Table 1 Devices used in the investigation

Hand-held laser rangefinder

DE120 (DELIXI ELECTRIC, China)

① To measure the size of the mural

Accuracy: ± 1.5 mm; Laser type: 635 nm, < 1mW

Full-frame digital camera

EOS 5D Mark IV + EF 24–70 mm f/2.8L II USM (Canon, Japan)

① To take photos of the mural for marking cracks

Focal length: 24 mm; Shooting mode: aperture priority; Aperture: f/9; Light sensitivity: International Standardization Organization (ISO) -100

Scissor-type aerial working platform

A4076 (C-GA Machine, Shanghai, China)

① To directly observe the cracks’ features from a short distance

Load capacity: 480 kg; Maximum height:7500 mm

Industrial endoscope

LF28 (VISION MEDICAL, Zhuhai, China)

① To directly explore the interior structure of the mural

Field of view: 90°; Depth of view field: 3 ~ 50 mm; Range of tip bending: − 180° ~ 130°; Outer diameter of insertion tube: 2.8 mm; Working length: 600 mm

Infrared thermal imager

E384 (HAWK ELECTRONIC, Xi’an, China)

① To indirectly explore the mural’s interior structure

② To observe the heat conduction process between the mural and external environment

Focal length: 25 mm; Field angle: 15° × 11.5°; Detector: uncooled focal plane arrays (UFPA); Resolution: 384 × 288 ppi; Noise equivalent temperature difference (NETD): 60 mK

Wireless temperature and humidity sensor terminal

WEMS500-HT (Zhejiang University, Hangzhou, China)

① To monitor the continuous change of air temperature and humidity in the main hall

Temperature sensitivity: 0.01℃; Humidity sensitivity: 0.01%

Hand-held vane anemometer

410-2 (testo, Germany)

① To measure the air fluidity at the secondary cracks

Resolution: 0.1 m/s; Measuring range: 0.4 ~ 20 m/s

Three-dimensional (3D) laser scanner

Focus S350 (FARO, USA)

① To obtain the point cloud data for deformation analysis of the mural

Field of vision: 360° transverse/300° longitudinal; Accuracy: ± 1 mm (25 m away); Working resolution: 1/4; Point cloud quality: 4×; Average scanning time per station: 469 s