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Clement Kostov observed that in 2-D, even if the sampling in
offset is irregular, the matrix *L L*^{H} is Toeplitz, as long
as the sampling in ray parameter is uniform. For a Toeplitz
matrix, an efficient means of solving the system,
using Levinson recursion, exists with a cost proportional
to the square of the length of the unknown vector.
In the migration case, because of the extra axis in the
transform domain, the matrix is typically not Toeplitz.
Though it is often block Toeplitz, and solution methods exist that
take advantage of that form, we have elected to use
a simple conjugate gradient scheme to solve the least-squares
problem in these initial tests.

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Stanford Exploration Project

11/17/1997