Removing Fault Shadow Distortions by Fault Constrained Tomography.
Sergey Birdus, CGGVeritas
Summary
Fault Shadows manifest a serious challenge to successful
seismic imaging. The major part of this problem is caused
by rapid lateral velocity changes within fault zones.
Seismic rays traveling through fault areas experience
geometrical and traveltime distortions which result in poor
seismic images and non-hyperbolic moveout anomalies in
areas below such fault planes (Fault Shadow Zones). Fault
Constrained Tomography (FCT) is a special depth
processing technique developed to solve this problem by
building detailed high resolution interval velocity model for
such zones. Combined with Pre-Stack Depth Migration
(PSDM) this technique allows to remove Fault Shadow
distortions from seismic images.
In reality we always have a combination of these factors.
All these types of velocity variations exhibit strong and fast
lateral changes and can be described as short wave-length
velocity anomalies. We use "short wave-length" title for an
anomaly with lateral length smaller than the cable length.
These anomalies:
- cause non-hyperbolic moveout;
- cannot be restored from RMS-velocities by Dix-based
velocity transformations;
- seismic tomography is the only current tool capable of
building velocity model with short wave-length anomalies
using seismic data;
- PSDM with proper velocity model is the tool that can
remove their distortions from seismic images.
Fault related velocity anomalies
Depth-velocity modeling in Fault Shadow zones
Fault zones contain several different types of velocity
anomalies:
1. Model driven velocity anomalies are created when faster
velocity rocks contact slower velocity rocks across a fault
plane. This is the most obvious type of fault related
velocity anomalies which can be predicted and included
into depth-velocity model if sufficient amount of a-priory
geological information is
available (well based velocity data, interpreted horizons,
fault planes). In reality, this information is limited and can
give only limited solution to this part of the problem. For
many years, depth processing has been focusing on this
type of anomalies using
horizon based depth-velocity
modeling.
Normally, detailed depth-velocity model is built by the
following sequence:
A. Initial modeling. With regard to Fault Shadows we need
to put as much as possible a-priory geological information
into an initial model to accommodate "Model Driven
Velocity Anomalies".
B. PSDM with initial model;
C. Depth residual analysis to collect information about
residual moveout left after "B"; this residual moveout
shows how accurate our velocity model is and can be used
to improve the model;
D. Model update by seismic tomography (Zhou et al.,
2003). This step uses residual information gathered in "C"
to change interval velocity model. Avoiding complex
mathematical equations we can say that seismic
tomography distributes residuals along seismic rays and
explains measured residual field as a result of velocity
anomalies through which these rays have traveled.
Depending on input data and objectives, it can be very time
consuming and sometimes unstable procedure which
requires some data and model regularization to build
geologically plausible model.
Steps B-D are repeated iteratively until desirable result is
achieved.
2. Fluids and pore pressure related velocity anomalies are
very important. Faults serving as paths or seals for vertical
and horizontal fluids movement significantly change
"normal" fluids and pore pressure distribution and create
additional intensive interval velocity anomalies.
3. Imaging velocities used for PSDM always represent a
simplified smoothed copy of real interval velocity field.
Sonic logs may show a number of thin high contrast
velocity layers that cannot be included into imaging PSDM
velocity model because of their small thickness and lateral
inconsistency. Imaging model integrates these thin layers
into much larger velocity features. It works fine
everywhere except fault zones, where a fault can displace
high contrast velocity layers and create distortions to
seismic rays traveling through these areas. In order to fix
these distortions we need to put non-geological anomalies
into imaging PSDM velocity model.
Working within Fault Shadow zones our main challenge is
to build accurate depth-velocity model with sufficient
lateral and vertical resolution. Our experience shows that
under standard conditions we need velocity models with 50
m and higher resolution to successfully accommodate fault
related anomalies. Standard depth processing techniques
cannot achieve this level of resolution due to unacceptable
Removing Fault Shadow Distortions by Fault Constrained Tomography.
computation cost and problems with getting converging
tomographic solution.
Fault Constrained Tomography
Special technique called Fault Constrained Tomography
has been developed to build high-resolution interval
velocity models for fault zones. Distinctive features of this
technique (figure 1) are:
- Fault planes are picked and included into depth-velocity
model;
- Non-hyperbolic Residual Curvature Analysis (RCA)
carried out on dense grid of PSDM gathers;
- High-resolution 3D seismic tomography using fault
planes as a constrain to update velocity. It allows
unrestricted velocity variations and achieves required high
Figure 1. Fault Constrained Tomography Scheme.
velocity resolution but only within limited zones
corresponding to fault planes. This type of model
regularization (model constrain) corresponds to the nature
of fault related velocity anomalies.
Depending on geological settings, several iterations of
PSDM followed by RCA and Fault Constrained
tomography can be required. These iterations are run in
addition to standard iterative global tomography sequence
(Zhou et al., 2003), but because they effectively solve all
fault related problems the total turnaround time of depth
processing project can even be reduced.
Fault Constrained Tomography uses distortions in seismic
data cased by faults to restore the adequate velocity model
to allow PSDM to remove these distortions from seismic
images.
Removing Fault Shadow Distortions by Fault Constrained Tomography.
Examples
Fault Constrained Tomography has been employed on
several real 3D seismic datasets with severe Fault Shadow
imaging distortions
and successfully resolved these
problems (figures 2 , 3).
Analysis of fault related velocity variations revealed by
Fault Constrained Tomography (figure 1) showed presence
of all 3 types of velocity anomalies listed above.
Figure 2. a – PSTM image with typical Fault Shadow anomaly; b- PSDM section migrated with model built by Fault
Constrained tomography. Data courtesy Talisman Energy, Petronas Carigali, PIDC and PetroVietnam.
Removing Fault Shadow Distortions by Fault Constrained Tomography.
Figure3. a – PSDM section after several iterations of standard tomographic model update before applying Fault Constrained
Tomography; b – the same after applying FCT. Data courtesy ExxonMobil and Petronas Carigali.
Conclusions
Acknowledgments
Fault Constrained Tomography takes into account the
nature of velocity variation in fault zones. Combined with
PSDM it can successfully remove fault shadow distortions
from seismic images.
Author would like to thank Talisman Energy, ExxonMobil,
Petronas Carigali, PIDC, PetroVietnam and CGGVeritas
for carrying out these projects and permission to present the
results