## 3 Results
### Hotspot Monitoring: Site-Scale Assessment of Looting in Apamea
In particular, on 6 April 2016 higher reflectance was found in the fields located south-east of the theatre (Figure 4c), close to a cluster of looting pits that were already visible in November 2015. The remarkable increase in surface reflectance is compatible with new holes being dug and the associated brighter terrain being exposed by the excavation. Indeed the increase of reflectance due to earth movement and excavation of toposoil with the consequent exposure of the underlying ground is well known and reported in the literature [6; 20]. In the eastern portion of the archaeological site, along the second main Decumanus, we also detected an expansion of the previously looted area from 20 August 2015 to 16 February 2016 (Figure 4g,h), as well as an intensification of looting starting on 6 April 2016 (Figure 4i). At this date, new looting also became apparent in the area of the Market (Figure 4e) where it expanded until 05 June 2016 (Figure 4f shows the situation on 11 May 2017). What was observed with Sentinel-2 anticipates the evidence gathered through an ad hoc DigitalGlobe image acquired on 20 June 2016 and used by ASOR to report the ongoing looting at Apamea [31]. Except for the different spatial detail in boundary delineation, as expected due to the different spatial resolution, the two satellite-based assessments at HR and VHR mutually match. Thus, proving the complementarity of these two Earth observation systems for hotspot mapping. Thanks to the high temporal frequency of its acquisitions, Sentinel-2 brings in a better capability to temporally log observations of looting. Going through the Sentinel-2 time series up to the end of 2017, we found evidence that looting did not stop, but instead continued with new episodes. As captured on 11 May 2017, new looting had clearly manifested in the archaeological site west of the Cardo Maximus and the Agora (Figure 4f) and above the Decumanus Maximus (Figure 4j). The subsequent Sentinel-2 images did not highlight any apparent further expansion of the looting-affected areas. On the contrary, unexcavated sectors within the Justinian Walls that had not been previously looted (such as the north-western and south-eastern fields) did not appear to have been affected by looting until the end of the Sentinel-2 time series that we analysed, that is, mid-December 2017 (Figure 4b). Owing to the short revisiting time of Sentinel-2, we were also able to observe the progressive weathering and 'ageing' of older looting clusters. This is a known effect and is described in the specialist literature using both optical and SAR satellite imagery [19; 20]. In dry seasons, these holes may also become less visible due to low tone contrast with regard to the surrounding brown soil. Consequently, in absence of frequent satellite images, the spatial zoning of looted areas may be affected by an underestimation. In this regard, an example is provided in Figure 4h-j. A sub-rounded cluster of looting holes was timely detected in February 2016 (bottom left in Figure 4h) and was still visible a couple of months later (bottom left in Figure 4i). More than one year after, in May 2017, although the expansion and reworking of looting were clearly detected and mapped, the older cluster of looting holes was much less distinguishable from the brownish surrounding land (Figure 4j). The weathering of looting holes and subsequent underestimation of looted areas are environmental constraints that can affect both HR and VHR optical imagery, but the high temporal frequency offered by Sentinel-2 can help to cope with this inconvenience owing to the higher probability of timely detection and logging under conditions of good visibility and tone contrast.