三维地震共炮检距向量道集

3D seismic common offset vector gather

(CGGVeritas公司, 加拿大阿尔伯塔卡尔加里T2P 5A2)

common offset vector gather; inline offset; crossline offset; cross spreads

共炮检距道集中覆盖次数以及方位角的不规则变化是造成偏移噪声的主要原因之一，构造共炮检距向量（COV）道集是解决偏移噪声问题的有效方法。COV道集是传统二维共炮检距道集概念在三维地震数据中的延伸，在一个规则的三维地震观测系统中，每个COV道集是一个单次覆盖整个数据观测范围的数据子集。COV道集也可以通过连接三维地震交叉排列的特定子集而构成。一个简便实用的构造COV道集的方法是：在给定检波线的走向后，计算出每一地震道的inline炮检距和crossline炮检距；用观测系统中一个具有代表性的炮点的检波器排列范围来确定整个数据的inline炮检距范围和crossline炮检距范围（由负到正），用两倍的炮线间距把inline炮检距的范围分成若干份，用两倍的检波线间距将crossline炮检距的范围也分成若干份，一份inline炮检距和一份crossline炮检距就构成一个炮检距向量道集。在炮线与检波线不垂直时，可由实际的炮线方向及间距计算出垂直于检波线方向的有效炮线间距，再使用这个有效炮线间距和原检波线间距构造出合理的COV道集。最后对COV道集技术在实际应用中存在的炮检距范围较大和观测系统不规则问题给出了客观的分析。

The fold number and the irregular variation of azimuth angle within an offset gather is one of the main reasons causing migration noise. Therefore, the construction of common offset vector (COV) gather is an effective approach to solve this problem. This paper summarizes the basic concept of COV gathering and provides a practical scheme to construct COV gathers from a regular and orthogonal 3D seismic geometry. The COV gather is the extension of the concept of traditional 2D common offset gather in 3D seismic data. In a regular 3D geometry, each COV gather is a data subset which covers the whole observation range in a single time. COV gathers can also be constructed through selecting traces from different cross-spreads of a 3D survey. A simple and practical way of constructing COV gathers can be described as that 1) when a dominant receiver-line direction is determined, the inline offset and crossline offset of every trace can be calculated; 2) the inline offset and crossline offset ranges from negative to positive for the whole dataset can be well represented by the range of the receiver spread of a typical source; 3) the full range of inline offsets can be divided into several sub-ranges using twice the (typical) source line spacing and the full range of crossline offsets can be divided using twice the (typical) receiver line spacing; and 4) one such sub-range inline offset and one such sub-range crossline offsets form a COV gather. When receiver lines are not perpendicular to the source lines, one can calculate the effective source line spacing along the direction perpendicular to the receiver lines, and such effective spacing should be used to form COV gathers. At last, some practical considerations dealing with the large offset range within a COV gather and the survey geometry irregularity are also discussed.