Geodaysit 2023

Raffaele Castaldo


The use of satellite data for the knowledge of the territory: geological applications
Giuseppe Solaro, Andrea Barone, Raffaele Castaldo, Vincenzo De Novellis, Antonio Pepe, Susi Pepe, Pietro Tizzani

The geological processes that occur several kilometers below the earth's surface, such as displacement along a seismogenic fault, pressure variation in magma reservoirs and landslides, in many cases cause deformations of the earth's surface that can be measured with geodetic methods and remote sensing techniques, such as differential SAR interferometry (DInSAR). DInSAR is a consolidated microwave remote sensing technique which, by exploiting two satellite images acquired at different times, makes it possible to estimate the surface deformations that occurred between the two acquisitions with centimeter precision. DInSAR systems are able to revisit the same area at regular intervals, providing very high spatial resolution information of the observed scene. For example, ESA ERS 1/2 and Envisat satellites, active since 1992, have a revisit time of 35 days, the sensors of the Italian COSMO-SkyMed constellation, have a revisit time of 8 days, and finally this time was reduced to 6 days for the "Sentinels" of the European Copernicus programme. These measurements are indicated by a series of colored bands, the so-called fringes or interferograms. The electromagnetic waves used are characterized by an alternation of crests spaced a few cm apart. By "counting" these crests, the radar is able to figure out how far the object being observed has moved; if the object is hundreds of kilometers away, moving only a few centimeters, the number of crests that characterize the electromagnetic waves will change, allowing the displacement to be accurately detected and measured with centimeter accuracy.
Interferometric techniques produce not only surface deformation maps measured along the sensor's line of sight; indeed, by exploiting a series of images acquired over time, it is possible to follow the temporal evolution of the deformation. This information can be particularly valuable, for example, for measuring ground deformation in volcanic areas, as this parameter can be a precursor to the resumption of eruptive activity or the increase of the unrest phenomena. If we consider that the first satellites (ERS-1) used for this purpose have been collecting data since 1992, the history of deformation of a volcano over the last 30 years can be analyzed in previously unimaginable detail.
The main results obtained in recent years in various geological contexts will be presented. For example in the volcanic context, using the DInSAR technique and benefiting from the availability of long-term SAR archives, it was possible to detect and monitor the evolution of the surface deformation of the Campania volcanoes (Campi Flegrei, Vesuvio and Ischia) and, with geophysical inversion, identify and analyze the deep sources responsible for the observed deformation. In the context of hydrogeological instability, by way of example, we will present a study conducted on the Ivanchic landslide in Umbria characterized by a relatively slow movement, which starting from satellite data and with geological, geotechnical and geophysical knowledge, has allowed us to characterize the geometry and the detachment surface of the landslide. Finally, some examples of applications of these techniques to identify deformations of infrastructures, such as buildings, dams, viaducts, will be illustrated in the urban context.

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