1. Birch, F., J. Geophys. Res., 57, 227 (1952).
2. Al'tshuler, L. V., Trunin, R. F., and Simakov, G. V., Bull. Acad. Sci. USSR, Earth Phys. Ser., No. 9, 657.
3. Ahrens, T. J., and Syono, Y., J. Geophys. Res., 72, 4181 (1967).
4. Anderson, D., Science, 157, 1165 (1967).
5. Doran, D. G., Poulter Lab. Tech. Rep., 004–65 (Stanford Research Institute, 1965).