Fig. 1. The city walls experimental plot in Desheng Fortress

Fig. 2. Schematic diagram of the Desheng Fortress distribution. (a) Adobe brick masonry repaired section. (b) Well-preserved eastern section. (c) The traditional layer-by-layer ramming repaired section

Fig. 3. Terahertz time-domain spectrometer. (a) Inside of the THz-TDS system. (b) Schematic diagram of THz detection and sampling. (c) Sample holder for transmission measurements

Fig. 4. THz spectra of the soil samples from Desheng Fortress. (a) Time-domain spectra. (b) Absorption coefficient spectra. (c) Refractive index spectra. (d) Average refractive index spectra

Fig. 5. Visible light images and infrared images of three randomly selected sub-regions in the No.1 test region. (a) and (b): Area 1. (c) and (d): Area 2. (e) and (f): Area 3

Fig. 6. Visible light images and infrared images of two minimally weathered sub-regions in the No.2 test region. (a) and (b): Area 1. (c) and (d): Area 2.

Fig. 7. Visible light images and infrared images of three randomly selected sub-regions in the No.3 test region. (a) and (b): Area 1. (c) and (d): Area 2. (e) and (f): Area 3.