The ocean offers exciting opportunities, but also poses terrifying dangers. Harnessing ocean-wave energy, for example, is a promising environmental solution; yet the world is threatened by tsunamis that can take the lives of thousands in one fell swoop. Dr. Usama Kadri of the University of Haifa’s Hatter Department of Marine Technologies studies what is called acoustic-gravity waves. These are sea waves that have properties of both surface waves (gravity waves) and sound (acoustic) waves. The presence and power of acoustic-gravity waves, as Kadri’s research is discovering, could influence approaches to developing technologies for wave energy generation and tsunami alert systems.
Surface-gravity waves are the waves visible to the eye and create dynamic pressure down to a few meters below the surface. As their name suggests, they are governed by the restoring force of gravity. Acoustic waves are longitudinal waves that travel in water by means of compression and decompression. Kadri explains that science traditionally relates to surface-gravity waves and acoustic waves as separate entities. His research focuses on the interaction between the two wave types.
A theory suggested by Kadri establishes a concrete relationship between acoustic and gravity waves. His research has found that 20 to 90 percent of the energy initially within the surface-gravity waves can travel below as an acoustic-gravity wave, which is capable of traversing the depths. “The propagating acoustic-gravity waves result in drifting which may explain the movement of deep-sea marine biology, salts, nutrients, carbon-dioxide, and ocean circulation, which are all necessary for a healthy ocean life and sustaining the environment,” he explains.
One implication of this relates to harnessing energy from gravity waves. He cautions that “harnessing energy from
gravity waves may reduce the generation of acousticgravity waves, in turn affecting the ocean’s ecosystems.” But we are ahead of the game, he reassures. “While harnessing energy from the sea is still in its infancy, the system should be designed to prevent damage and maintain harmony in the sea’s depths.”
A second research project examines the propagation and detection of acoustic-gravity waves, which travel hundreds of times faster than surface-gravity waves, and span the entire ocean depth. Here too, practical implications are immense. Take the Indian Ocean earthquake of 2004, for example, which radiated acoustic-gravity waves along with a destructive tsunami. Three minutes before the resulting tsunami hit the shores of Indonesia, killing tens of thousands, acoustic-gravity waves had already traveled over 1,000 km from the earthquake’s epicenter carrying information about the approaching tsunami. Early detection of acoustic-gravity waves coupled with a warning alarm could have increased evacuation time and potentially saved lives. For Sri Lanka, which was hit by the tsunami over an hour later, such an early warning system could have significantly reduced the number of casualties.
Kadri has set up a collaborative project with the Woods Hole Oceanographic Institution, and more recently with peers from the European scientific community, preparing the ground for the first early detection warning system based on monitoring acoustic-gravity waves. “We would like to be able to develop this application globally, to protect all high-risk tsunami zones. About two dozen stations would provide a global warning system and monitoring of deep water transportation. In fact, many such stations already exist, but only need to be operated accordingly,” he says.
After earning his PhD at Delft University of Technology in the Netherlands, Kadri completed a post-doctoral fellowship at MIT. At present, Kadri is dividing his time between the University of Haifa and MIT. His newly constructed state-of-the-art laboratory at the University was built with the generous support of the Helmsley Charitable Trust. “There is a good opportunity here. The University is encouraging local and global collaboration, and there’s lots of room for scientific teamwork within the University that will positively impact our environment.”