Abstract
In archaeological remote sensing, space-borne Synthetic Aperture Radar (SAR) has not been used so far to monitor ‘looting’ (i.e. illegal excavations in heritage sites) mainly because of the spatial resolution of SAR images, typically not comparable to the ground dimensions of looting features. This paper explores the potential of the new TerraSAR-X beam mode Staring Spotlight (ST) to investigate looting within a workflow of radar backscattering change detection. A bespoke time series of five single polarisation, ascending mode, ST scenes with an unprecedented azimuth resolution of 0.24 m was acquired over the archaeological site of Apamea in western Syria, from October 2014 to June 2015 with a regular sampling of one image every two months. Formerly included in the Tentative List of UNESCO, the site has been heavily looted from at least early 2012 to May 2014, as confirmed by Google Earth Very High Resolution (VHR) optical imagery. Building upon the theory of SAR imaging, we develop a novel conceptual model of ‘looting marks’, identify marks due to occurrence of new looting and discriminate them from alteration (e.g. filling) of pre-existing looting holes. ‘Looting marks’ appear as distinctive patterns of shadow and layover which are visible in the ground-range reprojected ST image and generated by the morphology of the holes. The recognition of looting marks within ratio maps of radar backscatter (σ0) between consecutive ST scenes allows quantification of the magnitude, spatial distribution and rates of looting activities. In agreement with the estimates based on Google Earth imagery, the ST acquired in October 2014 shows that ~ 45% of the site was looted. In the following eight months new looting happened locally, with holes mainly dug along the margins of the already looted areas. Texture values of ~ 0.31 clearly distinguish these holes from the unaltered, bare ground nearby. Hot spots of change are identified based on the temporal variability of σ0, and colour composites indicate where repeated looting and alteration of existing holes occurred. Most looting marks are observed north of the two main Roman decumani. Looting intensified almost steadily from December 2014, with over 1500 new marks in February–April 2015. The estimated rates of looting increased from 214 looting marks/month in October–December 2014 to over 780 marks/month in April–June 2015, and numerically express the dynamic nature of the phenomenon to which Apamea is still exposed. The method of identifying looting marks in VHR radar images therefore proves a reliable opportunity for archaeologists and image analysts to measure remotely the scale of looting and monitor its temporal evolution.
Highlights
In archaeological heritage science, the term ‘looting’ refers to unauthorised excavations without any scientific purpose that aim to remove goods of historical or cultural value, frequently used to feed the clandestine market trafficking antiquities
In this work we focus on looting holes and identify how these are imaged by the side-looking viewing geometry of the TerraSAR-X Staring Spotlight (ST) acquisition mode
~80% is found over the previously excavated archaeological areas, while the remainder falls over the agricultural fields that were formerly unexcavated by archaeologists
Summary
The term ‘looting’ refers to unauthorised excavations without any scientific purpose that aim to remove goods of historical or cultural value, frequently used to feed the clandestine market trafficking antiquities. ⁎ Corresponding author at: British Geological Survey, Natural Environment Research Council, Nicker Hill, Keyworth NG12 5GG, UK. Looting even affects some western countries with long standing tradition of archaeological conservation (Proulx, 2013). D. Tapete et al / Remote Sensing of Environment 178 (2016) 42–58 heritage are further constrained by the inaccessibility of the site due to security considerations. The methods (e.g., hand-digging or using machines) and rates (e.g., systematic and steady or occasional and sudden) with which looting occurs contribute to determine the degree of destruction of the archaeological context. It is to be acknowledged that looting causes irreversible damage to the landscape and the anthropogenic environment
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