This paper owes a great deal to John Burg. We would like to thank him
and Francis Muir for many useful and stimulating discussions. The
first author also thanks Jim Berryman for explaining the variational
derivation of the biharmonic and tension-spline equations.
Ralf Ferber and Ali Özbek provided helpful reviews.
The financial support for this work was provided by the sponsors of
the Stanford Exploration Project.
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