Evaluation of a laser-based measurement system for the detection of subsurface anomalies

Tunnel inspections are critical for ensuring structural integrity and safety and are therefore required in regular intervals, but they are often a time-consuming, labor-intensive task. We present a remote sensing technique that offers a promising solution to the challenge of detecting subsurface anomalies by other means than manual hammering. Our system utilizes a strong, pulsed laser that generates a plasma-induced shockwave on the concrete surface, which excites characteristic resonant vibrations indicative of subsurface anomalies like delamination. These vibrations are detected remotely using a customized laser Doppler vibrometer setup. Both lasers raster the surface in a predefined pattern while automatically being focused to the correct distance. We present the results of recent case studies in the laboratory as well as in a tunnel, that demonstrate the systems capability to deliver objective and reproducible data. Based on the findings we also discuss the systems limitations.