自由观测条件下共中心点抽道集叠加方法的误差分析

Error Analysis of Common-midpoint Sort Stacking under the Condition of Free Observation

常规共中心点抽道集叠加方法是以水平观测面,水平反射界面、均匀介质为条件,当前提条件不能完全具备时,会带来两个方面的误差,其一是CMP道集整体离散误差,其二是CMP道集内部各道之间的离散误差。通过建立相应数学模型并进行运算分析,界定了误差(离散量)的大小范围及分布规律:离散量与观测面反射界面之间的夹角、观测面高程、反射界面倾角及同道集炮检距等相关,其变化值越大,离散距离越大;相反离散距离会随观测面与反射点空间距离的增大而变小;另外介质波速越小,其垂直离散距离越大;中心点自激自收其离散量为0。对于CMP道集中诸道△t_0离散量常存在大于目的层反射波T/4的现象;炮距越大、观测界面倾角越大、介质波速越小,其△t_0离散值越大,其中△t_0离散值与炮检距呈非线性关系;相反反射界面埋深越大,其△t_0离散越小。根据离散量对资料影响程度的分析,指出了目前共中心点抽道集叠加方法存在着局限性。

The preconditions of conventional common-midpoint sort stacking have horizontal observation surface, horizontal reflecting boundary and uniform medium. If the conditions are not complete, two errors will occur, the one is CMP gather integral discrete error and another is CMP in gather discrete error between traces. To establish corresponding mathematical model and carry out operational analysis, define size, range and distribution pattern of the error （discrete data）： inclination between observation surface and reflecting boundary, observation surface elevation, reflecting boundary dip angle and geophone distance in same gather are correlated each other, the larger variation, the larger discrete distance; but the discrete distance will getting smaller along with spatial distance between observation surface and reflecting point getting larger; the smaller wave velocity in media, the larger vertical discrete distance; if center point shot and receiving itself, the discrete will be 0. For CMP gather, discrete of traces △t0 is often larger than target reflecting wave T/4; the larger geophone distance and observation boundary plane dip angle, the smaller wave velocity in media, the larger At0 discrete, the relation between At0 discrete and geophone distance presents nonlinearity; by contrast, the deeper reflecting boundary, the smaller At0 discrete. According to the analysis of discrete influencing extent on data, the paper pointed out the limitation of common-midpoint sort stacking method at present.