Side-Scan Sonar also known as Bottom Classification Sonar and Side Imaging Sonar is a method of mapping an underwater environment by emitting a wide perpendicular pulse towards the seabed, the acoustic reflections of this pulse is then mapped to create an image of the search area. By reading this image the operator can identify indentations and anything protruding from the seabed, sometimes the items and reasons for these protrusions and indentations can be positively identified where a particularly good image is achieved, however in the majority of cases these must be further investigated by use of divers to identify what they are. Sonar has many uses within the commercial world including debris identification to highlight obstructions for shipping, fisheries research, environmental studies as well as within the military for mine detection. The use of Side-Scan sonar within the search for missing persons is a relatively new use of this technology and therefore any technological uses in its infancy need to be adapted to the situations they find themselves in, this is an area where the IUSRU has had to do a lot of research into the best deployment of side-scan sonar and through many trials of our equipment and software we have identified how it is best used in the search for missing persons.
The IUSRU uses the "Starfish 990F" Side Scan sonar which is a High Resolution sonar device produced by "Tritech International" specifically for use in shallow water in the search for missing persons. The device is a pole mounted fish that is towed by one of our boats just below the surface, it is connected to a laptop onboard the vessel where the operator views and records live images of the seabed as they are transmitted. The Starfish 990F possesses a 70m beam coverage capability, however the IUSRU typically only scan an area of 10m to ensure the highest resolution image of the seabed we are scanning and to ensure no anomaly is left undetected and uninvestigated. The actual Sonar device (Also Known as "The Fish") can be either pole mounted or towed behind one of our vessels, however due to the environments we typically find ourselves searching in we find the pole mounted option to be the most favourable due to underwater obstructions and snags being an ever present danger. "The Fish" is made of a reinforced polyurethane rubber which makes it durable and flexible, it weighs 1KG in fresh water and 2kg's on land and is 378mm in length, 110mm in width and 97mm in height, it is a small but powerful device which is stable within the water and cuts the water well to ensure a fluid image on the operators screen.
The IUSRU generally operates the side-scan Sonar from our 4.5m Valiant RIB with a 50hp Mercury engine, which is a small but powerful and responsive vessel with sufficient deck space in the forward section for the equipment, operator and 1 crew member. This vessel is used in waters in excess of 4M depth and is perfectly designed for searches in our rivers nationwide. For shallower searches we have the option of using our Inflatable Tender Boat and when in stronger tidal conditions or when operating around our coasts we also have the use of larger RIB's built for manoeuvrability in these conditions. For more information on our boats, visit our boats section here.
We use our side-scan sonar to eliminate large portions of our target search area by ensuring that there are no anomalies that cannot be positively identified by the operator, where there is an anomaly that cannot be identified we will then use our divers to further investigate. This piece of technology further increases the safety of our dive team, by identifying any underwater obstructions or dangers that may pose a risk to the diver and therefore allowing us to take measures to diminish this risk that would have otherwise been unknown. In the past searching large areas by hand with divers was a long and arduous activity, however by using side-scan sonar technology we have dramatically decreased our search times allowing us to cover more areas in a quicker time frame. Our Sonar team has been designed to operate completely independent of the rest of the unit, it requires 3 members (Coxswain, Sonar Operator and Crew member) who can travel to a search site in advance of a full unit turnout and can set up and be operational in a short period of time, the equipment and vessel is fully independent requiring no reliance on other factors of the unit.
Our Side Scan Sonar was purchased in October 2012 from funds raised by the Riordan Family. Monica Riordan went missing on 20th of December 2011 in Dublin City Centre, our Side Scan Sonar is dedicated in the memory of Monica. We are eternally grateful to the Riordan Family and friends who facilitated us in the purchase of this much needed piece of equipment which has and will in the future support searches for missing persons by the IUSRU.