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The Envisat C‐band satellite was deorbited on October 2010 and was no longer available for monitoring the range changes over the injectors at In Salah. Fortunately, several other satellite systems were functional as replacements before this date. In particular, the COSMO‐SkyMed (CSK) X‐band satellite pair were launched and operational before the loss of the Envisat satellite. Similarly, the TerraSAR X‐band satellite was also put in orbit, with a repeat cycle of 11 days. Thus, three X‐band satellites were accessible for estimating range changes at the In Salah site due to the injection of carbon dioxide. The repeat time attainable for these three satellites was 8 to 11 days, much better than the best possible Envisat return time of 35 days. The down side of this transition to X‐band data was the increased cost of data acquisition because the satellites have to be tasked to gather data at a particular site. However, with the help of British Petroleum, we were able to acquire X‐band SAR scenes from both ascending and descending acquisition geometries from 2009 until 2012.

Due to the favorable geometry of the two‐satellite systems, it was possible to obtain both quasi‐east‐west and quasi‐vertical components for all time intervals during this period. An example is shown in Figure 2.6 for displacements above well KB‐501. Note that there is a hint of a double‐lobed pattern in the vertical component of displacement. As at KB‐502, this pattern suggests the possible opening of a subvertical tensile feature, such as a fault or damage zone. The existence of such a damage zone is supported by the narrow, linear, high‐permeability trend shown in Figure 2.4, found by the diffusive travel time inversion of Vasco et al. (2008). Furthermore, a recent inversion of a decomposition of C‐band data into quasi‐vertical and quasi–east‐west components found that is was not possible to fit both data sets simply with reservoir volume change. A contribution from a subvertical tensile feature provided an acceptable fit to the two components (Rucci et al., 2013). Based upon these results, a steeply dipping (86^o) tensile damage‐zone model was adopted to explain the surface displacement observed in Figure 2.6. The subvertical damage zone, with an orientation of 135^o, was divided into a 20 x15 grid of surface elements. Each grid element could undergo a distinct aperture change in order to fit the two components of data. The resulting model (Fig. 2.7) contains an elongated region of aperture change from the depth of injection to about 100 m above the well. Significant aperture change extends roughly 6 to 7 km to the northwest of the injection well and about 5 km to the southeast.

The aperture change estimates, shown in Figure 2.7, provide a reasonable fit to the InSAR observations, even reproducing the small secondary lobe at the eastern edge of the vertical displacement anomaly (Fig. 2.8).