G022 Multi-azimuth Seismic Data Imaging in the Presence of Orthorhombic Anisotropy

Transcription

1 G0 Multi-azimuth Seismic Data Imaging in the Presence of Orthorhomic Anisotropy Y. Xie* (CGGVeritas), S. Birdus (CGGVeritas), J. Sun (CGGVeritas) & C. Notfors (CGGVeritas) SUMMARY The presence of orthorhomic anisotropy can seerely affect the imaging of multi- and wide- azimuth data. Analysis of multi-azimuth (MAZ) data often reeals noticeale fluctuations in moeout etween different acquisition directions, preenting constructie summation of MAZ images. Vertical transerse isotropy (VTI) effects can also cause well misties and higher order moeout. We hae deeloped an approach for imaging in the presence of orthorhomic anisotropy. Both synthetic and real data from offshore Australia show that our approach can take into account the co-existing /VTI effects of orthorhomic anisotropic media, reduce the structural discrepancies etween seismic images uilt for different azimuths, producing a constructie summation of MAZ dataset, resoling well misties, and deliering a step-change in the final seismic image quality. 73 rd EAGE Conference & Exhiition incorporating SPE EUROPEC 0 Vienna, Austria, 3-6 May 0

2 Introduction Orthorhomic anisotropy is considered to e the simplest realistic symmetry for many geophysical prolems (Tsankin, 997a). Orthorhomic anisotropy exhiits oth (Horizontal Transerse Anisotropy) and VTI (Vertical Transerse Anisotropy) effects, making seismic elocity arying with azimuthal direction as well as polar direction. In many cases, these effects are important y themseles and can e a target of special studies. The presence of orthorhomic anisotropy poses challenges in wide azimuth imaging. Analysis of multi-azimuth (MAZ) data often reeals noticeale fluctuations in moeout etween different acquisition directions, preenting constructie summation of MAZ images (Dickinson et al 00). VTI effects can also cause well misties and higher order moeout. Orthorhomic anisotropy takes into account the co-existing /VTI effects. Thus, orthorhomic anisotropic PSDM can reduce the structural discrepancies etween seismic images uilt for different azimuths, producing a constructie summation of MAZ dataset and resoling well misties. In the following, we first descrie our approach for imaging in the presence of orthorhomic anisotropy. Our orthorhomic PSDM approach is ased on Tsankin s work on anisotropy parameters and P-wae elocity for orthorhomic anisotropy. We deelop our ray tracing method suitale for oth weak and strong anisotropy, which applies to oth Kirchhoff and Beam PSDM (Notfors et al, 006). In order to demonstrate the aility of our method to perform multi-azimuth imaging in the presence of orthorhomic anisotropy, we apply our method to a 3D multi-azimuth data set acquired offshore Australia, which exhiits oth azimuthal anisotropy and ertical transerse isotropy. By using our orthorhomic PSDM approach, we managed to account for oth and VTI effects present in the field seismic data. Theory and Method Tsankin (997a) introduced the notation for orthorhomic anisotropy along the lines of Thomsen s parameters for VTI media. A typical orthorhomic symmetry model is shown as Figure (Tsankin, 997a). The ertical symmetry planes are defined y the directions parallel and normal to the cracks. For P-wae imaging, the required parameters for an orthorhomic model include: V P 0, the ertical elocity of the P-wae;,, anisotropy parameters in the symmetry plane x x 3, normal to symmetry axis x ;,, anisotropy parameters in the symmetry plane x x 3, normal to symmetry axis x ; 3, the VTI parameter of plane x x, where x plays the role of the symmetry axis, and is the azimuth angle of symmetry x. Following Tsankin s work, we propose to compute the phase elocity in the following form, which remoes the weak anisotropy restriction: V ( ) sin ( ) sin (, ) ( ) sin V p 0 4 cos cos 4 ( ) sin ( ( ) sin 3 ) cos 8( ( ) ( )) sin cos. () where V (, ) is the phase elocity with as ray polar angle, as the azimuth angle of ray relatie to symmetry axis x. Equation () gies an azimuth angle and polar angle dependent phase elocity. Under acoustic approximation for orthorhomic media, we define the eikonal G as G ( )( p p ) p (( ( ))( p p ) p ) 8( ( ) ( ))( p p ) p () 73 rd EAGE Conference & Exhiition incorporating SPE EUROPEC 0 Vienna, Austria, 3-6 May 0

3 where p, p, p are the normalized slowness components (Zhou et al, 003, Huang et al, 007) x y z p x p. /, p p /, p p / x p 0 y y p 0 z z p 0 By defining the Hamiltonian H ( x, p ) V ( x)( G ) / 4 p, we uild the orthorhomic ray tracing 0 system, which honours oth azimuthal angle and polar angle dependency of wae propagation. The ray tracing system can e used for oth Kirchhoff and Beam PSDM. Multi- and wide azimuth sureys proide the elocity information along different azimuths. Hung et al (006) and Womell (006) descrie how to estimate azimuthal anisotropy parameters for conentional narrow azimuth marine streamer data. Moreoer, with ertical check shot sureys, ertical elocity at well locations can e accurately otained. By making use of linear slip theory (Schoenerg et al, 997, Bakulin et al, 000), the effectie anisotropy parameters of an orthorhomic model can e related to the set of parameters and the set of VTI parameters as follows:. The symmetry plane x x 3, orthogonal to the symmetry axis x, is purely VTI, solely determined y the estimated set of VTI parameters,, VTI VTI (3). In the symmetry plane x x 3 orthogonal to the symmetry axis x, we hae where and and,, (4) VTI VTI are related to the conentional rotated Thomsen s parameters as following (Tsankin, 997), /( ), ( ( )) /( ) (5) The parameters take into account and correct for the azimuthally dependent elocity ariation. The VTI parameters proide the measure for slowing down the apparent elocity to match with geology and correct for the higher order moeout when necessary. Examples The first example compares the impulse responses of isotropic, VTI, and orthorhomic PSDM respectiely. Input impulse sits at inline 0, crossline 0. Tale lists the properties of the medium. In oth and orthorhomic cases, the azimuth angle of the slowest elocity direction (normal to fracture) is along the inline direction. Figure compares the migrated sections of inline 0. It clearly shows that the presence of causes azimuthally dependent wae propagation; VTI causes a polar angle dependent wae propagation, making eents shallower for this case with 0. Orthorhomic effects cause oth azimuthally and polar dependency of wae propagation. In the plane along the crossline direction, orthorhomic PSDM produces the same result as that y VTI PSDM as VTI is the only anisotropy in this plane. The plane along inline direction is determined y the comination of oth and VTI effects. This shows that y introducing orthorhomic anisotropy, we can account for oth azimuthally dependent and polar dependent wae propagation. In this synthetic test, it images reflectors at shallower depths and makes elocity azimuthally dependent. Orthorhomic PSDM was applied to a real dual azimuth 3D seismic dataset that had een acquired in an area of the North-West Australian shelf with known strong horizontal elocity anisotropy. As expected, the data were affected y oth horizontal azimuthal and VTI elocity anisotropy. Figure 4 shows the initial isotropic PSDM gather and moeout at the same common reflection point, ut of different sureys (with 35 and 89 degree acquisition azimuths respectiely). The azimuthally dependent ariation of residual moeout clearly shows the presence of azimuthal anisotropy. Figure 5 shows the gather and moeout y PSDM. By taking into account, azimuthally dependent residual moeout was taken care of, ut the gather still has weak non-hyperolic residual moeout, indicating the presence of VTI anisotropy. An orthorhomic elocity model was then uilt ased on 73 rd EAGE Conference & Exhiition incorporating SPE EUROPEC 0 Vienna, Austria, 3-6 May 0

4 the estimated and VTI models. Figure 6 shows the orthorhomic PSDM results. Orthorhomic modeling and imaging takes into account the co-located VTI and effects, further flattens the gathers of oth sureys, makes eents shallower as expected. Figure 7 compares the gamma (residual elocity analysis) map from initial isotropic PSDM and orthorhomic imaging. Orthorhomic imaging effectiely flattens the gathers of different azimuth sureys as well as non-hyperolic moeout due to VTI (warmer color: more residual moeout, cooler color: less residual moeout). Conclusions We hae deeloped and demonstrated an effectie method for P-wae imaging in the presence of orthorhomic anisotropy. Synthetic and real data results show that our method can e used to effectiely take into account oth azimuthally and polar angle dependent wae propagation, which is of particular importance for multi- and wide azimuth sureys which hae large azimuth coerage for a etter constructie stack of image migrated from different azimuth surey and for etter tie with well. Extending this work to orthorhomic systems tilted relatie to the horizontal direction (the orthorhomic analog of tilted transerse isotropy) is straightforward. References A. Bakulin, V. Grechka, and I. Tsankin, 000, Estimation of fracture parameters from reflection seismic data - Part II: Fractured models with orthorhomic symmetry: Geophysics, 65, D. Dickinson, T. Ridsdill-Smith, 00, Benefits of Multi-azimuth Depth Migration oer the Tidepole Field, North West Shelf, Australia: 7 nd Annual International Coference and Exhiition, EAGE, Extended Astracts, B044. T. Huang, S. Xu, Y. Zhang, 007, Anisotropic Estimation for Prestack Depth Imaging - A Tomographic Approach: 77th Annual International Meeting, SEG, Expanded Astracts, 4-8. B. Hung, F.M. Zhang, J. Sun, M. Stanley, A. Osadchuk, 006, An Automated 3D Method for Azimuthal Anisotropy Analysis in Marine Seismic Data, 68th Annual International Coference and Exhiition, EAGE, Extended Astracts, H035. C. Notfors, Y. Xie, and S. Gray, 006, Gaussian Beam Migration: A Viale Alternatie to Kirchhoff?: 68th Annual International Coference and Exhiition, EAGE, Extended Astracts, G046. M. Schoenerg, K. Helig, 997, Orthorhomic media: Modeling elastic wae ehaior in a ertically fractured earth: Geophysics, 6, I. Tsankin, 997a, Anisotropic parameters and P-wae elocity for orthorhomic media, Geophysics 6, I. Tsankin, 997, Reflection moeout and parameter estimation for horizontal transerse isotropy, Geophysics 6, R. Womell, 006, Characteristics of azimuthal anisotropy in narrow-azimuth marine streamer data, 68th Annual International Coference and Exhiition, EAGE, Extended Astracts. H. Zhou, D. Pham, S. Gray, and B. Wang, 003, 3-D tomography analysis in TTI media: 73rd Annual International Meeting, SEG, Expanded Astracts, Tale Medium property for impulse response tests Medium m/s Anisotropy parameters using Thomsen p 0 and Tsankin s notation Isotropic 000 NIL VTI , /, 0 Figure An orthorhomic model caused y parallel ertical cracks emedded in a medium Orthorhomic , 0.043, 0., 0 3 composed of thin horizontal layers. Orthorhomic media hae three mutually orthogonal planes of minor symmetry. From Tsankin 997a. 73 rd EAGE Conference & Exhiition incorporating SPE EUROPEC 0 Vienna, Austria, 3-6 May 0

5 a c d Figure Inline 0 of impulse responses (in color) y a) isotropic PSDM, ) VTI PSDM, c) PSDM, and d) orthorhomic PSDM, all oerlain with isotropic PSDM result (in grey), in depth domain. a c d Figure 3 Time slice of impulse responses y a) isotropic PSDM, ) VTI PSDM, c) PSDM, and d) orthorhomic PSDM, conerted to time domain, at.6sec a a Weak non-hyperolic moeout Figure 4 Isotropic PSDM, gather and semlance display of the Figure 5 PSDM, gather and semlance display of the same same CRP location. a) surey with 89 degree acquisition azimuth, CRP location. a) surey with 89 degree acquisition azimuth, ) ) surey with 35 degree acquisition surey. effect can clearly surey with 35 degree acquisition azimuth. Weak non-hyperolic e seen. residual moeout indicates the existence of VTI a Non-hyperolic moeout was taken into account Figure 6 Orthorhomic PSDM, gather and semlance display of the same CRP location. a) surey with 89 degree acquisition azimuth, ) surey with 35 degree acquisition azimuth. Takes into account collocated and VTI effects. Figure 7 Gamma map at depth 900m. a) Isotropic PSDM, surey with 89 degree acquisition azimuth; ) Isotropic PSDM, surey with 35 degree acquisition azimuth; c) Orthorhomic PSDM, surey with 89 degree acquisition azimuth; d) Orthorhomic PSDM, surey with 35 degree acquisition azimuth. Gamma olume measures the residual moeout. Warm color: more residual moeout, cool color: less residual moeout. a c d 73 rd EAGE Conference & Exhiition incorporating SPE EUROPEC 0 Vienna, Austria, 3-6 May 0