The surveillance of structures which are prone to damage or critical for a system are usually done through AE monitoring. One of the principal advantages of AE monitoring of a structure is that the AE source can be located from the difference of the arrival times of AE waves from the source to different sensors placed at different points on the structure. Two kinds of acoustic waves are usually observed in AE signals. One of them is of continuous type and other is of burst type. Usually AE source location is identified from the difference of the arrival times of the burst type signals. However, there are methods to locate source through continuous type of AE also but these techniques will not be discussed in this paper.

In case of one dimensional structure, only one independent coordinate with respect to a fixed origin is necessary to identify the location of the source. For that reason, only two sensors will be necessary on the structure to locate the position of the source. In case of a plane surface where AE source can have two independent coordinates, at least three sensors will be necessary on the structure to identify the coordinates of the source. The question now arises whether the location of the source identified from the difference of arrival times of AE waves to the sensors is unambiguous. In this context, several papers and reports were published [1-5] but none of them have given any mathematical formalism for the cases of ambiguity.

In the paper published by Tobias [1], computer plots of different points on a plane surface on which ambiguity will occur, have been identified but no mathematical justification was given. In this paper, mathematically the condition for the unambiguous solution for AE source location in case of plane surface is discussed. Location of AE source in a plane is detected from the difference of the arrival times of the AE signal to at least three sensors placed on it. The detected location may not be unique in all cases. Here, the condition for the unambiguous solution for the location of the source has been deduced mathematically in terms of arrival times of the AE signal, the coordinate of the three sensors and the acoustic velocity. REFERENCES

1. A. Tobias, "Acoustic-emission source location in two dimensions by an array of three sensors" Nondestructive Test, Vol. 9, No. 1, pp. 9-12, 1976
2. M. Asty, "Acoustic emission source location on a spherical or plane surface", NDT Intern, Vol. II, No. 5, pp. 223-226, 1978
3. Dong-Jin Yoon, Young H. Kim and Oh-Yang Kwon, "New algorithm for acoustic emission source location in cylindrical Structure", J Acoust Emiss, Vol. 9, No. 4, pp. 237-242, 1992
4. J. A. Baron, and S. P. Ying, "Acoustic emission source location: Nondestructive Testing Handbook, (edited by P. McIntire), American Society for NDT, 1987
5. P. Barat, P. Kalyanasundaram, and B. Raj, "Acoustic Emission Source Location on Cylindrical Surface" NDT Intern., Vo. 26, No. 6, pp. 295-297