Part of this work was done while WBB was on a two week research visit to SEP in July 1993. We thank Jon Claerbout and Elf GRC for sponsoring WBB's visit and providing an arrangement in which to make this possible. We thank Dave Nichols for some productive discussion regarding Hessian operators and their diagonalization. Dave Nichols and Thorbjorn Rekdal provided careful reviews of the paper.