Summary. Introduction

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1 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

2 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.

3 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

4 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

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