Two-dimensional resistivity surveys of the moisture content of historic limestone walls in Oxford, UK: implications for understanding catastrophic stone deterioration

Catastrophic deterioration of limestone facades occurs where areas of stonework become rapidly hollowed out. It affects many historic buildings in Oxford, especially where soot-rich gypsum crusts have accumulated. In order to understand the processes of catastrophic deterioration we need to understand the microenvironmental conditions, especially the moisture distributions in the deteriorating walls. Geoelectric methods, in the form of two-dimensional (2D) resistivity surveys, have been used to study the distribution and amount of water stored in deteriorating limestone walls within the historic centre of Oxford. Fifteen vertical profiles, each 2–2.5 m in length, have been monitored at five sites using 50 medical electrodes and GeoTom equipment. Calculated moisture contents and distributions are presented for those profiles that extend up to 40 cm into the wall. The data indicate the diversity and complexity of moisture distributions within these often heterogeneous walls, which have also had long histories of decay and conservation. Replacement stone patches show consistently higher moisture conditions than the surrounding stone. Most profiles indicate the presence of wetter patches 5–10 cm behind the wall face under blackened crusts. Catastrophically decayed sections of profiles often exhibit wetter near-surface conditions than surrounding stonework, whilst areas with shallow but active decay are often much drier than surrounding crusted stone. In conclusion, the results give preliminary confirmation of a simple model of catastrophic decay and illustrate the complexity of moisture regimes in historic walls.