多次覆盖地震资料的叠前偏移

Gazdag 和 Sguazzero(1984)利用相移加插值方法(PSPI)对叠后资料进行偏移。此法也可用来偏移叠加前的地震资料。首先把-个分布震源定义为函数S(x,t),它由位于x处、延迟时间为t以及振幅为S的-些点震源组成。在此条件下所测得的记录用m(x,t)表示。利用 PSPI方法由S(x,t)构成下行波场S(x,z,t),由 m(x,t)构成上行波场M(x,z,t)。在(x,z)处的偏移图像可以用(x,z)处的时间函数M和S的零延迟互相关系数来定义。然后用参数已知的、按比例设计的物理模型进行此种方法试验。该例表明通过选择-定形状的分布震源S(x,t),使下行波场能够几乎同时到达某-反射界面上的所有的点,可以精确地使此反射界面偏移成像。用合成记录进行试验表明,深度偏移图像中的子波出现了拉伸现象,伸长程度跟入射角成正比。在实际的海洋勘探中,由于船后的地震电缆长度有限,所以可产生只包含-定入射角范围的下行波场,仅对这个入射角范围的反射波场,才能有效地被电缆上的检波器接收。文中提出的叠前偏移方法的意义,在于能够获得没有叠加误差的深度偏移剖面,此外还可用于研究振幅随炮检距的变化关系及速度分析。

Migration of pre-stack mufti-fold seismic data

The phase-shift-plus-interpolation (PSPI) method for migrating stacked data (Gazdag and Sguazzero, 1984) is adapted in this paper to migrate pre-stack data. An extended source is defined as a function S(x,t) which consists of point sources at locations x with time delays t and amplitudes S.The corresponding measured data are denoted by m(x,t).PSPI is used to construct a downward moving wave-field S(x,z,t) from S(x,t) and an upward moving wave field M(x,z,t) from m(x,t).The migrated image at (x,z) is defined as the zero-lag cross correlation coefficient of the time histories of S and M at(x,z).The performance of this method is illustrated with scaled physical model data for which the correct answer is known. It is shown that the extended source can be tailored to image a particular horizon accurately by arranging that the downward traveling wave field arrives almost simultaneously at all points of the horizon.Tests with synthetic data show that the wavelet in the depth migrated image is stretched in proportion to the angle of incidence.In practice, since the cable covers a finite interval behind the ship, downgoing wave fields can be generated to cover a range of angles of incidence for which the reflected wave field is adequately sampled.The significance of the method is that depth migration is achived without making stacking errors and that it might be used to examine amplitude versus offset or for velocity analysis.