25 years of satellite InSAR monitoring of ground instability and coastal geohazards in the archaeological site of Capo Colonna, Italy

Abstract

There is a copious literature investigating the natural and human-induced land subsidence along the eastern coast of Calabria region in Southern Italy, as well as its correlation with geological and tectonic setting, temporal evolution and impact on the landscape. Earliest evidence of ground instability and coastal subsidence were found in the archaeological records of the Ancient Greek sites distributed along the Ionian coast. Among these, Capo Colonna is of particular interest for its cultural value, as it was known for centuries for the glorious sanctuary of the Greek Goddess Hera Lacinia. Although only one monumental column is preserved, the site is nowadays the archaeological icon of the region and, for this reason, a focus for the heritage bodies responsible for its preservation. The site has been imaged by space-borne Synthetic Aperture Radar (SAR) sensors over the last 25 years, resulting in an unprecedented SAR time series, starting from ERS-1/2 (1992-2000), ENVISAT ASAR (2003-2010), TerraSAR-X StripMap (2008-2010), COSMO-SkyMed StripMap (2014-2015) and Sentinel-1A Interferometric Wide Swath (2014-2016) scenes. In this work, we analyse annual rates and temporal evolution of ground motion across the site, estimated through Interferometric SAR (InSAR) processing with Permanent Scatterer InSAR (PSInSAR), Coherent Pixel Technique (CPT), Persistent Scatterer Pairs Differential InSAR (PSP-DIFSAR), Small Baseline Subset (SBAS) and Intermittent SBAS (ISBAS). The goal of our multi-sensor, multi-temporal and multi-processing approach is to test different spatial resolutions, radar wavelengths and InSAR routines and provide an improved understanding of the performances of these data and methods over an archaeological area that is known to have been uninterruptedly exposed to land subsidence and coastal processes of instability, but challenging to monitor with InSAR methods due to its chiefly rural setting. We integrate InSAR ground motion information for natural reflectors present across the archaeological site with a recently installed network of artificial corner reflectors. This now enables the analysis of sectors where land cover constraints prevented a dense coverage of persistent scatterers and coherent targets to be obtained when processing historical ERS and ENVISAT data. The spatially extended coverage achieved by processing the Sentinel-1 data with the novel ISBAS method also allows us to further fill the knowledge gap and to retrieve information on land motion for 2014-2016 over the whole site, including rural and semi-vegetated areas.