H Seismic imaging in and around salt bodies: problems and pitfalls. Ian Jones 1, Ian Davison 2. Summary

Transcription

1 H1-2-3 Seismic imaging in and around salt bodies: problems and pitfalls Ian Jones 1, Ian Davison 2 1 ION, Chertsey, UK, 2 EarthMoves, Egham, UK Summary Salt movement often results in steeply-dipping complex structures, which pose significant challenges for model building and migration. In recent years, advances in seismic imaging algorithms have permitted imaging of steep structures by exploiting the two-way wave equation via the introduction of reverse time migration (RTM). With such imaging algorithms, double bounces and turning wave reflections can be imaged, thereby enabling the imaging of vertical and overturned salt flanks. However, despite advances in the migration algorithms, the derivation of a suitable earth model incorporating the anisotropic behaviour of the velocity field remains a significant challenge, requiring tight integration of geological interpretation, and geophysical skills. A major contributing factor to the successful execution of a complex salt imaging project, is the understanding of the many and varied pitfalls involved at every stage of the process. Here we describe and discuss some of these issues, building on the observations made at the recent EAGE salt workshops. The various aspects described in this review paper all contribute to the overall uncertainty associated with creating and understanding images of complex structures. Introduction: Physical properties of evaporites Pure halite has a velocity of 4500 ms -1 and this often is assumed to be a constant in velocity modelling and processing. However, many salt bodies contain varying amounts of anhydrite, (velocity 6500ms 1 ),or K- Mg-rich salts such as sylvinite, carnallite, and tachyhydrite (velocities as slow as 3500ms -1 ).. All deformed salt bodies contain mineral grains which are preferentially elongated in the flow direction; vertically in salt diapirs and horizontally in source layers. The inherent seismic velocity anisotropy has been measured at up to 7% faster in the flow direction (Kendall and Raymer, 1999, Landrø et al. 2011). However, there has been little attempt to incorporate salt anisotropy into velocity model building. Figure 1 shows a section through the Epsilon salt diapir, demonstrating the complex nature of the evaporite layering, in this case especially with fast anhydrite layers on the diapir flanks that can confound attempts to build a salt model. Figure 2 shows a mapped geological vertical section through the Riedel salt dome, clearly indicating the impracticality of attempting to characterize the polar axis of any anisotropy that could result from the emplacement flow.

2 Figure 1. Left Seismic image of the Epsilon salt diapir, Norwegian North Sea and detailed interpretation up of the diapir stem showing the well trajectory. (Jackson and Lewis, 2012). Reproduced with permission from Geological Society of London. Right Figure 2. Vertical cross section of the Reidel Salt Dome NW Germany. From Schachl The blue lines indicate the complex repeatedly reflecting raypath that sound may take from a surface source to a receiver during seismic acquisition. The black arrows indicate the localised fast sound speed directions, which follow the flow structure. Velocity Model Building This remains the least well addressed issue in contemporary imaging today (Jones 2010). There is great promise for dealing with the very near surface offered by waveform inversion, but deeper problems are still very challenging. Even for laterally invariant velocity fields, the reduction in rayangle coverage associated with a velocity inversion (such as often occurs at the base of salt) severely limits velocity estimation procedures based on observed residual moveout in CRP gathers. Given that the complex raypaths associated with salt bodies often involve laterally propagating energy, in order to construct a reliable image, we must have a good understanding of the relationship between the vertical and horizontal components of velocity: in other words, the anisotropy parameters. Characterizing anisotropy associated with salt movement, even in the neighbouring sediment layers can be difficult. For example, to define the polar axis for anisotropy, we often take the reflectivity structural axis to be acceptably representative. However, as indicated in the cartoon of Figure 3, if we have salt movement during deposition of the overlying sedimentary layers, then we can have decoupling of the polar and structural axes. In addition, the ambient stress field will further alter the anisotropic behaviour, as can be observed when fractures with a preferred orientation are predominantly opened in a given direction (producing orthorhombic rather than simple TTI anisotropy, e.g. Li 2012; Zdraveva 2012, as indicated in Figure 4).

3 Figure 3. Left: for structural deformation after deposition, the orientation of platy minerals (or diagenetic mineralization) will parallel the underlying bedding, and with subsequent deformation the polar axis of the mineral grains will still be conformable to the bedding axis. However, if the underlying strata are being continuously deformed during deposition, then the grains will initially be deposited locally flat lying, with a vertical polar axis, but this will be tilted by further ongoing deformation. Right Figure 4: for a TTI description, the fast direction is nominally parallel to the bedding planes, or at least related to them. However, if we have fracture sets orthogonal to the bedding planes, as may occur due to salt-related deformation or external stresses, then this will slow-down sound propagation along the beds, and an orthorhombic description of anisotropy may be better suited to describe the problem (from Jones and Davison, 2014). Migration It is now widely accepted that reverse-time migration (RTM) is most appropriate for complex imaging (Leveille et al. 2011), but perhaps the reasons why, are not generally understood. The main advantages of RTM are that a high-order finite difference (FD) operator can handle rapidly varying velocity fields such as those associated with salt bodies, and that solving simultaneously for both the downgoing and upcoming wavefields facilitates imaging of double bounces and turning arrivals. Failure to adequately deal with any of these issues can result in spurious and misleading images. Complex arrivals in the data (such as double bounces, and through salt reflection travel paths) can produce many confusing features, which if not correctly addressed will create classes of image noise further confusing interpretation. Figures 5 and 6 show the possible ray paths in a simple salt-wall model and an associated shot gather. The plethora of non-intuitive arrivals in the gather can produce artefacts if not correctly imaged. Figure 5. Left: selection of raypaths in a simple salt model. Figure 6 Right: events in the shot record corresponding to the raypaths outlined show on the model. a) Simple reflection of the flat layer. b) Double bounce from the flat and vertical interfaces. c) Limiting case for the farthest double bounce and simple reflector. d) Upcoming headwave. e) Upcoming headwave after reflection from the vertical salt wall. f) Nearest reflected refraction. g) Upcoming scatter at the top salt corner from downgoing direct energy (we also have similar scatter from upcoming double bounce arrivals). h) Downward scattered energy reflecting back up from the flat reflector. i) Base salt corner upcoming scatter. j) Base salt upcoming scatter travelling in the salt. Some of the artefacts produced in RTM need to be addressed in a pre-stack migrated domain (not inherently produced by RTM). Hence various additional techniques need to be adopted to suppress image contamination (e.g Jones 2014). Figures 7 indicated the wave paths created during an RTM for both the downgoing and upcoming wavefields: as part of the imaging condition (multiplying the two wavefields) an unwanted atefact contribution is produced that imposes a low frequency background noise on the image. In addition, very rapid lateral velocity change, such as encountered where steeply dipping flank sediments abut a salt wall, also create undesired events. Figures 8 and 9 show an extended-imaging condition time-shift gather before and after filtering in the angle domain. Figures 10 and 11 show the corresponding RTM images (the yellow arrow is the

4 location of the artefact and of the gathyers shown in Figures 8 and n9). one such example where near-vertical artefacts (that confuse interpretation of the salt flank) have been removed by filtering in an angle domain. Figure 7, Left:. two-way shot migration imaging condition for a simple reflector. The downgoing source-side wavefield has energy on the downgoing path, but also creates a contribution back up along the upgoing path. Likewise, the upcoming receiver-side wavefield propagated back into the earth towards the reflector also reflects back upwards towards the source. Consequently, as well as creating the desired image contribution for the reflector, the imaging condition from multiplying both wavefields together produces an unwanted image contribution that has to be removed (the grey region along coincident portions of the ray paths). Figure 8: Centre transformed time shift gather at the location indicated by the yellow arrow in Figure 10. The central trace from this gather (at shift=0) is the actual seismic trace at that location (as seen in Figure 10): only flat-lying events in this domain constitute real image energy. Right: gathers after filtering in the angle domain. Figure 10. Near vertical RTM artifact emanating from a strong reflector termination indicated by yellow arrow. Figure 11 Right:. Image after filtering of RTM angle gathers. (GXT RTM image shown courtesy of Talisman Sinopec Energy UK and partners GdF-Suez, EON, & Idemitsu. Input data courtesy of CGG). In addition, for conventional marine streamer acquisition, we routinely ignore the existence of shear mode wave propagation. However, for salt bodies we need to concern ourselves with PSSP and PSPP/PSPP arrivals because energy propagating on these mode-paths will contaminate a conventional image, and occasionally, if we migrate the data with a shear-wave velocity model, we can perhaps obtain a useful shear image of the base salt (e.g. Lewis 2006) to aid with interpretation and reduce structural uncertainty. Figure 12 shows an example of migrating to form a converted mode imagine of the base salt to assist in interpretation.

5 Figure 12. Pre-SDM performed using S-wave velocity in the salt geobody. This gives us a differently illuminated image of the base salt to assist in its picking. Whereas in a conventional migration with the P-wave salt velocity model, the P-wave base salt image will be in the correct location, and the S-wave image too deep, when we migrate with an S-wave salt velocity, the S- wave image should be at the correct location and the P-wave image too shallow. Picking both can sometimes yield a more complete base salt interpretation. Discussion Interpretational confusion exists in assessing images of complex structures if great care is not taken in building an acceptable representative subsurface parameter model or if the migration scheme used is not carefully adapted to avoid production of artefacts. References Jones, I.F, 2014, Tutorial: migration imaging conditions. First Break, 32, no.12, Jones, I.F, and Davison, I., 2014, Seismic imaging in and around salt bodies. SEG Interpretation, 2, no.4, SL1-SL20. Jones, I.F., 2010, An introduction to velocity model building, EAGE, ISBN , 296 pages. Kendall R.R., and Raymer, D.G., 1999, Processing and Interpretation of VSP Data to Determine Salt Anisotropy - Mahogany Field, Gulf of Mexico, EAGE 61 st Conference and Technical Exhibition 1999 Landrø, M., Puigdefabregas, C., and Arntsen, B, 2011, Anisotropy in the salt outcrop at Cardona, Catalonia implications for seismic imaging; First Break, 29, no.8, Leveille, J.P., Jones, I.F., Zhou, Z-Z., Wang, B., Liu, F., 2011, Subsalt Imaging for Exploration, Production and Development: A Review. Geophysics, 76, no.5, WB3-WB20. Lewis, J., 2006, The potential of mode-converted waves in salt interpretation. SEG/EAGE summer research workshop, Utah. Li, Y, 2012, Velocity Model Building for Tilted Orthorhombic Depth Imaging, Proceedings of the SEG/EAGE Summer Research Workshop. Charleston: Velocity Model Building in Complex Geology. Zdraveva, O., 2012, Anisotropic Model Building in Complex Media: VTI, TTI, or Orthorhombic: Proceedings of the SEG/EAGE Summer Research Workshop. Charleston: Velocity Model Building in Complex Geology.

6