Multimodal remote sensing for the monitoring and identification of moisture and salts on heritage buildings

Abstract

Historical buildings are prone to deterioration due to various reasons including environmental conditions, humidity and structural failures. The main factors of their degradation are moisture and salt activity. Salt weathering affects the appearance of the monuments but also causes chemical and mechanical degradation. The effect of salts in building deterioration is well-known, with several laboratory-based studies focusing on understanding the formation mechanisms. Here, we introduce a new methodology for the non-invasive monitoring and identification of moisture and salts following a complementary remote sensing approach. The study is based on ground-based remote short-wave infrared (SWIR) imaging and remote Raman spectroscopy from standoff distances of 3 to 15m [1]. The remote SWIR spectral imaging system covers the spectral range between 1 and 2.5 μm, with a spectral resolution of 5.5 nm and spatial resolution of 150 μm at a distance of 3m. The in-house developed mobile standoff Raman system operates with a continuous-wave (CW) excitation laser source at 780 nm. The laser beam can be focused at different distances resulting in a spot size of ~1 mm on the target. In our approach, SWIR imaging is used for scanning large wall surfaces. The post-processing of the acquired spectral imaging data using our novel machine learning-based clustering methods highlights the material variations across the wall. The detailed examination of the mean SWIR spectra for each cluster, allows a primary identification of moisture and salts, indicating also variations in volume concentration. The salts identification is then confirmed by remote Raman spectroscopy. The new method is presented through the examination of the historical building in Fort Brockhurst, an English Heritage monument in Portsmouth, UK.