3. 3D data geometries

Subsurface geological features of interest in hydrocarbon exploration are three dimensional in nature. Examples include salt diapirs, overthrust and folded belts, major unconformities, reefs, and deltaic sands. A two- dimensional seismic section is a cross-section of a three-dimensional seismic response. Despite the fact that a 2D section contains signal from all directions, including out-of-plane of the profile, 2D migration normally assumes that all the signal comes from the plane of the profile itself. Although out-of-plane reflections (sideswipes) usually are recognizable by the experienced seismic interpreter, the out-of-plane signal often causes 2D migrated sections to mistie. These misties are caused by inadequate imaging of the subsurface resulting from the use of 2D rather than 3D migration. On the other hand, 3D migration of 3D data provides an adequate and detailed 3D image of the subsurface, leading to a more reliable interpretation.

A typical marine 3D survey is carried out by shooting closely spaced parallel lines (line shooting). A typical land or shallow water 3D survey is done by laying out a number of receiver lines parallel to each other and placing the shotpoints in the perpendicular direction (swath shooting). Because of practical and economical considerations 3D surveys are never acquired with full regular sampling of the spatial axes. The problem of designing 3D surveys presents many more degrees of freedom than the design of 2D ones and it has no standard or unique solution [4].

The design of 3D acquisition geometries is the result of many trade-offs between data quality, logistics, and cost. Further, nominal designs are often modified to accommodate the operational obstacles encountered in the field.

Acquisition design and processing are becoming more and more connected because the characteristics of the acquisition geometry strongly influence the data processing. Understanding the principles of acquisition design is necessary to the understanding of many data processing issues. The main goal of conventional acquisition design is to obtain an adequately and regularly sampled stacked cube that can be accurately imaged by post-stack migration.

Other important design parameters are the minimum and maximum offsets. The minimum offset must be small enough to guarantee adequate coverage of shallow targets. Maximum offset must be sufficiently large to allow accurate velocity estimates that are necessary for both stacking and post-stack imaging. However, in common acquisition geometries the sampling of the offset axes may be inadequate when the data require more sophisticated pre-stack processing than simple stacking. Even the application of standard dip move out (DMO) may be problematic with some commonly used acquisition geometries. In these cases, the requirement that the midpoint axes are adequately sampled is not sufficient, because the offset and azimuth sampling play an important role as well [5].

Another important emerging issue in modern 3D survey design is subsurface illumination, as we image targets under increasingly complex overburden, and as we demand more control on the amplitudes of the images. Complex wave propagation associated with large velocity contrasts (e.g. salt or basalt bodies) may cause the data to illuminate the target only partially, even if the wavefield is sufficiently sampled at the surface. Because target illumination is strongly related to the data offset and azimuth, a careful survey design can dramatically improve the image quality. Survey-modeling tools that analyze the target illumination by ray tracing through an a priori model of the subsurface geology help to guide the design process towards successful acquisition geometries. However, the usefulness of these tools is limited by how accurately the subsurface geology and velocity model are known [3].

Although there are no preset solutions to the problem of acquisition design, there are a few acquisition schemes that are commonly used as templates to be adapted to individual surveys.