3D Diagnosis Of A Tunnel Through Infrared Thermography Combined With Ground Penetrating Radar

In 1994, BRGM began a research project based on the combined use of infrared thermography and ground<br>penetrating radar (GPR) for the non-destructive testing of civil engineering works. This project aims to<br>combine these two techniques to enable: a). a more-precise knowledge of the nature and three-dimensional<br>extension of each anomaly, and b). a reduction in time spent on the measurement stage, and c). a better<br>interpretation.<br>The first measurements were taken during the survey of a railway tunnel, 1601 m long, in the east of France in<br>August 1994. The two entrances to the tunnel were tested over a distance of a few hundred metres.<br>A thermal imaging radiometer was used to take infrared scenes, 4 m wide and positioned by thermal markers,<br>whilst six parallel radar profiles (or radargrams) were recorded at three different frequencies.<br>A large amount of anomalies are detected when both techniques are combined, with correlation between the<br>total anomalies located during the present survey being 55%. In addition, this high level of correlation between<br>anomalies confirms both their existence and dimension. GPR systematically detects voids in masonry and<br>country rock whereas the depth penetration of infrared thermography is 1 m. On the other hand, the latter<br>technique is far more sensitive to problems of spoiled subsurface areas and humidity.<br>Finally, combined use of the two techniques provides a much clearer picture of the extension of each anomaly<br>in three-dimensions. Much work remains to be done however, specifically the calibration of radar profiles by<br>systematic velocity analysis. In addition, a better understanding of the ditTusion effect of each thermal anomaly<br>is required for a more precise estimation of the true extension.