Mapping the ocean floor

Press, 30 August 1985, Page 18

Mapping the ocean floor

Finding out what lies beneath the ocean floor around our continents is no easy task, especially when the depth of water may be more than 100 metres. Any information about the drowned edges of continents is important in deciphering their long geological history, and how each continent relates to its neighbour. When petroleum exploration moved from the land to offshore regions, there was a dramatic improvement in the geophysical equipment used which enabled the scientist to learn more about what lay below the sea without actually going down there. Basins where sediment had accumulated for many millions of years were located offshore, some far thicker than those known on the adjacent land. These basins are regarded as potential sources of oil, and have led to further refinements in geophysical equipment. In early 1984, a United States Geological Survey ship, the S.P. Lee, went south to Antarctica with some of the most sophisticated

geophysical apparatus ever developed aboard. Dr Frederick Davey, now director of New Zealand Geophysics Division, who had previously done geophysical work in the Antarctic, accompanied the cruise. The voyage was in two parts. Leg 1 surveyed the coast off Wilkes Land south of Tasmania, gathering information about an area adjacent to one surveyed by the French Petroleum Institutes in 1982. Leg 2 was in western Ross Sea south of New Zealand, an area of great interest to many New Zealand geologists. The S. P. Lee began and ended its Antarctic cruise in Lyttelton. Further seismic work off the east coast of the North Island was undertaken with Dr Jarg Pettinga of Canterbury University and other New Zealand scientists. Both cruises were regarded as a great success, and the information gained will stimulate research for many years.

The geophysics undertaken by the S. P. Lee in the Antarctic involved seismic profiling, gravity,

geomagnetic and heatflow measurements, side-scan sonar, and bottom sampling by coring and dredging.

When the S. P. Lee was in Lyttelton the Canterbury Museum’s geologist had the good fortune to visit the ship, and subsequently the museum has been given two dredged samples and a 2.5-metre long multichannel seismic profile of the Iselin Bank, a major Ross Sea feature. These gifts were recently put on display in the Antarctic Hall of Discovery, with photographs of the S. P. Lee and some of its equipment. The multichannel seismic profile shows not only the topography of the sea floor, but also the attitude of the rocks below it. The profile is

produced when a continuous series of sound wave pulses emitted by six airguns towed behind the ship are bounced off both the sea floor and reflector horizons within the rock below. They are received sequentially by 48 hydrophones located in a “streamer” towed behind the ship. The streamer is a large plastic tube 2.4 km long, that enclosed masses of wire which connect the hydrophones with a series of computers on board the ship. Controllable metal “fish” fitted at .intervals along the streamer keep it at a constant depth of 10 metres. The image of the sea floor and the rocks below is processed by the multichannel computers and is printed out as a continuous profile.

By

MARGARET BRADSHAW