Catching currents with a cable.
Whenever a geomagnetic storm stirs up the ionosphere, electrical currents surge throughout the earth. These currents provide scientists with a means of learning about the conductivity of various regions of the planet.
Most of the conductivity studies done with conventional techniques have been limited to small areas in the Pacific Ocean. in the last few years, however, researchers at AT&T Bell Laboratories in Murray Hill, N.J., have taken advantage of a telecommunications cable laid across the Atlantic Ocean for measuring, in another way, both earth and ocean electrical currents on very large scales.
Last year, Louis Lanzerotti, David Thomson and their co-workers reported on studies using a section of the TAT-7 cable, which stretches from Tuckerton, N.J., to Lands End, England, to search for currents leaking from the earth's core (SN:7/6/85,p.5). Now they've used the cable to explore the electrical structure of the eastern U.S. coast and to measure ocean tides.
In studying the continental margin of the East Coast, Lanzerotti's group monitored the voltage (from which current can be calculated) across 1,200 kilometers of the generally east-west-trending cable in February 1983. For six days the cable was not operated by AT&T and so could pick up currents from the natural environment. At the same time the group measured changes in the magnetic field at the Tuckerton end of the cable, and at another site along the coast. The researchers expected to find a strong correlation between the cable voltage and the north-south component of the magnetic field. Instead, they discovered that the cable voltage was most strongly correlated with the east-west magnetic field readings.
As reported in the June 10 JOURNAL OF GEOPHYSICAL RESEARCH, Lanzerotti's group decided that the most plausible explanation for the correlation is that the continental margin is underlain by an extensive north-south-trending current channeled in the crust and upper mantle. The researchers suspect that this current may be associated with what is known as the East Coast Magnetic Anomaly, a northwest-southeast-trending region that bears an unusually strong magnetic field.
Other scientists have suggested, among other things, that this anomaly may be caused by a buried fault or a body of highly magnetized rocks. "We don't think that the anomaly itself is making the current flow," says Lanzerotti. "But whatever exists in the earth that causes the magnetic anomaly may also be a region of enhanced conductivity."
In the June GEOPHYSICAL RESEARCH LETTERS, the researchers also report on the use of a long segment of the undersea cable to measure the flow of electrically conductive seawater. In particular they detected ocean tides, which are caused by the gravitational tugs of the moon and sun. However, Lanzerotti says the strength and frequency of a few of the cable signals differ from what tidal models predict. "It's not yet clear if those differences are due to inadequacies of the models per se or if it's the way we are testing the model," he says.
The researchers think the cable will eventually provide a test for tidal models that is different from, and in some cases more rigorous than, the tidal guage data used now.
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|Title Annotation:||telecommunications cable used to measure earth and ocean currents|
|Date:||Jun 21, 1986|
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