复杂条件下速度分析与叠前成像技术研究
Research for velocity analysis and amplitude-preserved imaging under complex conditions
  • 基本信息
  • 报告类型:  最终报告 编制日期:  2021-03-08 公开范围:  公开 全文页数:  15
    中文关键词:  复杂构造;速度分析;叠前偏移;保幅成像
    英文关键词:  complex stucture;velocity analysis;prestack migration;amplitude-preserved imaging
  • 中文摘要

叠前数据的信噪比对叠前成像道集的提取及成像影响很大,若数据信噪比太低很难得到同相轴清晰的成像道集与成像剖面,这会给速度分析带来困难。因而提高叠前实际数据的信噪比是叠前处理的重要环节。偏移可以看成是绕射波收敛,倾斜同相轴向上倾方向移动,消除地震波在地下介质中的传播效应的一种成像过程。而反偏移则相当于地震数据建模,它又重新恢复了地震波在地下介质中的传播效应。在反偏移过程中,由于可以任意选取建模参数,所以我们可以将偏移后的结果反偏移到任意参数介质下,得到任意参数介质下的满足不同偏移算法的地震数据。 基于共反射面(CRS)叠加方法是另一项提高资料信噪比的关键技术。在形成的共中心点(CMP)道集中对零偏移距剖面中的每个样点通过相关分析和优化算法确定参数,得到能够最大程度聚焦该反射界面局部形态的走时关系,然后在相应叠前空间中沿走时关系进行叠加,从而实现该反射点的最佳成像,实现叠前数据规则化并提高信噪比。 叠前深度偏移是目前高精度地震数据处理的重要环节,保幅偏移不仅可以得到地下成像结果还可以对地下的岩性信息进行准确描述。反射地震信号的振幅是多种因素的复合效应,包括采集因素和传播、散射因素等。在波场反传播过程中基本完成几何扩散校正等主要振幅处理,成像振幅基本与地下反射系数相等或成正比关系的偏移方法称为真振幅偏移。对于真振幅偏移而言,波场传播算子、波场透射效应、成像条件是必须要考虑的关键问题。 常规处理流程的理论基础是假设地下介质以碎屑岩水平层状介质。在此假设条件下CMP道集对应的时距曲线表现为双曲型。此时由常规的速度分析可以准确地确定叠加速度(近似等于均方根速度-RMS速度),然后由Dix公式,可以把RMS速度转化为层速度。对于常规速度分析(即基于双曲线型时距曲线的速度分析)而言,主要的影响因素是,速度垂向梯度和强横向变速的存在,这两种因素与倾斜反射层是连在一起的。而倾斜反射层又会引起反射点弥散现象。常规速度分析方法无法满足叠前偏移的要求。 常规速度分析或者一种方法的速度分析方法很难取得理想的效果,为此考虑多道集相结合的速度分析方法。多道集速度分析方法以初始共散射点(CSP)道集速度分析结果为基础,通过基于角度域共成像点道集(ADCIGs)层析速度分析来更新速度场,最终建立满足精度保幅叠前深度偏移要求的叠前偏移速度场。

  • 英文摘要

Extraction of prestack imaging gathers and imaging are seriously influenced by the signal to noise ratio of seismic prestack data. If the data’s S/N is too low, it’s hard to obtain satisfying imaging gathers and imaging section composed of clear events, which may do harm to velocity analysis. As a result, improving the S/N of prestack data is very important for prestack processing. Migration can be regarded as a process of imaging, including the convergence of diffracted wave, upward movement of sloping events, elimination the effect of wave propagation in subsurface media. While demigation amounts to modeling of seismic data, it restores the propagation effect of seismic wave in underground media. In the process of demigation, parameters of modeling can be arbitrarily chosen, thus we can do demigation for the migrated data to arbitrary medium parameters, acquiring seismic data when different migration algorithms are used. Stack based on CRS is another crucial technique of improving seismic data S/N. Through correlation analysis and optimization algorithm for every sample in zero-offset section within CMP gathers, we determine the parameters, get the travel-time relationship that focusing local shape of reflectors to the greatest extent. Then stack the data along travel-tine curve in the corresponding prestack space. Thus, optimal imaging of reflection point and data regularization of prestack data are accomplished and the SNR is improved. Prestack depth migration is an important method for high-accuracy seismic data processing. Amplitude-preserved migration not only can lead to the underground imaging, but also accurate description of lithological information. Amplitude of reflection seismic data is affected by kinds of factors, such as acquisition, propagation effect and scattering. When seismic waves are propagated backwards, main amplitude processing (geometric spreading correction, etc) are completed. Imaging amplitude is thus almost equal or proportional to reflection coefficient, this is so-called true amplitude migration. What’s more, for true amplitude migration, wavefield propagation operator, wavefield transmission effect and imaging condition are key issues which must be taken into considerations. Conventional processing flow is based on the hypothesis that underground medium is horizontally layered medium made up of clastic rocks. According to this, time-distance curve is hyperbolic for the CMP gathers. Stacking velocity (approximately equal to RMS velocity)can be accurately determinated through conventional velocity analysis and converted to interval velocity using Dix formula. However, conventional velocity analysis(based on hyperbolic time-distance curve assumption) is mainly influenced by velocity vertical gradient and severely lateral variation of velocity which are both connected to dip reflectors. Besides, sloping reflectors cause dispersion of reflection points. Consequently, conventional velocity analysis cannot meet the requirements of prestack migration. Conventional velocity analysis or a single velocity analysis method seldom reaches to ideal results. As a result, we introduce multi-gathers velocity analysis method combining different gathers. Initial CSP gathers velocity analysis result is the basis of this method. Taking advantage of ADCIGs tomography velocity analysis to update the velocity field, finally we obtain the prestack migration velocity field which meet the accuracy requirement for amplitude-preserved prestack depth migration.