Introduction to 3-D seismic exploration
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 (2-D) seismic section is a cross-section of a three-dimensional (3-D) seismic response. Despite the fact that a 2-D section contains signal from all directions, including out-of-plane of the profile, 2-D 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 2-D migrated sections to mistie. These misties are caused by inadequate imaging of the subsurface resulting from the use of 2-D rather than 3-D migration. On the other hand, 3-D migration of 3-D data provides an adequate and detailed 3-D image of the subsurface, leading to a more reliable interpretation.
A typical marine 3-D survey is carried out by shooting closely spaced parallel lines (line shooting). A typical land or shallow water 3-D 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).
In marine 3-D surveys, the shooting direction (boat track) is called the inline direction; for land 3-D surveys, the receiver cable is along the inline direction. The direction that is perpendicular to the inline direction in a 3-D survey is called the crossline direction. In contrast to 2-D surveys in which line spacing can be as much as 1 km, the line spacing in 3-D surveys can be as small as 25 m. This dense coverage requires an accurate knowledge of shot and receiver locations.
The size of the survey area is dictated by the areal extent of the subsurface target zone and the aperture size required for adequate imaging of that target zone. This imaging requirement means that the areal extent of a 3-D survey almost always is larger than the areal extent of the objective.
A few hundred thousand to a few hundred million traces normally are collected during a 3-D survey. In a modern marine 3-D seismic survey, typically, more than 100 000 traces per square km are recorded. Most 3-D surveys are aimed at detailed delineation of already discovered oil and gas fields. Additionally, 3-D surveys are repeated over the same area at appropriate intervals, say every few years, to monitor changes in fluid saturation which may be inferred from changes in seismic amplitudes. By mapping changes in fluid saturation, changes in fluid flow directions may also be inferred and used for planning of production wells. To make use of seismic amplitudes for reservoir monitoring, however, data from all vintages must be processed consistently using a processing sequence aimed at preserving relative amplitudes. Seismic monitoring of oil and gas reservoirs by using time-lapsed 3-D surveys has come to be called the 4-D seismic method (4-D seismic method).
The basic principles of 2-D seismic data processing still apply to 3-D processing. In 2-D seismic data processing, traces are collected into common-midpoint (CMP) gathers to create a CMP stack. In 3-D data processing, traces are collected into common-cell gathers (bins) to create common-cell stacks. A common-cell gather coincides with a CMP gather for swath shooting. Typical cell sizes are 25 × 25 m for land surveys and 12.5 × 25 m for marine surveys.
Conventional 3-D recording geometries often complicate the process of stacking the data in a common-cell gather. Cable feathering in marine 3-D surveys can result in traveltime deviations from a single hyperbolic moveout within a common-cell gather. For land 3-D surveys, azimuth-dependent moveout within a common-cell gather is an issue.
After stacking, the 3-D data volume is migrated. Before migration, the data sometimes need to be trace-interpolated along the crossline direction to avoid spatial aliasing. The migrated 3-D data volume then is available to the interpreter as vertical sections in both the inline and crossline directions and as horizontal sections (time slices). The interactive environment with powerful 3-D visualization tools provides an efficient means for interpretation of the sheer volume of 3-D migrated seismic data. Fault correlation, horizon tracking, horizon flattening, and some image processing techniques can be adapted to the interactive environment to help improve interpretation.
- The need for imaging in three dimensions
- 3-D survey design and acquisition
- Processing of 3-D seismic data
- 3-D poststack migration
- 3-D prestack time migration
- Interpretation of 3-D seismic data
- Mathematical foundation of 3-D migration