New Photoacoustic Airborne Sonar System Helps Image Underwater Objects

By integrating both sound and light, Stanford University researchers have successfully designed a novel airborne technique for imaging underwater objects and eventually break through the apparently impenetrable barrier at the air-water interface.

According to the researchers, their new hybrid optical-acoustic system would be used in the future to conduct large-scale aerial searches of sunken planes and ships, perform aerial drone-based biological marine surveys, and plot the depths of oceans with an analogous speed and level of detail as the landscapes of the Earth.

The “Photoacoustic Airborne Sonar System” developed by the team has been described in a new study published in the IEEE Access journal.

Airborne and spaceborne radar and laser-based, or LIDAR, systems have been able to map Earth’s landscapes for decades. Radar signals are even able to penetrate cloud coverage and canopy coverage. However, seawater is much too absorptive for imaging into the water. Our goal is to develop a more robust system which can image even through murky water.

Amin Arbabian, Study Lead and Associate Professor of Electrical Engineering, School of Engineering, Stanford University

Energy Loss

While oceans cover approximately 70% of the surface of the Earth, only a small portion of their depths have been exposed to high-resolution mapping and imaging.

The major obstacle is tied up with physics—for instance, sound waves cannot travel from the atmosphere into water or the other way round without losing most—over 99.9%—of their energy in the form of reflection against the other medium.

A system that attempts to visualize the underwater environment, through sound waves passing from the atmosphere into water and back into the atmosphere, loses this energy by two-fold, leading to an energy reduction of 99.9999%.

In a similar way, electromagnetic radiation—an umbrella name that comprises radar, light, and microwave signals—is also subjected to energy loss when traveling from one physical medium into another, even though the mechanism is not the same as that for sound.

“Light also loses some energy from reflection, but the bulk of the energy loss is due to absorption by the water,” stated Aidan Fitzpatrick, the study’s first author and a graduate student in electrical engineering at Stanford University.

Incidentally, such an absorption is also the reason why sunlight cannot enter the depths of oceans and why a smartphone—which depends on cellular signals, a kind of electromagnetic radiation—is unable to receive calls in the underwater environment.

The bottom line is that it not possible to map oceans from space and the air in the same way as that for the land. So far, a majority of the underwater mapping has been realized by fixing sonar systems to ships that search for a specified area of interest. However, this method is not only slow but also inefficient and costly for covering large regions.

An Invisible Jigsaw Puzzle

This is where the Photoacoustic Airborne Sonar System (PASS) comes into play. The system integrates sound and light to break through the interface of air and water. The concept for it came from another study in which microwaves were used to conduct “non-contact” characterization and imaging of underground plant roots.

Certain instruments of the PASS were initially developed for that purpose in association with the laboratory of Butrus Khuri-Yakub—an electrical engineering professor at Stanford University

At its core, the PASS plays to the individual strengths of sound and light.

If we can use light in the air, where light travels well, and sound in the water, where sound travels well, we can get the best of both worlds.

Aidan Fitzpatrick, Study First Author and Graduate Student in Electrical Engineering, Stanford University

To achieve this, the system initially fires a laser from the atmosphere that gets absorbed at the surface of the water. Then, after the absorption of the laser, the system produces ultrasound waves that travel down via the water column and then reflect off the underwater objects before speeding back toward the water surface.

While the returning sound waves are still devoid of most of their energy when they breach the surface of the water, the researchers can still prevent the energy loss from occurring twice by creating the sound waves underwater with the help of lasers.

We have developed a system that is sensitive enough to compensate for a loss of this magnitude and still allow for signal detection and imaging.

Amin Arbabian, Study Lead and Associate Professor of Electrical Engineering, School of Engineering, Stanford University

The ultrasound waves, which are reflected, are captured by instruments known as transducers. Subsequently, a software is used to organize the acoustic signals back together, similar to an imperceptible jigsaw puzzle, and rebuild a 3D image of the underwater object or feature.

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

Drone Ocean Surveys

Traditional sonar systems can enter the ocean depths of hundreds to thousands of meters, and the team believes that their new system will sooner or later be able to reach analogous depths.

So far, PASS has only been validated in laboratory settings in a container similar to the size of a huge fish tank.

“Current experiments use static water but we are currently working toward dealing with water waves,” added Fitzpatrick. “This is a challenging but we think feasible problem.”

According to the team, the next step will be to perform tests in a bigger setting and, ultimately, open-water surroundings.

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

Ajay Singhvi, a graduate student from Stanford University, is also the study’s co-author. The study was funded by the Advanced Research Projects Agency-Energy (ARPA-E) and the U.S. Office of Naval Research.

An airborne sonar system for underwater remote sensing and imaging. Video Credit: Stanford University.

Journal Reference:

Fitzpatrick, A., et al. (2020) An Airborne Sonar System for Underwater Remote Sensing and Imaging. IEEE Access. doi.org/10.1109/ACCESS.2020.3031808.