Sonar, which measures the time it takes for sound waves to bounce off objects and travel back to a receiver, is the best way to visualize underwater terrain or inspect marine-based structures. Sonar systems, though, have to be deployed on ships or buoys, making them slow and limiting the area they can cover.
However, engineers at Stanford University have developed a new hybrid technique combining light and sound. Aircraft, they suggest, could use this combined laser/sonar technology to sweep the ocean surface for high-resolution images of submerged objects. The proof-of-concept airborne sonar system, presented recently in the journal IEEE Access, could make it easier and faster to find sunken wrecks, investigate marine habitats, and spot enemy submarines.
“Our system could be on a drone, airplane or helicopter," says Amin Arbabian, an electrical engineering professor at Stanford University. “It could be deployed rapidly…and cover larger areas."
Airborne radar and lidar are used to map the Earth's surface at high resolution. Both can penetrate clouds and forest cover, making them especially useful in the air and on the ground. But peering into water from the air is a different challenge. Sound, radio, and light waves all quickly lose their energy when traveling from air into water and back. This attenuation is even worse in turbid water, Arbabian says.
So he and his students combined the two modalities—laser and sonar. Their system relies on the well-known photoacoustic effect, which turns pulses of light into sound. “When you shine a pulse of light on an object it heats up and expands and that leads to a sound wave because it moves molecules of air around the object," he says.
The group's new photoacoustic sonar system begins by shooting laser pulses at the water surface. Water absorbs most of the energy, creating ultrasound waves that move through it much like conventional sonar. These waves bounce off objects, and some of the reflected waves go back out from the water into the air.
At this point, the acoustic echoes lose a tremendous amount of energy as they cross that water-air barrier and then travel through the air. Here is where another critical part of the team's design comes in.
Image: Aidan Fitzpatrick
To detect the weak acoustic waves in air, the team uses an ultra-sensitive microelectromechanical device with the mouthful name of an air-coupled capacitive micromachined ultrasonic transducer (CMUT). These devices are simple capacitors with a thin plate that vibrates when hit by ultrasound waves, causing a detectable change in capacitance. They are known to be efficient at detecting sound waves in air, and Arbabian has been investigating the use of CMUT sensors for remote ultrasound imaging. Special software processes the detected ultrasound signals to reconstruct a high-resolution 3D image of the underwater object.
An animation showing the 3D image of the submerged object recreated using reflected ultrasound waves. Gif: Aidan Fitzpatrick
The researchers tested the system by imaging metal bars of different heights and diameters placed in a large 25cm-deep fish tank filled with clear water. The CMUT detector was 10cm above the water surface.
The system should work in murky water, Arbabian says, although they haven't tested that yet. Next up, they plan to image objects placed in a swimming pool, for which they will have to use more powerful laser sources that work for deeper water. They also want to improve the system so it works with waves, which distort signals and make the detection and image reconstruction much harder. “This proof of concept is to show that you can see through the air-water interface" Arbabian says. “That's the hardest part of this problem. Once we can prove it works it can scale up to greater depths and larger objects."
This article appears in the February 2021 print issue as “Laser Sonar Scans Ocean Depths."