Stingrays demonstrate mapping potential as they skim the seafloor
While we're pretty well across the topography of the Earth's surface, understanding that of the ocean floor is a whole other ball game. It is said that we know more about the surface of Mars than the seafloor, and scientists are developing all sorts of technologies to help us fill in the blanks. Among them is a new technique that uses deep-dwelling rays to survey the seabed for us, and possibly even power the required hardware themselves.
Something like 80 percent of the ocean remains unmapped as of today, so there is considerable interest in expanding our understanding of what lies beneath the water's surface. Unmanned vessels that autonomously map the seafloor with multi-beam sonars, or airborne platforms that bounce sonar signals off underwater objects could form part of the solution, as could rays fitted with the appropriate hardware.
"Electric rays and sting rays are benthic animals, meaning that they spend most of their time swimming around the ocean floor in deep places," explains study author Yo Tanaka, from Japanese scientific institute Riken. "By combining simple pinger technology and digital cameras with this natural behavior, we think we can use rays to map the ocean floor, and at the same time collect meaningful data about ocean wildlife, biota, and resources."
The pinger technology referred to here is a small device that emits ultrasonic sound, which can then be picked up by special receivers. The position of the receivers and the time it takes for them to detect the ultrasonic sound can be used to pinpoint the pingers, which can be combined with camera footage to create an accurate map of a space.
Tanaka and his team explored this idea by first fixing sets of cameras to electric and sting rays and having them swim around a large tank. The images gathered by the animals were of adequate quality for the scientists to move ahead with the next phase of the project – experiments in the ocean.
This took them to a relatively flat seabed off the coast of Okinawa in Japan, around 20 m (60 ft) deep. Both sting and electric rays were fitted with pingers and sent to the ocean floor along with four receivers. Over the course of two hours, the scientists recorded the position of the pingers as the rays frolicked about in the ocean, and were able to use this data to compile a map of the seabed.
This was then compared to an existing map of the seabed, with the data gathered by the rays found to be largely in line, down to an accuracy of around 10 cm (4 in). Importantly, the team was able to distinguish between when the stingrays were swimming through the water at speed or skimming slowly over the seabed, which will be key to gathering reliable mapping data as they move through the ocean depths.
Through the separate demonstrations showing that the rays can swim with the camera gear attached, and that the pingers attached to the animals can be used to build a reliable map of an underwater area, the team believes it as demonstrated the feasibility of its ocean-mapping approach.
"In our ocean experiment, in addition to the pinger positioning, we were able to confirm that electric rays actually move around the seabed," says Tanaka. "In the near future we will test the system for long-term monitoring."
These follow-up experiments will involve electric rays using their own electricity to power the pingers, which is something the team says it has already demonstrated in previous experiments. They will also make use of wearable battery packs for the sting rays, with the animals to be tasked with mapping a more uneven seabed over longer periods.
The research was published in the journal SN Applied Sciences.