Transcript Slide 1
Prestack Multi-parameter Scanning for Average Vp/Vs and h in Multicomponent Seismic Data Christopher Ogiesoba* Jim Gaiser** Robert Stewart* * University of Calgary ** WesternGeco, Denver Outline • Introduction to anisotropic velocity analysis • Review of traveltime equations • Validity and sensitivity tests • Numerical model and real data results • Conclusions, Future work, Acknowledgments Introduction • Average Vp/Vs plays a crucial role in multicomponent seismic analysis • Some VTI parameters depend on it; for example: h 0 eff 1 2 0 eff h • But how should we recover it? Traveltime Equation t ps t ps 0 2 2 x2 A4 x 4 2 v ps 1 A5 x 2 (1) (Tsvankin and Thomsen, 1994; Thomsen, 1999) 2 0 2 4 2 ( 1) A4 4( 0 1) t ps 0 v ps A4v ps A5 2 v ps (1 2 ) vp (2) 2 v ps 0 (1 0 ) D 0 (1 2 0h ) (3) 2 vp2 2 (4) Traveltime Equation t ps t ps 0 2 2 x2 ( 0 1)2 0 (1 2h ) D 4 2 ( 2 2 4 2 2 2 )x 2 v ps 4 0 (1 2h ) D ( 0 1)t ps 0 v ps ( 0 1) ( 0 0 )v ps x D where 2 (5) 1 1 2 0 D 1 depending on the value of 0 But if D is 1, then the traveltime equation becomes, x2 ( 0 1) 2 0 (1 2h ) 1 4 2 ( 2 2 4 2 2 2 )x 2 v ps 4 0 (1 2h ) 1( 0 1)t ps 0 v ps ( 0 1) ( 0 0 )v ps x 2 t ps t ps 0 2 2 (6) Validity of traveltime equation Model Parameters Thickness = 533m Vp = 1600 m/s e = 0.137 -0.012 Thickness = 1300m Vp = 4000 m/s e = 0.0360 -0.039 Thickness = 427m Vp = 2700 m/s Thickness = 1000m Vp = 5500 m/s e = 0.1280 0.0780 Vs = 400 m/s r = 2.2gm/cc h 0.1527 Vs = 1429 m/s r = 2.5 gm/cc h 0.0813 Vs = 900 m/s r = 2.3 gm/cc e = 0.1720 = 0.0000 h = 0.1720 Vs = 2500 m/s r = 2.75 gm/cc h 0.0433 Validity of traveltime equation Anivec Synthetic: Four Layer Anisotropic Model P-wave E PS-wave 1 PS-wave 2 PS-wave 3 PS-wave 4 Black line is an offset-depth ratio of one. Validity of traveltime equation Comparison between Anivec Synthetic and Equation (2) Pwave PS-wave 1 PS-wave 2 PS-wave 3 PS-wave 4 Shot record from anisotropic Anivec Plot from equation (2) Validity of traveltime equation Comparison between Anivec Synthetic and Equation (2) Offset (m) Offset (m) P-wave PS-wave 1 Time (s) PS-wave 2 Time (s) ANIVEC PS-wave 3 PS-wave 4 Shot record from anisotropic Anivec Plot from equation (2) Sensitivity Test Traveltime curve at constant Vps and constant h Constant Vps = 1675 m/s Constant h = 0.2 0 = 4.0 0 = 2.8 0 = 3.0 0 = 2.2 Sensitivity Test Traveltime curves at constant Vps and constant 0 Constant Vps = 1675 m/s Constant 0 = 2.2 h = -0.2 h = -0.1 h = 0.1 h = 0.2 Velocity Analysis . . . . Vps Vp/Vs Analysis: Summed over all h • • • • 2D semblance obtained for vertical velocity ratio Dual Parameter Scan: h and 0 Layer 1 . 0, h = 2.75, 0.1535 Timeslice at 1.666 secs after rescaling colorbar Dual Parameter Scan: h and 0 Layer 2 . 0, h = 3.75, 0.1945 Timeslice at 2.900 secs after rescaling colorbar Dual Parameter Scan: h and 0 Layer 3 . 0, h = 3.6, 0.1985 Timeslice at 3.533 secs after rescaling colorbar Dual Parameter Scan: h and 0 Layer 4 . 0, h = 3.4, 0.1884 Timeslice at 4.11500 secs after rescaling colorbar Model vs Scanned Parameters: h0 Error analysis from model 3 results Model 0 4.00 2.80 3.00 2.2 Average 0* 4.00 3.41 3.30 3.10 Scanned 0** 2.75 3.75 3.6 3.4 Error in 0 -31 +10 +9 +9.7 Table showing error analysis in 0 hint 0.157 0.0813 0.172 0.0433 heff* 0.1527 0.1942 0.1936 0.1734 heff** 0.1535 0.1945 0.1985 0.1884 Error in heff +0.5 +0.2 +0.5 +8.6 Table showing error analysis in h scan Results from the Blackfoot Area, Western Canada Scanned Vp/Vs values range from 1.8 to 2.3 Conclusions • We can find h and values from moveout analysis. • The derived equation is adequate to describe converted wave 0 but scans slightly higher vp/vs values. • It is inaccurate at shallow depths where the offset to depth ratio is greater than 1.5 • Accuracy increases with depth • Post-critical angle events degrade analysis from shallow levels. • We need to modify traveltime equation to make use of the far offset data Future work • Improve traveltime equation to target the far-offset events. • Improve the algorithm so that velocity ratio-time log will be displayed while picking velocities. Acknowledgements CREWES sponsors WesternGeco for a summer internship CREWES personnel: Kevin Hall Richard Bale Linping Dong Carlos Nieto Dr. Charles Ursenbach Dr. Mehran Gharibi Thank you for your attention Traveltime Equation t ps 2 2 4 x A x 2 t ps 0 2 4 2 v ps 1 A5 x (1) ( 0 eff 1)2 8(1 0 ) eff A4 2 4 4t ps 0 v ps 0 (1 eff )2 (2) (Li, 2001) eff (heff 0 eff eff ) (3) A5 1 v ph A4 1 2 v ps 2 (4)