Sensors to Increase the Security of Underwater Communication Cables: A Review of Underwater Monitoring Sensors <s> Cable Tapping versus Cable Cutting: Potential Solutions to Identify These Consequences <s> The economic importance of the Dutch rivers is very high as they are heavily used for inland waterway transport between the Netherlands and their neighbouring countries. A minimum depth must be guaranteed to keep the rivers navigable but also to ensure that the ships can carry maximum cargo. An attractive system for obtaining information about the riverbed bathymetry is the multi-beam echo-sounder (MBES). Furthermore, the MBES received echoes due to acoustic backscatter from the sediments in theory also allow for discriminating between different sediments. The aim of the research presented in this thesis was to develop methods for discriminating between different river sediments using MBES measurements. In order to fulfil this aim, MBES surveys were performed in the Rhine river and the Meuse river between 2007 and 2010. The research shows that indeed the MBES system can be used for discriminating between the different sediments present in the river areas. In addition an important finding is that areas differing in sediment type require different classification approaches. <s> BIB001 </s> Sensors to Increase the Security of Underwater Communication Cables: A Review of Underwater Monitoring Sensors <s> Cable Tapping versus Cable Cutting: Potential Solutions to Identify These Consequences <s> This chapter covers current commercial developments and major research activities for underwater vehicle-manipulator systems. It briefly describes the dynamics of underwater vehicle-manipulator systems to show the level of its complexity. The teleoperation of the underwater vehicle-manipulator system and sensor-based underwater vehicle manipulator controls, such as force control and visual servoing are discussed. Furthermore, it describes coordinated motion controls of underwater vehicle and manipulator systems and presents the results of recent developments in autonomous manipulation for autonomous underwater vehicles. <s> BIB002 </s> Sensors to Increase the Security of Underwater Communication Cables: A Review of Underwater Monitoring Sensors <s> Cable Tapping versus Cable Cutting: Potential Solutions to Identify These Consequences <s> To characterize seafloor substrate type, seabed mapping and particularly multibeam echosounding are increasingly used. Yet, the utilisation of repetitive MBES-borne backscatter surveys to monitor the environmental status of the seafloor remains limited. Often methodological frameworks are missing, and should comprise of a suite of change detection procedures, similarly to those developed in the terrestrial sciences. In this study, pre-, ensemble and post-classification approaches were tested on an eight km2 study site within a Habitat Directive Area in the Belgian part of the North Sea. In this area, gravel beds with epifaunal assemblages were observed. Flourishing of the fauna is constrained by overtopping with sand or increased turbidity levels, which could result from anthropogenic activities. Monitoring of the gravel to sand ratio was hence put forward as an indicator of good environmental status. Seven acoustic surveys were undertaken from 2004 to 2015. The methods allowed quantifying temporal trends and patterns of change of the main substrate classes identified in the study area; namely fine to medium homogenous sand, medium sand with bioclastic detritus and medium to coarse sand with gravel. Results indicated that by considering the entire study area and the entire time series, the gravel to sand ratio fluctuated, but was overall stable. Nonetheless, when only the biodiversity hotspots were considered, net losses and a gradual trend, indicative of potential smothering, was captured by ensemble and post-classification approaches respectively. Additionally, a two-dimensional morphological analysis, based on the bathymetric data, suggested a loss of profile complexity from 2004 to 2015. Causal relationships with natural and anthropogenic stressors are yet to be established. The methodologies presented and discussed are repeatable and can be applied to broad-scale geographical extents given that broad-scale time series datasets become available. <s> BIB003
Underwater communication cables are transferring important and sensitive data across the seas and oceans. Therefore, it could be more interesting for an intruder to have access to the data by tapping a cable instead of destroying it. Tape and cable cut detection requires comparison of the seabed, where the cable is buried, at different times in order to identify any changes on the seabed environment. As mentioned in Section 4.1 and shown in Table 23 , divers, ROVs and AUVs (with manipulating arms) could tape or cut underwater cables. For identifying tapping or cutting, an efficient approach is to produce high-quality acoustic images of the seabed using towed SSS or SAS (tow fish or AUVs) close to the seabed. For successful comparison of the DTMs (digital terrain models), it is mandatory to collocate at the time of the survey the bathymetry in order to geo-reference the measurements. SBP could be used in conjunction with the other proposed sonars where the cable is buried. Depending on the frequency used by an SBP (Table 9) , it could penetrate the seabed up to 150 m in sandy areas. For the frequency ranges of 4-15 kHz, the penetration would be around 40 m and the vertical resolution around 5 cm. In case any DTM differences are observed at the positions of the cables, AUVs with underwater still cameras and video cameras could be used for obtaining clearer pictures of the nature of the change and provide this information to the decision makers to assess the situation. In shallow waters, it could be possible to use also MBES. Regarding the backscatter component of MBES, its utilization to monitor the environmental status of the seafloor by performing repetitive surveys on the same area remains limited. Some methodological techniques of analysis of the differences of backscatter seabed maps of the same area at different times can be found in BIB001 BIB003 . Monitoring the cable area for tapping or cutting can be connected to the seabed coverage phase, which is an attribute for the search phase. The appropriate sensors for mapping the cable seabed are MBES, SAS and SSS. The swath covered by these systems depends on the water depth. So the limits X1 and X2 of the monitoring and surveillance zones (see Table 23 ), respectively, cannot be conveniently calculated here as the depth in the case study ranges from very shallow to 4000 m. Therefore, each water depth may require a different coverage regarding the selected systems and the number of them needed. The seabed topography is of paramount importance for the efficient operation of AUVs. When the cable's location and shape are given, the AUV has to navigate to the location, identify the cable, position itself, and then either monitor its status (for friendly AUVs) or cut/tape the cable (for hostile AUVs). At present, this task is not very simple BIB002 , so tapping is more probable to be performed by divers.