Abstract
Remote sensing techniques, such as LiDAR and photogrammetry, are used by researchers exploring the spatial distribution of weathering features in historic masonry. These well-established tools provide users with a perspective of the processes affecting the surface of masonry blocks; however, they cannot provide information on the alteration occurring subsurface. Geophysical tools are being explored as a potential approach to observe the variation in material properties beneath masonry block surfaces and to examine the patterns of deterioration across wall sections. Applying such techniques inform the development of conceptual models of weathering at the block to building wall scale. In this study, ground-penetrating radar (GPR) was selected to inspect the subsurface condition of the wall section of an historic church wall, where areas of granular disintegration and flaking can be observed. 3DGPR was selected for this task, as its use of regular grids during data collection make it better suited for detecting features within an area. Three high-frequency antennas, 1.2 Ghz, 1.6 Ghz and 2.3 Ghz, were run across the study area in a series of 80 cm by 80 cm grids. The data were collated within GIS, where observed features were annotated onto a schematic of the wall surface. The 3DGPR outputs identified anomalies within this structure that could not have been as easily interpreted using a 2DGPR transect. However, as 3DGPR relies upon interpolative techniques to estimate the returns between observation transects, the validity of features detected in these locations need to be tested. The results of this application of 3DGPR identified variable weathering response across the wall section, relative to elevation. These observations were used to develop a conceptual model linking these findings to seasonal variation in the capillary rise of groundwater, upward from the base of the church wall. Through these findings it is possible to see how GPR can assist in developing our understanding of the processes threatening heritage buildings.
Highlights
The operation of weathering processes upon sandstone building facades leads to deterioration of block integrity and the loss of material
This paper aims to expand upon our earlier findings [11] that only used 2DGPR to explore the potential of using multiple radar frequencies on the smoothing sheet to aid in the application of 3DGPR
The 3DGPR results (Figure 5) identify both anomalies and a spatial distribution of features that would not have been distinguishable using only 2DGPR transects, such as we used in a previous publication to this one [11]). 3DGPR has the advantage of detecting features across the entire area
Summary
The operation of weathering processes upon sandstone building facades leads to deterioration of block integrity and the loss of material. Often the distribution of visible weathering features can appear to be chaotic in nature [2] For this reason, new tools need to be explored to establish links between the spatial variables within the stone decay system and the eventual response. A series of spatial tools, including GIS, terrestrial laser scanning and photogrammetry, have been integrated into studies of stone decay in historic buildings [3,4,5]. These approaches only provide information relating to surficial alteration and do not provide specialists with an understanding of the impact of weathering processes operating in the subsurface
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