Evaluation of restoration effects for city walls based on terahertz-infrared integrated technology

To scientifically evaluate the restoration performance of ancient city walls, Terahertz time-domain spectroscopy (THz-TDS) and infrared thermal imaging technology were applied to assess the Desheng Fortress (Ming Dynasty). Three representative sections were examined: adobe brick masonry repaired (Area 1), well-preserved original (Area 2), and layer-by-layer ramming repaired (Area 3). THz spectral data revealed significant differences between Area 1 (time delay: 3.72 ps; refractive index: 2.224) and Area 2 (time delay: 3.02 ps; refractive index: 2.107), while Area 3 (time delay: 3.12 ps; refractive index: 2.098) demonstrated nearly identical THz spectral data to Area 2. Infrared thermal imaging also showed that the Area 3 restored by layer-by-layer ramming exhibited greater uniformity with fewer instances of cracks, capillary phenomena, or biological diseases. The proposed point-surface integrated evaluation methodology synergistically combines infrared thermography mapping of heritage surfaces with THz spectral datasets acquired through in-situ micro-sampling, enabling quantitative restoration assessment and providing a novel approach for scientifically validating traditional conservation techniques.