Remote sensing of the subsurface between a Cistercian abbey and a medieval grange

Cistercian abbeys are known for their complex, subterranean networks of pipes, drains and tunnels. While much of this infrastructure would have been for the transport of water, and removal of waste, tunnels could have been used for other purposes – including concealment, evasion or escape. In this paper, we showcase the use of remote sensing to detect linear and geometric subsurface features between the Cistercian abbey of Aunay in Normandy and La Garenne, a nearby medieval grange. Analysis using statistical thresholds reveals anomalous multispectral reflectance behaviour from vegetation along a shared abbey-grange boundary wall, revealing the location of a previously unknown, underground passage from La Garenne. We then combine our two-dimensional subsurface anomalies from high-resolution, vegetation indexed satellite imagery with elevation anomalies in a very-high-resolution (VHR) hillshaded digital surface model (DSM). In doing so, we reveal several high-resolution reflection and micro-relief anomalies in the abbey’s largest field enclosure. Comparing these regular shaped anomalies with an eighteenth-century garden plan, we see several distinct similarities. We propose that these remote sensing observations are the surface expression of several near-surface, structural features from a large, formal, baroque garden, reinstated by the Cistercians during their seventeenth-century restoration of the abbey.