## 1 Introduction  
Wildfires are among the most destructive disasters. These catastrophes have an enormous impact in populated regions. It applies to the United States (Western states) [1]; Canada (South-Western) [2]; Mediterranean Europe (Portugal, Spain, France, Italy and Greece) [3] and South Eastern Australia [4, 5]. As can be seen, this is a worldwide problem and not just on less developed countries. The means available for fighting the wildfires are clearly insufficient for their efficient suppression. The answer to this problem must be not only reactive, but also proactive. The prevention of fires and the implementation of strategies to help the firefighting are imperative. One of the possible strategies is the implementation of a fire break network (FBN). A fire break (FB) is a strip of land that has been strategically and artificially modified, where vegetation density is reduced to break up the continuity of fuel. It acts as a barrier to slow or stop the progress of wildfire,thus improving fire control opportunities. Technically, in the Portuguese FBN, an FB is a land strip with 125 m wide composed by three regions: the road network, with a minimum width of 5 m; the fuel interruption with a minimum width of 10 m where all vegetation is cut; the fuel reduction composed by two zones, where a minimum distance between the tree tops is imposed (the complete information is available in [6]). The technical specifications are synthetized in Figure 1 and a ground observation of an FB can be seen in Figure 2.  
Since the vegetation is always growing, the monitorization of an FB is essential for its efficiency. The Portuguese Institute of Nature and Forest Conservation (Instituto de Conservacao da Natureza e das Flores, ICNF) planned a priority network of FBs to help control wildfires and decrease their burnt area. The implementation and maintenance of FB is crucial for its efficiency and should be ensured by the Local Authorities. The verification by the National Authorities (ICNF) of the previous premises can be made by ground observation, but it is expensive and time consuming if a wide network is implemented (the FBN was defined as having 11,125 km, having 1600 km already implemented). The current practice is the visualization of satellite imagery, pointing out the cases that need special attention. This technique is also a time-consuming process and prone to errors. In this paper, a remote sensing semi-automatic methodology for the detection of maintenance operations in an FB is presented. With the launch of the Sentinel 2 (the first satellite of the constellation in June 2015 and the second in March 2017) free satellite imagery with a spatial resolution of 10 m and a temporal resolution of five days was made available to the community. This new high spatial resolution allowed the analysis of the FB conditions, which in current work, was to detect when maintenance operations are performed. It is common to divide remote sensing applications into two groups: land cover classification [7; 8; 9; 10] and change detection [11; 12; 13; 14; 15; 16; 17]. The proposed methodology fits into the second group. In this kind of applications, the output gives the information of the occurrence of an event in the study area. In [PERSON] et al. [12], forest disturbances were detected with resource to two observations and spatio-temporal features and in [PERSON] et al. [13]; annual composites were  
Figure 1: Technical specifications of a fire break (FB) in the Portuguese fire break network (FBN) [6]. Figure 2: Ground observation of an FB. (Source: [[http://www2.icnf.pt/](http://www2.icnf.pt/)]([http://www2.icnf.pt/](http://www2.icnf.pt/))). generated to detect changes. In [15; 16; 17], different approaches were used to identify changes in the land cover and the kind of changes. In [16; 17], pixel-based methods were implemented, with three and six observations, respectively, while in [15], an object-based technique was used.