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Most downward continuation methods require that the data lie on a regular mesh. To map the irregular recorded seismic data onto the regular mesh is a far from trivial exercise. A common approach in industry is to think of the problems in the same way we approach Kirchhoff migration, namely to loop over data space and spread into our regular model space. The spreading operation is governed by something like AMO Biondi et al. (1998), which maps data from one offset vector to another. If we think of the AMO operator $\bf T$ as mapping from the regular model space $\bf m$to the regular data space $\bf d$, our estimation procedure becomes,  
\bf m= \bf T' \bf d
.\end{displaymath} (1)
This formulation suffers from all of the usual problems associated with applying an adjoint operation. We are spraying into a regular mesh, but the coverage is not regular. Areas with higher concentration of data traces will tend to map to artificially higher amplitudes in the model space. We can do some division by hit count to help minimize this effect but will still see some artifacts that come from approximating the inverse with an adjoint.

We can think of turning (1) into an inversion problem but, in addition to the high cost associated with the AMO operationm we face the same stability issues that setting up the migration problem as an inverse problem encounters. The null space of the imaging operator tends to put high frequency noise in the model space when cast as inverse problem.

Fomel (2001) suggested thinking of the problem more as a missing data problem. We can write the missing data problem in terms of the fitting goals
\bf d&\approx&\bf L\bf m\nonumber \\ \bf 0&\approx&\epsilon \bf A\bf m
where $\bf L$ is a simple interpolation operator (nearest-neighbor, linear, etc) and the real work is done by the regularization operator $\bf A$ which describes the relationship between the irregular data and the regular sampled model.

We can speed up the convergence of (2) by preconditioning the model with $\bf m=\bf A^{-1}\bf p=\bf B\bf p$ . Our new fitting goals become,
\bf d&\approx&\bf L\bf B\bf p\nonumber \\ \bf 0&\approx&\epsilon \bf p

Biondi and Vlad (2001) suggested following the approach of Claerbout and Nichols (1994) and Rickett (2001). Instead of solving the inverse problem, they suggest filtering the adjoint solution with a diagonal operator. We obtain our filtering operator by first noting the least squares inverse of the interpolation problem,  
\bf m= \bf B\left( \bf B' \bf L' \bf L\bf B+\epsilon^2 \right)^{-1} \bf B' \bf L' \bf d
.\end{displaymath} (4)
We can think of equation (4) as filtering the adjoint solution with the matrix $\bf W^{-1}$ where,
\bf W = \bf B' \bf L' \bf L\bf B+\epsilon^2 .\end{displaymath} (5)
The weighting matrix is (np x np) where np is the size of our preconditioned model space. This matrix will be generally diagonally dominant. We can think of estimating a diagonal filtering operator $\bf W_{{\rm diag}}$ by using a reference model (in preconditioned model space) $\bf p$ and applying
\bf W_{{\rm diag}} = \frac{ {\rm\bf diag} \left[( \bf B' \bf... p_{ref} \right]}
{ {\rm \bf diag}\left(\bf p_{ref}\right)} .\end{displaymath} (6)
We can then get an estimate of our model through  
\bf m= \bf B{\bf W^{-1}} \bf B' \bf L' \bf d
.\end{displaymath} (7)