Over The Horizon

Mapping Ocean Gravity

Marine gravity anomaly maps, plotted from satellite-derived information originally intended to guide cruise missiles, are proving of interest to commercial fishing operators and scientists alike.

They cover most of the Southern Hemisphere and show previously unknown shallower areas where fish congregate and breed. Fishing companies are increasingly using the maps to harvest previously-unfished areas, some of which lie outside the zone where New Zealand's quota management regime applies.

Marine geophysicist Dr Bryan Davy heads a team which uses specialised computer software to process the information which is derived from satellite measurements of sea-surface height.

Much of the information, prepared by scientists in the United States, has recently been made available on the Internet. Four years ago it was partly classified by the United States military, but over the past two years non-military scientists have been gaining greater access to it. The Institute of Geological and Nuclear Sciences have been compiling maps for New Zealand.

Davy says the new maps have been a revelation to oceanographers, marine geophysicists, oil exploration operators and others with an interest in the seabed and plate tectonics.

"It's like having a new road map that shows a whole lot of detail that hasn't been seen before," he says.

They show a number of large geological features, such as deep ridges and seamounts, some of which scientists were not aware of.

"There is a fresh wave of interest in seabed research from scientists internationally as a result of the new detail shown in the maps."

The raw information is gathered by a satellite picking up changes in sea-surface height which are influenced by density variations beneath the sea-surface. The main density variations are due to water depth and the type of rock under the seabed.

A submarine volcano, for example, exerts a gravitational pull on adjacent sea water causing a local elevation in the height of the sea surface.

Low-density, sediment-filled areas, or deep ocean trenches, exert comparatively less gravitational pull resulting in reduced sea-surface height directly above. US scientists correct the satellite information for the distorting effects of ocean tides, currents and wave motion before making it available to scientists internationally.

"The flood of high-resolution satellite information has been so rapid that scientists are still working through it."

The Louisville Ridge, a prominent line of seamounts in the northeast of the map, was already known to scientists as a volcanic "hot-spot trail", marking the motion of the Pacific Plate over a heated zone in the Earth's interior. It is particularly well defined by the gravity measurements.

However, an example of a structure which became apparent when the New Zealand map was compiled is a line crossing the Louisville Ridge at right angles and tending southwest toward the Chatham Islands.

Scientists have tentatively interpreted it as an extinct spreading-ridge -- perhaps 100 million years old -- where new seabed crust was formed as tectonic plates pulled apart.

Scientists are debating the impact that such a structure would have had on New Zealand's formation, as well as plate motion reconstructions worldwide. A collaborative marine survey, involving a US research ship and GNS scientists, has been proposed for 1998 to help resolve the nature and age of the structure.

Another feature highlighted in the map is an extinct spreading ridge to the west of New Zealand, on the extreme left of the map. This marks the point where New Zealand and Australia -- which were joined as one landmass until 80 million years ago -- stopped moving away from each other 58 million years ago. The gravity map shows previously unrecognised details of the spreading process.