Engineers combine light and sound to see underwater

Engineers at Stanford University have developed an airborne method for imaging underwater objects by combining light and sound to cross the seemingly impassable barrier at the interface of air and water.

Researchers envision that their hybrid optical-acoustic system will one day be used to conduct drone-based marine biological studies from the air, conduct large-scale aerial searches of sunken ships and planes, and map ocean depths. with similar speed and level. detail like the landscapes of the Earth. Their “photoacoustic airborne sonar system” is detailed in a recent study published in the journal IEEE Access.

“Airborne and space-based radars and laser systems, or LIDARs, have been able to map Earth’s landscapes for decades. Radar signals can even penetrate cloud cover and canopy cover. However, seawater is far too absorbent for imaging in water, ”said Amin Arbabian, study leader, associate professor of electrical engineering at the Stanford School of Engineering.“ Our goal is to develop a more robust system that can create images even through cloudy water. “

Subtitle: energy loss

Oceans cover about 70% of the Earth’s surface, but only a small fraction of their depths has been subjected to high-resolution imaging and mapping.

The main obstacle is related to physics: sound waves, for example, cannot pass from air to water or vice versa without losing most – over 99.9% – of their energy by reflection against the other middle. A system that tries to see underwater using sound waves traveling from air through water and back into the air is subjected to this loss of energy twice, resulting in reduced energy of 99.9999%.

Likewise, electromagnetic radiation – an umbrella term that includes light, microwaves, and radar signals – also loses energy as it moves from one physical medium to another, although the mechanism is different from that. sound. “Light also loses energy from reflection, but most of the energy loss is due to absorption by water,” said study lead author Aidan Fitzpatrick, a graduate student. from Stanford in Electrical Engineering. Incidentally, this absorption is also the reason why sunlight cannot penetrate the depths of the ocean and why your smartphone – which relies on cellular signals, a form of electromagnetic radiation – cannot receive calls. under water.

The result of all this is that the oceans cannot be mapped from the air and from space in the same way as the earth. To date, most underwater mapping has been done by attaching sonar systems to vessels trawling a given region of interest. But this technique is slow and expensive, and inefficient for covering large areas.

Subtitle: An invisible puzzle

Enter the Photoacoustic Airborne Sonar (PASS) system, which combines light and sound to pierce the air-water interface. The idea arose from another project that used microwaves to perform “non-contact” imaging and characterization of underground plant roots. Some of PASS’s instruments were originally designed for this purpose in collaboration with the lab of Stanford Professor of Electrical Engineering Butrus Khuri-Yakub.

At its heart, PASS plays on the individual strengths of light and sound. “If we can use the light in the air, where the light travels well, and the sound in the water, where the sound propagates well, we can get the best of both worlds,” Fitzpatrick said.

To do this, the system first fires a laser from the air which is absorbed at the surface of the water. When the laser is absorbed, it generates ultrasonic waves that travel through the water column and reflect off underwater objects before returning to the surface.

Returning sound waves are still wrecked with most of their energy as they pass through the surface of the water, but by generating the sound waves underwater with lasers, researchers can prevent energy loss from occurring. produce twice.

“We have developed a system sensitive enough to compensate for a loss of this magnitude while still allowing signal detection and imaging,” said Arbabian.

The reflected ultrasonic waves are recorded by instruments called transducers. Software is then used to reconstruct the acoustic signals as an invisible puzzle and to reconstruct a three-dimensional image of the element or the submerged object.

“Similar to how light refracts or ‘bends’ as it passes through water or any medium denser than air, ultrasound also refracts,” Arbabian explained. “Our image reconstruction algorithms correct for this bending that occurs when ultrasonic waves pass from water into the air.”

Subtitle: Ocean surveys by drone

Conventional sonar systems can penetrate to depths of hundreds to thousands of meters, and researchers expect their system to eventually reach similar depths.

To date, PASS has only been laboratory tested in a container the size of a large aquarium. “Current experiments use static water, but we are currently working to deal with water waves,” Fitzpatrick said. “It’s a difficult problem but we think it is doable.”

The next step, the researchers say, will be to perform tests in a larger setting and, possibly, in open water.

“Our vision for this technology is on board a helicopter or a drone,” said Fitzpatrick. “We expect the system to be able to fly tens of meters above water.”