Italy: Researchers at the University of Padua in Italy have compared old and new satellite imagery to confirm that the famous city of Venice is still sinking.
The problem was first recognised decades ago when researchers noticed that pumping of groundwater was causing the city to sink. In a recent study, Pietro Teatini, a hydraulic engineer at the University of Padua in Italy, and his colleagues used two sets of satellite measurements of Venice”s historical city centre and the surrounding area. The first dataset came from first-generation satellite sensors that collect data about once a month, whereas the second dataset comes from a newer satellite that takes measurements every 10 days. The satellites send signals to the Earth”s surface, where they reflect off land and buildings. The researchers measured differences in the signals returning from the city relative to those returning from nearby areas, a method called interferometry. The results revealed the city is naturally subsiding at a rate of about 0.03 to 0.04 inches (0.8 to 1 millimeter) per year, while human activities contribute sinking of about 0.08 to 0.39 inches (2 to 10 mm) per year. The researchers compared short-term changes in the city”s height measured by the new satellite from 2008 to 2011 with the average, long-term movement measured by the old satellite from 1992 to 2010. Then they subtracted the short-term changes in ground level from the long-term ones to determine the human contribution to the sinking.