Key Reports:

State-of-the-Art of Monitoring Methods to evaluate Storage Site Performance

2.2.3 Interferometric Synthetic Aperture Radar (InSAR)

InSAR is a satellite-based radar measurement technique able to survey large areas of the earth's surface and provide, at a minimum, monthly updates on ground deformation (McColpin, 2009). The user only has to ask for a satellite to be tasked to a project area and purchase a data subscription to begin acquisition. Images normally cover 2,500 to 10,000 square kilometres. Searches can be easily done for historical baseline data, which may exist in the industry's extensive archives.

InSAR works by taking readings at regular intervals and comparing changes from month to month (see Fig. 2-8). Accuracy with just two scenes is in the centimetre range but millimetre accuracy can be achieved by stacking many months of data to eliminate atmospheric errors. Accuracy can also be improved through integration of DGPS and tiltmeter data, which can provide finer resolution over smaller areas (McColpin, 2009). Differences between scenes are usually represented as coloured bands with each band representing an interval of ground movement.

Fig. 2-8: Principle of the InSAR method (<span data-ps-type= McColpin, 2009).">

Fig. 2-8: Principle of the InSAR method (McColpin, 2009).

InSAR works best in areas with consistent radar reflections. Areas with dense vegetation can be problematic, as well as areas with natural variations in the surface caused, e.g. by frost heave or wetting-drying cycles, which mask the changes that occur due to pressure changes (IPCC, 2005). Earthworks represent another type of "masking" effects in the survey area.

Results in the above-mentioned problematic areas can be improved by installation of simple sheet metal corner reflectors, which create a high surface-to-noise ratio. By properly placing and installing these reflectors, generation of accurate reflections from month to month can be ensured (McColpin, 2009). Another deficiency of standard InSAR measurements is that the method is unable to resolve the horizontal and vertical components of the recorded motion.

The technology is now being used in various modifications, of which in particular the DInSAR (Differential Interferometric Synthetic Aperture Radar) and PSInSAR (Permanent Scatterers) techniques have provided interesting results. DInSAR is based on pixel-by-pixel computation of interferometric phase using two satellite radar acquisitions: such differential phase is a measurement of what has changed in the time interval between the two satellite acquisitions (ERS: 35 days, Envisat: 35 days, Radarsat: 24 days and TerraSAR-X: 11 days). Apparent phase variations between two satellite images can be caused by actual ground displacement or by atmospheric effects that delay electromagnetic wave propagation. An example of DInSAR results is shown in Fig. 2-9.

Fig. 2-9: DInSAR results from In Salah showing deformation time series with respect to July 31, 2004 for six selected dates. Areas around the KB-501 and KB503 wells are swelling up, while KB-502 is slightly subsiding.

Fig. 2-9: DInSAR results from In Salah showing deformation time series with respect to July 31, 2004 for six selected dates. Areas around the KB-501 and KB503 Wells are swelling up, while KB-502 is slightly subsiding. Contour interval is 5 mm (Onuma et al., 2011).

The PSInSAR^TM technique uses a temporally complete set of SAR scenes obtained over an identical target area. The approach is based on a few basic observations: atmospheric artefacts show strong spatial correlation within individual SAR acquisitions but are uncorrelated in time and, conversely, target motion is usually strongly correlated in time, exhibiting varying degrees of spatial correlation depending on the type of deformation (e.g. subsidence due to water pumping, fault displacements, localised sliding areas, collapsing buildings, etc.). In order to separate these phenomena, atmospheric effects are estimated and then removed by combining data from long time series of SAR images (such as those available in the ESA-ERS archive), which were acquired from late 1991.

In order to improve the accuracy of ground displacement estimations, only scatterers minimally affected by temporal and geometrical decorrelations are selected for processing (Ferretti et al., 2001; Tamburini et al., 2010). The PSInSAR^TM technique was introduced by the Politecnico di Milano (POLIMI) in the late nineties, and later improved by Tele-Rilevamento Europa (TRE). Its main advantages are the extension of area that can be monitored (up to thousands of square kilometres) and the accuracy of the obtained measurements (deformation rates accurate to millimetres/year).

Due to the geographical characteristics of the area, the In Salah CO2Carbon dioxide storage site in the Algerian part of the Sahara desert has become a field laboratory for testing of the InSAR technology for the purposes of CO2Carbon dioxide storage monitoring. Results of several independently working groups have been published (e.g. ùOnuma et al., 2011; Vasco et al., 2010). Moreover, the InSAR data were used for geomechanical, reservoir and fluid-flow analyses of processes occurring within the reservoir during CO2Carbon dioxide injection (e.g. Rutqvist et al., 2010; Morris et al., 2011; Shi et al., 2012) or CO2Carbon dioxide distribution simulations (Cavanagh and Ringrose, 2011).

Surface deformation monitoring techniques are generally applicable only onshore. The only exception is tiltmeters: underwater tiltmeters are commonly deployed to monitor sea bottom-based structures. They have not been, however, deployed in larger arrays yet. The issues connected with the use of underwater tiltmeters embrace on-site deployment, power supply and transfer of measured data. Nevertheless, this method could prove to be cost-effective in future compared to other offshore monitoring techniques (McColpin, 2009).