\begin{thebibliography}{1}

\bibitem{SEG-1994-0980}
Kenneth~P. Bube and Robert~T. Langan.
\newblock A continuation approach to regularization for traveltime tomography.
\newblock In {\em 64th Annual International Meeting, SEG, Expanded Abstracts},
  pages 980--983. Soc. Expl. Geophys., 1994.

\end{thebibliography}
