THE MAKING OF A 3-D SEISMIC DATA VOLUME
by A. Aming - Trintoc
INTRODUCTION
Seismic surveys have been a part of exploration programs for more than half a century. Major events in the
development of the seismic tool include: the divining rod approach of the 1920's, the introduction of single
fold data in the 1930 5 to 1950 5, multiple fold and common depth point (CDP) stacking of the 1960's, and migration
of 2-D data in the 1960 5 & 1970 5. Rapid advances in computer technology over the last fifteen (15) years
have led to the development of the 3-D seismic technique and today 3-D seismic surveys have become an important
tool for exploitation and exploration programs. Good 3-D seismic data should collapse diffractions, restore reflections
from out of the plane of the section, ensure better geometrical corrections, more accurate map constructions,
extraction of 2D seismic lines in any orientation and overall better
quality seismic data. A 3-D seismic survey consists of four stages: planning, acquisition, processing and interpretation.
PLANNING
This is the most important stage of the survey and usually requires three (3) months to as much as one (1)
year to complete. The following points should be considered at this stage:
1. The geological objective, as this is the whole reason for doing the survey. Relevant factors include velocity
and depth to the objective, the area extent, the lithology of the overlying and the objrctive formations,
the dip and strike and the trapping mechanism.
The survey should generally be laid out in a strike-dip orientation. The size of the survey would depend on the
extent of the objective, the dip and trapping mechanisms (migration aperture) and the recording geometry (roll-on/roll-off).
2. The data acquired over the area of interest should be full-fold and fully migrated. This implies that in addition
to the actual target coverage required roll-on, roll-off and migration aperature zones would be necessary. The
roll-on and roll-off zones are the same as in 2-D seismic data and their size depends on the acquisition geometry.
The migration aperture has both space and time attributes; the spatial migration aperture depends on the dip and
depth of the geological objective while the time migration aperture depends on the maximum depth of penetration
required. These zones would occur on the perimeter of the objective areas.
3. The spatial sample interval or bin size (CDP in 2-D seismic data) depends on the dip and depth of the geological
objective and should be chosen to provide the resolution necessary to image the geological oblective without aliasing.
4. The most optimum geometry should be chosen. There are two types of geometry : the 'swath type' and the "real
3-D type" (Figure 1). In the ·'swath
type" the shot and receiver lines are usually parallel and restricted to one or two of each. This is to ensure
limited source to receiver azimuthal variations. The bin size is determined by the spacing between the geophone
lines in the crossline direction and spacing between the geophones in the inline direction. These surveys are generally
oriented in the dip direction with the fold being calculated as in the 2-D case. In the "real 3-D type"
geometry shot and receiver lines are generally perpendicular to each other and the number ot each is limited by
the recording instrumentation. This geometry ensures maximum source to receiver azimuthal variations. The bin size
is determined by the smaller of the shot or geophone line spacing in the dip direction and the geophone or shot
line spacing in the strike direction. The fold is calculated as the product of the 2--D fold in each direction.
5. The told of recording would determine the data quality and generally the higher the fold the better is the data
quality. If 2-D seismic data are available and the quality is fair to good then adequate fold for the 3--D data
volume would generally be half of the 2~D data told.
6. The type ot navigation equipment used determines the accuracy of the survey; hence special consideration
should he given to this aspect.
7. The type of source and receivers would depend on ~he location of the data volume.
8. The types of geophone and source arrays would depend on the coherent source generated noise characteristics
of the area.
9. The environment or location nf the survey would be a limiting factor to many of the above. Whether the survey
is land or marine could limit the possible geometry Cultural effects and topography can determine the
choices of sources. Water depths, obstacles and currents all affect the equipment requirements.
10. The choice of acquisition, processing and interpertation contractors would be influenced by availability,
capability and cost.
11. Quality of the data should never be sacrificed be-cause of budget constraints. The data volume may be reduced
in size keeping in mind the roll-on, roll-off and migration aperture zones. If this is not feasible then the survey
should be postponed until economic conditions are more favourable.
AQUISITION
Once the planning stage has been well executed the aquisition becomes a matter of quality control. A company representative
should be present as often as possible to ensure the following:
1. Geophysical instrumentation is in order and within specification.
2. All equipment requested is available and functioning.
3. Navigation or surveying of the source and receiver locations is kept within specifications.
4. The source size and synchronization are kept with-in specifications.
5. Recordings are made only when noise levels are low.
6. Proper documentation and reports.
7. Quality control field records.
8. Tapes and support data are sent to the processing centre as often as possible.
All the above would ensure acquisition of the best possible quality data. There is no processing technique that
can make good data from bad data, hence the data must be properly acquired.
PROCESSING
Seismic data are usually processed in two stages: the navigation or survey data and the actual seismic data. Proper
processing of the navigation or survey data and the accurate positioning of every shot and receiver point are critical
to the success of the survey.
There are two types of processing flows available in the industry today: the 2-D seismic flow converted to the
3-D seismic flow and the "real" 3-D flow. The former type can only be used on data acquired in a swath
type" geometry. In this case each line is processed as a normal 2-D seismic line and at the end 2-D
migration is applied in each of the inline and crossline directions. This is the only real 3-D step in this processing
flow.
"Real" 3-D seismic processing involves amalgamating the data into a 3-D data volume then analysing for
velocity, dip and dip azimuth on small cuboids of the data. Velocities, dip and dip azimuths are then applied to
the data. Finally a 2-D by 2-D migration is done. The latter processing flow works well on all types of 3-D data
volumes and is favoured to the "real" 3-D geometry. When this reprocessing is done less fold data will
suffice and generally all cases of reprocessing of old data using this technique show considerable improvement.
INTERPRETATION
A 3-D seismic data volume consists of an enormous amount of data. Conventional interpretation techniques are inefficient
and time consuming. Interactive interpration work stations like the one shown on the cover are now routinely used.
These stations have the following capabilities:
1. 3-D interpretation (horizon tying, correlation across faults, map generation etc.).
2. 2-D interpretation.
3. Calculation of complex trace attributes and acoustic impedances for stratigraphic interpretation.
4. Tying of synthetic seismograms to seismic data.
The main advantage of the 3-D data volume is greater accuracy. This is afforded by closely spaced strike dip seismic
lines and horizontal time slices. (Fig.2).
CONCLUSION
Application of the 3-D seismic tool results in be structural and stratigraphic definition by eliminating side swipe
and diffractions. It also improves the solution of the seismic data and provides the abi to examine the entire
prospect area on one seismic section (horizontal time slice).
In figure 3 a portion of a line taken
from offshore California shows a conventionally processed filte stack, the stack migrated with normal 2-D migration
and the stack migrated with 3-D migration. It obvious that the diffractions in the upper section have been collapsed
better with the 3-D migration than the 2-D migration. A reflection from out of the plane the section between 2.0
and 2.4 seconds has been moved on the 3-D migrated section but is still present in the 2-D migrated section.
Four (4) 3-D seismic surveys have been acquired in Trinidad to date: Teak (Amoco), Ibis (S.E.C.C.), Pelican (S.E.C.C.)
and Cassia (Amoco). During 1984/1985 two (2) new surveys were acquired: West/South WE Soldado (Trinmar) and an
Amoco survey on the South East Coast. All of the above surveys, with the exceptin of the Trinmar survey, are "swath
type".
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