An Application of Wavelet Transform Signal Analysis to Acoustic Tapping Survey of Ground Cavities behind the Wall of Underground Structures.

Abstract

To judge if there exists any cavity by a tapping test is very easy but lacks objectivity. In order to attain more objective results, we have developed a method to judge by inspecting a scalogram (spectrum) of Wavelet transformed signals, rather than auditory evaluation, of recorded sound. With this method, a clear distinction could be made between scalograms of a solid ground and the one with cavities. No special measurement is required with this method. It is only necessary to hit the wall of a structure being surveyed in succession with a hammer, just like an auditory judgment. The principle and theoretical background of Wavelet Transform, the instrumentation, and the relationship between vibration and sound are explained in this paper. In addition, application tests using real-sized models, basic laboratory experiments of sound generation by hitting and some successful field examples are to be introduced. These are principal results from our works: 1) This method at present is especially effective against dry or water-saturated grounds. A cavity behind concrete pipe walls could be detected, if it is of considerable size. 2) This method has been applied successfully to underground structures like a power station to examine the quality of cement grouting between the machine housing and its surrounding base rock. 3) Location of cavities and their width (area) could be obtained with this method. As for their thickness (depth), however, decisive conclusion must be postponed until further experiments are conducted. 4) The factory produced pipes, of thickness 30cm or less, are especially favorable for this method. The acoustic response of their wall itself is generally stable due to well-controlled manufacturing, and enables us to recognize abnormal sound clearly, if cavities exist behind the wall.