A history of high resolution observations of near-surface features due to archaeological looting in Apamea (Syria) combining radar and optical satellite time series

Since the Syrian civil war started in 2011, the archaeological site of Apamea in Syria has been devastated by a systematic campaign of looting, i.e. unauthorized excavations carried out on a massive scale without any scientific purpose to remove goods of historical or cultural value. This process pockmarks the landscape with individual pits and clusters of holes, often surrounded by deposits of the excavated material and going as much deep in the ground as looters opportunistically dig to find objects to sell in the clandestine market trafficking antiquities. In the situation of site inaccessibility and impossibility to undertake field surveys and ground investigations, archaeological, geophysical and remote sensing research scholars drew their earliest reports of archaeological looting in Apamea based on very high spatial resolution, but discontinuously distributed in time, optical images that are freely accessible in Google Earth. As very distinctive anthropogenic targets, the looting holes offer effective proxies to infer spatial and temporal patterns of looting. However, the challenge is to have consistent series of images of adequate spatial and temporal resolution allowing for a timely and accurate mapping of these features. In this paper, we present a detailed account of satellite-based observations that we have made by exploiting the high spatial resolution of X-band Synthetic Aperture Radar images (up to 1 m or even less) and the high temporal revisit of optical Sentinel-2 time series (up to 5 days), to track the evolution of looting in Apamea since its onset. Following an experimental campaign of amplitude change detection undertaken with very high spatial resolution TerraSAR-X time series showing an intensification of looting from the end of 2014, we screened 57 Sentinel-2 images acquired from 2015 to 2017 to prove that new incidents of looting occurred both in previously looted and new sectors of the site. While our results offer the most comprehensive reconstruction of looting history of Apamea, they provide a proof of concept of the value of combining radar and optical satellite time series to observe and characterize near-surface anthropogenic features.