Two different methods have been used in recent
years for seismic data acquisition offshore.
- The more common of these uses hydrophones deployed in a streamer
or streamers towed behind a vessel at a depth of a few meters,
while the vessel moves at a speed of four or five knots.
- In the other technique, the recording sensors are deployed
on the ocean floor and are connected to a stationary recording
vessel. These bottom-referenced systems are called ocean bottom
cable (OBC), or ocean bottom seismic (OBS).
This article concentrates on marine 3-D survey design
using towed streamers. A prior article by Mike
Galbraith (September EXPLORER) dealt with the geophysical issues
related to spatial sampling (bin size) and fold, so these will not
be repeated here.
Typical data acquisition uses from four to 16 streamers per vessel, with either one or two airgun source arrays. Most modern boats can tow six or more streamers with a total length of streamers between 50 and 75 kilometers.
With long streamers, fewer are deployed (e.g., six
streamers of 8,500 meters each), while with shorter streamers many
more are possible (e.g., 16 of 4,500 meters each; figure 1).
Longer streamers are being used more frequently today
because of both deeper targets and improved imaging requirements
in areas with complex geology, such as sub-salt structures in the
Gulf of Mexico. Both increased streamer length and the number of
streamers increase the amount of time taken for line changes.
Because most marine surveys are recorded with the
boat traveling in straight lines, survey orientation is still problematic.
In areas with rapidly varying velocity fields, conventional wisdom
now recommends the longest source-to-receiver axis being aligned
in the strike direction. This minimizes the raypath complexity —
and thus makes the normal moveout more hyperbolic.
Subsurface illumination also is normally improved
with strike acquisition. However, because the natural spatial sampling
of streamer marine systems is typically finer in the inline direction
than in the crossline direction, strike acquisition results in coarser
spatial sampling in the dip direction. This spatial sampling must
be adequate to sample the geology or aliasing will occur.
This may mean that overlapping, interleaved boat
passes may be necessary in order to achieve fine enough crossline
Another problem is that, in complex geology, there
are not necessarily true dip and strike directions, and therefore
any survey orientation may result in imaging difficulties in the
data processing stages. Higher density surveys with improved wavefield
sampling can provide significant improvements in the imaging processes.
The natural inline spatial sampling of most streamers
is fixed at 12.5 meters or less, which is adequate for almost all
geologic environments. Varying the crossline sampling to allow for
geologic dip has a significant impact on survey costs; a reduction
in streamer spacing means more boat passes will be required. Therefore,
the decision to acquire the survey in the dip or strike direction
is a major factor in determining the cost of the survey, with dip
acquisition normally being preferred from a cost standpoint.
However, the size and shape of the survey and additional
issues related to tides, currents and obstructions (e.g. platforms,
shallow water along coastlines and reefs) also will complicate the
In general, fewer boat traverses through the area
with longer lines is preferable to more, shorter lines, since the
ratio of recording time to line change time is greater.
Towing multiple streamers close together can lead
to operational difficulties.
A common method to achieve smaller crossline sampling
has been to use two source arrays with wider streamer separation.
Since the source arrays are fired alternately, the shotpoint sampling
on each shotline is doubled, and the recorded fold for each common
mid-point line is halved.
This leads to coarser offset sampling in each bin,
which can result in degraded performance of some data processing
algorithms, such as multiple attenuation. Single source acquisition
provides higher trace density and more uniform offset distributions.
Another imaging consideration for marine surveys
is the difference in source to receiver azimuths at the boundary
between data recorded on adjacent boat passes when recorded with
traditional "race-track" shooting (figure
2a) It has been shown that these can result in shadow zones
with inadequate subsurface illumination, leaving both structural
and amplitude errors in the data volume.
Alternative recording methodologies, such as anti-parallel
recording (figure 2b) can help minimize
Because of the need for higher resolution images
and improved structural and stratigraphic interpretations, higher
density surveys are being acquired more frequently. These surveys
typically have smaller spatial sampling and higher fold, with much
better offset distributions.
For time-lapse 3-D surveys, often called 4-D, it
has been shown that minimizing the differences in the pre-stack
offset and azimuth attributes between the base and monitor surveys
is very important in reducing the seismic differences caused by
the data acquisition:
- One method to achieve this is the use of steerable streamers
to better match the streamer feathering of the monitor survey
to that of the base survey.
- Another method repeats source locations, which, together with
an overlapped shooting configuration using additional outer streamers,
improves azimuth preservation. The use of additional outer streamers
on the base survey with overlapped recording contributes to more
uniform offsets and azimuth distributions.
Also, by using a more closely spaced streamer configuration,
source-receiver azimuths and offsets can be repeated very accurately.