Acknowledgment

I would like to thank Karl Schleicher, Shuwei Gan, Zhaoyu Jin, Sergey Fomel and Jianwei Ma for fruitful discussions. I also thank Jean Virieux and two anonymous reviewers for very constructive suggestions that improve the manuscript greatly. The paper was prepared in the Madagascar (Fomel et al., 2013) open-source software platform and all the examples shown in the paper are reproducible.