P-S migration using EO method

Thais Guirigay, John C. Bancroft

November, 2012

1

Outline

• Objective • Examples Hussar Synthetic model NEBC

• Summary • Acknowledgments

Objective

After finding an estimation of the converted wave velocity Vc and shear wave velocity Vs, from P-P and P-S wave data, show the results using EOM.

Hussar

Geophone 3C (SM7 10 Hz) Vibroseis low-dwell sweep

Geometry specification

First breaks picks

Refraction Statics analysis

Geometry Spreading compensation

Residual Statics

Stack

Trace edit

Datum/Elevation correction

NMO correction

Bin definition/ Header Update

Velocity analysis

Gabor Deconvolution

Migration

EOM

Processing flow for P-P data

Processing flow for P-S data Rotation into radial component

Import P-wave shot static and applied

Calculate and applied receiver elevation S statics

Geometry Spreading compensation

Repeat S-wave receiver statics

Check the CRS for additional significant receiver statics

Edit bad traces

Common Receiver Brute Stack (CRS)

Estimate S-wave receiver statics from CRS

Asymptotic CCP binning

Velocity analysis, Initial Vc

Gabor Deconvolution

Noise attenuation: Radial filter

PS wave Stack EOM

Surface wave noise attenuation

Migration

EOM processing flow

Generation of CSP gathers

Stack

NMO correction

Estimation of velocities

• Initial Vc1 with Vp and • Limited range EO gathers • Pick new Vc2

• Estimate Vs

• Full EO gathers with Vp and Vs • Pick new Vc3

• Moveout correction with Vc3 • Stack to complete the prestack migration

2 rms p rms sc

rms p rms s

V VV

V V− −

− −

=+

γ

Estimation of the velocities

Initial estimate of Vc1 2γ = 2.5γ =

0 1000 2000 0 1000 2000

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Tim

e t

(sec

.)

Initial estimate of Vc1 x =

x =

Stacked line (Supergather)

Stacked line (EO method)

LCCSP gather x = 100 m

Second estimation of Vc2

0 1 2 3 4 5 60

1000

2000

3000

4000

5000

Time T0 (sec)

Vel

ocity

(m

/s)

Vp-rms, Vc-rms(G) for γ = 2 and picked Vc-rms(P)

Vp-rms

Vc-rms(G)

Vc-rms(P)

0 2000 4000 6000 8000 100000

1000

2000

3000

4000

5000

6000

Depth (m)

Vel

ocity

(m/s

)

Comparison of interval velocities in depth

Vp-int(z)Vc-int(Gz)Vc-int(Pz)

0 0.5 1 1.5 2 2.5 3 3.5 40

1000

2000

3000

4000

5000

6000

Time T0 (sec)

Vel

ocity

(m

/s)

Comparison of interval velocities

Vp-intVc-int(G)Vc-int(P)

Estimating the S velocities

0 1 2 3 4 50

1000

2000

3000

4000

5000

Time T0 (sec)

Vel

ocity

(m

/s)

Comparison of RMS velocities, with Vs-rms in Tp time

Vp-rmsVc-rms(P)Vs-rms(P)Vs-rms(P) in Tp time

0 1 2 3 4 50

1000

2000

3000

4000

5000

6000

Time T0 (sec)

Velo

city

(m

/s)

Comparison of interval velocities in time

Vp-intVc-int(P)Vs-int(P)

0 2000 4000 6000 8000 100000

1000

2000

3000

4000

5000

6000

Depth (m)

Velo

city

(m

/s)

Comparison of interval velocities in depth

Vp-int(z)Vc-int(Pz)Vs-int(Pz)

Full EOM

With the Vp and the estimated S velocity, new Vc3 is calculated picked from the new CCSP gathers

2 22 20 0

0 0

( ) ( )4 ( ) 4 ( )

p sc s r

rms p p rms s p

t tx h x ht t tV t V t− −

+ −= + = + + +

2 20

1 1 ˆrms srm p

es

tV

zV

h−−

= + +

Full EO gathers with Vp and Vs

Interval velocities

0 2000 4000 6000 8000 100002000

3000

4000

5000

6000

7000

Depth (m)

Vel

ocity

(m/s

)

Comparison of P interval velocities

Vp-int-Log(z)Vp-int-Picked(z)

0 1000 2000 3000 4000 5000 6000 7000500

1000

1500

2000

2500

3000

3500

4000

Depth (m)

Vel

ocity

(m/s

)

Comparison of Shear interval velocities

Vs-int-Log(z)Vs-int-Estimated(z)

Gamma functions

0 2000 4000 6000 8000 100001

1.5

2

2.5

3

3.5

4

4.5

Depth (m)

Gam

ma

Comparison of interval Gamma functions

Gamma from Vp-rms(Gz)Gamma from picked Vc-rms(Pz)Gamma from well-log

Final stack after Full EOM

0.0

2.0

1.0

2.0

1.0

0.0

3.0

1.0

0.0 P-P section P-S section

Final stack after post-stack migration

0.0

2.0

1.0

2.0

3.0

1.0

0.0 P-P section P-S section

Final stack after post-stack migration Final stack after Full EOM P-S section

Synthetic seismogram for PP

Belly River

Basal Belly River

Viking

Medicine River Coal

Mannville

Colorado

Synthetic seismogram for PS

Belly River

Basal Belly River

Viking

Medicine River Coal

Mannville

Colorado

Final stack after Full EOM

Belly River

Basal Belly River

Viking Medicine River Coal Mannville

12-27 12-27

Colorado

Results Synthetic model

(Lloyd and Margrave, 2010)

Estimating the velocities

0 500 1000 1500 2000 25001.5

2

2.5

3

3.5

Depth (m)

Gam

ma

Comparison of Gamma functions in depth

Gamma from Int. Vel.Gamma from RMS Vel.Gamma from Table 4.1 Layer Vp Vs γ

1 1500 500 3

2 2750 1100 2.5

3 3287 1700 1.93

4 3487 1800 1.94

5 4090 2000 2.03

6 4175 2050 2.04

P-P section

P-S section

Results NEBC

0 500 1000 1500 2000 2500 3000 3500 40001000

2000

3000

4000

5000

6000

Depth (m)

Vel

ocity

(m/s

)

Comparison of P interval velocities

Vp-int-Log(z)Vp-int-Picked(z)

Interval velocities

0 500 1000 1500 2000 2500 30000

500

1000

1500

2000

2500

3000

3500

Depth (m)

Vel

ocity

(m/s

)

Comparison of Shear interval velocities

Vs-int-Log(z)Vs-int-Estimated(z)

Estimating the velocities

0 500 1000 1500 2000 2500 30001

2

3

4

5

Depth (m)

Gam

ma γ

Gamma functions in depth from picked velocities

γ: Int. Vels. γ: RMS vels. γ: log

Summary

• This initial velocity Vc was estimated from RMS velocities Vp and an initial constant value for the Vp/Vs ratio .

• A reasonably accurate estimate of converted wave velocities Vc is required to form CCSP gathers as part of the Equivalent Offset Migration of converted waves.

• These converted wave velocities (second Vc) were then used to make an initial estimate of the shear wave velocities Vs. These velocities were then used with the Vp velocities for prestack migration.

Summary (continued)

• The common converted wave scatterpoint (CCSP) gathers were then used to create accurate estimates of Vc in order to improve Vs and .

• The quality of the method is demonstrated for the case of Hussar, synthetic model and NEBC dataset. The results show superior imaging when compared with alternative migration algorithms.

• The estimated interval velocities of Vp and Vs are compared with velocities obtained from well-logs and compare favorably.

γ

Acknowledgments

• All CREWES staff. Dr. Margrave Dr. Isaac

• All CREWES students • All CREWES sponsors Gedco Halliburton Husky Energy GeoKinetics Inova Nexen

Thanks