INTRODUCTION…………………………………………………
1 3D SEISMIC SURVEYS…………………………………………
2 THE FACTORS INFLUENCE ON DESIGN OF 3D SEISMIC SURVEYS………………………………………………………………..
3 3D DATA GEOMETRIES………………………………………
3.1 Data coordinates………………………………………………
3.2 Marine-data geometries……………………………………….
3.3 Land-data geometries………………………………………...
3.3.1 Wide-azimuth geometries…………………………………
CONCLUSION………………………………………………………
GLOSSARY………………………………………………………..
LITERATURE……………………………………………………..
АННОТАЦИЯ……………………………………………………..
Seismic reflection surveys have been performed in oil exploration to delineate subsurface structure since the 1930. The early surveys (2D, single fold, continuous coverage profiling) provided large-scale structural information about the subsurface, but forced oil exploration teams to drill without a completely accurate image of the reservoir. As the use of seismic surveys became more accepted and as funds were available for research, the technique evolved until it became an effective way to view and interpret large-scale subsurface geologic structural features. The advent of the 2D, multi-fold, common-depth-point surveying techniques, along with advances in instrumentation, computers, and data processing techniques, greatly increased the resolution of seismic data and the accuracy of the subsurface images.
However, the technique still yielded little information on the physical properties of the imaged rocks, or the pore fluids within them.
It was not until the introduction of 3D reflection surveying in the 1980's that seismic images began to resolve the detailed subsurface structural and stratigraphic conditions that were missing or not discernable from previous types of data. Today potential oil reservoirs are imaged in three dimensions, which allows seismic interpreters to view the data in cross-sections along 360° of azimuth, in depth slices parallel to the ground surface, and along planes that cut arbitrarily through the data volume. Information such as faulting and fracturing, bedding plane direction, the presence of pore fluids, complex geologic structure, and detailed stratigraphy are now commonly interpreted from 3D seismic data sets.
The essence of the 3D method is a data collection followed by the processing and interpretation of a closely-spaced data volume. Because a more detailed understanding of the subsurface emerges, 3D surveys have been able to contribute significantly to the problems of field appraisal, development and production as well as to exploration. It is in these post-discovery phases that many of the successes of 3D seismic surveys have been achieved. The scope of 3D seismic for field development was first reported by Tegland.
In the late 1980s and early 1990s, the use of 3D seismic surveys for exploration has increased significantly. This started in the mid-1980s with widely-spaced 3D surveys called, for example, exploration 3D. Today, speculative 3D surveys, properly sampled and covering huge areas, are available for purchase piecemeal in mature areas like the Gulf of Mexico. This, however, is not the only use for exploration. Some companies are acquiring 3D surveys over prospects routinely, so that the vast majority of their seismic budgets are for 3D operations.
The primary goal of any 3D is to achieve the desired S/N at the target. In complex areas, feasibility studies are routinely done to establish suitable parameters of a 3D seismic survey and calculate an initial budget.
To ensure the best image, the best sampling method that can be used to recreate the various spatial wavelengths in X, Y and Z must be chosen. The symmetric sampling – whatever is done to shots must also be done for receivers.
Noise attenuation may be just as important as recording the signal. The «best» geometry is the one that addresses the specific local problems of improving signal at a chosen target, while identifying and reducing the various sources of noise. If the CMP stack for one geometry attenuates the noise by 6 dB, when compared to the CMP stack for another geometry, the fold has been effectively quadrupled. Today’s best geometries for noise attenuation seem to be wide azimuth slanted geometries with 18 degrees often emerging as the winning angle. The small departure from orthogonal (18 degrees instead of zero) does not dramatically affect the imaging properties.
For noise reduction (both linear, backscatter and multiples), it can be noted, that wide azimuth surveys will be better than narrow – simply because of the preponderance of long offsets. Other factors like unequal shot and receiver line spacing and slanted lines are currently under investigation.
Arrays are also making a comeback. Since 2D gave way to 3D, arrays have been largely ignored. All too often bunched phones and single holes have been the norm. Recently many acquisition geophysicists have made attempts to reduce linear and backscatter noise before it reaches the recorder.
Arrays play a crucial role in wavefield filtering and resampling to the group interval. In the absence of a geophone array, the wavefield samples collected at each surface station (group) will be aliased for all wavelengths less than the group interval. The geophone array will essentially “filter” the wavefield prior to sampling and remove this spatially aliased energy.
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