Tracking human-induced landscape disturbance at the Lines and Geoglyphs of Nasca and Palpa UNESCO World Heritage site in Peru with satellite radar

The 'Lines and Geoglyphs of Nasca and Palpa' in Peru are a globally well-known UNESCO World Heritage site, where heritage assets cannot be separated from their natural and anthropogenic environment. The site is exposed to interactions between nature and human presence. The Nasca Lines have long been studied using surface and near-surface geophysical methods including satellite Synthetic Aperture Radar (SAR) for identification of anthropogenic targets and change detection. In this work, we present some experiments carried out with satellite SAR imagery to track events of human-induced landscape disturbance. Very high resolution (3 m) HH-polarised SAR scenes acquired in StripMap HIMAGE mode by the X-band COSMO-SkyMed constellation are exploited to track two recent events. The first occurred in December 2014, when environmental activists accessed the area near the Hummingbird geoglyph, marked the route followed to access the geoglyph and disturbed the surrounding terrain. The second occurred in January 2018, when a truck diverted off the Pan-American Highway, entered the protected area and crossed over 3 geoglyphs. Pre-, cross- and post-event Interferometric SAR (InSAR) pairs with small temporal and normal baselines allow the detection of temporal decorrelation associated with the two events, the extent and time reference of which match with photographic and video evidence, literature, web news and press releases by the Ministry of Culture. Other SAR-based investigations relied on medium resolution ERS and ENVISAT C-band SAR data to assess their potential to analyse regional to site scale landscape changes, and establish the condition of heritage assets - either at surface or brought back to light by archaeological excavations. Recent studies also trialled very high resolution (1 m) data acquired by the X-band mission TerraSAR-X, and the shorter repeat cycle (12 days) of the Copernicus Sentinel-1 C-band constellation. These experiments confirm that SAR data archives, such as those being built for Nasca by COSMO-SkyMed and Sentinel-1, prove valuable for retrospective analysis and digital recording of landscape disturbance events from space, and therefore act as essential sources of geophysical and geospatial information on the condition and conservation history of archaeological assets.