3D beam prestack depth migration with examples from around the world

A Publication of Petroleum Geo-Services Vol. 8 No. 8 August 2008 3D beam prestack depth migration with examples from around the world Introduction In 1999 AGS specialized in 2D seismic depth processing. It was commercially necessary to progress into 3D depth migration, but it seemed unrealistic to utilize Kirchhoff or wave equation methods and be able to compete with large companies, their compute resources, and their economies of scale. Consequently, it was decided to attempt the commercial development of an alternative approach that would hopefully provide similar or improved technical quality, but with capital outlay and economics that were more appropriate to a small company. A very straightforward approach referred to as Beam Pre-Stack Depth Migration (BPSDM) was adopted, this even having its origins in work performed as long ago as the 1930 s by Frank Rieber. The anticipated merits of BPSDM were overall simplicity, economy, flexibility and future development possibilities. The unknowns were the migration accuracy and quality achievable. BPSDM was essentially developed independently, but aspects of it obviously are related to much recently published work on other beam migrations and stereotomography Method The input seismic data is assumed to have been prepared with conventional preprocessing. Thereafter, BPSDM consists of three important steps: decomposition, migration, and reconstruction. Decomposition A multidimensional slant stack decomposes the data into a basis of seismic wavelets, from 100-200 ms in time duration, within local surface spatial superbins each being centered on a uniform Cartesian grid of CDP xline and inline co-ordinates (x,y) and S-R half offset co-ordinates (hx,hy). Each wavelet should thus have a center location (x,y,hx,hy,t) and determined dip components (dt/dx, dt/dy, dt/dhx, dt/dhy), these contributing to the properties of the wavelet. For each spatial axis the grid interval between superbin centers is typically around 200 to 400 m, and the superbin width is chosen somewhat larger in order to have overlap. Summary In 1999 AGS (now part of PGS) started development of Beam Pre-Stack Depth Migration (BPSDM). The anticipated merits were simplicity, economy, flexibility and future development possibilities. The unknowns were the migration accuracy and quality. BPSDM consists of three important steps; decomposition, migration, and reconstruction. Decomposition is a multidimensional slant stack which decomposes the data into a basis of seismic wavelets. Each wavelet has a center location and determined dip components. These contribute to the properties of the wavelet. Migration computes a point to point mapping between the unmigrated and the migrated wavelet centers. Reconstruction composites each wavelet into its local region in migrated space and outputs seismic depth traces. Initial results in 1999 were poor, but with further development BPSBM proved to have some superior aspects to Kirchhoff and wave equation migration. The quality and flexibility of BPSDM includes capability for handling steep and overturned dips, demultiple options, the ability to reject coherent noise, incorporation of anisotropy, speed for migration iteration, calculation of residual 3D RNMO, extension to multi- and wide azimuth data acquisition, and the capacity to handle land and marine data. Data examples from around the world illustrate the quality and flexibility of the technique. Figure 1 is a variable density display of typical seismic trace behavior in time and one spatial dimension, x. It is evident that these data are a superposition of several wavelets with both positive and negative dips components, dt/dx, all of which can be estimated with a Continued on next page

TechLink August 2008 Page 2 Method Continued from Page 1 slant stack method. A schematic representation of one such wavelet with time dip, px=dt/dx, is displayed in Figure 2. It is significant to note that this decomposition performs a desirable uniform spatial gridding or binning of the data, the result being essentially independent of minor variations in the data acquisition geometry (minimized acquisition footprint). Also, the individual wavelets should not exhibit any aliasing effects, even for very steep time dips. Thus, the BPSDM procedure bypasses the aliasing issues that are a very significant problem in both Kirchhoff and downward continuation, wave equation migration. Note also that the wavelet amplitude is preserved, a vital issue for any future AVO or inversion operations, and that some random noise suppression is achieved at this early stage. severe steep dip and turning wave limitations of typical wave equation migrations. Economics often force aperture limits in Kirchhoff migration and can result in not imaging steep dips and turning waves. The BPSDM independent point to point mapping of each wavelet clearly has no migration aperture issue. Figure 3: Schematic representation of the point to point mapping between a wavelet s recording parameters (surface location, dip components, and time) and its migrated reflector location and dip. Figure 1: Variable density display of typical data in time and a single spatial dimension, x.. Figure 2: Schematic representation of a single wavelet in time and a single spatial dimension, x. Migration Given a wavelet s center (x,y,hx,hy,t) and dip components (dt/dx, dt/dy, dt/dhx, dt/dhy), plus a current earth velocity model, it is possible to 3D ray trace from source and receiver locations and determine some corresponding best reflector migration location (xmig,ymig,zmig), together with ancillary properties such as reflector dip, reflector azimuth, angle of incidence at reflector, shot and receiver wave front curvatures, local interval velocity, etc. It is of paramount significance that a point to point mapping exists between the unmigrated and the migrated center of each seismic wavelet. This means that the seismic wavelet decomposition forms a basis in the migrated domain as well as the unmigrated domain, the mapping function being the earth velocity model. Note that migration is applied directly to each coherent wavelet without creating a travel time table in Cartesian coordinates. This bypasses the need to select a single raypath when gridding raypaths; the source of the multipath traveltime problem encountered in Kirchhoff migration. Also, note that BPSDM does not suffer from the Figure 3 is a schematic for an ideal primary turning wave reflection, where the ray paths with estimated dips at the shot and receiver intersect at the associated reflector at the correct two way traveltime. However, since shot and receiver rays corresponding to a wavelet s properties normally will not intersect to give a model two way traveltime exactly equal to the wavelet center time, t, it is also sensible to estimate a focusing quality factor, q. This is an additional valuable wavelet property. If the wavelet is truly a primary reflection then q is representative of traveltime discrepancy along the computed ray path, and can be included in a tomography method. Alternatively, a poor focus value can be used to recognize a multiple reflection wavelet, or a converted wave event with its NMO offset dip (dt/dhx, dt/dhy) significantly different from that of a corresponding primary reflection. In summary, for a current earth velocity model the migration operation determines and stores reflector location and associated properties along with each wavelet and its surface location and dip parameters. This stored point to point mapping between unmigrated and migrated space is very valuable, and is not directly available with the Kirchhoff and wave equation methods. Reconstruction A seismic wavelet can easily contributed to its local region in either unmigrated space or migrated space. In particular, the local nature of a wavelet and its associated migration properties enables a very limited wavefront Kirchhoff migration contribution to a 3D migrated depth volume (x,y,z) for the common offset (hx,hy). This yields certain improved signal-to-noise characteristics over normal full wavefront Kirchhoff migration, where data from

Page 3 A Publication of Petroleum Geo-Services millions of seismic traces do not necessarily cancel in an output quiet reflector area, such as salt. Figure 4 illustrates the contribution to the migrated reflector space of the wavelet depicted in the point to point mapping in Figure 3. routine and enables the updating of the earth interval velocity model. The migration, reconstruction, and tomography steps are then iterated until a satisfactory velocity model is developed for which the common reflection point depth gathers are adequately flat. The final common offset volumes are reconstructed on an appropriately fine (x,y,z) grid. Since this operation is reasonably economic, it is normal to output volumes over the entire (x,y) common mid-point range of the input data. Figure 4: A single wavelet contributes to a local region of the output migrated depth volume. Relevant ray path spreading properties enable amplitude correction of each primary reflection for its actual propagation path through the interval velocity model. This facilitates later AVO or inversion operations. Note that any wavelet can be excluded or weighted down based upon a variety of individual or joint criteria for the wavelet properties, for example the quality of focus, thereby providing a powerful flexibility for coherent noise reduction in the unmigrated or the depth migrated data. Inline, xline or full volumes are output on appropriate grids, both for quality control and for residual moveout analysis on common reflection point depth gathers. The residual moveout field is interpreted either manually or automatically, depending on the complexity of the data. This information is supplied to a tomography Results Initial results within 1999 were poor, but after one or two years of further development proved to be superior to Kirchhoff and wave equation results in several respects. The quality and flexibility of BPSDM has been illustrated in many ways, such as its unique capability for handling steep and overturned dip, its demultiple options, the ability to handle extraneous coherent noise, the adaptability to anisotropic velocity earth models, its speed for iteration and the calculation of residual 3D RNMO for input to 3D tomography, the extension to multi- and wide azimuth data acquisition, and the capacity to handle both land and marine data. Figure 5 is a comparison of Kirchhoff time migration and BPSDM applied to data from the Beaufort Sea (both plotted in time to help direct comparison). BPSDM was very successful at imaging steep dips where Kirchhoff PSTM had failed. Based on these images the structure is interpreted as an inversion anticline, not a shale diapir. The interpretation is thus utterly different. Figure 6 shows CRP offset gathers and stacks created using BPSDM on data from the KG-D4 basin from southwest India. After several iterations of migration and Continued on next page Figure 5: Kirchhoff prestack time migration (left) and BPSDM (right) on Beaufort Sea data. The beam migration imaged steep and overturned events.

TechLink August 2008 Page 4 Results Continued from Page 3 tomography there was still residual moveout due to unresolved local velocity anomalies associated with a rugose water bottom, slump sequences, and a strong velocity increase at the basement. After our efforts to solve the problem using velocity estimation and depth migration, further improvement was achieved using a non-hyperbolic residual moveout. The migrated offset gathers output by BPSDM allowed the flexibility required to address these data problems. Figure 7 is a line processed with BPSDM in the central Gulf of Mexico. Several known fields are indicated on this cross section. Play 1 appears as a high amplitude that correlates with structure. Subsalt play 2 and the recent discovery are truncations against the base salt. There are some interesting large amplitudes near the Play 2. Play 3 appears as high amplitudes associated with an anticline, a fault, and pinchouts on the top salt. Subsalt Play 4 is associated with truncations on the base salt near the right edge of the section. Figure 6: BPSDM from the KG-D4 basin, southwest of India. The stack and migrated gathers show residual moveout due to local unresolved velocity anomalies associated with a rugose water bottom and slump sequences. Residual moveout can be used to flatten the gathers and improve the stack. Figure 7: BPSDM of a line from the deep water Gulf of Mexico through proven hydrocarbon fields and a recent discovery.

Page 5 A Publication of Petroleum Geo-Services Figures 8 through 12 show BPSDM results from southwest India. Volcanics were expected in this area and a strong velocity increase was identified below an unconformity identified in Figure 8. Seismic analyses indicate the velocity is significantly faster below the unconformity, but not fast enough to be basalt. Depth migration velocity of approximately 4000 m/s indicates this is potentially the top of a carbonate zone. Deeper, potentially volcanic Figure 8: BPSDM of a line from southwest India. Velocity analysis indicates the unconformity is structures are identified on the inline potentially the top of a carbonate zone. There are deeper, potentially volcanic structures. and cross-line sections (Figures 8 and 9). The depth structure map of the horizon we identified as potentially the top of a carbonate zone (Figure 10) shows several faults or channels. One fault or channel shows offset when it crosses another fault. A structural high appears above the volcanics identified in Figures 8 and 9. Amplitude extraction maps on the right half of the structure map in Figure 10 are shown in Figure 11. The plot on the left is the amplitude 112 m above the horizon and the one on the right is 114 m above the horizons. Two interesting high amplitude anomalies are labeled Amp 1 and Amp 2. These amplitudes are high on structure (see Figure 12). Current water depth (1500 m) is very deep for reefs. Perhaps the area was much shallower water at the time of deposition and these amplitudes are hydrocarbon indicators. Continued on next page Figure 9: Xline section from southwest India project with unconformity and volcanic structures identified. Figure 10: Potential Top of Carbonate Depth Structure Map from the southwest India project. Several faults or channels are identified. One fault or channel shows offset when it crosses another fault. A structural high appears above the volcanics identified in Figures 8 and 9.

TechLink August 2008 Page 6 Results Continued from Page 5 Figure 13 shows the BPSDM result from a line in the Arabian Sea. The depth migrated section is corendered with the velocity estimated with depth migration tomography. Blocks can be seen that slip down a large extensional fault. The blocks detach producing a series of toe thrusts. The low velocity (blue and green) beneath the toe thrusts may indicate shale. Conclusions BPSDM does have the anticipated merits of simplicity, economy, flexibility, and future development possibilities. Migrated images have excellent accuracy and quality, especially in areas of poor signal-tonoise ratio and steep dip. BPSDM s relative economy makes it an excellent velocity estimation tool to use prior to other more compute intensive depth migration methods. Figure 11: Amplitude extraction maps on the right half of the structure map in Figure 10. Amplitude extracted 112 m above the depth mapped in Figure 10 (left plot) shows a high amplitude anomaly in blue labeled Amp 1. A similar high amplitude on the amplitude extracted 144 m above the horizon depth (right plot) is labeled Amp 2. These amplitudes are high on structure (see Figure 12). Current water depth (1500 m) is very deep for reefs. Perhaps the area was much shallower water at the time of deposition and these amplitudes are hydrocarbon indicators. Acknowledgements The authors thank Devon Canada for permission to show the Beaufort Sea data, PGS MultiClient group for permission to show the deep water Gulf of Mexico data, Reliance for permission to show the KG-D4, southwest India, and Arabian Sea data. We also thank the AGS data processing and interpretation staff for their truly dedicated efforts applying BPSDM to numerous datasets. Figure 12: Cross-line through the amplitude anomalies identified on Figure 11. These amplitude anomalies are high on structure and there is interesting signal attenuation below the high amplitudes. CONTACT Petroleum Geo-Services London Tel: 44-1932-260-001 Fax: 44-1932-266-465 Oslo Tel: 47-67-52-6400 Fax: 47-67-52-6464 Houston Tel: 1-281-509-8000 Fax: 1-281-509-8500 Singapore Tel: 65-6735-6411 Fax: 65-6735-6413 2008 Petroleum Geo-Services. All Rights Reserved Figure 13: BPSDM from the Arabian Sea. The depth migrated section is co rendered with the velocity estimated with depth migration tomography. Blocks can be seen that slip down a large extensional fault. The blocks detach producing a series of toe thrusts. The low velocity (blue and green) beneath the toe thrusts may indicate shale. For Updates on PGS Technological Advances, visit www.pgs.com More TechLinks at www.pgs.com/techlink