Venice, Italy, more of an island than part of a country, is gradually being inundated by waters from the Adriatic Sea. This is not news. However, despite efforts once thought to have halted the drenching of the city, combined technology shows that the problem continues.
GPS along side Interferometric Synthetic Aperture Radar (InSAR) is providing more precise measurements of the rate at which Venice is sinking below the waves. The combined subsiding of the city and the rising of the oceans accelerates the flooding of the streets and drenching of lower floors of buildings in the Renaissance city. Venice is sinking at the rate of about 2 millimeters a year while the sea level in the Venetian lagoon is rising at the same rate, putting Venice at 80mm or 3.2 inches below sea level in 20 years according to Yehuda Bock, a research geodesist with Scripps Institution of Oceanography at the University of California, San Diego.
InSAR is a cloud-penetrating microwave imaging system. Measurements of travel path variations as a function of the satellite position and time of acquisition allow generation of digital elevation models and measurements of centimetric surface deformations of the terrain. They are beneficial in showing changes in the earth?s surface surrounding volcanoes and earthquake fault lines.
How InSAR works
Tele-Rilevamento Europa who took part in the study says that SAR satellites acquire images of the Earth?s surface by emitting radar signals and analyzing the reflected signal. A typical SAR image is 100 x 100 km and is regularly acquired over the same area. It is the measurement of signal phase change, or interference, over time. When a point on the ground moves, the distance between the sensor and the point on the ground also changes and so the phase value recorded by a SAR sensor flying along a fixed orbit will also be affected.
The GPS measurements provide absolute elevations, while the InSAR data are used to calculate elevations relative to other points. The GPS instruments enabled the researchers to take absolute readings of the city and its surrounding lagoons. "Our combined GPS and InSAR analysis clearly captured the movements over the last decade that neither GPS nor InSAR could sense alone" said Shimon Wdowinski, associate research professor of Marine Geology and Geophysics at the University of Miami.
InSAR recorded changes between two points, but GPS picked up on something the satellite images did not. GPS has shown that, like the Tower of Pisa, the area is tilting. The Eastern side is sinking more than its Western counterpart leaving the area of city of Venice proper a tad drier by about 2mm per year.
One controllable reason Venice was subsiding was the pumping out of ground water. An uncontrollable reason is that plate tectonics affect Venice wherein the Adriatic plate dips beneath the Apennines Mountains.
As a potential partial solution to the problem, Pietro Teatini, at the University of Padova in Italy with other researchers is looking into injecting saltwater into the aquifers deep below the city, reversing the sinking caused by groundwater pumping. Other problems contributing to Venice?s high water mark are not so not easily solved.