Combination of Space and Terrestrial Geodetic Techniques to Monitor Land Subsidence

We discuss a multidisciplinary approach that combines observations derived from continuous Global Positioning System (GPS), Interferometric Synthetic Aperture Radar (InSAR) and terrestrial gravimetry in combination with times series of local environmental parameters to estimate subsidence in the Southeastern Po Plain in Italy. The multi-technical approach takes advantage of the techniques’ complementary aspects, proving to be a powerful mean to observe and study vertical deformation with reliable spatial and temporal continuity; it provides a contribution to the realization of a local vertical reference frame and it adheres to the principles of the Global Geodetic Observing System (GGOS) presently being developed by the International Association of Geodesy (IAG). We demonstrate the importance of numerous co-located techniques to obtain reliable vertical velocities at fiducial sites. Moreover, by using the Permanent Scatter (PS) technique of InSAR, we can extend our knowledge of the spatial velocity field. This observational approach could have value for understanding the geodynamic processes driving vertical crustal movements in other regions of the world. We have studied the seasonal oscillations and the interannual variability of the GPS height time series as well as those of the superconducting gravimeter data series at Medicina. Environmental models describing the observed seasonal oscillations were compared to the mean observed seasonal cycles. Removal of the environmental models epoch by epoch reduces the standard deviation of the GPS and gravity time series by about 1 mm and 1 μGal respectively. In absence of a good environmental model, stacking of the daily solutions for a mean annual signal proves to be a reliable methodology for the estimate of the seasonal effect when this cannot be correctly represented by a sinusoidal function. The proper removal of the seasonal oscillations from the data series is important because they contaminate the estimate of the long-term trends. By combining GPS, InSAR and gravity, subsidence in the southeastern Po Plain has been mapped, revealing a noticeable spatial variability. Large vertical rates, mostly of anthropogenic nature, on the order of 20 mm/yr have been observed in and around the city of Bologna. The transition between a subsiding region and the uplift of the adjacent Apennines occurs over a narrow zone as evidenced by the analysis of the InSAR PS results. This is in agreement with a preliminary result provided by the GPS height data from the Loiano station located in the Apennines. As for the gravity data, the measurement of mass and height variations rather than height alone introduces an additional parameter beyond geometry to be assessed. The gravity measurements combined with the geometrical information allows us to distinguish between the different sources and mechanisms driving the observed height changes. At Medicina, the comparison of the secular gravity trend and the long-term linear height variations seems to indicate that there are no significant mass redistribution processes involved in this region. The remarkable consistency found between the time series derived from the GPS, InSAR and gravity observations in this study provides significant confidence in our interpretation of the geodynamical processes at this region. In addition, this study indicates that a systematic and synergetic combination of these technologies appears to be a valuable approach for monitoring and understanding surface deformation.