Fig. 1. Distribution of hollow area in Damaidi petroglyph

Fig. 2. Two typical hollow regions in Damaidi petroglyph. (a) Hollow image 1; (b) hollow image 2

Fig. 3. Schematic diagram of hollow region boundary and characteristic line

Fig. 4. Schematic diagram of simplified heat conduction model in hollow region during heating process

Fig. 5. Temperature distribution curve of characteristic line for hollow

Fig. 6. Schematic diagram of the hollow region model

Fig. 7. Comparison of numerical simulation and experimental temperatures

Fig. 8. Sensitivity analysis diagram of heat sensitive parameters with change of hollow width

Fig. 9. Variation in hollow heat accumulation ratio with heating time under different hollow widths, illustration shows a magnification of the heating time of 0‒500 s

Fig. 10. Heat conduction stage division during the heating process

Fig. 11. Relationship between heat accumulation ratio and hollow width in different stages

Fig. 12. Variation in error rate of heat accumulation ratio with heating time under different hollow widths, illustration shows a magnification of the heating time of 0‒1600 s

Fig. 13. Flowchart of thermal infrared hollow calculation method

Fig. 14. Measurement layout and method for hollow boundary detection using hardness tester

Fig. 15. Comparison of infrared thermal imaging and hardness testing in the hollow detection of rock paintings

Fig. 16. Relative error rate of hollow width between thermal infrared imaging and hardness testing methods

Fig. 17. Variation in relative error rate with hollow width