through a large amount of experiments,I find that half of the p-wave length of the PML layers can absorb the boundary reflection effectively,and then,I do comparisons of two different boundary conditions and two kind of different modeling accuracy. After numerical simulation, source illumination is computed.Based on two kinds of lens with high velocity and low velocity and acoustic wave equation, source illumination is computed, which reveals the defocusing effect of high velocity and focusing effect of low velocity. The multiple sources illumination of the model is even. Adopting poynting's theorem,I compute the elastic illumination vector in the first time.Two-way elastic equation can describe wave propagation and seismic illumination more accourately. The illumation changes obviously as the source position.So in order to acquisite high quality seismic data, we should change the position with even and high illumation to excite and receive seismic wave.According to the above research, conclusions are as follows: the one-way wave equation can provide synthetic records with high SNR, including only reflected waves or diffracted waves, which is useful for wave field recognition and analyzation. The computation efficiency is also very high, while the coefficient of reflection is approximate. Two-way acoustic wave equation can deal with the coefficient of reflection correctly and provide accurate wave field. The two-way elastic wave equation gives the most realistic results and can model many kinds of waves in the cost of computation. Thus, we can choose the proper type of wave equation to use in the practical work.