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| IWAVE Structure and Basic Use Cases | |
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Next: Interlude: The Internal Grid
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Previous: Born Approximation
One version of Reverse Time Migration (RTM) is simply the adjoint of
Born modeling. IWAVE provides adjoint computations for every
derivative mapping (first, second,...) using the optimal
checkpointing method of reverse time propagation
(Symes, 2007; Griewank, 2000; Blanch et al., 1998; Plessix, 2006). Other approaches to time reversal can be more efficient in
special cases, especially when the interior dynamics are conservative
(acoustics, elasticity) (Clapp, 2009; Dussaud et al., 2008). However
none are more effective in general, in particular when energy
attenuation is significant part of wave dynamics, as is the case for
all realistic models of seismic wave motion.
Figure 10 displays the migration of the single
Born ``shot'' gather (really, OBS receiver gather) located at
m from
the left edge of the model. No effort has been made to remove the
low-frequency noise caused by the sea bottom reflection.
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migr12000
Figure 10. Reverse-time migration of Born data
from Figure 9.
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The parameters required for this job are
deriv = 1
adjoint = 1
nsnaps = 10
csq = ../csq_4layer.rsf
csq_b1 = ../migr12000.rsf
source = ../wavelet12000.su
data = ../born12000.su
The adjoint key flags the adjoint computation. The checkpointing
algorithm requires allocation of workspace for checkpoints (copies of
wavefield Cauchy data, consisting of all dynamic arrays). The number
of checkpoints allocated is the value for key nsnaps. The
appropriate number of checkpoints depends on the number of time
steps. Reasonable numbers to achieve a cost ratio of adjoint to
forward computations of around 5 are
- up to 1000 time steps: 5 checkpoints
- up to 5000 time steps: 10 checkpoints
- up to 10000 time steps: 20 checkpoints
- up to 20000 time steps: 30 checkpoints
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| IWAVE Structure and Basic Use Cases | |
|
Next: Interlude: The Internal Grid
Up: Single Shot Examples
Previous: Born Approximation
2015-04-20