In the protection and restoration practice of murals, investigating the distribution range and severity of hollowing disease, as well as evaluating the grouting reinforcement effect of hollow murals, has always been a technical problem. After attempting to use the air-coupled ground penetrating radar method to detect the internal hollowing disease of Dunhuang Grottoes murals without coating, this paper based on the theory of thermal conduction switched to using an infrared thermography camera to detect under active heating conditions, achieving good results.
Simulated the construction of Tibetan temple murals and Dunhuang Grottoes murals, and pre-set regular hollows within varying depths and thicknesses. Through forward simulation experiments, the collection parameters of the ground penetrating radar were determined, experience in data processing was accumulated, and the performance of different radar antennas was compared.
Based on forward simulation experiments of the ground penetrating radar, on-site detection was conducted for the space hollowing disease in the murals of Potala Palace, Roblinka, and Sakya Monastery. The results showed that the space hollowing disease of these three monasteries' murals was severe and manifested as a feature of enhanced negative amplitude on the radar image.
Basing on the discontinuity of heat conduction inside the mural and viewing space hollowing disease as a thermal resistance-type defect, under the far-field point source thermal radiation, an infrared thermography camera was used to analyze the temperature difference on the surface of the mural's west wall of Cave 12 in Dunhuang's Xiqianfodong. Affected by the shape of the cave and the external environment, the spatial distribution difference of surface temperature at the beginning of the mural varied, the temperature of the controlled air flow approached the highest temperature in the cave's history, increased the temperature gradient along the depth direction of the mural, and weakened the horizontal heat diffusion.
The on-site detection results showed that after the air in the cave was continuously heated for a period of time, the average temperature on the mural's surface increased from 11℃ to 18℃, and the temperature of a specific area of the wall surface reached up to 19℃. Considering the on-site detection conditions, it was found that the area with relatively high surface temperature of the mural was basically consistent with the distribution range of the hollowing disease, and the border where the hollowing area met the non-hollowing area was relatively consistent with the hollowing ruptured boundary on the mural surface.
Through comprehensive analysis of the living environment, construction materials, and surface conditions of Tibetan temple murals and Dunhuang Grottoes murals, the ground penetrating radar is suitable for detecting internal hollowing disease in Tibetan temple murals and can be used to evaluate the grouting reinforcement effect of hollow murals; while the infrared thermography camera is suitable for detecting internal hollowing disease in Dunhuang Grottoes murals.
