Geodetic monitoring

What is geodetic monitoring for?

The activities of extraction/storage of hydrocarbons and re-injection of fluids underground can induce surface deformation. Through geodetic satellite monitoring (DInSAR), it is possible to measure the trends over time of ground displacements with an accuracy of one centimetre, and in some cases even a few millimetres, on spatially extensive areas (from tens to tens of thousands of square kilometres). The ability to measure the deformations of the ground over large areas compared with the size of the deposit and above all to follow their temporal evolution allows highlighting any variations associated with the storage activities with respect to the pre-existing deformation scenario.

What are ground deformations, and why does the ground deform?

The ground can be deformed either by the deformation of the Earth's surface crust or by local phenomena of various kinds. The deformation of the Earth's crust occurs mainly due to tectonic movements, i.e., the relative displacements of continents that create zones of deformation with varying intensity. However, the ground also deforms due to other natural causes, such as seismic events, volcanic eruptions, faults, and landslides, and as a consequence of human activities (e.g., extraction of water from groundwater, of oil or gas from hydrocarbon deposits, and of mining material and gas storage).

What is a GNSS system, and what does it detect?

The Global Navigation Satellite System (English acronym GNSS) is a system for geo-radiolocation and navigation on land, sea or air, using a network of artificial satellites in orbit and pseudolites.
The geolocation systems provide a geo-spatial positioning service with global coverage that allows small, purpose-built, electronic receivers to determine their geographical coordinates (longitude, latitude and elevation or altitude) at any point on the Earth's surface or in the atmosphere with an error of a few metres (and up to millimetres in precise realizations such as permanent stations for geodetic use), by processing radiofrequency signals transmitted in the line of sight by such satellites.

What are DInSAR data, and what do they detect?

DInSAR data are measurements of the deformation of the Earth's surface (expressed in centimetres or millimetres) obtained using radar data acquired by satellite. In particular, we use image sequences obtained using systems called SAR (synthetic aperture radar) acquired in a certain time interval by one or more satellites in an area of interest; through processing techniques called DInSAR (differential interferometry SAR), we obtain maps of the ground deformation in the area of interest, and for each point on these maps, we determine the evolution of the deformation in the time interval considered (i.e., how the deformation varies over time).
The DInSAR analysis natively generates measurements of the ground deformation projected along the line of sight (LOS) of the sensor, which, in fact, "looks" at the globe not perpendicularly but at a certain angle. Taking advantage, however, of the fact that the sensors acquire data by covering both ascending orbits (i.e., moving from south to north) and descending orbits (moving from north to south), it is possible to combine the DInSAR results of the analysis performed for the different orbits and obtain the measurements of the vertical and east-west components of the ground movements.

What are the consequences of ground elevation changes?

The effects of elevation changes depend primarily on the location of the site where they occur. In many situations, the most important factor to consider is the differential elevation variation, i.e., the relative elevation variation between two "neighbouring" points (the term "neighbours" means a distance of interest with respect to the object whose effect is to be estimated). For masonry buildings with more than one floor, it is estimated that the maximum acceptable deformation is within 5 × 10-4 mm, i.e., 5 mm for every 10 m [Skempton and McDonald, 1956]. For reinforced concrete buildings and steel structures, this value can be 10-15 times larger.

Does storage generate deformations or changes in ground elevation?

Gas storage can generate deformations of the order of magnitude of a few millimetres both horizontally and vertically. The deformations detected or estimated are much lower than those that can cause consequences for people and, in general, for the environment. However, it is important to monitor surface displacements both to have objective evidence of the effects caused by storage and to improve the interpretation of events.

Are there natural variations in ground elevation around Cornegliano?

Yes, it is natural for this phenomenon to possibly occur with shifts on the order of a few millimetres in an area that includes the storage volume. In fact, it is estimated that the induced deformations may be very limited and far below the values that can be perceived or cause any consequence for both humans and the environment. Continuous monitoring will, however, allow constant verification of the situation.

What is the extent of ground deformations measured in the Cornegliano Laudense area?

To date, monitoring does not show any position changes or deformations that exceed the natural background values.

How can I read the results of GNSS and DInSAR analysis?

The main results of DInSAR analysis are the average ground deformation velocity maps, expressed in cm/year or mm/year, and, for each point displayed in these maps, the time series of the deformation calculated in centimetres, i.e., the deformation trend during the time interval in which SAR data were acquired.
In particular, the maps of average terrain deformation velocity generated by the DInSAR analysis of the Cornegliano Laudense area are geocoded maps; therefore, they can be superimposed on existing maps of the area and are represented in false colours, which correspond to the displacements of the ground. Usually, green represents stable zones (which do not deform over time), yellow and red indicate subsidence zones, and light blue and blue mark uplifted zones.