The videos below describe some of the SONAR systems discussed above. Click on the pictures to open them: • Manufacturer: RTsys - SonaDive Video title: “Diver held navigation and sonarsystem SonaDive”. • Manufacturers: Blueprint Subsea Video title: “Artemis diver held SONAR system”. • Manufacturers: Blueprint Subsea Video title: “Blueprint oculus SONAR”. • Manufacturer: Tritech Video title: “Diver mounted display dmd webinar”. • Manufacturer: Tritech Video title: “Helmet mounted sonar demonstration”. • Manufacturer: Tritech Video title: “Introducing the diver mounted display”. • Manufacturer: Northern Diver Video title: “Nimrod-V2 presentation”. • Manufacturer: Reeds sonar consultancy Video title: “Mannequin demonstration”. Note that these videos are also available on the manufacturers’ websites.

Divers operating in ports, estuaries, rivers, lakes, enclosed spaces, and industrial plants are familiar with turbid waters. Such diving conditions result in limited or no visibility and inefficient lighting due to the absorption or reflection of the light on matter particles. Interventions in such conditions require the divers to be adequately trained and experienced. However, even though divers familiar with working in such environments acquire the ability to move toward the target and perform the planned task in the total black, the absence of visibility results in a lot of time being lost to move prudently in areas where numerous debris may be present, and carry out the requested task without seeing his hands and the object on which the operation is performed. Also, providing a picture of what the diver works on is impossible. Such conditions can be improved today by using the last generation of diver-held SONARs, providing monochrome pictures of the surrounding, and, thus, the ability to see objects, SONAR is the acronym for “Sound Navigation and Ranging". This name describes a family of tools used to measure distances and detect objects underwater by acoustc emission and reception. The early models of such devices were voluminous and extremely expensive, restricting their use to big ships. However, the progress of the electronic industry regarding prices and miniaturization has resulted that they can be sold at reasonable prices and used today by divers to provide quality pictures, such as the images below of a leaking flange situated in an area of the Surabaya strait where the visibility equals zero. SONARs are commonly classified into single or multibeam systems (beam = sound wave): • Single beam systems emit pings, read the return echoes, and calculate the time from the emission of each ping to provide distances. They are commonly used by echo sounders. • Multibeam systems emit multiple acoustic waves in a fan shape. They can be used to collect distances and backscatter. Like single-beam systems, the distances are computed by measuring the time it takes for the sound to leave the transducer, hit the seafloor or the object, and return. Backscatter is a measurement of the intensity of the sound echo that reflects. Backscatter can provide information about the geology of the seafloor or objects on it. That results in a monochrome picture with shadows behind the object detected. The shadows result from sound waves reflecting on the object when they hit it and not passing behind it. Note that high frequencies (> 400 kHz) are commonly used for diver systems. The reason is that such frequencies provide higher resolution. However, the distance ranges of high-frequency sound waves are very short compared to lower frequencies that are used to detect objects situated at long distances. Three types of equipment are currently available on the market: • Portable single beam SONARs • Multiple beam SONARs without a helmet/mask mounting system and being part of multifunction devices. • Multiple beam SONARs with a helmet/mask mounting system. 1. Portable single beam SONARs: These stystems are limited to only distance measurement. An example of such systems is the “Dive Scan”, a basic sonar designed by “RJE International, Inc”, a company based in Irvine, California - United States of America. Website: https://www.rjeint.com/diver-sonar/ Phone: (949) 727-9399  Screen size: < 2 inches  Weight in air: 0.2 kg  Operational depth: 150 m  Sonar type: Single beam 200 kHz  Distance of detection: 60 m  Battery: 9v alcaline (500 ops), or Lithium (1000 ops) 2. Multiple beam SONARs without a helmet/mask mounting system and being part of multifunction devices: These systems are initially designed for military and police scuba divers. However, they can be used by surface-supplied divers for commercial diving applications. The following models can be taken as references: Echologger DS 900:  Manufacturer: EofE Ultrasonics Co., Ltd. Address: Goyang-Si, Gyungki-Do, 10540, Korea Website: https://www.echologger.com/ E mail: info@echologger.com Phone: +82-2-3158-3178  Screen size: Approx. 10”  Weight in air & size: 4.3 kg / 230 x 290 x 160 mm  Operational depth: 100 m  Sonar type: multiple beam 900 kHz  Distance of detection: 60 m  Battery duration: 3 hours  Additional functions: Depth sensor, timer, temperature sensor, compass, camera (1280x720), recorder, & light. Nimrod V2:  Manufacturer: Northern Diver Address: Appley Bridge - Lancashire - UK Website: https://www.ndiver-commercial.com/ E mail: info@ndiver.com Phone: +44 (0) 1257 25 6944  Screen size: 120 x 80 mm  Weight in air & size: 3.75kg / 169 x 134 x 76 mm  Operational depth: 80 m  Sonar type: multiple beam 700 kHz  Distance of detection: 75 m  Battery duration: Not indicated / Power cable availble  Additional functions: GPS, Compass, Depth sensor, Clock+stop watch, & Positioning. SonaDive:  Manufacturer: RTsys Address: Caudan, Brittany - France Website: https://rtsys.eu/diverheld-system-sonadive E mail: enquiries@rtsys.fr Phone: +33 297 898 580  Screen size: 12” diagonal  Weight in air & size: 20 kg / 270 x 372 x 560 mm  Operational depth: 100 m  Sonar type: multiple beam 450 kHz / 900 kHz  Distance of detection: O to 300m, depending on the mode.  Battery duration: 6 hours  Additional functions: Pressure sensor, GPS, Low light camera Communications to divers, surface, and beacon units. Light, Gradiometer.

3. Multiple beam SONARs with a helmet/mask mounting system: Some models are initially designed for military and Police scuba divers' purposes. However, some others are initially designed for surface supply diving. The head mounting allows reading the SONAR images in very turbid waters. Also, systems with the emitter/receptor mounted on the helmet or mask provide pictures corresponding to what is in front of the diver's eyes, thus, a view of what the diver would see in not turbid waters, which provides the advantage he can use his two hands and see what he is doing during the operation. The following models can be taken as references: Artemis M750d, M1200d, or M 3000d:  Manufacturer: Blueprint Subsea Address: Ulverston, Cumbria - United Kingdom Website: https://www.blueprintsubsea.com/ E mail: enquiries@blueprintsubsea.com Phone: +44 (0)1539 531536  Screen size: 12” & head mounting screen 7”  Weight in air & size: 10.5 kg / 365 x 340 x 215 mm  Operational depth: 100 m  Sonar type: Multibeam M750d: 750 kHz/1200 kHz M1200d: 1200 kHz/2100 kHz M3000d: 1200 kHz/3000 kHz  Distance of detection: M750d: 120m/40m, M1200d: 40m/10m, M3000d: 30m/5m  Battery duration: 6 to 20 hours, depending on the type.  Topside control system: Optional supervisory kit  Additional functions: Depth sensor, Diving planner, Navigation system, Dive light, Camera. Defender 1800 & 3000:  Manufacturer: Reeds Sonar Consultancy Ltd Address: Bourne - United Kingdom Website: https://www.reedssc.com/diver-held-sonar/ E mail: sonar@reedssc.com Phone: +44 (0)7881384957  Screen size: Head Mounted Display 800 x 600  Weight in air & size: 11 kg / 43 x 22 x 24 cm  Operational depth: 100 m  Sonar type: Multibeam dual frequency Defender 1800: 1800 kHz & 1100 kHz Defender 3000: 3000 kHz & 1800 kHz  Distance of detection: Depending on the mode used Defender 1800: 35 & 15 m Defender 3000: 15 & 5 m  Battery duration: 6 hours / Optional power cable input  Topside control system: Optional supervisory kit  Additional functions: Adaptable to ROV Tritech - Diver Mounted Display (DMD):  Manufacturer: Tritech Address: Westhill - Scotland Website: https://www.tritech.co.uk/product/dmd E mail: sales@tritech.co.uk Phone: +44 (0)1224 744111  Screen size: Head mounted display through a monocle  Weight in air & size: 8 kg  Operational depth: 100 m  Sonar type: Multibeam dual frequency 1200KHz and 720 KHz, , depending on the mode used  Distance of detection: 50 to 120 m, depending on the mode used.  Battery duration: 4 to 19 hrs, depending on the model.  Topside control system: Optional supervisory kit  Additional functions: No additional functions To conclude As for many technical tools proposing several variants, the model selection depends on the operation to be performed and the supporting tools already available to the diving team. For example, systems such as the "Tritech diver mounting display" are well adapted to surface-supplied diving operations (using umbilicals) from Diving Support Vessels equipped with mapping and positioning systems. However, systems such as the "Echologger DS 900", "Nimrod V2", & "SonaDive", more thought for military SCUBA diving operations from light surface supports, should not be neglected, as they provide valuable additional functions, such as positioning, navigation, and communication, that can also be useful for surface- supplied diving, particularly from surface supports where precise navigation and mapping systems are not present. Note that the "Artemis" from Blueprint Subsea seems to propose a mix of the two design philosophies. However, the selection of the device should not be limited to only this aspect, as many other points are to be considered, such as the prices (which are only communicated confidentially), the encumbrance of the system, the operational duration of the batteries, the protection of the head-mounted systems (that may be damaged in enclosed space areas), the easiness of implementation, and the robustness of these systems. For all these points, it would be interesting to have customer feedback. A last point to consider is that the devices presented have not been thought for saturation diving, as they are limited to depths between 80 and 100 m. a complementary investigation will be to see whether they can easily be adapted to this purpose. Of course, the list of models should not be limited to only those presented in this article.