Sensors to Increase the Security of Underwater Communication Cables: A Review of Underwater Monitoring Sensors <s> Sensors to Detect Multiple Threats in the Sabotage Scenario under Study <s> The undisputed remote sensing tool for detailed sea floor object detection is the side scan sonar. From pipelines, to downed aircraft, to mines, a side scan sonar’s unique characteristics effectively ensonify and subsequently display these objects. For bathymetry, the Multibeam Echosounder (MBES) has quickly proven its superior capabilities. An MBES’s unique characteristics allow it to provide 100 per cent ensonification of the sea floor while meeting or even exceeding IHO specifications. During the 1997 Coastal Multibeam Sonar Training Course in Victoria, BC, inert mines were laid at different depths and then ensonified by a side scan sonar and an MBES. The aim of this exercise was to compare the detection characteristics of the two sonars using one of the inert mines. This paper is a discussion of that comparison and the lessons learned. Comparative imagery is used to illustrate the success of each sonar in illuminating the mines to the operator. Unaided visual detection is not always the most efficient method for finding objects in large quantities of MBES data; therefore, subtractive temporal analysis was implemented in order to enhance the MBES object detection process. As well, a simple automated detection algorithm was used successfully on the temporally differenced images. The background, implementation and results of the MBES subtractive analysis performed in the area of the inert mine will be compared to the detection achieved using the side scan sonar. Finally, conclusions will be drawn as to the efficacy of the MBES as an object detection sonar. <s> BIB001 </s> Sensors to Increase the Security of Underwater Communication Cables: A Review of Underwater Monitoring Sensors <s> Sensors to Detect Multiple Threats in the Sabotage Scenario under Study <s> Recent concerns about underwater intruder attacks on ships in port and harbour infrastructure raised strong interests in detecting divers. This paper reviews current research and systems on diver detection using acoustic means and the factors affecting system performance. Previous published data on diver acoustic target strength is discussed and seem to be plausibly explained by considering the acoustic scattering from a diver’s body and exhaled bubbles. <s> BIB002 </s> Sensors to Increase the Security of Underwater Communication Cables: A Review of Underwater Monitoring Sensors <s> Sensors to Detect Multiple Threats in the Sabotage Scenario under Study <s> In this thesis I investigate, describe and demonstrate new platform technology and its application in fisheries research. The first task was to prepare an Autonomous Underwater Vehicle (AUV) for pa ... <s> BIB003
Underwater threats, which are considered in this case study (see Table 23 ) are listed below, for which recommended sensors are provided in the following subsections: -Threat group 1-Intentional man-made threats: divers, submarines, ROVs and AUVs. -Threat group 2-Intentional man-made threats: anchors, fishing trawls. -Threat group 3-Intentional man-made threats: submarines. Underwater threats, which are considered in this case study (see Table 23 ) are listed below, for which recommended sensors are provided in the following subsections: On the other hand, ROVs and AUVs can reach very deep waters (thousands of meters). The length of AUVs and ROVs varies. An assumption is made that threats are vehicles of substantial size (size greater than divers) able to have manipulator arms. The most efficient solution for diver detection, which can also be used for the detection of underwater vehicles (ROVs, AUVs), is the DDS (see Table 11 ). The sonars operate at approximately 70 kHz having an acoustic coverage of 360 • and a maximum detection range that can reach approximately 700 m for closed-circuit divers and almost 1000 m for open-circuit divers. They can be installed along the water column and are routinely used for asset protection in ports, where the vessel traffic is high and the number of detection targets (threats) large. The maximum operating depth of these devices is around 50 m. Therefore, there is a margin of approximately 10 m (from 50 to 60 m water depth) where a typical DSS cannot detect divers. For water depths greater than 50 m, divers, and also ROVs and AUVs, could be detected using fishery SBES (see Table 5 ) or MBES (see Table 6 ). The advantage of these systems is that they can operate from a surface platform and detect an intruder along the whole water column. In the case of fishery SBES, the maximum detection range of a target can be from 270-1100 m for frequencies 200 Hz-18 kHz, respectively. The bottom detection (for monitoring the environment where the cables are laid or buried) can be from 70 m up to 10,000 m for frequencies 710 Hz-12 kHz, respectively. SBES is usually mounted on surface vessels (USVs as well), but they can also be installed on AUVs. MBES offers high resolution due to its large number of beams with small width. MBES can sweep large corridors across the cable route. These systems have been designed for seabed mapping but their ability to display and log the whole water column along with their high-resolution can make them potential candidates for intruder detection when the beam footprint is small enough to achieve the necessary resolution. The use of MBES backscatter in target detection on the seabed is questionable, so it is better to use its high-resolution bathymetry mode BIB001 . In this case, the important factors that define its potential bathymetry target resolution are: (a) the sounding solution spacing and (b) the quality of the bottom detection for each solution BIB001 . As an example, for the maximum operation depth of divers at 58 m, for a MBES with 1 • beam width at 20 • beam angle, both the across-and along-track resolutions are approximately 1 m. Therefore, divers, as well as ROVs and AUVs, could be detected at this water depth. As depth increases, the beam footprint becomes larger. So for example: at 100 m depth, both resolutions are approximately 2 m; at 200 m depth, both resolutions are approximately 4 m; and at 400 m depth, both resolutions are approximately 8 m. For water depths greater than the GCS, it is more efficient to detect ROVs and AUVs-if they perform seabed tapping or cable cutting-using high-resolution towed systems, such as SSS (see Table 7 ) and SAS (see Table 8 ), or MBES mounted on an AUV (so part of the water column can still be covered). The operating frequency ranges of SSS and SAS are from 75 kHz to 1800 kHz. These systems are by default part of AUVs, so they can be used up to the maximum depth of the sabotage scenario (4000 m). As an example, if a very high frequency operation mode is selected (e.g., 600 kHz), then the resolution is less than 4 cm for a range of 50 m (distance from the seabed). This means that any object of significant size, including a communication cable on the seabed (if not buried), could be detected. OAS (see Table 10 ) are also a default part of any AUV and could be used for the detection of any underwater asset which falls inside its scanned sector. Regarding passive systems, it has been shown that open circuit divers can be detected from hydrophone arrays installed on the seafloor  BIB002 . An additional advantage of using seabed hydrophone arrays is that they can capture noise from the excavation activity needed for reaching the buried cable. Table 15 shows that the operating depth of hydrophones is about 1700 m. The cable is supposed to be buried up to 1000 m. Therefore, it would be useful to have a hydrophone array along the cable for the buried section. Since the cable can be reached only through excavation for the first 1000 m of water depth, the threat activity could be indirectly traced by ADCPs (see Table 17 ) and/or turbidity sensors (see Table 22 ). ADCPs' advantage is that they have usually a vertical beam and 4 slanted beams at 20 • or 25 • and a range that starts from 30 m and can reach up to 1000 m. So ADCPs could cover the whole or at least a large part of the water column covering a conical-shape area around the vertical. Their operation depth can reach up to 6000 m. So, they could be used at any part of the seabed of the multi-threat scenario under study. ADCPs have been used to observe the movement of fishes (e.g., migration speed of herrings) and animals BIB003 . So, they could also be considered as detection tools for intruders. Turbidity sensors installed on AUVs or UUGs could detect significant changes in the vertical water column due to suspended solids that may occur from any excavation activity. Underwater cameras (video and still, see Tables 20 and 21 , respectively) could be used on stationary platforms close to the seabed and in shallow waters (maximum: 20 m depth, if the visibility is optimal) for intruder detection. They could also be installed on AUVs or UUGs for monitoring the underwater environment. Finally, USVs or UUGs having oceanographic sensors (see Table 22 ) could be used for detecting potential anomalies in the underwater environment due to the presence of ROVs or AUVs.