This document describes challenges with imaging hydrocarbon reservoirs in offshore peninsular Malaysia that are affected by shallow gas clouds. Conventional P-wave seismic data suffers from issues like attenuation, scattering, and velocity inversions due to the gas-charged sediments. The document outlines improvements achieved in imaging a 3D-4C seismic dataset from the area through steps like S-wave receiver statics, receiver line interpolation, and iterative converted wave velocity analysis. This resulted in a higher quality PS-wave time image and PS-wave depth image that provides clearer imaging of structures and faults beneath the gas clouds compared to previous P-wave data.

1. 76th
EAGE Conference & Exhibition 2014
Amsterdam RAI, The Netherlands, 16-19 June 2014
Tu D202 03
3D-PS Converted Waves – Solving 3D-imaging
Challenges under Gas Clouds - Offshore Malaysia
M.F. Akalin* (PETRONAS Carigali Sdn Bhd), A.A. Muhamad (PETRONAS
Carigali Sdn Bhd), Y.C. Tan (PETRONAS Carigali Sdn Bhd), Y.B.M. Yusoff
(PETRONAS Carigali Sdn Bhd), N.A.M. Radzi (PETRONAS Carigali Sdn Bhd),
S.F.M. Zohdi (PETRONAS Carigali Sdn Bhd), M.H. Hashim (PETRONAS
Carigali Sdn Bhd), M. Ghazali (PETRONAS Carigali Sdn Bhd), S. Maitra
(CGG), A. Wardoyo (CGG), M.L. Ghazali (CGG), J.V.S. Murthy (CGG), G.
Wang (CGG) & X.G. Miao (CGG)
SUMMARY
In offshore peninsular Malaysia there are strings of major hydrocarbon bearing fields which are affected
by shallow gas clouds. This poses a major problem in imaging using the P-wave data from conventional
streamer or OBC 2C surveys which suffer from frequency dependent attenuation, multipathing, scattering,
internal multiples, velocity inversions and mode conversions due to the gas charged sediments. 2D-4C
data is proven to be useful in areas affected by shallow gas where 2D-PS-wave imaging provides clearer
image as compared to 2D-P-wave imaging. Imaging of 3D-PS-converted-wave data presents its own
unique challenges which discourage companies from processing the converted wave component of 3D-4C-
OBC data. Through this case study we showcase three processes which have brought about significant
improvements in the 3D PS-wave time-imaging and outline the results of 3D-PS-PSDM depth-migration,
rendering the data to be highly useful for interpretation and inversion work inside the gas cloud affected
area.

2. 76th
EAGE Conference & Exhibition 2014
Amsterdam RAI, The Netherlands, 16-19 June 2014
Introduction
In offshore peninsular Malaysia there are strings of major hydrocarbon bearing fields which are
affected by shallow gas clouds. Shallow partially gas charged sands create severe problems in
imaging of conventional streamer or OBC 2C surveys and in interpretation of hydrocarbon reservoirs
and faults. Conventional P-wave imaging, in such areas, is known to suffer from frequency dependent
attenuation, multipathing, scattering, internal multiples, velocity inversions, time sagging, loss of
continuity and abnormal amplitude terminations due to the overlying gas charged sediments.
A 3D PSTM image (Figure 1a) from a survey, acquired in 2002 from the east coast of peninsular
Malaysia, illustrates the problems of imaging through such gas clouds. The survey was acquired along
the strike direction of the major E-W anticlinal structure, shooting over the multi-level stacked gas
sand layers. Reprocessing this data in 2009 through depth imaging showed improvements in the
image (Figure 1b), especially in correcting for time sags, event continuity and imaging of the E-W
and N-S trending major faults. In 2009 also, a 2D 4C pilot line was shot in the dip direction of the
structure passing through the worst affected gas wipe-out zone. The PP data showed encouraging
results because of the low frequency penetration and healing of time-sags by undershooting of the
shallow gas bodies. The PS data, however, was even more exciting as it showed for the first time, the
structural continuity of events through the gas wipe-out zone (Figure 1c).
Encouraged by the success of the 2D pilot survey, a new 3D-4C-OBC multi-component survey was
shot in 2012. In this paper, several key steps in the processing of this 3D-4C dataset are described.
Through this case study, we demonstrate processes which have brought about significant
improvements in the 3D PS-wave time-imaging and outline the results of 3D-PS-PSDM depth-
migration, rendering the data to be highly useful for interpretation and inversion work inside the gas
cloud affected area.
Method
In 2012, about 200sqkm of 3D-4C-OBC was acquired with an inline shooting geometry along a N-S
shooting direction and with a receiver line spacing of 300m. Eleven nodes were also deployed to fill
in the offsets close to an FPSO located at the center of the survey. The 4C-OBC data had hydrophones
to record pressure (P) and DSU channels to record acceleration in X, Y and Z directions. 10 seconds
of live data was recorded using 11 km of active cable length in a split-spread mode (5500 m
maximum offset) and a nominal fold of 110.
Figure 1 Example sections comparing 2002 3D PSTM (1a), 2009 3D PSDM (1b), and 2009 PS-PSTM
(1c) showing gas wipe-out effect and the utility of PS data in imaging beneath gas.
Starting from common initial pre-processing for all components, the PP data went through denoise,
PZ summation (Soubaras, 1996), de-multiple, surface consistent statics (Le Meur et al., 2011) and
amplitude corrections, and finally though 3D ray-bending anelliptic Kirchhoff pre-stack time
migration (PSTM).
(a) (b) (c)

3. 76th
EAGE Conference & Exhibition 2014
Amsterdam RAI, The Netherlands, 16-19 June 2014
For PS data, the initial processing was followed by rotation of the X and Y components to radial (R)
and transverse (T) components. After rotation, the transverse component was discarded since the
energy on the transverse component receiver gathers was weak. The radial component was processed
through noise attenuation, shear-wave statics, demultiple, vertical Vp/Vs (γ0) estimation (Thomsen,
1999), migration-velocity estimation and PS-PSTM.
Generating a good PS image required three main steps:
1) Shear wave receiver statics,
2) Receiver line interpolation, and
3) Iterative converted wave velocity (VC) and conversion point location Vp/Vs
determination (Thomsen, 1999).
Figure 2 S-wave statics map (Figure 2a) clearly showing near surface channel pattern. Figure 2b
and 2c show receiver stack before and after static correction (both 2b and 2c are in PS time).
In marine environment, S-wave statics are mainly caused due to heterogeneous nature of sediments in
the near-seabed sediments. Unconsolidated sediments, in shallow channels for example, can cause
large static values. Since P-waves are sensitive to water saturation, P-wave statics are generally small,
up to 4 or 6ms. The S-wave statics on the other hand depend on the shear strength (rigidity) or
compaction of the sediments. S-wave velocities, which can drop to as low as a few hundred meters
per second, result in large S-wave statics on the order of 100ms. Figure 2a shows a map of S-wave
statics for the survey, and comparison of a stack before and after S-wave static correction (Figures 2b
and 2c). Note the significant contribution of the static correction to an improved PS image quality.
The 300m receiver line spacing caused only a few issues in PP imaging but was found to be too sparse
to get a usable 3D-PS image. Since the upgoing S-leg of the PS data is considerably slower than the
downgoing P-leg, the CCP (common conversion point) image points are shifted very close to the
receiver. As a result, the illumination area is limited to only a small region under the receiver. To
counter this effect, a 4:1 receiver line interpolation (from 300m to 75m) was necessary. This was done
using a four dimensional, anti-leakage Fourier transform (Poole, 2010), with the four dimensions
being inline, crossline, offset, and time. Using the four dimensions simultaneously for the
interpolation helped to overcome the sampling problem in the crossline direction. The effect of this on
the final image is clearly seen in Figures 3a and 3b. Without receiver line interpolation, PS image in
the first two second (PS-time) of data would have been entirely unusable.
The positive and negative offsets of the same CCP gather experience asymmetric PS raypaths, thus
exhibiting imaging differences known as a diodic effect (Thomsen, 1999). To account for this, is
used in the migration equation by Li and Yuan (2001). The and VC fields were scanned for and
picked on PS time-migrated gathers in an iterative manner which helped to focus positive and
negative offset images and give better fault definition. Figure 4 shows comparison of stacks migrated
using initial and VC versus stacks migrated with the final and VC. The improvement in
image quality, coming from the proper focusing of the positive and negative offset data, is apparent.
The improvement in the final 3D PS PSTM stack volume is most emphatic in the focusing of events
in the first one second of data and also in the imaging of the two main faults over the crest.
(a) (b) (c)

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EAGE Conference & Exhibition 2014
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Figure 3 Effect of receiver line interpolation on PS PSTM crossline image a) without interpolation b)
with interpolation.
Figure 4 Solving for Diodic effect and focusing error due to incorrect Vp/Vs ratio. Figure 4a and 4b
show PS migrated stacks before and after iterative VC and picking.
To get the final PS image in PP time a joint interpretation of the PP and PS stack volumes is essential
to generate a final, smooth, average field. This registration process was very challenging but
yielded the final piece of the puzzle for PP and PS images to be compared and interpreted together.
Figure 5 shows a comparison between the 2009 conventional 3D-P-wave PSDM data with the present
3D-4C-OBC PP and PS volumes. As can be seen, the biggest improvements are in corrections for
time sagging, and interpretability through the central gas cloud.
Figure 5 Comparison of PP (Figure 5a and 5b) and PS (Figure 5c) stacks in PP time showing clear
improvements in continuity and fault imaging beneath gas cloud in the PS dataset.
In gas-cloud affected areas PSDM is the only final solution. An optimal PP-PSDM and especially PS-
PSDM imaging requires joint P & S wave velocity and anisotropic (VTI) parameters determination
with high accuracy. The PS-PSDM accuracy can be checked by degree of similarity between the
forward & reverse inline-shooting PS-images. Figure 6 shows the 3D-4C-OBC PP-PSDM vs PS-
PSDM comparison.
(a) (b)
(a) (b)
(a) (b) (c)

5. 76th
EAGE Conference & Exhibition 2014
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Figure 6 Comparison of PP-PSDM and PS-PSDM stacks in depth domain.
Conclusions
The PS image from the 2009 pilot study had set very high expectations from the present survey.
However, it had also given a critical understanding of what could be done at an acquisition level.
Shooting cross-dip immediately allowed the P-wave data to undershoot the shallow gas bodies to a
large extent. Also, careful processing of the PS data and developing solutions for specific challenges,
coming from the 3D nature of the survey, has resulted in a high quality image of the subsurface. This
level of detail has not been seen in more than three decades since the field was first discovered. The
PP and PS PSDM volumes from this survey are already helping to de-risk future drilling programs,
and PP-PS joint inversion, which is to follow soon, will lead to an even greater understanding of the
reservoirs. Being the first successful large scale 3D multicomponent survey in this region, this survey,
the processes adopted and the results achieved will give more confidence in multicomponent
technology as a viable solution for gas cloud imaging in the future.
Acknowledgements
We thank PETRONAS and PETRONAS Carigali Sdn. Bhd. Management and CGG Management for
permission to present this work.
References
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azimuth data. 81st
Annual International Meeting, SEG, Expanded Abstracts, 30, 1328-1332.
Li, X. and Yuan, J. [2001] Converted-wave imaging in inhomogeneous, anisotropic media—Part I
Parameter Estimation. 63rd
EAGE Conference & Exhibition.
Poole, G. [2010] 5D data reconstruction using the anti-leakage Fourier transform. 72nd
EAGE
Conference & Exhibition.
Soubaras, R. [1996] Ocean bottom hydrophone and geophone processing. 66th
Annual International
Meeting, SEG, Expanded Abstracts, 24-27.
Thomsen, L. [1999] Converted-wave reflection seismology over inhomogeneous, anisotropic media.
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