Sensors to Increase the Security of Underwater Communication Cables: A Review of Underwater Monitoring Sensors <s> Proposed Sensors for Threat Group 2: Anchors and Fishing Trawls <s> Introduction - Why Submarine Cables? Douglas Burnett, Tara Davenport, Robert Beckman PART I: BACKGROUND Chapter 1 - The Development of Submarine Cables Stewart Ash Chapter 2 - The Submarine Cable Industry: How Does it Work? Mick Green PART II: INTERNATIONAL LAW ON SUBMARINE CABLES Chapter 3 - Overview of the International Legal Regime Governing Submarine Cables Douglas Burnett, Tara Davenport, Robert Beckman PART III: CABLE OPERATIONS - LAW AND PRACTICE Chapter 4 - The Planning and Surveying of Submarine Cable Routes Graham Evans, Monique Page Chapter 5 - The Manufacture and Laying of Submarine Cables Keith Ford-Ramsden, Tara Davenport Chapter 6 - Submarine Cable Repair and Maintenance Keith Ford-Ramsden, Douglas Burnett Chapter 7 - The Relationship between Submarine Cables and the Marine Environment Lionel Carter Chapter 8 - Out-of-Service Submarine Cables Douglas Burnett PART IV: PROTECTING CABLESHIPS AND SUBMARINE CABLES Chapter 9 - Protecting Cableships Engaged in Cable Operations Mick Green, Douglas Burnett Chapter 10 - Submarine Cables and Natural Hazards Lionel Carter Chapter 11 - Protecting Submarine Cables from Competing Uses Bob Wargo, Tara Davenport Chapter 12 - Protecting Submarine Cables from Intentional Damage: The Security Gap Robert Beckman PART V: SPECIAL PURPOSE SUBMARINE CABLES Chapter 13 - Power Cables Malcolm Eccles, Joska Ferencz, Douglas Burnett Chapter 14 - Marine Scientific Research Cables Lionel Carter, Alfred H.A. Soons Chapter 15 - Military Cables J. Ashley Roach Chapter 16 - Submarine Cables and Offshore Energy Wayne Nielsen, Tara Davenport) PART VI: APPENDICES AND KEYWORD INDEX Appendix 1 - Timeline of the Submarine Cable Industry Appendix 2 - Overview of the Major Submarine System Suppliers (1850 -2012) Appendix 3 - Excerpts of Most Relevant Treaty Provisions Keyword Index <s> BIB001 </s> Sensors to Increase the Security of Underwater Communication Cables: A Review of Underwater Monitoring Sensors <s> Proposed Sensors for Threat Group 2: Anchors and Fishing Trawls <s> Beam trawling causes physical disruption of the seabed through contact of the gear components with the sediment and the resuspension of sediment into the water column in the turbulent wake of the gear. To be able to measure and quantify these impacts is important so that gears of reduced impact can be developed. Here we assess the physical impact of both a conventional 4 m tickler-chain beam trawl and a “Delmeco” electric pulse beam trawl. We measure the changes in seabed bathymetry following the passage of these gears using a Kongsberg EM2040 multi-beam echosounder and use a LISST 100X particle size analyser to measure the concentration and particle size distribution of the sediment mobilized into the water column. We also estimate the penetration of the gears into the seabed using numerical models for the mechanical interaction between gears and seabed. Our results indicate that the seabed bathymetry changes between � 1 and 2 cm and that it is further increased by higher trawling frequencies. Furthermore, our results suggest that the alteration following the passage of the conventional trawl is greater than that following the pulse trawl passage. There was no difference in the quantity of sediment mobilized in the wake of these two gears; however, the numerical model introduced in this study predicted that the tickler-chain trawl penetrates the seabed more deeply than the pulse gear. Hence, greater alteration to the seabed bathymetry by the tickler-chain beam trawling is likely to be a result of its greater penetration. The complimentary insights of the different techniques highlight the advantage of investigating multiple effects such as sediment penetration and resuspension simultaneously and using both field trials and numerical modelling approaches. <s> BIB002
Anchors and fishing trawls constitute similar threats. Both harshly invade and disturb the underwater habitat. Fishing trawls cause physical disruption of the seabed through contact of the gear components with the sediment and the resuspension of sediments into the water column in the turbulent wake of the gear BIB002 . Regarding anchors, an asymmetrical act could happen due to a vessel dropping a heavy anchor and dragging it in an area where underwater communication cables lay BIB001 . As discussed in Section 2 (Figure 4 ), this type of threats is prevalent in shallow waters (typical maximum depth of 200 m), normally within the GCS. Anchors have different dimensions, mainly depending on the vessel that carries it. The study performed in  used anchors weighing 8.4 and 11.7 tons, with approximate dimensions of 2.8 × 3.6 m, and with an anchor chain of 60 m. The maximum penetration depth was 0.88 m. Although this is lower than the minimum recommended burial depth (1 m), it is assumed that this depends on anchor (type of ship) and seabed type. Otherwise, the percentage of cable faults due to anchors would not have been too high (see Figure 3 ). Furthermore, BIB001 states that for soft to firm sediments, ship anchors may penetrate 2-3 m into the seabed. Fishing trawls mainly affect the upper layer of the seabed, usually penetrating it around 0.5 m BIB001 . In the case of the cable not being sufficiently buried, any strike of a trawl would damage it. For example, the impact force of a 1900 kg (weight in water) trawl door, when towed at 2.9 knots is calculated to be about 11 tons  . Fishing beam trawls are made of steel and they are up to 12 m long. Based on the dimensions and properties of the anchors and fishing trawls described above, they could be detected using towed magnetometers (see Table 19 ). As an example, an anchor of 20 tons could be detected by a magnetometer at a range of 120 m. Their maximum depth rating (2730-6000 m) exceeds by far the required maximum depth of 200 m. Multiple magnetometers could be installed on a pole in order to simultaneously cover larger areas. As previously discussed, dangerous anchors and fishing trawls are of significant size and weight. As they operate on the seabed, they could be also detected using high-resolution seabed mapping systems, like MBES (see Table 6 ), SSS (see Table 7 ) and SAS (see Table 8 ). As discussed in the previous sub-section, for the MBES, the beam footprint at 200 m water depth, both across-and along-track resolutions, is approximately 4 m. Therefore, it would be able to detect large anchors and fishing trawls that could damage the cable and at the same time monitor the water column for detecting other types of intruders. The towed SSS and SAS systems can operate close to the seabed (where the object of interest is). Operating at very high frequencies (e.g., 1800 kHz) and from a distance of 25 m from the seabed, they could reach a resolution of 0.4 cm, being able to detect the above-mentioned anchors and fishing trawls. A significant advantage of towed systems is that it is possible to install a SSS (or SAS) and a magnetometer in the same tow fish. So the detection accuracy and confidence would significantly increase. Anchors and fishing trawls heavily disturb the seabed sediment. Therefore, ADCPs (see Table 17 ) and turbidity sensors (see Table 22 ) can be used to detect them. As mentioned in the previous Section 4.2.1, ADCPs usually have 5 beams (4 slanted at 20 • , and one vertical) and can detect changes in the water current velocities due to suspended particles in a conical shape-area around the water column. ADCPs can be installed either on the seabed or on USVs, AUVs and UUGs, and monitor effectively the underwater environment. Turbidity sensors installed on AUVs or UUGs could also detect the impact of anchors and fishing trawlers on the seabed through sensing the changes in the vertical water column due to suspended solids. AUVs equipped with underwater cameras (video and still, see Tables 20 and 21 , respectively) could be used to monitor the underwater environment and identify possible intruders that are inside the monitoring zone of the cables.