Sensors to Increase the Security of Underwater Communication Cables: A Review of Underwater Monitoring Sensors <s> Permanent Surveillance Network (Permanent SN) <s> Abstract In order to study the Earth's structure and subduction zone tectonics, seismic data from the oceanic region are extremely important. The present seismograph distribution in the oceanic region, however, provides a very poor coverage. To improve this poor seismic coverage, a cable OBS system using a retired submarine telecommunication cable is proposed. The GeO-TOC cable runs from Ninomiya, Japan, to Guam through the Izu-Bonin forearc and the Marina Trough. The total length of the cable is 2659 km. An OBS, IZU, using the GeO-TOC cable, was successfully installed at the landward slope of the Izu-Bonin Trench in January 1997. The IZU OBS is located approximately 400 km south of Tokyo. The installation method is similar to repair work on submarine cables. The IZU OBS is equipped with three accelerometers, a hydrophone, a quartz pressure gauge, and a quartz precision thermometer with a few temperature sensors to monitor overheating of the internal electronics. After installation, the voltage increase is 90 V when the current is maintained at a constant 370 mA. Data from accelerometers are digitized by 24-bit A/D converters and sent to Ninomiya at 9600 bps for each component. Hydrophone data are sent to Ninomiya as analog signals using the AM (Amplitude Modulation) method for safety reasons. Hydrophone data are digitized at the shore station. Other slow-rate data are multiplexed and sent to the shore at 9600 bps. The instrument can be controlled by a shore computer. All data will be transmitted from Ninomiya to Tokyo and combined with other existing seismic data. <s> BIB001 </s> Sensors to Increase the Security of Underwater Communication Cables: A Review of Underwater Monitoring Sensors <s> Permanent Surveillance Network (Permanent SN) <s> Seafloor networks of cables, pipelines, and other infrastructure underpin our daily lives, providing communication links, information, and energy supplies. Despite their global importance, these networks are vulnerable to damage by a number of natural seafloor hazards, including landslides, turbidity currents, fluid flow, and scour. Conventional geophysical techniques, such as high-resolution reflection seismic and side-scan sonar, are commonly employed in geohazard assessments. These conventional tools provide essential information for route planning and design; however, such surveys provide only indirect evidence of past processes and do not observe or measure the geohazard itself. As such, many numerical-based impact models lack field-scale calibration, and much uncertainty exists about the triggers, nature, and frequency of deep-water geohazards. Recent advances in technology now enable a step change in their understanding through direct monitoring. We outline some emerging monitoring tools and how they can quantify key parameters for deepwater geohazard assessment. Repeat seafloor surveys in dynamic areas show that solely relying on evidence from past deposits can lead to an under-representation of the geohazard events. Acoustic Doppler current profiling provides new insights into the structure of turbidity currents, whereas instrumented mobile sensors record the nature of movement at the base of those flows for the first time. Existing and bespoke cabled networks enable high bandwidth, low power, and distributed measurements of parameters such as strain across large areas of seafloor. These techniques provide valuable new measurements that will improve geohazard assessments and should be deployed in a complementary manner alongside conventional geophysical tools. <s> BIB002 </s> Sensors to Increase the Security of Underwater Communication Cables: A Review of Underwater Monitoring Sensors <s> Permanent Surveillance Network (Permanent SN) <s> The ocean is key to understanding societal threats including climate change, sea level rise, ocean warming, tsunamis, and earthquakes. Because the ocean is difficult and costly to monitor, we lack fundamental data needed to adequately model, understand, and address these threats. One solution is to integrate sensors into future undersea telecommunications cables. This is the mission of the SMART subsea cables initiative (Science Monitoring And Reliable Telecommunications). SMART sensors would “piggyback” on the power and communications infrastructure of a million kilometers of undersea fiber optic cable and thousands of repeaters, creating the potential for seafloor-based global ocean observing at a modest incremental cost. Initial sensors would measure temperature, pressure, and seismic acceleration. The resulting data would address two critical scientific and societal issues: the long-term need for sustained climate-quality data from the under-sampled ocean (e.g., deep ocean temperature, sea level, and circulation), and the near-term need for improvements to global tsunami warning networks. A Joint Task Force (JTF) led by three U.N. agencies (ITU/WMO/UNESCO-IOC) is working to bring this initiative to fruition. This paper explores the ocean science and early warning improvements available from SMART cable data, and the societal, technological, and financial elements of realizing such a global network. Simulations show that deep ocean temperature and pressure measurements can improve estimates of ocean circulation and heat content, and cable-based pressure and seismic-acceleration sensors can improve tsunami warning times and earthquake parameters. The technology of integrating these sensors into fiber optic cables is discussed, addressing sea and land-based elements plus delivery of real-time open data products to end users. The science and business case for SMART cables is evaluated. SMART cables have been endorsed by major ocean science organizations, and JTF is working with cable suppliers and sponsors, multilateral development banks and end-users to incorporate SMART capabilities into future cable projects. By investing now, we can build up a global ocean network of long-lived SMART cable sensors, creating a transformative addition to the global ocean observing system. <s> BIB003
Permanent SNs can be constructed based on the threat to be detected. It is important to mention that building a Permanent SN represents a one-time investment in instrumentation deployment and infrastructure. The sensors could be installed on non-moving platforms like buoys, moorings and seafloor bottom mounts. Some ideas of potential Permanent SNs for the multi-threat scenario are: -Arrays of multiple hydrophones (uniformly spaced) could be installed on moorings at various depths to address different threats. The installation could be in shallow waters (50 m) for diver and/or AUV detection. It could also be in deeper waters to detect different threats, similar to SOSUS for the detection of submarines  . Passive acoustic monitoring systems are most effective when operating in not noisy multipath environments (e.g., low ship traffic). -When digging into the seabed for searching a cable or when anchors and fishing trawls are used, the seabed is disturbed. In these cases, an array of ADCPs or turbidity sensors on the seabed could be used to detect changes in the water properties due to suspended particles. -For diver detection, DDS could be installed at regular intervals along the cable route up to the depth of 50 m. The DDS could be installed on mooring lines all along the water column. -Underwater cameras could be installed on mooring lines in shallow waters (less than 20 m depth) for the detection of potential intruders (e.g., divers, ROVs, AUVs). -A very effective solution to detect surface threats is to use buoys equipped with above-water sensors such as radar and/or EO/IR cameras. The buoys can be placed at regular intervals along the borders of the cable monitoring zone (see Figure 7b ) to detect and recognize suspicious ships. Then they could send early warnings to C2 reach-back centers for triggering further actions. USVs or AUVs could then be deployed for assessing the exact nature of the threat. -Nowadays, the idea of using underwater cables as platforms for carrying different types of sensors for environmental monitoring is growing BIB003 BIB002  . Various types of sensors could be installed on the cables for monitoring geo-hazards (e.g., earthquakes, tsunamis). The Geophysical and Oceanographycal-Trans Ocean Cable (GeO-TOC) was the earliest example of a submarine cable-based observatory. It was installed in 1997 midway between Guam and Japan using the retired TPC-1 communication cable BIB001 . This idea could be extended for enhancing the security of the cables  by using them as platforms that carry sensors able to detect possible threats or anomalies in the underwater environment. The ideas proposed here for creating Permanent SNs could be used to support decision-makers while developing networks to secure underwater communication cables.