VSP 1.5. Main Menu. SEG/Houston 2005 Annual Meeting 2625

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1 3C VSP Tomogrphy Inversion for Subsurfce P- nd S-Wve Velocity Distribution Yingping Li, Xiomin Zho, Rn Zhou, nd Dvid Dushmn, VSFusion, A Bker Hughes CGG Compny Peter Jnk, TOTAL E&P USA, Inc. Summry First brek times (FBT) of P-wve nd S-wves picked from three-component (3C) VSP dt were used to produce tomogrphic velocity profiles nd Vp/Vs rtio distribution. 3C Eigenvlue rottion is criticl for correctly identifying nd ccurtely picking FBTs of S-wves. The resultnt tomogrphic imges provide importnt informtion for formtion evlution nd produce suitble velocity models for improving migrtion imging of subsurfce structures. Introduction Seismic tomogrphy is method for finding the subsurfce velocity distribution from multitude of observtions, such s VSP, cross-well, nd surfce seismic dt. The VSP trveltime tomogrphy cn provide vluble informtion bout complicted geologicl structures nd produce suitble velocity model for depth migrtion imging (e.g., Stewrt et l., 1987; Slo nd Schuster, 1989; Zhou nd Hou, 2000). Lpin et l. (2003) crried out joint VSP nd surfce seismic tomogrphy. Li et l. (2003) successfully used VSP tomogrphy to delinete the slt-sediment boundry. Tomogrphic Method nd Dt The trveltime dt, T, cn be written s non-liner function, F, of the velocity model, m, T = F(m). (1) The gol of tomogrphic inversion is to find model (m) which exhibits the formtion chrcteristics nd cn reproduce the trveltime dt (T). An objective function, O, is defined s: O= T obs -F(m) 2 + ( D 1 C -1 (m) 2 + D 2 C -1 (m) 2 ), (2) where T obs is the dt, F(m) is the model predicted dt, D 1 nd D 2 re the first nd second order directionl derivtive opertors, respectively, C -1 (m) is the model covrince mtrix, is the reltive coefficient of the first order derivtive, nd is smoothing prmeter. The non-liner inversion problem is solved using n itertive conjugte grdient method (Zhng nd Toksoz, 1998). Direct Perpendiculr Tngentil P SV SH ZVSP WVSP OVSP1 OVSP2 ZVSP Well Direct Perpendiculr Tngentil P SV SH 3000 ft OVSP Direct + Perpendiculr Figure 1. Verticl section showing sediment velocity structure, well position, nd ZVSP, OVSP, nd WVSP survey geometry. P Perpendiculr Although VSP trveltime tomogrphy using P-wves hs extensive pplictions, few cses re reported for S-wve trveltime tomogrphy. In this pper we present cse study in which both P- nd S-wve trveltimes recorded by multifrious 3C VSP survey in West Texs were used for tomogrphic inversion of the distribution of subsurfce velocities nd lterl vrition of Vp/Vs rtio. WVSP SV Figure 2. Seismic trces of 3C VSP dt from zero offset VSP (top), n offset VSP2 (middle), nd wlkwy VSP (bottom). Red lines re the first brek times (FBT) of P, SV, nd SH wves picked from direct, perpendiculr, nd tngentil components, respectively. SEG/Houston 2005 Annul Meeting 2625

2 3C VSP Tomogrphy with P- nd S-Wves The VSP survey ws cquired within verticl well on lnd, with Vibroseis source nd downhole 3C geophones in West Texs, long line running pproximtely West to Est. This VSP dtset includes zero offset VSP (ZVSP), two fr offset VSP (OVSP1 nd OVSP2), nd wlkwy VSP (WVSP) recorded using 13-level geophone rry set up t two depth rnges. The survey geometry is shown in Figure 1. Offsets of ZVSP, OVSP1 nd OVSP2 re 294, 6084, nd 7624 ft, respectively. Distnces from the WVSP sources to the wellhed rnge from ft (West) to 8772 ft (Est). Depth level occupied by 3C geophone receivers rnges from 365 to 8815 ft (TVD from dtum). Geophone depth intervls re 50 ft for the VSP dt. Exmples of the VSP wveform dt used in this study re shown in Figure 2. This dtset consists of the following: 3C, 170-level ZVSP, 3C, 104-level OVSP1, nd 49-level OVSP2, nd 3C, 2 x 47-shot x 13-level WVSP (with top geophones t 6265 ft nd 8215 ft, respectively). 3C wveforms of the ZVSP, OVSP1, nd WVSP (Figure 2) clerly show sher wves. Velocity nd hodogrm nlyses by Zho et l. (2005) confirmed tht these sher-wves were generted very close to the erth surfce nd referred to them s qsv nd SH. For simplicity, we will nme them SV nd SH herefter. Accurtely picking first brek time (FBT) of S-wves is criticl for this study. 3C Eigenvlue rottions ply n importnt role in seprting nd enhncing P, SV, nd SH wve fields. Processing steps include rrivl time picking, generting the initil velocity model, nd tomogrphic inversion. The FBTs of P wves were initilly picked on the verticl component nd refined subsequently using the direct component. The SV nd SH FBTs were directly picked on perpendiculr nd tngentil components. A totl of 4635 (3 x 1545) FBT picks were used for the tomogrphic inversion velocity distribution, for both P & S wve fields. Tomogrphic Inversions nd Vp/Vs Rtio Distribution Figure 3 shows two initil P-wve velocity models: ) flt lyer model bsed on ZVSP only nd b) complex model with structurl dips which ws creted using both ZVSP nd surfce seismic dt. P-wve velocity Tomogrphic inversion imges using FBTs fter 25 itertions re shown in Figures 3c nd 3d. Seismic ry pths were clculted for the two resultnt velocity models nd were superimposed on the two tomogrphy velocity profiles (Figures 3e nd 3f), showing the ry coverge for the inversions. These tomogrphy imges revel reltively simple nd flt velocity structures t the shllower depths, but complex structures with bnorml velocities nd fults below depth of 4000 ft. Tomogrphic inversions were performed using the FBTs of both SV- nd SH-wves nd n initil S-wve velocity model in Figure 4 s the strting model. The tomogrphy solutions for SV nd SH wves obtined fter 25 itertions re displyed in Figures 4b nd 4c, respectively. Although the SV nd SH tomogrphy imges hve some differences, they shre mjor common fetures of velocity bnorml spots below depth of 4000 ft. We observed low S-wve velocity region close to the surfce long the borehole on both SV nd SH tomogrphic imges (Figures 4b nd 4c). Figure 5 displys the RMS errors s function of the number of itertions with four different tomogrphic inversions. For P-wve tomogrphic inversions with the initil flt (nd complex) models, the RMS misfit decrese lmost monotoniclly from 16.7 (38.9) to 2.40 (2.41) ms fter 19 (22) itertions, indicting the verge velocities of initil models re not fr from those of the true model. For the SV-wve (nd SH-wve) tomogrphy inversions, RMS misfits reduced from 99.7 (107.4) ms to 13.4 (14.7) ms, fter 25 (19) itertions. The reltively lrger RMS residuls cn be ttributed to much less ccurcy of the FBT pickings for S-wves thn those for P-wves. One of most importnt results of this study is tht sptil distribution of Vp/Vs rtio cn be clculted using P- nd S-wve velocity distributions derived from tomogrphic inversions. The lterl vritions of Vp/VS rtio for SV nd SH re shown in Figures 6 nd 6b, respectively. The sptil distribution of Vp/Vs rtio my provide importnt informtion bout formtion properties in the reservoir re. Along the well pth, the Vp/Vs rtio, for both SV nd SH, monotoniclly dropped from 3.3 to 1.8 s the depth (TVD) incresed from se level to 6500 ft, which is identicl with the velocity nlysis result by Zho et l (2005). The rtio tomogrphy imges revel two distinct regions with high rtio of 3.3 t shllow depths (Figure 6). Discussion nd Conclusions It hs been demonstrted tht multifrious 3C VSP dt cn be successfully used to perform tomogrphic inversions for P- nd S-wve velocity distributions nd lterl vritions of Vp/Vs rtio. The P-wve tomogrphic imge, within the boundries of the ry pth coverge, revels the complex structures nd fults (Figure 7) tht re consistent with the sme fetures on the surfce seismic profile. Both P- nd S-wve tomogrphic imges long with the Vp/Vs rtio lterl vritions indicte two distinct regions with high Vp/Vs rtio of up to 3.3. However, the bnorml region t fr offset is cused by the high P-wve velocity in the region, nd the higher rtio region close to the wellhed is chrcterized by lower S-wve velocity re on both SV nd SH tomogrphic imges (Figures 3, 4, & 6). The complicted ptterns of Vp/Vs rtio distribution (Figure 6) below 5000 ft seem to be ssocited with those smll fults SEG/Houston 2005 Annul Meeting 2626

3 3C VSP Tomogrphy with P- nd S-Wves Distnce (ft) c e West Est b d f P-Wve Velocity (ft/s) Figure 3. Initil models nd tomogrphy imges. ) nd b): Initil flt nd complex P-wve velocity models. c) nd d): Correspondent reconstructed P-wve velocity models. e) nd f): Seismic ry pths, superimposed on two reconstructed velocity models, show the ry coverge re, only within which tomogrphy imges were relibly updted. West Est Distnce (ft) b SV c SH S-Wve Velocity (ft/s) Figure 4. ) Initil S-wve velocity model. b) nd c) Reconstructed SV nd SH velocity distributions. SEG/Houston 2005 Annul Meeting 2627

4 Figure 7. VSP P-wve tomogrphy imge shows fults nd velocity bnorml regions corresponding to the mplitude bnorml res on the migrtion imges of surfce seismic dt. Figure 5. Comprison of RMS residuls for four different tomogrphy inversions. b Vp/Vsv Vp/Vsh Figure 6. Vp/Vs rtio distribution clculted for ) SV-wves nd b) SH- wves. very close to the wellbore. We note tht some Vp/Vs rtio bnorml spots pprently co-locted with some formtion tops. Although tying the Vp/Vs rtio bnorml ptterns to the formtion properties beyond the scope nd length limit of this pper, further integrted investigtion using 3D seismic volumes, well logs, nd ll vilble petrophysicl nd geologicl informtion is strongly recommended. Our current reserch indictes tht the resultnt tomogrphic velocity profiles my be useful not only for improving migrtion imging, but lso for formtion evlution. This study shows the power of the tomogrphic inversion with 3C VSP dt to imge subsurfce structures. References Lpin, S., Kisin, S., Zhou, H., 2003, Joint VSP nd surfce seismic tomogrphy, 73 rd Ann. Internt. Mtg., Soc. Expl. Geophys., Expnded Abstrcts, in CD-ROM. Li, Y., O Brien, J, Mllick, B., Zhng, J., Wng D., Brbern, C., Dushmn, D., 2003, Tomogrphic velocity determintion of the sedimentry nd slt flnk imges using ultifrious VSP dt, 8th Interntionl Congress of Brzilin Geophysicl Society, Rio de Jneiro, Brzil, in CD-ROM. Slo, E.L., nd Schuster, G.T., 1989, Trveltime inversion of both direct nd reflected rrivls in verticl seismic profile dt: Geophysics, Vol. 54, p Stewrt, R.R., Chiu, S.K.L., Wilcox, M.A., nd Hmpson, D.P., 1987, Tomogrphy-bsed imging using well logs, VSP, nd 3D seismic dt: 57 th Ann. Internt. Mtg., Soc. Expl. Geophys., Expnded Abstrcts, p Zhng, J., nd Toksoz, M.N., 1998, Nonliner refrction trveltime tomogrphy: Geophysics, Vol. 63, p Zho, X., Zhou, R., Li, Y., Jnk, P., nd Dushmn, D., 2005, Sher wves from ner-offset VSP survey nd pplictions, 75 th Ann. Internt. Mtg., Soc. Expl. Geophys., Expnded Abstrcts, submitted. Zhou, H., nd Hou, A., 2000, A reversed VSP tomogrphic velocity nlysis: 70th Ann. Internt. Mtg., Soc. Expl. Geophys., Expnded Abstrcts, p Acknowledgements We gretly pprecite TOTAL E&P USA, Inc. for permission to use their dt nd thnk VSFusion for supporting this study. SEG/Houston 2005 Annul Meeting 2628

5 EDITED REFERENCES Note: This reference list is copy-edited version of the reference list submitted by the uthor. Reference lists for the 2005 SEG Technicl Progrm Expnded Abstrcts hve been 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 Web. 3C VSP Tomogrphy Inversion for Subsurfce P- nd S-Wve Velocity Distribution References Lpin, S., S. Kisin, nd H. Zhou, 2003, Joint VSP nd surfce seismic tomogrphy: 73rd Annul Interntionl Meeting, SEG, Expnded Abstrcts, Li, Y., J. O Brien, B. Mllick, J. Zhng, D. Wng, C. Brbern, D. Dushmn, 2003, Tomogrphic velocity determintion of the sedimentry nd slt flnk imges using ultifrious VSP dt: 8th Interntionl Congress of Brzilin Geophysicl Society, Rio de Jneiro, Brzil, in CD-ROM. Slo, E. L., nd G. T. Schuster, 1989, Trveltime inversion of both direct nd reflected rrivls in verticl seismic profile dt: Geophysics, 54, Stewrt, R. R., S. K. L. Chiu, M. A. Wilcox, nd D. P. Hmpson, 1987, Tomogrphybsed imging using well logs, VSP, nd 3D seismic dt: 57th Annul Interntionl Meeting, SEG, Expnded Abstrcts, Zhng, J., nd M. N. Toksoz, 1998, Nonliner refrction trveltime tomogrphy: Geophysics, 63, Zhou, H., nd A. Hou, 2000, A reversed VSP tomogrphic velocity nlysis: 70th Annul Interntionl Meeting, SEG, Expnded Abstrcts,