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

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  • 応用地質における調査・解析技術の新展開　　ウェーブレット解析を応用した構造物周辺の地盤空洞調査
  • ウェーブレット カイセキ オ オウヨウシタ コウゾウブツ シュウヘン ノ ジバ

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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.<BR>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.<BR>These are principal results from our works:<BR>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.<BR>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.<BR>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.<BR>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.

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