Dec. 2000 Vol. 221 No. 12  Exploration

Perry A. Fischer,  Engineering Editor

It’s deep and hot, but maybe not

The idea that volcanic island chains are caused by mantle plumes was first proposed by J.T. Wilson 37 years ago. The plumes apparently remain stationary; upwelling molten material rises to the surface as the plate moves, thereby creating intraplate island chains such as the Hawaiian Islands. A decade later, W. J. Morgan further proposed that the source of those plumes was the core / mantle boundary. Two geologists from Southern Methodist University in Texas could have an answer as to why and where hot spots (mantle plumes) occur.

Geologists Douglas Oliver and Rebecca Ghent presented their findings in November at the 112th annual meeting of the Geological Society of America in Reno, Nevada. Oliver and Ghent noticed that the plumes were mostly concentrated in belts between 20° and 30° latitude north and south of the equator. Some scientists believe that mantle plumes are areas of relatively hot, upward convection. However, if buoyancy were the only factor involved, such hot spots would be more randomly distributed.

Ghent searched the literature on fluid dynamics. She found an explanation in a theory: If a liquid is trapped between two spheres that rotate at different speeds about a common axis, then the liquid will create eddies in the circulation pattern at latitudes of about 20° to 30°. At present, most evidence suggests that the Earth has a liquid outer core lying between a solid inner core and mantle. There is also evidence that the inner core rotates faster than the Earth’s surface.

"The spots do not occur at high latitudes, because the fluid has already cooled and is on its way back down," said Ghent. The two geologists said that their theory might explain the occurrence of clusters of mantle material, or superplumes, that arrive together at the Earth’s surface. Superplumes offer a possible explanation as to why Pangea – the last known supercontinent – split into the present continents.

However, Gillian Foulger of the University of Durham, UK, and Bruce Julian of the U.S. Geological Survey have a different idea. Their research was published in the September issue of Geophysical Journal International and summarized in the November issue of Geotimes.

Their conclusions are based on the seismic velocity contrast that occurs when a sound wave enters a much hotter region. Using that fact, a research team in Iceland found that the base of the hot spot fueling Iceland’s volcanism is only about 250 mi deep. Overall seismic aperture was limited to the island, so only raypaths within 30° to 40° of vertical were recorded. This limits resolution at great depths.

Although Iceland is not an intraplate island chain, the summary also points out that preliminary research at Hawaii and Yellowstone, Montana, indicate a similar conclusion: The 30-year assumption that hot spots originate from deep mantle / core plumes may not be correct. A new theory is emerging that volcanic activity is determined by lithospheric motion and regional extension. Exactly what forces cause movement of the Earth’s plates while allowing for stationary, lithospheric hot spots will undoubtedly be a research subject for decades.

To help settle the issue, John Orcutt of Scripps Institute of Oceanography – along with scientists from Woods Hole Oceanographic, Carnegie Institute of Washington and the University of Hawaii – plans to deploy 70 seismometers on the seafloor surrounding the Hawaiian chain. By extending the aperture beyond the islands, resolution at great depths should be possible. After 15 months of data collection, Orcutt believes they will be able to determine exactly where Hawaiian volcanism originates.

It looks brighter. There’s little doubt that data integration can result in data overkill, and an ordinary Cathode Ray Tube (CRT) monitor is insufficient for viewing huge data sets, especially exploration data in a regional context. The abundance of new visualization systems is a testament to that fact. However, most of these systems are based on CRT projection technology and, therefore, do not form large images as bright or clear as one might hope.

Christie Digital Systems teamed with Fakespace Systems and demonstrated a stereoscopic, digital projection system at the Supercomputing 2000 show in Dallas this November. The new system uses Texas Instruments’ 3-chip DLP technology – a chip that has an array of 786,432 (XGA) hinged, microscopic mirrors mounted on a standard logic device.

The 3-D system offers 10,000 lumens, XGA (1,024 x 768 pixels) resolution and 110-Hz refresh-rate capability. This is much brighter than ordinary CRT projectors. Christie also plans to have SXGA (1,280 x 1,024 pixels) resolution in the near future. Say goodbye to dimly lit rooms.

Changes at CGG. Aker Geo (subsidiary of Aker Maritime) has sold its two seismic vessels, Amadeus and Symphony, to CGG. Also included in the sale is 1,000 km² of recently acquired seismic data and operating continuity with respect to current Aker Geo clients and suppliers. Payment – subject to shareholder approval – comprises $25 million in cash and about 1.3 to 1.8 million shares of CGG stock, depending on share price at the time of transfer. Total transaction value is $118 million. The agreement is subject to successful completion of an offering of senior notes in aggregate principal of about $165 million.

In addition, CGG purchased a remaining 40% interest in CGG Marine that was owned by Louis Dreyfus Armateurs (LDA). Under the contract, a 50/50 joint-venture company will be created between CGG and an unnamed new owner of the seismic vessel Mistral – currently owned by LDA. The new JV company will upgrade the seismic vessel to 10 streamers. The transaction will be paid in CGG shares valued at $24.2 million.

Finally, the Borehole Services Division of CGG and Read ASA signed a cooperation agreement for providing VSP and related borehole seismic services in the UK and Norwegian sectors.

All of this comes on the heels of CGG’s sale of its Flagship division to Paradigm Geophysical and CGG’s acquisition (through Sercel) of Mark Products – a geophysical equipment manufacturer.

Comments? Write: fischerp@gulfpub.com