A method for stacking 3-D swath data from a 3-D dipping horizon, without the need for a stacking velocity analysis

Abstract

The Generalised 3-D Normal Moveout (GMO) equation has been developed for a single, three-dimensionally dipping horizon, in which the velocity term has been separated from the dip term. The velocity term is the medium velocity above the chosen horizon whereas the dip term is partitioned into orthogonal dip directions. If a method could be found to establish the dip terms in the equation, then a single velocity term could be used to stack 3-D swath data, without the need for a stacking velocity analysis. Firstly, a 3-D field method was devised in which the true dip of a reflecting horizon and its azimuth could be measured within a survey area. The method required a single horizon whose velocity was known, assuming a constant dip throughout the area. Two orthogonal swath surveys were then recorded, generating two orthogonal Common Midpoint (CMP) lines. The GMO equation then corrected data to a zero-offset coordinate origin point. An optimum value of dip and strike provided the best reflection line-up at the origin, producing the value of true dip and its azimuth for that horizon. A physical model was used to demonstrate the method, which was able to predict dips and their azimuths to within half a degree. The two orthogonal CMP lines were then correctly stacked using the variable 3-D dip information only, without a need to perform a stacking velocity analysis, and the method shows potential for use in stacking low-fold data.