Velocity and anisotropy parameter scan 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 Christopher Ogiesoba
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: But how should we recover it?
Traveltime Equation (1) (Tsvankin and Thomsen, 1994; Thomsen, 1999) (2) (3) (4)
Traveltime Equation where depending on the value of (5) where depending on the value of But if is 1, then the traveltime equation becomes, (6)
Validity of traveltime equation Model Parameters Thickness = 533m Vp = 1600 m/s Vs = 400 m/s r = 2.2gm/cc h = 0.1527 Thickness = 427m Vp = 2700 m/s Vs = 900 m/s r = 2.3 gm/cc e = 0.1720 d = 0.0000 h = 0.1720 Thickness = 1300m Vp = 4000 m/s e = 0.0360 d = -0.039 Vs = 1429 m/s r = 2.5 gm/cc h = 0.0813 Thickness = 1000m Vp = 5500 m/s e = 0.1280 d = 0.0780 Vs = 2500 m/s r = 2.75 gm/cc h = 0.0433 e = 0.137 d = -0.012
Black line is an offset-depth ratio of one. Validity of traveltime equation Anivec Synthetic: Four Layer Anisotropic Model P-wave PS-wave 1 PS-wave 3 PS-wave 2 PS-wave 4 E 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 3 PS-wave 2 PS-wave 4 Shot record from anisotropic Anivec Plot from equation (2)
Validity of traveltime equation Comparison between Anivec Synthetic and Equation (2) P-wave PS-wave 1 PS-wave 3 PS-wave 2 PS-wave 4 ANIVEC Time (s) Offset (m) 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 g0 = 4.0 g0 = 2.8 g0 = 3.0 g0 = 2.2
Sensitivity Test Traveltime curves at constant Vps and constant g0 Constant Vps = 1675 m/s Constant g0 = 2.2 h = -0.2 h = -0.1 h = 0.1 h = 0.2 Traveltime curves at constant Vps and constant g0
Velocity Analysis . Vps
Vp/Vs Analysis: Summed over all h 2D semblance obtained for vertical velocity ratio
. Dual Parameter Scan: h and g0 Layer 1 g0, h = 2.75, 0.1535 Timeslice at 1.666 secs after rescaling colorbar
. Dual Parameter Scan: h and g0 Layer 2 g0, h = 3.75, 0.1945 Timeslice at 2.900 secs after rescaling colorbar
. Dual Parameter Scan: h and g0 Layer 3 g0, h = 3.6, 0.1985 Timeslice at 3.533 secs after rescaling colorbar
. Dual Parameter Scan: h and g0 Layer 4 g0, h = 3.4, 0.1884 Timeslice at 4.11500 secs after rescaling colorbar
Model vs Scanned Parameters: h-g0 scan Error analysis from model 3 results Table showing error analysis in g0 Table showing error analysis in h
Results from the Blackfoot Area, Western Canada Scanned Vp/Vs values range from 1.8 to 2.3
Conclusions We can find h and g0 values from moveout analysis. The derived equation is adequate to describe converted wave 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 (1) (2) (Li, 2001) (3) (4)