Maarten V. de Hoop,1 Robert D. van der Hilst2 and Peng Shen3 1Center for Computational and Applied Mathematics, Purdue University, 150 N. University Street, West Lafayette IN 47907, USA 2Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Rm 54-517A, Cambridge, MA 02139, USA. E-mail: [email protected] 3Total E&P USA, 800 Gessner, Suite 700, Houston TX 702...