## 2 Monitoring Protocols and Methods
### Regulatory Aspects
#### 2.7.2 Regulations Governing Injection Monitoring  
The baseline data were then used--in accordance with French Administration guidelines--to define threshold levels for a subset of gas species (CO\({}_{2}\) soil-gas concentration and CO\({}_{2}\) soil flux). The baseline data were compared with literature data in soils under a temperate climate [50; 51; 52; 53; 54; 55; 56]. This analysis--by the site operator--allowed for defining concentration and flux levels that can be considered as normal or anomalous ones (threshold levels); French regulations mention that the operator shall determine how information is processed. In particular, the operator determines the threshold values that enable detecting an anomaly in the environment that may be caused by a leak from the reservoir. The operator then decides if further investigations are necessary, or even if the injection process must be modified. The thresholds were defined based on simple statistical considerations as there were little data available for defining universal threshold values that could be applied univocally to the Lacq-Rousse site. For example, in 2009, the potential leakage reported at the Kerr Farm, Weyburn oilfield, was not known [57; 58], so that the criticality of threshold definition was not as clear. Nonetheless, it was agreed with the French authorities that these thresholds could be debated later and that they probably needed adaptation during the pilot experiment. The selected approach thus defined a \"vigilance\" mode as the mean value (M) plus twice the standard deviation (\(\sigma\)) of the baseline dataset (\"vigilance\" mode = M + 2\(\sigma\)), and an \"anomaly\" mode as the mean value plus three times the standard deviation of the dataset (\"anomaly\" mode = M + 3\(\sigma\)). The first attempts at defining the thresholds used the September 2008 to September 2009 dataset, as the December 2009 data were then thought to postdate the start of injection. The result was a set of high \"vigilance\" and \"anomaly\" levels (respectively 8.3% vol. and 11.2% vol. for CO\({}_{2}\) concentration and 16.6 cm\({}^{3}\cdot\)min\({}^{-1}\cdot\)m\({}^{-2}\) and 21.5 cm\({}^{3}\cdot\)min\({}^{-1}\cdot\)m\({}^{-2}\) for CO\({}_{2}\) flux). The operator then refined these levels by only considering the data acquired during \"low season\", but this meant that the redefined threshold levels were only valid for the \"low season\" monitoring. This was approved by the administration. The question of what would happen if leakage was reported during the \"high season\" was not specifically addressed; further work would have been necessary, such as required when a threshold is exceeded during \"low season\". Fortunately, soil-gas investigations and the use of a process-based method (defined as such by [PERSON] et al. [59], but used before) would certainly have provided sufficient evidence of the origin of the gas to reject any allegations of leakage. From analysis of the \"low season\" baseline data, the following cases were defined and used for data acquired during the injection monitoring period:  
* \"Vigilance\" mode (M + 2\(\sigma\)): This mode is activated if 5 different locations of the 35 show CO\({}_{2}\)-gas concentrations >5.4% vol. and CO\({}_{2}\) fluxes >7.4 cm\({}^{3}\cdot\)min\({}^{-1}\cdot\)m\({}^{-2}\). When this happens, the measurements are repeated to check if the threshold levels are permanently exceeded or not. The regulations did not provide for repeating the measurements, and repeat measurements were scheduled to be done during a different part of the day than the first one. If a vigilance situation occurred in the morning, the measurement was repeated during the afternoon or the day after if necessary, and if it happened during the afternoon, the repeat took place the day after.