AN ALIEN SEEKING a name for the third planet from the sun might reasonably plump for “Sea” or “Ocean”, rather than “Earth”. Two-thirds of its surface is covered by salt water, and its predominant colour, viewed from far away in space, is blue. What underlies all this brine, though, remains surprisingly mysterious to the planet’s ape-descended inhabitants. As recently as 2019, for example, researchers found several thousand new underwater mountains, known as seamounts, by measuring the effects of their gravity on the ocean’s surface. More such discoveries almost certainly await.

One important reason for ignorance about the seabed is the lack of a tool that can easily map its topography from an aircraft flying above the water. That, though, is about to change. Researchers at Stanford University, led by Amin Arbabian, an electrical engineer, have developed what they call the Photoacoustic Airborne Sonar System, PASS. This makes it possible to scan the ocean floor rapidly, from a helicopter, rather than relying on a slow-moving ship or submarine.

The problem to be solved is that sound waves, in the form of sonar, are the only reasonable way to accomplish such mapping. Both light beams and the radio waves of radar are rapidly absorbed by water. Sound, by contrast, propagates well. What it does not do well is cross the boundary between water and air. When this happens its amplitude is diminished a millionfold. That diminution applies in both directions, so a pulse of sonar broadcast from an aircraft and reflected back to it from the sea floor would have a trillionth of its original amplitude. Expecting to detect such a reflection would be a fool’s errand.

PASS partly overcomes the air-sea boundary problem by circumventing the first of those crossings. It does so by generating the sonar pulses not on board the aircraft but rather in the water itself, using intense bursts of laser light fired at the water’s surface. These heat the water, causing rapid expansion. That generates a sound wave which propagates to the sea floor, whence it is reflected back to the surface. Only then does it cross the energy-sapping interface between water and air. Though that still weakens the signal a lot, the other part of PASS, a device called a CMUT, is sensitive enough to detect it.

CMUT stands for capacitive micromachined ultrasonic transducer. As its name suggests, it is a species of electrical capacitor, and, like all such, it is composed of two parallel plates. Any disturbance of these plates, such as the vibration induced by a sound wave, changes the capacitor’s properties in a way that is easily detected.

CMUTs were developed at Stanford two decades ago. They are widely used in ultrasonic medical scanners and are made in the same way as the micro-electromechanical deceleration sensors which trigger the deployment of car air bags, so they can be mass produced. PASS employs CMUTs tuned to resonate at the exact frequency of the sonic pulse generated by the laser. This has the double benefit of improving reception and filtering out background noise.

Preliminary tests in a university fish tank used a laser weighing 50kg, but this was a general-purpose device and the apparatus could, the team reckon, be scaled down to weigh just a few kilograms. That would fit on commercial camera-carrying drones. A device this size would be able to “see” through tens of metres of water, making it suitable for use above rivers, lakes and coastal waters. A larger version for deep-sea operations would fit on a manned helicopter or a larger drone and would be able to peer down to depths of hundreds, and eventually thousands, of metres. The team’s researchers imagine fleets of such drones making short work of the task of charting the abyss.

Deep thoughts

Besides the scientific value of mapping the seabed with the sort of resolution normal for terrestrial cartography, PASS will also be able to locate the wrecks of missing ships and aircraft, and engage in commercial tasks such as monitoring underwater infrastructure like oil and gas pipelines.

There are military applications, too, particularly for the detection of submarines. In this context it is no surprise that the project is being sponsored by America’s Office of Naval Research. But for inhabitants of the third rock from the sun—or, at least, for those of them interested in hidden aspects of the orb they inhabit—the generation of the first high-resolution map of all of that orb’s solid surface will be a matter of moment in its own right.

This article appeared in the Science & technology section of the print edition under the headline “Full fathom five”

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