Long-Range Over 25m Detection of Internal Defects of Shotcrete by SSE Analysis for Noncontact Acoustic Inspection

In our noncontact acoustic inspection, a target surface is acoustically excited by strong aerial sound waves, and its vibration velocity distribution is measured by a scanning laser Doppler vibrometer. This time, by a remote measurement over 25m, the internal defects of shotcrete was detected and visualized. With SSE analysis using spatial spectral entropy, the resonance frequencies of the internal defects can be detected. By narrowing the imaging frequency band, clear imaging of the defects was achieved.