4d seismic survey
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PGS has recently devoted
significant resources to the
improvement of 4D technology. A
strong effort has been made to establish
a thorough, consistent product
including leading technology and tools
as well as the right expertise. PGS has
defined four Ds that characterize our
products and strengths with respect to a
4D project:
Detailed planning
Dense acquisition
Distinguished processing
Dedicated QC
This newsletter focuses on Dense
Acquisition and Distinguished
Processing.
When we design a seismic survey,
we would ideally like to have:
Dense sampling to avoid aliasing and
get optimum resolution.
High fold to decrease random noise
and get clear data.
Multiple azimuths to improve
imaging and map and control
anisotropy.
Efficient acquisition to reduce costs.
Dense sampling implies the
sampling interval (in space and time) is
so small that aliasing is avoided in all
relevant processing domains. It also
means we get a sufficient number of
samples or traces within each bin.
Proper sampling and proper sub-surfacecoverage is the best way to avoid
harming the data during processing.
The Ds in 4D Seismic from PGSPart 2: Dense Acquisition & Distinguished Processing
A Publication of PGS Geophysical March 2002Vol. 2 No. 2
PGS has introduced HD3D
technology for onshore, seafloor
and streamer surveys. This
technology is well suited for 4D
seismic in all these environments
because:
Dense acquisition gives
adequate sampling of thewavefield, higher resolution and
better velocity analyses.
Dense acquisition gives higher
fold, reduced random noise and
increased repeatability.
Dense acquisition may also give
better azimuthal coverage,
which enables better matchingof two surveys due to:
- Flexibility in selection of
input gathers of monitor
surveys.
- Opportunity to remove
multidirectional noise.
- Better imaging.
The efficient, dense
acquisition facilitated by HD3D
makes this a very appropriate
technology for acquisition of 4D
data.
Dense Aqcuisit ion
Overview
Continued on next pageFigure 1: Comparison of conventional 3D survey with 166400 traces per km 2 (lower part) and
HD3D survey with 576000 traces per km2 (upper part) - Courtesy BP.
The Advantages of Dense
HD3DTM for 4D Seismic
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PGS has introduced HD3D
technology for onshore, seafloor and
streamer surveys. Bin sizes in the range
of 10m x 10m can be efficiently
acquired using this technology.
An example of improvement in
data quality achieved by HD3D
streamer seismic is shown in Figure 1.
HD3D technology enables the
small bins/high resolution and high
fold/high S/N we need for detailed
reservoir studies. The higher trace
density resulting from HD3D will allow
for improved repeatability because
processing artifacts originating in
transforms or data interpolation are
reduced. The dense acquisition
improves the velocity analyses which
normally have a strong impact on data
quality. HD3D surveys are therefore
ideal for base line purposes for a 4D
project.
We frequently face the problem
that the monitor and baseline surveys
do not have the same survey
configurations and we
consequently have to
compare two surveys
with varying source-
receiver azimuths and
different sampling. For
streamer surveys, the
efficiency and
flexibility of HD3D
makes it feasible to
acquire swaths that are
overlapping, which
again makes available a
larger range of azimuths
for all source - receiver
configurations.
If we are acquiring
a monitor survey,
HD3D technology will
normally give more hits
per bin than the base
line survey. The surplus
of data makes it
possible to achieve a
better match between the data that are
used in each bin from the monitor and
base line surveys respectively. Hence, a
monitor survey acquired with HD3D
allows us to pick the traces that have
optimum match to the base survey's
offset and azimuth. In
addition, the level of
non-repeatable noise
will be reduced through
use of directional noise
suppression. Many
noise suppression
processes suffer
because the data are
sampled only in one
direction.
Again, HD3D can
allow superior
directional noise suppression when a
wider range of azimuths have been
acquired.
Distinguished Processing
High-quality 4D processing
requires an integrated processing
sequence that is specifically focused on
both structural fidelity and primary
amplitude preservation. The 4D
friendly processing flow must be highly
repeatable between base and monitor
surveys. Among the many vital
components of a 4D friendly processing
flow are pre-stack full or partial
migration, coherent and incoherent
noise removal, and consistent
amplitude recovery. As is clear from the
processing flow in Figure 2, it is crucial
to do QC after each step!
TechLink March 2002 Page 2
Continued from Page 1
The Advantages of Dense
HD3DTM for 4D Seismic
Figure 2: QC of the processing flow.
Figure 3: (a) Comparison of AVO response for Radon
demultiple using an aliased and an un-aliased Radon
transform. (b) Improvement in amplitude using the well-
parameterised high-resolution transform. The solid line
indicates the true AVO trend of a primary event. The long-
dashed line shows the AVO response of the event after
standard (least-squares) Radon demultiple. The short-
dashed line shows the AVO response after demultiple using a
high-resolution Radon transform.
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Page 3 A Publication of PGS Geophysical
4D Optimized
Amplitude Recovery
It is important to recover the
amplitudes from the data in a consistent
and accurate manner to obtain a good
4D signal. Three of the most important
considerations in this process are: (i)
source/receiver directivity, (ii)
geometrical spreading and (iii) the
effects of Q. The source and receiver
arrays used in typical marine surveys
produce responses that depend on the
emission/emergence and azimuth
angles of each source-reflector-receiver
ray-path. These effects are time and
offset dependent for a given source-
streamer combination and should be
corrected for at an early stage in the
processing sequence.
Absorption results in a progressive
loss of high frequency signal as a
function of travel-time. Dispersion
results in progressive wavelet delay as a
function of travel-time. The main effect
is that the imbedded wavelet becomes
non-stationary and this, in turn, distorts
the true amplitude (and thus true 4D)
effect. A robust approach is to apply
scalars (that are functions of Q, time,
velocity and offset) to try and recover
peak amplitude. Computation of these
scalars is based on dominant frequency
and bandwidth of the data at various
travel-times.
Multiple Removal
In general, removing coherent
noise and multiple energy from the
seismic dataset is one of the most vital
processing steps. Of the many
demultiple techniques available, two
stand out as being generally good at
preserving pre-stack primary
amplitudes. Surface related multiple
elimination (SRME, Figure 4) shows
very good primary amplitude
preservation as long as care is taken
during the design of the adaptive
subtraction component of the process.
However, the most commonly used
demultiple algorithm, in cases where
pre-stack primary amplitude
preservation is critical, is Radon
demultiple.
When Radon demultiple is used, it
is vital that the Radon transform not
generates alsiasing. Figure 3(a) shows a
comparison of AVO behaviour for a
synthetic event after Radon demultiple
with both an aliased and unaliased
Radon transform.
The unaliased Radon transform
improves the amplitude preservation at
both far, and in particular, near offsets
(Figure 3(b)). Hence, a well-
parameterised high-resolution
transform is necessary for 4D
processing.
Figure 4: Seismic section before (left) and after application of SRME (right).
Continued on next page
PGS has developed a series of
algorithms specially suited for 4D
processing: Consistent amplitude recovery
compensating for
source/receiver directivity,
geometrical spreading, and
absorption.
High-fidelity DMO.
Multiple removal by SRME or
Radon demultiple using high-resolution transforms.
Pre-stack full or partial
migration using true-amplitude
pre-stack time migration that
takes into account velocity
variations as a function of
depth.
As a result, PGS has a total
processing flow that comprises 4D
friendly-processing.
Dist inguished
Processing Overview
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TechLink March 2002 Page 4
Although pre-stack depth/time
migration is rapidly becoming a
conventional tool, it is noticeable that
DMO still retains a useful role as a
preprocessing step rather than as a
partial imaging step. DMO is often a
critical step to regularize the input data
set to different offset planes.
The main objective of such a
processing sequence is to obtain an
optimal migration velocity field that
can also be used to develop a model for
pre-stack depth migration or a full time-
migration before stack. It is obvious
that any DMO implementation used in
such a processing sequence must be
able to preserve amplitudes at all
offsets.
PGS has implemented a HiFi DMOthat makes an efficient practical
application of near offset amplitude-
preserved processing possible. Figure 5
shows two CMP DMO gathers of real
marine seismic data with 16 streamers
before and after the application of HiFi
DMO. A significant improvement of
DMO on near offsets has been
achieved.
For a full imaging solution, true-
amplitude pre-stack time migration
(TA-PSTM) is available. The
conventional 3D Kirchhoff pre-stack
time migration (KPSTM) assumes
straight raypaths and constant velocity
media for the traveltime calculations.
TA-PSTM uses an amplitude
preserving KPSTM algorithm suitable
for a media where velocity can vary as
a function of depth.
The migration results for model
and raw data demonstrate that this
curved ray migration technique
produces better images and migration
velocities than those obtained using the
straight ray KPSTM. Moreover, the
amplitude preservation makes it ideal
for use in 4D and AVO processing.
For Updates on PGS Technological Advances, visitwww.pgs.com
More TechLinks atwww.pgs.com/techlink
C O N T A C T
PGS Geophysical
London
Tel: 44-1932-260001
Fax: 44-1932-266465
Oslo
Tel: 47-67-52-6400
Fax: 47-67-52-6464
Houston
Tel: 1-281-509-8000
Fax: 1-281-509-8500
Singapore
Tel: 65-6735-6411
Fax: 65-6735-6413
2002 Petroleum Geo-Services. All Rights Reserved
Figure 5: DMO gather: Left panel - conventional DMO is applied. Right panel - HiFi DMO is applied to the maximum offset 1200 m. In
particular, compare the amplitudes on the nearest offset traces (arrowed).
Pre-Stack Imaging - High Fidelity DMO and TA-PSTM