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一个形状不规则的反射界面被一个具有常数速度V_T的物质所覆盖。多次反射数据集合在反射面上方的平面上,而此反射面首先通过迭加然后偏移反射数据而成象。在此过程中遇到三种速度函数:测量的迭加速度V_(NMO);覆盖层的真速度V_T及剖面偏移速度V_M。这些是目前点成象的偏移程序所要求的。确定V_(NMO)及随后的V_T的方法是众所周知的,然而由V_T确定V_M以前并未讨论过。通过研究线成象的偏移程序,我们发现V_M不仅取决于剖面的真速度,而且取决于与剖面的构造方向有关的某些几何因子。V_M和V_T之间的关系与V_(NMO)和V_T之间已知的关系相似,但不应相混。正确的剖面偏移速度总是等于或大于覆盖层的真速度,但可能小于、等于或大于最佳迭加速度。当剖面取了与二维构造走向成(90—θ)的角度时,近似的偏移速度是V_T/cosθ,并与所存在的任阿倾角大小无关。此外,如果二维构造沿走向倾伏成γ角度,则正确的偏移速度就由V_T/(1-sin~2θcos~2γ)~(1/2)求得。倾伏二维构造情况的偏移剖面的时间轴必须乘上一个因子[(1-sin~2θcos~2γ)/cos~2θcos~2γ]~(1/2)来重算。重新计算剖面的构造必须沿着对角线投影至地面来找出它们的真位置。当集中三维数据自动进行三维偏移时,几何因子已随同编入。在那种情况下,不管构造是二维的、倾伏二维的或是三维的,偏移速度就总是等于真速度。处理后的模型资料证实了这些结论。上面提出的方程式是用于常规的迭加后偏移的。把迭加前偏移与速度分析结合起来的最新方案直接给出V_M,此时,上面的方程式提供了由V_M确定V_T的方法。
An irregularly shaped reflective interface is covered by a material with constant velocity V_T. Multi-reflection data is collected on a plane above the reflective surface, which is first imaged by superimposing and then deflecting the reflected data. Three speed functions are encountered in this process: the measured superposition velocity V NMO; the true velocity V_T of the overburden and the cross-sectional velocity V_M. These are required by the current point-imaging offset procedure. Methods for determining V NMO and subsequent V_T are well known, however, V_M was not previously discussed by V_T. By studying the line imaging migration procedure, we find that V_M depends not only on the true velocity of the section, but also on some of the geometric factors associated with the direction of the section’s construction. The relationship between V_M and V_T is similar to the known relationship between V_ (NMO) and V_T, but should not be mixed. The correct section offset velocity is always equal to or greater than the true velocity of the overburden, but may be less than, equal to, or greater than the optimal overlap velocity. When the profile is taken at an angle of (90-θ) to the two-dimensional structure, the approximate velocity of migration is V_T / cosθ irrespective of the magnitude of any existing tilt angle. In addition, if the two-dimensional structure is tilted at an angle of γ along the strike, the correct offset velocity is obtained from V_T / (1-sin ~ 2θ cos ~ 2γ) ~ (1/2). The time axis of the offset profile of a dip two-dimensional structure must be multiplied by a factor [(1-sin ~ 2θ cos ~ 2γ) / cos ~ 2θ cos ~ 2γ] ~ (1/2). The recalculated sections must be diagonally projected to the ground to find their true position. When focused 3D data is automatically 3D-offset, the geometry factor has been programmed. In that case, the offset speed is always equal to the true speed, regardless of whether the construction is two-dimensional, dip two-dimensional or three-dimensional. The processed model data confirm these conclusions. The equation presented above is for the conventional post-stacking offset. V_M is given directly from the latest scheme that combines the superposition of prestack migration and velocity analysis. In this case, the above equation provides a way to determine V_T from V_M.