First Break - Volume 6, Issue 6, 1988

It is known that the quality of marine 3D data is strongly dependent on the shooting direction. In particular, on a line perpendicular to that directon, resolution and continuity are generally reduced. This can be partly explained by inaccuracies in controlling the acquisition parameters, mainly the location of the receivers. The geophysical industry is presently making an effort to improve this accuracy. Another part of the problem is inherent to the acquisition method itself, which remains essentially anisotropic, as the population of shooting azimuths is restricted to a small angle around the acquisition direction. Here too, new methods have been proposed and tested: for instance, orthogonal grids or multi-azimuth grids. This paper presents CGG's experience with solutions to the first part of the problem. Positioning in marine surveys covers four major areas: positioning of the sources, positioning of the heads of the streamers, especially in dual streamer acquisition, determination of the shapes of the streamers, and finally orientation of the images of the streamers. For each of these four points, we will explain the inherent inaccuracies of traditional positioning methods and present a set of recent solutions implemented by CGG. Throughout the paper, a priori assumptions are avoided as the only valid criteria are measurements. In nearly all 2D surveys using conventional positioning techniques, the source is not located by an instrument. lts position is calculated by making the assumption that it is at a given distance behind the vessel. As moderate distances ranging from 10 to 100 m are involved, an error of 1° introduces maximum lateral positioning error of some 2 m, which is acceptable. The remaining question is the angle between the source and the vessel. Fig. 2 indicates one possible choice, which is to align the source along the course followed. Another choice could be to plot the source along the ship's heading. This choice would be physically valid if the angle of drift of the vessel depended only on currents, but in fact the vessel's angle of drift also depends on sea state and wind. Modern survey vessels have bridges and superstructures which offer a wide surface to the wind and in some cases the angle between the ship's heading and the heading of the cable reaches 10°. Measurements are then required to determine along which direction the layback method should be applied. Without being over-pessimistic, we could very well estimate the lateral error on the location of the source as about 10 m, which would no longer be negligible. Especially in multistreamer acquisition, this error would seriously affect the shooting distances of the near traces.

We present and discuss four of the most accurate methods for depth migration in complex structure. These include two frequency-domain methods (x-w summation and phase shift plus interpolation) and two time-domain methods (reverse-time finite-difference migration and Kirchhoff migration using accurately computed traveltime curves). These methods can all handle steeply dipping structure and large lateral velocity variations. Comparing their implementations shows advantages and disadvantages of each of the methods; none of the methods emerges as clearly the best, even for two-dimensional (2D) post-stack migration.