Multi-layer tomography and its application for improved depth

Multi-layer tomography and its application for improved depth

Multi-layer tomography and its application for improved depth imaging
Patrice Guillaume, Steve Hollingworth*, Xiaoming Zhang, Anthony Prescott, Richard Jupp, Gilles Lambaré,
Owen Pape, CGGVeritas
Summary
The application of a new ray-based multi-layer approach
for reflection tomography overcomes many of the
limitations of tomographic methods typically employed on
PSDM projects. Global tomography is unable to accurately
represent major velocity and anisotropy contrasts within the
framework of a cell based model representation and will
not correctly reposition layer boundaries during the
inversion process. These limitations necessitate the use of a
layer stripping workflow on areas with a complex layered
geology. However, such a workflow is time consuming and
prone to velocity errors being propagated into deeper layers
as the model building progresses. Multi-layer tomography,
utilizing a new hybrid model representation, allows layer
boundaries to be accurately represented and repositioned by
map migration within the inversion. This allows all units in
a complex layered model to be updated simultaneously
without the need for a layer stripping workflow. This
results in a more efficient velocity inversion workflow and
improved quality of both models and imaged seismic data.
of meters, in order to limit the amount of null space in the
inversion matrix and to keep memory requirements within
acceptable limits. This fundamentally limits the ability of
the cell model to accurately represent layer boundaries. In
addition, matrix inversion schemes typically rely on the cell
dimensions being fixed during the tomographic inversion
Introduction
Pre-stack depth migration for seismic imaging is now
commonplace within the oil and gas industry and the use of
grid based reflection tomography is generally accepted as
the standard for velocity model estimation (Woodward et
al., 1998; Guillaume et al., 2001; Zhou et al., 2003).
Deriving a velocity model which yields the optimum image
for reservoir level studies can be a very time consuming
process, particularly in areas with a complex layered
geology. Improving the efficiency of pre-stack depth
migration projects to permit earlier prospect evaluation has
become increasing important. However, maintaining the
desired level of model quality within an acceptable time
frame is a significant challenge for existing tomography
techniques. Here we describe the application of a multilayer tomography method which is both efficient and
produces a dramatic improvement in model and image.
Layer-Stripping Tomography
Many regions of the world exhibit a complex layered
geology with significant velocity and anisotropy contrasts
which present a major challenge for standard grid based
tomography methods. Most tomography methods employ a
cell based representation to define the model velocities,
where the cell dimensions are held constant or are allowed
to vary at different depth levels within the model. Typically
the cell dimensions (xyz) are on a scale of tens to hundreds
Figure 1: Input and output velocities from multi-layer tomography
showing residual moveout (RMO) facets and horizons.
a) Initial model showing error in RMO facets (blue = too slow, red
= too fast, white = optimised) and starting horizons.
b) Final model showing re-migrated and predicted minimized
RMO facets and map migrated horizons
Multi-Layer Tomography and its application for improved depth imaging
process and therefore do not change dynamically to
preserve the travel time of velocity boundaries represented
in the cell model. Using reflection tomography to globally
update a model which contains velocity contrasts can
introduce significant errors into the inverted model in the
vicinity of the layer boundaries. Typically the positions of
the layer boundaries in the model become uncoupled from
the corresponding reflector positions in the updated seismic
image.
In order to accommodate these limitations, layer stripping
approaches are used to enable accurate repositioning of
velocity and anisotropy boundaries (Evans et al., 2005;
Jones et al., 2007). The model is divided into a set of layers
defined by the major velocity boundaries, which are
updated iteratively in a top-down manner. For the inversion
of a particular layer, the velocity and anisotropy from the
target layer are allowed to “flood” through the boundary
position. Residual move out from PSDM image gathers is
used only for the layer to be updated. Following the
tomographic update of the layer velocities, the data are remigrated and the correct position of the base of layer
boundary re-interpreted prior to final calibration to the
wells. For a typical North Sea project, as many as six
iterations of layer stripping may be required to correctly
handle all the major velocity and anisotropy contrasts
exhibited by the geology.
As well as being time consuming, layer stripping is also
prone to serious velocity errors. Since the method only
treats one layer at a time in a top-down manner, it precludes
any intercommunication between connected model layers
during the inversion process. In addition, it is common
practice to “freeze” a layer once it has been updated. While
this avoids corrupting the depth position of carefully
interpreted layer boundaries, it also results in residual
velocity errors being propagated into deeper model layers.
layers contributes to the global inversion scheme, resulting
in a significant improvement in overall model stability.
Furthermore, the integrated horizon information in the
hybrid model allows each model layer to be uniquely
parameterized and constrained to reduce the null space and
to achieve the best possible inversion result. Layers no
longer need to be frozen during the inversion process, since
the method will allow any layer to be updated during model
building without compromising the result. In addition,
since the entire initial model is updated during each pass of
multi-layer tomography, improvements to the imaging at
deeper reservoir levels can be monitored at all stages of
model development.
The minimization of residual move out error predicted by
the non-linear tomography and the spatial repositioning of
both the picks and horizon interfaces can be visualized
interactively. Figure 1 shows the initial and final models
from a multi-layer inversion, with both re-migrated
horizons and residual move out facets displayed. It can be
seen that the horizons remain registered to the boundaries
in the updated velocity model and residual move out error
is minimized. The reimaged picks and horizons can be used
to determine the optimum parameters for the inversion
without the need to generate a large number of PSDM tests.
Improvements in model accuracy and stability should
naturally translate into improved seismic images with less
reflector distortion. Discarding the traditional layer
stripping workflow and returning to a global approach for
complex layered geology should also yield significant
efficiencies in model building and interpretation effort.
Multi-Layer Tomography
Multi-layer tomography is an extension of non-linear slope
tomography (Guillaume et al., 2008; Montel et al., 2009). It
uses a new hybrid model format which uniquely defines the
velocity and anisotropy parameters for each model layer as
a mesh while also carrying the precise information for the
layer boundaries as horizons. The non-linear inversion
scheme performs kinematic de-migration and re-migration
of both the residual move out picks and the layer
boundaries.
Since the layer boundaries are repositioned by map
migration during the inversion process, preserving their
travel time, the traditional layer stripping workflow can be
discarded and all layers in the model updated
simultaneously. Residual move out information from all
Figure 2: Model schematic showing layers with significant
differences in velocity. The traditional layer stripping workflow
divided the model into five iterations.
Multi-Layer Tomography and its application for improved depth imaging
Example
In this example we compare model and image results
obtained using standard layer stripping and multi-layer
tomography on a PSDM project from the Q7/Q10 area of
the Dutch North Sea. Velocities within the survey area are
strongly controlled by the stratigraphy, with large velocity
contrasts between layers. Large-scale faulting has displaced
the high velocity Chalk, so that it is adjacent to the slower
Middle and Lower Cretaceous. This geology is very
challenging for layer stripping tomography since the
workflow is unable to fully utilize the pick information in
adjacent layers due to the presence of large lateral and
vertical velocity contrasts (Figure 2).
updated with a single pass of tomography, as all layer
boundaries are repositioned by integrated map migration.
Residual move out from all model layers can contribute to
the inversion result producing improvements in both
velocity model stability and seismic imaging. Traditional
layer stripping can be replaced with an efficient global
inversion without compromising on quality.
Acknowledgments
The authors would like to thank Smart Energy Solutions
B.V, PA Resources UK Ltd and EBN B.V. for permission
to present the Q7/Q10 results and CGGVeritas for
permission to publish this work.
Model building and imaging of the area was initially
completed using a traditional layer stripping workflow.
This consisted of five iterations of velocity inversion,
structural re-interpretation and residual well calibration.
The initial model was constructed from supplied horizons
and velocity profiles estimated from well data within the
area. A 1D horizon based update was carried out for the
first iteration in order to introduce the long wavelength
velocity variations for each of the model layers. This was
followed by four iterations of layer stripping (Figure 2).
In order to assess the potential benefit of multi-layer
tomography on the area, the initial model and initial RMO
picks were used to perform 2 passes of velocity inversion,
updating the entire model from top to bottom. Layer
boundary repositioning was handled within the tomography
and results were calibrated to the well data post inversion.
Comparison of the models from the two approaches (Figure
3) clearly demonstrates an increase in stability and
geological consistency with the multi-layer approach. This
is particularly noticeable within the Middle Cretaceous and
Jurassic, where velocity anomalies are clearly present in the
layer stripping result due to the propagation of shallow
velocity errors from layers which have been “frozen”.
Conversely the multi-layer inversion produces a much
simpler and more geologically consistent velocity
distribution. Comparison of the PSDM results also reveals
a significant improvement in imaging. Reflectors located
within the Middle Cretaceous and beneath the thrust fault
show less image distortion and a marked improvement in
continuity sub-BCU (Figure 4). Imaging of the complex
faulted section within the Middle/Lower Cretaceous is also
significantly improved with better sub-fault reflector
continuity down to the Jurassic (Figure 5).
Conclusions
The application of Multi-layer tomography for imaging of
complex layered geology provides significant advantages
over traditional layer stripping. The entire model can be
Figure 3: Final velocities from the different tomography methods
a) Layer stripping tomography showing model instabilities at the
Middle Cretaceous and Jurassic levels. b) Multi-layer tomography
showing improved stability and geologic consistency
Multi-Layer Tomography and its application for improved depth imaging
Figure 4: PSDM imaging beneath a thrust fault and sub-BCU a) Layer stripping tomography shows distortion Mid Cretaceous and poor
reflector continuity sub-BCU. b) Multi-layer tomography shows reduced reflector distortion and improved continuity.
Figure 5: PSDM imaging in a complex faulted region a) Layer stripping tomography struggles to image the faulting and sub-fault reflectors.
b) Multi-layer tomography provides an improved image of both faults and reflector continuity down to the Jurassic
References:
Evans, E., M. Papouin, S. Abedi, M. Gauer, P. Smith, I. F. Jones, 2005. Southern North Sea PreSDM imaging using hybrid
gridded tomography: 75th Annual International Meeting, SEG, Expanded Abstract, 2530–2533.
Guillaume, P., F. Audebert, P. Berthet,, B. David, A. Herrenschmidt, and X. Zhang, 2001. 3D Finite-offset tomographic
inversion of CRP-scan data, with or without anisotropy, 71st annual SEG meeting, Expanded Abstracts, 718-721, INV2.2.
Guillaume, P., G. Lambaré, O. Leblanc, P. Mitouard, J. Le Moigne, J. P. Montel, A. Prescott, R. Siliqi, N. Vidal, X. Zhang,
and S. Zimine, 2008. Kinematic invariants: an efficient and flexible approach for velocity model building: 78th Annual
International Meeting, SEG, workshop “Advanced velocity model building techniques for depth imaging”.
Jones, I. F., M. J. Sugrue, and P. B. Hardy, 2007, Hybrid gridded tomography: First Break, 25, no. 4, 15–21.
Montel, J.-P., N. Deladerriere, P. Guillaume, G. Lambaré, A. Prescott, J. P. Touré, Y. Traonmilin, and X. Zhang, 2009.
Kinematic invariants describing locally coherent events: an efficient and flexible approach to non-linear tomography: 71st
Annual International Meeting, EAGE, – Workshop WS 1: Locally Coherent Events – A New Perspective for Seismic
Imaging.
Woodward, M., P. Farmer, D. Nichols, S. Charles, 1998. Automated 3D tomographic velocity analysis of residual move-out
in pre-stack depth migrated common image point gathers, 68th annual SEG meeting, Expanded Abstracts, 1218-1221.
Zhou, H., S. Gray, J. Young, D. Pham, and Y. Zhang, 2003, Tomographic residual curvature analysis: The process and its
components: 73rd Annual International Meeting, SEG, Expanded Abstracts, 666–669.