【作者】 雍运动；

【导师】 王德利；

【作者基本信息】 吉林大学 ， 地球探测与信息技术， 2007， 硕士

【摘要】 在精细的地震勘探工作中,特别是在实际储层研究工作中,所涉及的介质往往是局部非均匀的。在这些局部非均匀介质中,小的异常存在而且分布极不规则,在油气预测、储层研究中需要研究储层的精细结构,这些小尺度的异常会明显地影响地震波的传播,需要加以考虑。另外,地震波在地下介质中传播途经小尺度非均匀体时发生散射,从而对地震记录产生影响,再使用常规地震的理论和方法,效果必然会不好。实际岩层对在其中传播的地震波有吸收作用,使其能量发生损耗,实际的地球介质性质更接近于粘弹性体,单纯应用理想弹性介质模型时不能解释许多实际问题。为了解决这些问题,应从地下介质的描述和相应的波场理论等基础工作开始进行研究。建立三维粘弹随机介质模型并对其波场模拟是十分必要的,然而三维正演模拟需要较大的内存和计算速度,该问题可通过并行计算可解决。通过网络把PC机连接成为一个PC cluster,将计算所需内存分配到PC cluster中的每个PC机(节点)上,并使多个CPU同时并行运算,从而使可计算规模大大增加,计算时间显著缩短。通过建立和正演模拟三维粘弹随机层状含溶洞介质模型,能较好地模拟复杂非均匀随机介质中的地震响应,有助于对溶洞裂隙型油气藏地震波场的了解和指导其实际数据的解释。更多还原

【Abstract】 Traditional seismic prospecting is based on theory of homogeneous medium or layered homogeneous medium, but in accurate seismic exploration, especially in reservoir research, medium which related usually is local heterogeneous. This local heterogeneous medium contains some distributed irregularly small-scaled anomaly geologic bodies, which will effect seismic propagation obviously, so this effect must be considered in oil gas pre-estimate and reservoir research which need study the exact structure of reservoir. In addition, seismic waves will be scattered by way of small-scaled anomaly geologic bodies, and scattering will attenuate direct wave, which caused seismograms be influenced, so conventional seismic prospecting theory and method will not get a good result. In order to solve this problem, we need to do some fundamental research such as description of underground medium and propagation of seismic wave.Because there are more scattered elements in 3-D random medium, seismic wave scatting and attenuation in 3-D and 2-D random medium make a great difference, so it is necessary to construct and forward model 3-D random medium. 3-D random medium forward modeling needs more memory and higher computing speed, single PC or workstation can’t meet the requirement, but this problem can be solved by connecting PCs into a cluster and dividing memory requirement into every PC, and let them parallel compute. Using this method, we can forward modeling larger medium model and save computing time.By constructing and forward modeling 3-D viscoelastic random medium model, seismic response in inhomogeneous complex medium will be better modeled, this will be useful in understanding the seismic wave field of karst cave reservoir and helpful in guiding data processing and interpretation.Main research contents and conclusions in the article are following.First, complex heterogeneous medium was described using statistical method. Many kinds of two-dimensional (2-D) random medium model and 3-D random-homogeneous included solution cavity medium model were constructed. There independent autocorrelation lengths (a, b and c) in random medium described the mean scale of heterogeneous anomaly in x, y and z directions respectively. The corresponding relationship between autocorrelation lengths and the scale of geologic bodies was studied by velocity variation and variance.Second, influence of viscoelasticity in forward modeling was reflected by using GSLS (generalized standard linear solid), GSLS shows no dispersivity in both high frequency and low frequency, so a GSLS can be used to model Q of every frequency. .Based on this,3-D viscoelastic wave equation and its discretization by finite-difference method using staggered grid were introduced. The explicit finite difference equations can be derive by using a second-order centred difference scheme to approximate the time derivatives, and a fourth-order staggered scheme with centred differences to approximate the spatial derivatives.Third, a PC 1inux cluster based on MPI was constructed. Based this platform, 3-D random medium model can be divvied into sub volumes ,the number of subvolumes equals to pc numbers, each PC just compute wave field of subvolumes owned to itself. For the communication at the internal edges a two grid thick padding layer has to be introduced, data exchange at the internal edges has to be performed after each time step. After wave field updating, each nodes submitted results to master, master received the data and terminated the program. Using parallel algorithm forward modeling larger scale 3-D random medium model, the problem of high running speed and large memory, which were required in forward modeling 3-D medium, can be easily solved, meanwhile dispersed computing resource will be well used and wealth can be saved.Fourth, 3-D single layered random medium model and random- homogeneous multi-layered medium model were forward modeled. For3-D single layered random medium model, the results showed that scattering wave produced by random perturbations was obviously displayed in seismograms, with autocorrelation lengths becoming larger, scattering event in section became longer, and with dominant frequency of source becoming higher events in section became fine and short. For 3-D random-homogeneous multi-layered medium model, scattering wave produced by random perturbations was obviously displayed in manner of wide and short events in seismograms in middle random medium layer as in single layered random medium, scattering wave appeared under the second reflecting event which was caused by upper layer random perturbations. With dominant frequency of source becoming higher, backward scatting becomes weak and forward scatting becomes strong, forward scatting event also becomes coherent. With autocorrelation lengths becoming larger, backward scatting becomes weak but forward scatting also strong, which influences reflecting event of boundary greatly.Fifthly, 3-D random-homogeneous included solution cavity medium mode was forward modeled. It is obviously to see many scatting wave around cavities in wave field snaps, the wavelength of scatting wave and source wavelet were different. Scatting waves stacked and interfered each other, and formed elementary wave front, perturbations observed from new wave front was the results of each point in elementary wave front stacked and interfered in observing point. Seismic wave field of karst cave reservoir was composed of differences scatting waves, which produced by variety inhomogeneous distributed low speed bodies. So, processing method researcher based on scatting theory should be intensified in order to solve the problem of karst cave reservoir.更多还原