Summary. Introduction

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Crystal Betty Houston
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1 Improved suslt imging using TTI nisotropy nd reverse time migrtion scns Duryodhn Epili*, George Cloudy Jr., Jun Ci, Quincy Zhng, Roy Cmp nd Simon Lopez-Mor, TGS Summry We present n dvnced velocity model uilding nd imging methodology tht resulted in significnt enhncements in defining the slt flnks with overhngs nd suslt structures. The key technologies used were () True Azimuth Multiple Elimintion (TAME), () Tilted Trnsverse Isotropy (TTI) model uilding (FAN), (c) TTI Reverse Time Migrtion (RTM), (d) RTM Delyed Imging Time (DIT) scns nd (e) post-migrtion multiple ttenution. Approximtely 39 OCS locks of the Len nrrow-zimuth survey locted in the Mississippi Cnyon re of the Gulf of Mexico were imged. The re ws previously imged with the Verticl Trnsverse Isotropy (VTI) Kirchhoff migrtion. Significnt improvements hve een chieved y reprocessing the sme dt with the new imging methodology. Introduction It is well known tht nisotropy must e tken into ccount for successful imging of Gulf of Mexico seismic dt. Whiteside et l. (2008) noted tht typicl depthing errors using the isotropiclly derived model in this region rnged from 5% to 12% t depth of 6 km. Trditionl VTI imging ssumes the velocity chnges re symmetric long the verticl xis. However, the VTI ssumption often reks down for dipping lyers in minisins nd steep dip trunctions ginst se of slt. TTI honors the trend of the depositionl systems nd incorportes geologicl informtion, thus providing etter velocity model nd seismic imge. The current study re, the Len project, is locted in the Mississippi Cnyon re of the Gulf of Mexico. The PSDM re comprised 39 OCS input locks (909 sq km). The input dt to the 3D pre-stck depth migrtion ws preprocessed to improve the signl to noise rtio. This flow included swell noise interference removl nd 3D surfcerelted multiple ttenution. Deule nd wter column sttic correction steps were lso pplied. The TTI model uilding workflow used for this dt set is given in Figure 1. There re five spects of this project descried elow tht mke it unique: (1) TAME, modified version of SRME, where the prediction of multiples is performed using the true zimuth, (2) FAN to define TTI nisotropic prmeters, (3) TTI RTM s model uilding tool s erly s possile in the work flow to clerly define the slt geometries, (4) RTM DIT scns to updte the suslt velocities nd (5) post-migrtion ttriute sed residul multiple ttenution. Focusing Anlysis for delt & epsilon Figure 1: TTI model uilding work flow. Multiple ttenution Build initil isotropic velocity model from check shots Isotropic depth migrtion Convert verticl velocity Vz to V0 TTI migrtion Updte nisotropy models if needed Updte V0 y Tomogrphy TTI Migrtion Slt Interprettion Suslt Velocity updte Finl TTI Migrtion Dip nd zimuth ngles We used TAME nd high-resolution residul multiple Rdon filter for multiple ttenution. TAME is dt driven, convolution sed surfce relted multiple prediction nd ttenution technology. The detils of the technique re descried in Ci et l. (2010). It includes three mjor components: (1) dt interpoltion/ regulriztion, (2) multiple prediction nd (3) sutrction. Figures 2 nd 2 show the input nd output from the TAME processing. SEG Sn Antonio 2011 Annul Meeting 243

Suslt imging using TTI nd RTM scns interpolting nd smoothing long the horizons. The estimted δ nd ε models were used for ll susequent itertions of nisotropic migrtion.

Using velocity perpendiculr to the edding (V0, computed from Vz), ε, δ, dip nd zimuth, first pss of TTI Kirchhoff migrtion ws performed to check the gther fltness, focusing nd well ties.

TTI Focusing Anlysis (FAN) Thirty-one check shots were nlyzed in the survey re.

2 Suslt imging using TTI nd RTM scns interpolting nd smoothing long the horizons. The estimted δ nd ε models were used for ll susequent itertions of nisotropic migrtion. The nisotropy hd mximum of 6% for δ nd 9% for ε. Using velocity perpendiculr to the edding (V0, computed from Vz), ε, δ, dip nd zimuth, first pss of TTI Kirchhoff migrtion ws performed to check the gther fltness, focusing nd well ties. Figures 3 nd 3 show the gthers from isotropic nd nisotropic migrtions. Figure 2: Shot record () input () fter TAME dptive sutrction. TTI Focusing Anlysis (FAN) Thirty-one check shots were nlyzed in the survey re. The seismic velocity model from prior isotropic Kirchhoff migrtion ws hevily smoothed nd clirted to generte the initil isotropic velocity model (Vz). The resulting verticl velocity model ws used to generte isotropic PSDM imge gthers (using Kirchhoff migrtion) nd to estimte the TTI nisotropy prmeters epsilon (ε) nd delt (δ). These fields were derived using n utomted FAN methodology. The detils of the FAN pproch re descried in Ci et l. (2009) nd He et l. (2009), therefore; only rief summry is given here. The specific steps for the FAN nlysis re s follows: (1) Tke zero-offset migrtion imge point in Common Imge Gther (CIG) s focl point nd perform ry sed offset dependent demigrtion to get the correct focusing opertor in the time domin. (2) Construct the clculted focusing opertors for the current nisotropy model from the focl point for different ε nd δ vlues. The serch for the correct ε nd δ is done utomticlly using L1 optimiztion criteri (to minimize the difference etween the clculted nd the true focusing opertors). The vlidity of ε nd δ is evluted y the fltness of the imge gthers. (3) Construct volume of epsilon nd delt through interpoltion long key horizons. We interpreted seven key horizons to define the xes of symmetry nd uilt nisotropic model volumes (δ, ε) y Figure 3: CIGs ner well loction efore () nd fter () nisotropic Kirchhoff migrtion. Velocity Model Building nd Slt Geometry Definition Three itertions of volume sed high-resolution grid tomogrphy were performed to updte the supr-slt sediment velocity model. For ech of the tomogrphy itertions, 3D TTI nisotropic pre-stck Kirchhoff depth migrtion ws run. Automtic residul curvture nlysis on the resulting imge gthers nd dip estimtion on the PSDM stck volume were performed for use in the tomogrphy. Cre ws tken to msk out rys pssing through slt during the ry trcing stge. V0 ws updted from the inversion results. Using multi-scle itertive pproch, long wvelength fetures of velocity nomlies were derived first. The short wvelength nomlies were grdully dded in the susequent itertions. Gther fltness, event focusing nd well ties were checked fter ech itertion, nd it ws noted tht further compenstion for ε nd δ ws not needed. The finl sediment velocity model ws vlidted ginst check shot velocities nd slt top picks. The slt top picks from wells mtch with the PSDM seismic with less thn 0.1% error indicting the ccurcy of the velocity model. SEG Sn Antonio 2011 Annul Meeting 244

Suslt Velocity Model Updte (RTM DIT Scn) DIT scn is set of RTM imges, typiclly 21 imges creted during the regulr RTM run y pplying the regulr zero-time imging condition nd time-shifted imging

3 Suslt imging using TTI nd RTM scns TTI RTM nd TTI Kirchhoff migrtion were run to define the slt geometry (top, se nd overhngs). After the slt model definition, grid sed suslt tomogrphy ws run to enhnce the suslt events followed y RTM DIT scns. Suslt Velocity Model Updte (RTM DIT Scn) DIT scn is set of RTM imges, typiclly 21 imges creted during the regulr RTM run y pplying the regulr zero-time imging condition nd time-shifted imging condition. The detils of this technique re given in Wng et l. (2009). The min steps for RTM DIT scns suslt velocity updting re s follows: 1) Post-Migrtion Demultiple A post-migrtion demultiple technique ws performed which is sed on multiple prediction nd dptive sutrction method (Guo et l., 2008; Wng et l., 2010). The multiple of the se of slt (BOS) surfce is predicted y ry trcing. Adptive sutrction is used to ttenute the residul multiples ccording to the predicted multiple surfce. Figures 5 nd 5 show the effectiveness of the post-migrtion demultiple technique. Run RTM to crete 21 DIT scn imges, perform gther conditioning nd form semlnce pnels. Use non-liner serch to find the est DIT vlues, which mximize the semlnces. Compute the residul velocity using the picked DITs. Updte the suslt velocity y pplying smoothed residul velocities. Compute the composite imge using the ckcomputed DIT vlues to check the result. 2) 3) 4) 5) Composite imges re formed y selecting the DIT pnels which produce the est focused, coherent imges for qulity control purposes. Figures 4 nd 4 show the enhncements due to the DIT scns model updte. The se of the turtle structure is improved fter running RTM on the DIT updted velocity model. Figure 5: TTI Kirchhoff PSDM stck efore () nd fter () post-migrtion demultiple processing. Imge Improvements We uilt geologiclly constrined nisotropic models using focusing nlysis sed TTI prmeter estimtion methodology nd volume sed grid tomogrphy tht tie the well informtion. The fult imging improved through detiled nisotropic model uilding. We uilt slt models with itertive ppliction of TTI Kirchhoff nd TTI RTM. This helped us delinete the correct slt geometry wheres the previous imged Kirchhoff VTI volumes filed to do so. Figure 4: Imge enhncement due to DIT scn velocity updte efore () nd fter (). Figures 6, 6 nd 6c re the inline comprisons of VTI Kirchhoff PSDM (previously depth processed), current TTI SEG Sn Antonio 2011 Annul Meeting 245

4 Suslt imging using TTI nd RTM scns Kirchhoff PSDM nd TTI RTM PSDM over complex slt ody. Note the clrity of the sediment trunction ginst the steeply dipping slt flnk nd the suslt reflections in the TTI RTM imge. 6 Conclusions The improvements to the imging were ccomplished y () removing s much noise s possile from the input dt without ffecting the qulity of the signl, nd () utilizing new technologies such s true zimuth multiple elimintion, TTI focusing nlysis for nisotropic prmeter estimtion, TTI Kirchhoff, TTI RTM, RTM DIT scns for velocity updte nd post-migrtion demultiple. A comintion of enhnced model uilding nd migrtion methodology is the key to the success of the Len project. Acknowledgments The uthors would like to thnk their collegues Itze Chng, ZhongYng Dong, Smpth Gjwd, Mnhong Guo, Yng He, Jen Ji, Zhiming Li, Xinyi Sun, Xuening M, Gry Rodriguez, Boedi Soelistijo, Bin Wng nd Kwngjin Yoon for their vlule contriution during vrious phses of velocity model uilding, interprettion nd depth migrtion processing. We lso thnk Lurie Geiger nd Wilfred Whiteside for reviewing the mnuscript nd Simon Bldock nd Rndy Heert for their support on this project. The uthors wish to thnk TGS mngement for support nd relese of the mteril for puliction. 6c 6 Figure 6: Inline comprisons of () VTI Kirchhoff, () TTI Kirchhoff nd (c) TTI RTM PSDMs over complex slt ody. Note the improvements in slt ody geometry, overhngs nd the suslt reflections in TTI RTM section. SEG Sn Antonio 2011 Annul Meeting 246

5 EDITED REFERENCES Note: This reference list is copy-edited version of the reference list sumitted y the uthor. Reference lists for the 2011 SEG Technicl Progrm Expnded Astrcts hve een copy edited so tht references provided with the online metdt for ech pper will chieve high degree of linking to cited sources tht pper on the We. REFERENCES Ci, J., M. Guo, S. Dong, R. Cmp, G. Ar, nd B. Wng, 2010, True zimuth surfce multiple elimintion: 80 th Annul Interntionl Meeting, SEG, Expnded Astrcts, 29, Ci, J., Y. He, Z. Li, B. Wng, nd M. Guo, 2009, TTI/VTI nisotropy estimtion y focusing nlysis, Prt I: Theory: 79th Annul Interntionl Meeting, SEG, Expnded Astrcts, 28, Guo, M., K. Young, S. Sen, J. Xu, J. Xie, nd B. Wng, 2008, A new multiple sutrction method using the ttriutes of predicted multiples: 78th Annul Interntionl Meeting, SEG, Expnded Astrcts, 27, He, Y., J. Ci, W. Whiteside, Z. Li, M. Guo, B. Wng, J. Xie, K. Yoon, nd E. Nessler, 2009, TTI/VTI nisotropy estimtion y focusing nlysis, Prt II: ppliction: 79th Annul Interntionl Meeting, SEG, Expnded Astrcts, 28, 311. Wng, B., C. Mson, M. Guo, J. Ci, S. Gjwd, nd D. Epili, 2010, Interctive demultiple in the postmigrtion depth domin: 80th Annul Interntionl Meeting, SEG, Expnded Astrcts, 29, Wng, B., C. Mson, M. Guo, K. Yoon, J. Ci, J. Ji, nd Z. Li, 2009, Suslt velocity updte nd composite imging using reverse-time migrtion sed delyed-imging time scn: Geophysics, 74, no. 6, WCA159 WCA166. Whiteside, W., W. Xu, Z. Li, A. Lundy, nd I. Chng, 2008, Enhnced nisotropic model uilding methodology nd prestck depth imging in deep wter Gulf of Mexico: A cse history: 80th Annul Interntionl Meeting, SEG, Expnded Astrcts, 27, SEG Sn Antonio 2011 Annul Meeting 247

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