Geosciences

Victor Schmidt • Houston

Mid-ocean ridges

Recent work at Columbia University's Lamont Doherty Earth Observatory has altered the traditional thinking about mid-ocean ridge development. The traditional thinking was that magma flows of differing size welled up at the ridges to create varying ridge heights. The new work demonstrates that the height and width of the underwater mountains comprising the ridges is correlated with the direction that individual ridges and their plates are moving.

Suzanne Carbotte, Chrisopher Small, and Katie Donnelly of the observatory examined ridges in five regions of the Pacific and Indian Oceans. They found that the ridges are higher in the direction that the ridge plate is moving. This means that as the ridges are built, more magma gathers on the advancing ridge than on the trailing ridge. The work has implications for crustal tectonics and the structures they form.

IODP cores

The drillship JOIDES Resolution left from Astoria, Oregon, in late June on the first expedition of the Integrated Ocean Drilling Program (IODP). Its eight-week voyage was the first of a series of six ocean research missions to gather fundamental scientific knowledge of the seabed. On this voyage the scientists returned to an area about 120 mi off Canada's British Columbia coast to drill and collect cores.

During the expedition an international team of scientists investigated how fluid flows through rock formations beneath the seafloor to the eastern flank of the Juan de Fuca Ridge. The Juan de Fuca Ridge, west of the drill site, is where new oceanic crust is being built as two oceanic plates spread apart. To the east, the Juan de Fuca Plate dives beneath the edge of the North American Plate.

The holes will be capped with in-well observatories, which will enable researchers to monitor processes beneath the seafloor and conduct experiments. Four observatories will be installed.

This latest effort continues and extends the work done earlier during the Offshore Drilling Program (ODP) that collected seabed core from areas across the globe. Results from those earlier expeditions are shedding light on many aspects of the Earth's history.

This spring, ODP core provided resear-chers with information to identify length of magnetic field reversal. By examining cores that crossed four magnetic reversals, the duration of magnetic reversal was found to average 7,000 years. The polarity change took twice as long at mid- and high latitudes than at low latitudes.

EXPLORATION

WesternGeco gathered two Q-Marine 4D monitor seismic surveys over the Heidrun and Norne fields on the Halten Terrasse, Norwegian Sea, for Statoil ASA. The Heidrun project repeats a baseline survey acquired by the company in 2001.

The Norne survey repeats the Q-on-Q surveys performed in 2001 and in June 2003 after 22 months of oil and gas production. On these data, 4D signal was clearly visible on 2D brute stacks. "Our first Q surveys on Norne confirmed that the repeatability was accurate enough to detect subtle 4D effects and resulted in changed drilling plans. We expect the 2004 surveys to similarly increase our knowledge of these reservoirs," said Hans A. Aronsen, project leader, Statoil Harstad.