c.r.ranero ,’v.’sallares,’’ e.’jimenez,’d ... · pdf...
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Barcelona Center for Subsurface Imaging
Travel Time & Full Waveform Tomographic Inversion
C. R. Ranero, V. Sallares, E. Jimenez, D. Dagnino, A. Melendez,
C. Gras, S. Begovic
Work Funded (largely) by
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FWI
TTT
Target
PSDM• Depth migration of MCS data
• Excellent definition of boundaries and geometry (e.g. RTM)
• Limitations to determine physical properties > need velocity model building (TTT)
• Travel Time Tomography (Inversion) using arrival time of refracted + reflected phases.
• Ray theory > Resolution ~(λd)1/2 ≈103 m
• Moderately non-‐linear > robust; moderate computational cost; limited resolution.
Why using Full Waveform Inversion to explore the subsurface structure?
Image & Properties
• Full waveform inversion (phases and amplitudes) > Advantages TTT & PSDM.
• Wave equation > Resolution ~λ/2 ≈ 101-‐102 m (similar to MCS+PSDM).
• Strong non-‐linearity > Initial model, low freq, noise, source, computational cost.
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shots
receivers
Acquisition with OBC Maximum offset: 13 km
Is FWI worth implementing?
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FWI
TTT
Sirgue et al., BP (2010)
Slice at 150 m depth
3-‐7 Hz
Is FWI worth implementing?
Slice at 1050 m depth
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Is FWI worth implementing?
TTT-RTM FWI-RTM
Tommeliten - North Sea OBC - 6km 11 km offset
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• Assuming that we can reproduce the physics of wavefield propagation with the required accuracy, the key remaining problem is “non-linearity”
• Minimization of objective function using J-based (CG…) or “approximate H”-based approaches (L-BFGS…)
• Most schemes use/assume L2-norm as objective function (see SEG 2013 abstracts)
• L2-norm is highly nonlinear (comparison point by point of the trace) > subject to “cycle skipping”
Operto & Virieux (2009)
L2-norm: ||u(t) - d(t)||2
Non-linearity in FWI
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Non-linearity in FWI: cycle skipping
Shah et al., IC (2013)
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First apply TTT and then use the result as initial model for FWI (e.g. Pratt et al., 2002)
Pratt et al. (2002)
TTT
FWI
TTT+FWI
Strategies to overcome cycle skipping
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9Meléndez et al., (GJI 2015)
Joint 3D refraction/reflection travel-time tomography (tomo3D)
Parallelized Code is available for download
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10 10Sirgue & Pratt (2003)
1) Multi-scaling: Add data sequentially, proceeding from low to high frequencies (Bunks et al., 1995)
Long acquisition offset Short acquisition offset
Strategies to overcome cycle skipping
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11Bi & Lin CGG (76th EAGE Conference & Exhibition 2014)
Industry 2D Example: 2 ship 2 streamer with a total 18 km offset
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Academia 3D Example: Whole Mantle Tomography
September 2015
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13Singh (EOS, 2012)
Long-offset streamers
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14Singh et al., (2013)
Long-offset streamers
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What seismic information is needed for FWI?
Bi & Lin CGG (76th EAGE Conference & Exhibition 2014)
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The longest offsets with diving waves
Sallares et al 2013
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Conclusion
Our group view: A (small?) revolution in seismic imaging is underway but requires the proper acquisition tools.