The ocean is Earth’s final frontier. Satellites have mapped 100 percent of the seafloor with a resolution of about a mile, but less than a third has been mapped with modern high-resolution sonar. The multibeam sonar used by ships is suitable for wide scans, but even the best shipboard systems produce maps with a resolution of only 49 to 164 feet once you get down to a mile or two in depth. Underwater drones can bring sonar closer to the seabed and significantly refine that image. Unfortunately, the shape of the drone could blur what the sonar is there to discover. Chinese scientists think they have a surprisingly simple solution. They developed a custom acoustic lens that brings sound waves back into focus.
Sonar works by sending acoustic energy outward, where it hits objects, creating echoes that can be analyzed. When attached to underwater drones, sonar transducers are placed behind a smooth, curved dome that protects the sonar and reduces vehicle drag. There is, however, a problem. Because the dome is curved, it distorts the sonar wavefront, creating a wider, weaker beam. It’s like having 20/20 vision and looking through someone else’s prescription glasses. Current solutions use complex electronics and computing components to direct and shape sound. It’s expensive, heavy and requires too much energy to operate. The new lens physically corrects the distortion before the sonar beam passes through the dome. It’s similar to how NASA scientists corrected distortion to make Hubble Telescope images much better.
How Chinese researchers improved sonar for underwater drones
Researchers at Xiamen University have developed an acoustic lens that manipulates sonar sound waves. They calculated exactly how the dome distorts the wave, then created a lens made of concentric silicone rings to bend it in the opposite direction. Each ring contains a different ratio of tungsten, which changes the speed at which sound travels through the silicone and reshapes the wavefront. Holographic lenses can be custom-made to match the aberrations created by the domes of various underwater drones.
It’s more than a theory. The team tested the lens in a lab and found that it helped the sonar beam shift from more than 65 degrees to between 16 and 30 degrees. After this success, the team launched into the real world: the bustling Jiulong River. They towed a hollow plastic sphere about six feet deep, then deployed an underwater drone to spot it with sonar. Without the lens, the environmental and boundary reverberations were too strong. After adding the lens, the researchers reduced the reverberation by 11.98 dB, making the sphere visible to sonar.
More precise sonar like this could open new frontiers in deep-sea exploration. More than 100,000 seamounts rise above 3,280 feet, but less than one-tenth of 1 percent have been explored. Scientists think they could be biological oases, harboring the type of life capable of surviving without sunlight. There could even be an unexpected necropolis like the one discovered in the depths of the Indian Ocean. The Xiamen University researchers next want to test their lens in real seawater, after trials in artificial salt water, and see how it behaves with the creatures that live there.
