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ONSHORE APPLICATIONS OF 3-D SEISMIC TECHNOLOGY
3D DATA ACQUISITION
Tom Davis - Colorado School of Mines
Steve Roche - Halliburton Geophysical Services, Inc.
Susie Mastoris - Landmark Graphics Corporation
The data acquisition of a 3D survey is critical for its success. Not only can nothing be produced until the data is gathered but many of the data processing algorithms are dependent on how the data is gathered. Algorithms such as migration require that the data be acquired over a certain areal extent with sufficient sampling. Other processes are statistical in nature and require a large enough base of samples to converge on the correct solution. It can be argued that the data acquisition is the first processing step, the sampling of the reflected waveform.
The design of a 3D survey needs to be target-oriented, that is, based on the objectives of the survey. Even when an acquisition program becomes "standard" through common use in a specific area, a review of project objectives is a valuable exercise. An example is the fairly low-fold surveys in use in the Horseshoe Atoll area in the Permian Basin of West Texas. These surveys are adequate for imaging the pinnacle reefs in most cases. But in areas where there is a lateral velocity gradient, caused by a later shelf edge, the "standard" design lacks the statistical base to solve the velocity field. A poor or incorrect Image may be the result.
To design a 3D survey, one considers the target depth, maximum frequency of interest, the velocity field and the dips (geometry) of the target horizon. The exercise may be viewed as defining the wavelengths, in time and space, or the reflected signal required to image the target. For simple structure, only simple equations describing the geometry of raypaths is required. For complex structure, the problem can be modeled on a computer. Once the exercise of understanding where the reflected signal arrives at the recording surface is complete, the acquisition parameters are selected.
In most cases there is more than one target horizon, often a second, shallower marker. This is desired for QC of the 3D data volume or for interpretation methods such as isocron mapping. Secondary considerations for project design include the number of individual trace contributors to a subsurface point, the "fold" of the data, and the offset and azimuth distribution that make up the fold.
As a geophysicist considers the many variables to find an optimum design, there are often cultural restrictions that compromise the efforts. The result is that a 3D project may involve several iterations of design/costs analysis. Geophysical contractors have developed sophisticated 3D design packages to aid in modifying acquisition parameters.
The surface area occupied by a seismic survey must be larger than the subsurface area to he imaged. The extra distance around the edges of the survey is called the migration aperture (or "fringe", "halo"). There are two main components to be considered in computing the required aperture, dip effect and fresnel zone width. The dip effect is the extra distance required to record information from dipping interfaces. For steep dips, 45 degrees or more, the increased aperture due to dip can be quite large. The fresnel zone width can be thought of as the minimum fringe required for the migration algorithms to operate correctly. The dimensions of the 3D acquisition geometry on the surface are the size and placement of the migration aperture.
The subsurface binsize is the sampling within the migration aperture. With the frequency content provided by the source parameters, the subsurface sampling is the major factor in the resolution capabilities of the resulting data volume. Both vertical thicknesses and areal extent are influenced by binsize. A hazard of undersampled data is the effect of aliasing. An example of aliased data could be steeply dipping, higher frequency data (often a desirable degree of resolution) that is aliased to appear as data with less dip and lower frequency (undesirable and misleading).
Several equations for general use are listed. The results for computing binsize and migration aperture are listed for a hypothetical survey.
The "fold" of the data is the number of individual trace contributors to a given subsurface bin. Not only the numerical amount of fold is important, but how it is composed. An even distribution of offsets is critical for velocity estimation, accuracy of statistical processes and improvement of signal/noise ratio. The azimuthal distribution of the fold is important depending on the complexity of the subsurface. For targets with gentle dip or a pronounced strike-dip orientation narrow range of azimuths will suffice (and may he preferable). For complex subsurface a wide range of azimuths is required so that more of the target reflectors are illuminated, that is there are no blind zones where no reflections points are recorded.
There are many techniques available to reduce the cost of a 3D survey, both acceptable an unacceptable. Every method employed to reduce the cost has an associated effect on the resulting data quality. It is part of the survey design exercise to understand what impact a cost-reduction technique will have on the data and whether or not the objectives of the survey will or will not be compromised.
Acceptable cost-reduction techniques, within constraints, included reduced fold, reduced source effort, the use of multiple vibrator source sets (alternate or simultaneous sweeping), reduced number of geophones per group and coarse binsize followed by interpolation.
Unacceptable cost-reduction techniques include:
- reducing the size of the survey (reduces the migration aperture which causes an incorrect image of the subsurface)
- increasing the grid spacing (results in insufficient fold at target depth, poor offset distribution and low S/N ratio)
Simultaneous sweeping use two sets of vibrator sources on different source lines. The two sets use complementary sets of coded sweeps such that two production records can be recorded at once. In practice upsweeps and downsweeps are used along with phase differencing. In areas where the vibrators have good access, a reduction in acquisition cost of 25% can be obtained. On large surveys, where there is a time limitation due to leasing or drilling requirements, two crews can use simultaneous sweep techniques to avoid interference.
Alternate sweeping also employs two sets of sources, but records one while the other set is moving up. Significant cost savings are possible if the access is good.
Reduced source effort is increasingly being used to lower acquisition costs. Many estimates of source effort are based on 2D techniques. More understanding of 3D methods will lead to finding the optimum balance between fold and source effort. Once an acceptable level of source effort has been established, there are different combinations of sweep parameters which may be more productive than others. As a general observation, the faster a crew can acquire the data, the lower the cost. This will be offset by personnel cost and equipment investment, but generally it is a sound observation.
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