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为了将逆时偏移应用于由速度可变介质获得的迭前、有限偏移距数据,标准(零时)成象条件需要进行推广,这是因为成象空间上的每一点都有一个或多个不同的成象时间。推广后的成象条件称为激发时间成象条件,其中每一点在源点至该点的单程旅时处成象。具激发时间成象条件的逆时偏移由三部分组成:(1)成象条件的计算;(2)记录波场的外推;(3)成象条件的确定。像中每一点成象条件的确定通过自源点作射线追踪来实现,这相当于源点波场在介质中的外推。记录波场的延拓则由有限差分算法来实现。有限差分外推成象中的每一步是通过从传播波场中提取该成象时间网格点上的振幅,并把它们加到成象空间中相应的空间位置上来完成。同一时间阶步中所有象点的轨迹为一个波前(等时或等相位轨迹)。以上迭前偏移算法是普遍适用的算法。其中,激发时间成象条件适用于各种炮点-检波点几何排列及变速介质,且当其应用于零偏移距地面数据时可以完全简化为常用的零时成象条件。该算法已被应用于合成及VSP数据。在处理合成数据时,最有意义和最有应用潜力的结果,就是当储层边界不能很好地成象时,储层内水平流体界面仍可被成象。
In order to apply the inverse time offset to the pre-frame, finite offset data obtained from the velocity-variable medium, the standard (zero-hour) imaging conditions need to be generalized because each point in imaging space has one or Multiple different imaging times. The extended imaging conditions are known as excitation time imaging conditions, where each point is imaged at the one-way trip from the source to the point. The inverse time migration with imaging time with excitation time consists of three parts: (1) the calculation of the imaging conditions; (2) the extrapolation of the recorded wavefield; and (3) the determination of imaging conditions. The determination of imaging conditions at each point in the image is achieved by ray tracing from the source point, which is equivalent to extrapolation of the source wavefield in the medium. The recording wavefield extension is realized by the finite difference algorithm. Each step in FD extrapolation is accomplished by extracting the amplitudes at the imaging time grid points from the propagating wavefield and adding them to the corresponding spatial locations in the imaging space. The trajectory of all the pixels in the same time step is a wavefront (isochronous or isochronous trajectory). The above prestack migration algorithm is a commonly applicable algorithm. Among them, the excitation time imaging conditions apply to a variety of shots - Geometrical Geometry and variable speed media, and when applied to zero-offset surface data can be completely reduced to the commonly used zero-hour imaging conditions. The algorithm has been applied to the synthesis and VSP data. As a result of the most significant and potential applications of synthetic data, the horizontal fluid interface in the reservoir can still be imaged when the reservoir boundaries are not well imaged.