If you've ever been asleep on a yacht in harbor when a submarine tests its sonar, you know that underwater sound is anything but trivial – one ping can send you out of your bunk and across the room. Small wonder that the major navies spend a fortune studying the impact of naval and civilian sonar systems on sea animals such as whales and dolphins, who live in a world of sound. Scientists at the University of Bath have developed a more cetacean-friendly sonar system called Acoustic Zoom that is not only less disruptive to marine life, but also improves resolution beyond that of current methods.

As civilization turns to the sea for new sources of energy and minerals, geological surveys to reveal what resources the seabed has to offer become increasingly important. One method is seismic imaging, which uses explosives or transponders to send sound waves through the various layers of soil and bedrock to discover its structure as the sound is reflected back to an array of sensors. The problem is that the resolution of such imaging is extremely poor and the sound used to create it can be disruptive to Marine life.

Developed by Jaques Guigné and Nick Pace at the University of Bath, Acoustic Zoom works on a different principle than conventional seismic imaging. Instead of reflecting sound energy back from the depths of the earth, Acoustic Zoom measures how the energy is scattered as a beam of sound scans through an area before being picked up by a 16-spoke array set on the ocean floor. The signal is then digitally processed, and from this, a much higher resolution image is built up.

An Acoustic Zoom image

According to Guigné and Pace, the result of Acoustic Zoom is that geologists can now see details such as fractures and fissures in strata that were previously invisible. In addition, the sound is created by a form of "marine trombone" that can emit sound at a much higher frequency over a longer time and at lower energies than conventional sonar methods. The scientists say that the lower energy and higher frequencies mean that Acoustic Zoom has less impact on marine life. In addition, they see the higher resolution as a way of reducing the need for unnecessary drilling in the future and therefore minimizing environmental effects.

"We’re really excited about this technology because it allows us to take virtual core samples, giving us a much more detailed understanding of subtle geological features, without any drilling," says Guigné. "It works by analyzing the scattered sound energy rather than the reflected energy that is normally recorded. The scattered signal is a lot weaker so it’s been quite tricky to do successfully. It’s a bit like driving at night trying to focus on the face of the driver of an oncoming car through the glare of their headlights."

The Guigné and Pace's results were published in Journal of Natural Gas Science and Engineering.

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