First Arrival Traveltime and Waveform Inversion of Refraction Data

1
First Arrival Traveltime and Waveform Inversion
of Refraction Data
Jianming Sheng and Gerard T. Schuster University
of Utah October, 2002
2
Outline

• Motivation
• First arrival traveltime and waveform inversion
• Numerical examples
• Summary

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Motivation
Traveltime and waveform of CDP refraction data
Given
Goal
High resolution tomogram
Problem Can waveform tomography
provide better resolution than
ray-based tomography?
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Ray-based Tomography vs. Full Waveform Inversion

Efficient and robust
Ray-based tomography
Resolution limited by high-freq. assumption
No high-freq. limitation
Full waveform tomography
Slow convergence and local minima problem
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First-arrival Traveltime and Waveform Inversion
Efficient and robust
Ray-based traveltime tomography
Initial model
No high-freq. limitation
First-arrival waveform inversion
Better convergence and mild nonlinear
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Outline

• Motivation
• First arrival traveltime and waveform inversion
• Numerical examples
• Summary

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First Arrival Traveltime and Waveform Inversion

• Step 1

Preprocessing the raw data band-pass, 3D
to 2D transform, trace normalization

• Step 2

Picking first-arrival traveltimes and muting out
other waves except first arrivals
8

• Step 3

First arrival traveltime tomography
Minimizes traveltime residual
Initial model
9

• Step 4 First arrival
• waveform inversion

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Multigrid Tomography

• Traveltime tomography

Dynamic smoothing scheme
(to attack local minima problem)
(Nemeth, T., Normark, E. and Qin, F., 1992)
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Outline

• Motivation
• First arrival traveltime and waveform inversion
• Numerical examples
• Summary

12
Numerical Examples

• Synthetic data I Three-layer
• Synthetic data II WesternGeco (Blind test)
• Redmond mine survey data

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Synthetic Model I
Suggested by Konstantin Osypov
Source Freq. 60 Hz
Avg. Velocity 2400 m/s
Source Wavelength 40 m
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Synthetic Model I
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Synthetic Data I

• Synthetic data set was calculated
• by 2-D FD acoustic wave equation
• solver

• Twenty-one shots and 51 traces
• per shot were used.

• Computational grid dimension was
• 401121.

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Synthetic Shot Gather
-80
120
Offset (m)
0.0
Time (sec.)
0.1
Air Wave
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Traveltime Tomogram
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Synthetic Model I
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Traveltime Residual
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Waveform Tomogram
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Synthetic Model I
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Waveform Residual
1
30
Iterations
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Numerical Examples

• Synthetic data I Three-layer
• Synthetic data II WesternGeco (Blind test)
• Redmond mine survey data

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True Velocity Model
Horizontal distance (km)
0.0
26
0.0
1000 m/s
20502500 m/s
Depth (km)
1.0
25
True Density Model
Horizontal distance (km)
0.0
26
0.0
Depth (km)
1.0
26
Recorded CSG 150
-3000
3000
Offset (m)
0.0
Time (sec.)
2.0
27
Guessed Density Model
3400
Density (kg/m3)
1400
5000
1000
Velocity (m/s)
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Source Wavelet
400
0
Amplitude
-600
0.0
0.25
Time (sec.)
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Waveform Matching
Offset (m)
-50
-25
Amplitude
0
25
50
0.0
0.2
Time (sec.)
30
Traveltime Tomogram
m/s
Horizontal distance (km)
0.0
26
2712
0.0
2284
Depth (km)
1856
1428
1.0
1000
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Traveltime Tomogram
m/s
Horizontal distance (km)
5.0
8.75
2409
0.0
2057
0.1
Depth (km)
1705
0.2
1352
0.3
0.4
1000
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Waveform Tomogram
m/s
Horizontal distance (km)
5.0
8.75
2700
0.0
2275
0.1
Depth (km)
1850
0.2
1425
0.3
0.4
1000
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Migration section
Horizontal distance (km)
5.0
8.75
0.0
0.1
Depth (km)
0.2
0.3
0.4
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Predicted CSG 150
-3000
3000
Offset (m)
0.0
Time (sec.)
2.0
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Recorded CSG 150
-3000
3000
Offset (m)
0.0
Time (sec.)
2.0
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Numerical Examples

• Synthetic data I Three-layer
• Synthetic data II WesternGeco (Blind test)
• Redmond mine survey data

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(No Transcript)
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Salt Diapir Data

• Thirty-one shots and 120 traces
• total 3188 traveltimes picked.
• Shot interval 20 m
• geophone interval 5 m

• Source frequency 40 Hz.

• Record length 1 sec.
• sample interval 0.5 millisecond .

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CSG for Field Data After Preprocessing
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CSG for Field Data After Muting
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Wavelet Extracted
0
Time (sec.)
0.1
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Traveltime Tomogram
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Traveltime Residual
1
30
Iterations
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Waveform Tomogram
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Traveltime Tomogram
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Waveform Residual
1
30
Iterations
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Predicted CSG
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CSG for Salt Data After Muting
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Logarithmic Amplitude Vs. Offset
2
0
Synthetic
Log10 Amplitude
-2
Observed
-4
0
400
Offset (m)
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Problems
Seismic attenuation
Surface wave noise
Source wavelet inversion objective function
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Outline

• Motivation
• First arrival traveltime and waveform inversion
• Numerical examples
• Summary

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Summary

• Synthetic results show that the waveform tomogram
  is much more resolved

• The preliminary results for the field data are
  not as good as expected, and further work is
  needed.

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Acknowledgment
I thank the sponsors of the 2002 University of
Utah Tomography and Modeling /Migration (UTAM)
Consortium for their financial support . I thank
Konstantin Osypov for providing the data set.