Improved subsalt imaging and salt interpretation by RTM scenario testing and image partitioning.

Transcription

1 Improved subsalt imaging and salt interpretation by RTM scenario testing and image partitioning. Michael O Briain*, Daniel Smith, WesternGeco; Claudia Montoya, Brian Burgess, Scott Koza, Marathon Oil Company; Olga Zdraveva, Margaret Ishak, Stephen Alwon, Ryan King, Dmitry Nikolenko, Sebastien Vautier, WesternGeco/Schlumberger Summary Advances in acquisition and processing technology help overcome imaging challenges in complex structural settings. The widespread adoption of wide-azimuth (WAZ) and the move towards full-azimuth (FAZ) acquisition geometries, both combined with increasing offsets, result in significantly improved illumination. Reduced compute cost and improved performance enabled reverse time migration (RTM) to emerge as the imaging algorithm of choice in such settings. Of course, an accurate velocity model is a key component in realizing the full potential of these acquisition geometries and algorithms. The trend is towards increasingly more complex anisotropic models, with a move from vertical transverse isotropy (VTI) to tilted transverse isotropy (TTI) and even orthorhombic. In the Gulf of Mexico (GoM), though the importance of defining an accurate anisotropic model in the supra-salt section cannot be understated, the largest contributing factor to a good image subsalt is often the correct delineation of the salt body itself. Without an accurate definition of the salt geometry, the subsalt image invariably remains distorted and poorly resolved. In this paper, we will focus on this portion of the depth imaging workflow and illustrate how the techniques of RTM scenario testing and image partitioning can be used in combination to both help define the salt geometry and improve the final post-migration image. We will describe a practical workflow and the key components that we feel are necessary for its success. In addition, we will illustrate a number of lessons learned during the course of recent projects executed in the GoM. Introduction In recent years, we have seen great advances in imaging technology, which, together with new wide-azimuth acquisition, have resulted in greatly enhanced images in even the most complex structural settings (Kapoor et al., 2007). RTM has clearly emerged as the algorithm of choice for such complex areas. With more advanced algorithms comes the requirement for earth models with even greater resolution and accuracy. TTI anisotropic model building workflows using well constrained tomography to build the supra-salt velocity model were described by Zdraveva et al. (2011), and O Briain et al. (2011) and play an important role in correctly positioning the top salt reflector. Fullwaveform inversion (FWI) can also be added to further refine the model (Vigh et al. 2011). These processes all contribute to a more accurate supra-salt velocity model. However, we may fail to reap the rewards of this supra-salt model building work, in terms of an improved subsalt image, if the salt body interpretation phase fails to accurately delineate the salt geometry. This paper focuses on this part of the workflow and emphasizes that localized seismic imaging (LSI), in combination with image partitioning, can be beneficial in helping to resolve areas of complex salt. LSI RTM and lessons learned In the more traditional GoM imaging workflow, a topdown approach is used to define the salt geometry. It comprises a number of iterations of migration followed by interpretation of the salt body interfaces. With this approach scenario testing is often left to later in the sequence and then is used to refine areas of the model that remain unresolved. However, this may not represent the most effective use of resources and, with ever-increasing demands to reduce interpretation cycle time, a trend is emerging towards workflows with more emphasis on LSI scenario testing as early in the sequence as possible ( Imtiaz et al., 2012). Important aspects of LSI include: a) Identification of the shots contributing to a particular volume of interest (VOI) b) Multi-z interpretation for complex geobodies. c) Geobody mesh modeling to stitch in new geobodies. d) Triggered localized reimaging from the interpreter s environment. Experience has shown that a large number of scenarios may be needed to successfully resolve complex salt areas. Starting the scenario testing process as early as possible, or at least in parallel with the traditional route, is often beneficial. Identification of the main challenging areas using a legacy, or fast-track gross salt body model can establish a baseline from which to improve, and may prove beneficial in the long run. SEG Houston 2013 Annual Meeting Page 3856

2 The importance of a knowledgeable team of interpreters with a thorough grounding in salt tectonics and the adoption of a systematic approach to the scenario testing cannot be overstated. Frequently a gunshot approach of multiple scenarios without any logical framework does not produce the best outcome and more is not necessarily better in such instances. A more painstaking and logical approach is often preferable. On the processing side, muting the input shots prior to RTM migration is an important consideration and should only be done with extreme caution. A very mild mute applied to the shots for imaging the top salt may produce a sharper image at the reflector itself, but can have a dramatic negative impact on the deeper base salt and subsalt sections. An example of such a case is illustrated in Figure 1. We see that the application of a very mild mute on the shots input to migration has attenuated some of the base salt and subsalt energy as seen in Figure 1b. Another important consideration is the choice of the normalization scheme within the RTM algorithm itself (Cogan et al., 2011). The use of a global normalization scheme can tend to produce a cleaner section subsalt; however, a shot-by-shot normalization scheme, though often noisier at depth, can greatly improve the image of weakly illuminated salt overhang energy. An example of this is shown in Figure 2. We see that the overhang event is clearly enhanced on the shot-by-shot image (Figure 2b) and appears missing on the global image (Figure 2a). Also the subsalt events are more continuous in this case on the shotby-shot image. Figure 2: a) RTM image global normalization scheme. b) RTM image with shot-by-shot normalization scheme. An example of scenario testing and its benefits is illustrated in Figure 3. Here, we see the RTM image before scenario testing a) compared with b) after, and we see clear improvements in the subsalt image as highlighted by the red circle. The area of main model changes is displayed in the white circle. Figure 3: a) Before scenario testing. b) After scenario testing. RTM vector image partition gathers RTM is the algorithm of choice for scenario testing in imaging complex salt geometries. Further image enhancement and an improved understanding of illumination in complex areas can be achieved by production of image partition gathers, rather than the more usual image-only output (Xu et al., 2011). Figure 1: a) RTM image with no pre-migration mute. b) RTM image with mild pre-migration mute. In this process, vector image partition (VIP) gathers, rather than a single image trace, are formed during the process of collection and summation over shots. The image is partitioned and each output location is assigned to one or more image partition bins based on a particular criterion: SEG Houston 2013 Annual Meeting Page 3857

3 X,Y coordinates relative to source (Cartesian VIPs). Distance from source and Azimuth (Polar VIPs). There are three key areas where image partitioning can help the depth imaging workflow. 1) Model building scenario testing (interactive); 2) Generating pseudo-angle gathers from VIPs for model QC; and 3) Image enhancement (automated). Figure 5 shows another example of this. We see an existing salt body interpretation in a challenging area. The edge of the lower salt body is poorly defined on the full-azimuth stack image in a) and b). We see in c) how restricting the image to azimuths from the west produces a clearer image of the deeper salt interface, which would benefit from reinterpretation. Model building with image partitioning VIPs can play a valuable role in providing additional information to help with interpretation of the complex salt. However, the large number of extra volumes may slow down the interpretation process unless there is any easy method that allows for interactive stacking of individual tiles and optimization of the stack image locally in an efficient way. Practice has shown that trying to optimize the image everywhere for salt geometry interpretation is nearly impossible due to highly variable illumination from different aziumths. What we are after is interactive local optimization to assist in particularly difficult, poorly illuminated overhang areas where we are struggling to extract the last bit of information from the image. In such instances VIPs can help us Locally optimize the stacked image. Understand illumination directions/effects. Differentiate noise from true signal. Validate and refine our interpretation. Figure 5: VIP partial stack of azimuths, a) Velocity model all azimuths, b) Stack all azimuths, and c) Stack W azimuth. Figure 4 is an example showing the sensitivity of the final image in a suture area to the azimuth. We observe that the best image of the left flank of the suture is seen while stacking a) NE-SW, while the best image of the base salt is seen on b) E-W. Figure 4: VIP partial stack of azimuths a) NE-SW, b) E-W, and c) SE-NW. Image enhancement and image partitioning Image partitioning also affords us the ability to do further enhancement and optimization of the migrated image for intermediate or final products. Different techniques exist to combine the partially imaged traces in such a way as to produce this improved image. These are automated techniques that rely on crosscorrelation and attribute generation approaches to design a series of weights to be applied to the individual traces prior to summation. Attributes used may include coherence or a similar attribute. Alternatively, a geological framework may be used to drive the process. Typically, a number of such enhanced volumes are produced with versions varying from mild to more aggressive, depending on taste and purpose. These can be very beneficial, particularly for enhancing the broad structural picture while the raw unaltered image is always saved and available as a final reference. An example of VIP image enhancement is shown in Figure 6 where we see the raw image a) compared with an enhanced image b). SEG Houston 2013 Annual Meeting Page 3858

4 far offsets that can be used for interpretation, validation, and model building. Figure 6: Image enhancement, a) Raw image, and b) Enhanced image. Cartesian or polar A question to consider when producing VIPs is whether to use Cartesian or polar partitioning. A second question is how many tiles to use. Some of the factors to consider are: polar tiles have the advantage of having a more intuitive representation of azimuth and, therefore, might be preferred when using VIPs for assisting model building and interpretation. Polar tiles with dense offset sampling are also preferred if one is hoping to do a further conversion of the offset azimuth traces to angle azimuth traces for velocity model QC. The decision on the number of tiles will also depend on their intended use. For assisting model interpretation, less is more in terms of keeping the number of stack volumes to a manageable level. However, for post-image enhancement, more is more up to a point, and a small number of tiles will tend to limit one s ability to optimize the image. Figure 7 shows a comparison of image enhancement using a small number of tiles b) compared with a larger number c) and d). We see that the larger number of tiles allows for more aggressive enhancement. A practical approach that we adopted on a number of projects is the use of polar partitioning into VIPs with 6 azimuths and 30 offsets giving a total of 180 tiles. This large number of tiles has the advantage of having more than adequate sampling for image enhancement, but also sufficient traces for the conversion of VIP offset azimuth gathers to VIP pseudo-angle gathers that can be used for final velocity model QC. Summing over 10 offsets produces a useful 18 tile VIPs representing near, mid, and Figure 7: Image enhancement examples: a) No enhancement, b) Enhanced 16 (4x4) Cartesian Tiles, c) Enhanced 49 (7x7) Cartesian Tiles and d) Enhanced 180 (6x30) Polar tiles. Summary and conclusions In this paper, we illustrated the use of localized seismic imaging to help resolve complex salt geometries on projects in the Gulf of Mexico and outlined some of the lessons learned while undertaking these projects. We also showed how image partitioning and VIPs can be a valuable aid in assisting the interpretation and model building process and how they can also afford one many options for further post-imaging enhancement. Acknowledgments We thank Marathon Oil Company for their collaboration and permission to show their data examples. We also thank Bob Vauthrin, Clive Gerrard, Alfonso Gonzalez, Dawit Desta, Lucia Mattiangeli Cara Montgomery and many other colleagues from WesternGeco for assistance with these case studies. SEG Houston 2013 Annual Meeting Page 3859

5 EDITED REFERENCES Note: This reference list is a copy-edited version of the reference list submitted by the author. Reference lists for the 2013 SEG Technical Program Expanded Abstracts have been copy edited so that references provided with the online metadata for each paper will achieve a high degree of linking to cited sources that appear on the Web. REFERENCES Cogan, M., R. Fletcher, R. King, and D. Nichols, 2011 Normalization strategies for reverse-timemigration: 81 st Annual International Meeting, SEG, Expanded Abstracts, Imtiaz, A., F. Ryan, K. Chandan, and F. Billette, 2012, Interactive velocity model validation (IVMV): Presented at the 82 nd Annual International Meeting, SEG. Kapoor, S., N. Moldevaneau, M. Egan, M. O Briain, D. Desta, I. Atakishiev, M. Tomida, and L. Stewart, 2007, Subsalt imaging: The RAZ-WAZ experience: The Leading Edge, 26, , O Briain, M., T. Jones, and T. Ho Wai-Ching, Kastner, D. Griffin, O. Zdraveva, M. Woodward, C. Ennen, 2011, Leveraging anisotropic workflows in changing times: Two case studies from the eastern Gulf of Mexico: Presented at the 81 st Annual International Meeting, SEG. Vigh, D., J. Kapoor, N. Moldoveanu, and H. Li, 2011, Breakthrough acquisition and technologies for subsalt imaging: Geophysics, 76, no. 5, WB41 WB51, Xu, Q., Y. Li, X. Yu, and Y. Huang, 2011, Reverse time migration using vector offset output to improve subsalt imaging A case study at the Walker Ridge GOM: Presented at the 73 rd Annual International Conference and Exhibition, EAGE. Zdraveva, O., M. Cogan, R. Hubbard, M. O Briain, and D. Watts, 2011, Anisotropic model building in complex media: Comparing three successful strategies in deep water Gulf of Mexico: 81 st Annual International Meeting, SEG, Expanded Abstracts, SEG Houston 2013 Annual Meeting Page 3860