next up previous [pdf]

Next: Interlude: The Internal Grid Up: Single Shot Examples Previous: Born Approximation

Reverse Time Migration

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 $ x_s = 12000$ 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.

migr12000
migr12000
Figure 10.
Reverse-time migration of Born data from Figure 9.
[pdf] [png] [scons]

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


next up previous [pdf]

Next: Interlude: The Internal Grid Up: Single Shot Examples Previous: Born Approximation

2015-04-20