1. Conference Proceedings Boston, MA;M. C. Bhardwaj,1987
2. Richardson, L. F.: Apparatus for warning a ship at sea of its nearness to large objects wholly or partially under water. British Patent Specification 11, 125, March 27, 1913.
3. Muhlhauser, O.: Verfahren zur Zustandsbestimmung von Werkstoffen, besonders zur Ermittlung von Fehlern darin. German Patent Specification 569, 598, January 10, 1931.
4. McGonnagle, W. J.: Nondestructive testing, pp. 1?244. New York: McGraw-Hill 1961.
5. Thompson, D. O., Chimenti, D. E. (eds.): Review of progress in quatitative nondestructive evaluation, vol. 5, pp. 1?56. New York: Plenum 1986.