Analysis of structure and infrastructure movements via PSIinSAR data over large areas.

An extensive study has been undertaken aimed to highlighting the potentiality and the limits of SAR interferometry technique devoted to monitoring soil and structure movements, particularly induced by slow motion landslide phenomena. The study has been performed on eleven sites in the Liguria Region, chosen as representative of different morphological and stability conditions. For these sites (each one covering an average area of some square Km) data elaborated by the Permanent Scatterers Interferometry SAR technique (PSInSARTM) were available. The images dataset have been acquired by ERS-1 and ERS-2 satellites from 1992 to 2001. The technique is based on the analysis of the movements of objects characterized by a stable and point-like behaviour (Permanent Scatterers) for which the average annual velocity is derived. Permanent Scatterers usually correspond to man-made structures such as buildings, steel elements and structures, retaining walls, roads, etc., as well as natural reflectors (e.g. exposed rock). In any case the targets maintain constant their geometrical characteristics (dimension and orientation) with time. The technique is virtually unaffected by temporal and geometrical decorrelation effects. In the selected areas almost 800 targets (on a number of over 1400) have been “materialized”, surveyed and listed; most of them referring to buildings, roads or part of them. All the data useful for the study purpose have been collected, especially as far as target description, geometrical and structural characteristics, construction/retrofit period, average movement velocity, cracks, damage degree, are concerned. By these information a data base has been obtained, useful for statistical analyses. Working over large areas, (even if on every single target) simplified damage criteria has been adopted to point out the average degree of damage to which a particular zone have been subjected to. Analyzing crack patterns and damage characteristics (on structural elements clearly subjected to past and/or current movements) direction on movements has been estimated and, when possible, related to landslide movement information. The obtained “damage maps” allows to identify the accuracy and reliability on the interferometric technique and, also, to localize areas of particular interest as far as landslide risk or new building permission are concerned. To sum up, the main gathered results allow the following: - The performed study has demonstrated the high reliability of the SAR technique, in that a good correlation between obtained velocity and degree of damage has been always verified. - The data need an accurate and critical interpretation, to avoid the simple conclusions “high velocity = high hazard” and “low velocity = no risk”). - The importance of information coming from local monitoring (e.g. inclinometers), from accurate geomorphological analyses, and, especially, from the knowledge of structural characteristics and structure response need to be stressed when passing from “large area scale” to “building scale” analyses. - As expected, masonry structures behave differently from RC structures. In addition, from a preliminary analysis of some case records, it appears a more clear correlation between recorded velocity and damage in the RC structure case; on the contrary, masonry structure damages are much more difficult to interpreter in relation to the average target velocity.