10-09-04

Huge seafloor deposits of frozen natural gas being eyed by many countries as a potential energy source may also pose a severe risk of creating tidal waves triggered by underwater landslides, a New Zealand scientist says.

Geophysicist Ingo Pecher will sail up the Hawke's Bay coast soon in the research ship Tangaroa, trying to detect "gas hydrate" deposits in the seafloor sediments. These "frozen" layers of methane gas are widespread in sediments beneath shallow seafloor around continental margins -- including some of New Zealand's.

Dr Pecher said the gas hydrates -- a crystal structure of methane gas surrounded by water molecules, held together by pressure and freezing temperatures -- were increasingly being seen as a potential energy source. Separating the water and the methane yields common natural gas, and global deposits are massive -- an estimated 10 mm tons of frozen methane -- about twice the total energy content of the earth's coal, oil and underground natural gas resources combined.

"It is becoming increasingly clear, however, that gas hydrates also represent a significant natural hazard," he said. "Melting" of hydrate would generate large volumes of gas that might weaken the seafloor, causing submarine landslides capable of triggering a tsunami, he said.

Dr Pecher received a Marsden Fund research grant to test new equipment built by his employer, the Geological and Nuclear Science institute, to detect the ice-like solid in which natural gas is trapped in cages of water molecules. He said New Zealand researchers had a unique opportunity to study the poorly understood link between gas hydrates and seafloor instability.

East of the North Island, in an area with abundant gas hydrates, an undersea ridge appeared to have been "chopped off" 600 m below sea level. He said that at this depth, water temperatures varied over time and hydrates beneath the seafloor would repeatedly "melt" and "freeze".

His team of researchers was working on the theory that such expansion and contraction might weaken the seafloor similar to the way "frost heave" threw up soil on land when sub-surface water froze. A piece of methane hydrate can release about 164 times its volume in methane gas.

The theory would be tested with a multidisciplinary study of the ridge and computer models, with the results expected to boost international understanding of how gas hydrates may cause submarine landslides and tsunamis. Other researchers assessing the geologic stability of the ice have said that hydrates sometimes break apart during earthquakes or underwater landslides, spilling out large amounts of methane gas which may have implications for global warming.

Source: Fairfax New Zealand