The National Geomagnetic Initiative (1993) / Chapter Skim
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A3: Electromagnetic Induction Studies in the Earth and Oceans
A3: Electromagnetic Induction Studies in the Earth and Oceans
Pages 119-133
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From page 119... ...
In addition, there are smaller-scale problems of great interest to society including energy and resource exploration, electric power grid reliability, water quality and waste management in which electrical conductivity of the target or its surroundings is a diagnostic physical variable. Electromagnetic methods are the only viable way to delineate conductivity structure from the Earth's surface.
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From page 120... ...
Therefore, electromagnetic measurements in the ocean are an extremely useful tool for probing large-scale dynamics and monitoring long-term, climatically important variability of the sea. Electromagnetic induction studies of the solid Earth involve either simultaneous measurements of time-varying magnetic and electric fields in orthogonal horizontal directions for the widely popular magnetotelluric (MT)
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From page 121... ...
Laboratory electrical conductivity studies provide the final critical step, linking models of conductivity and the physical and chemical processes occurring within the Earth. Because conductivity is sensitive to environmental parameters, interpretation of conductivity models requires a thorough understanding of the mechanisms that control conductivity under the limited conditions accessible in the laboratory.
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From page 122... ...
But how much of this signal is due to deep lateral heterogeneity and how much is due to biases associated with shallow structure such as the ocean and inadequately represented external source field morphology? Can lateral variations in conductivity in the upper mantle be mapped?
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From page 123... ...
· How is magma segregated from the mantle beneath a spreading midocean ridge? Model studies based on theoretical studies of this segregation process suggest that ocean bottom MT transects across the ridge can discriminate between major alternatives considerably more effectively than feasible seismic studies and will cover a range of spatial scales that seismic methods cannot resolve.
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From page 124... ...
Seismic date have frequently been interpreted to imply magma bodies in the crust, but electromagnetic methods are much more sensitive to the important variables. The growing ability to collect very high density electromagnetic data and to deal with complicated three-dimensional geometry has the potential of giving considerably more information than heretofore available.
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From page 125... ...
· Measurement of oceanic motions. Oceanic electromagnetism is having an increasing impact in studies of the water-velocity field based on measurements of motionally induced horizontal electric field in the deep ocean.
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From page 126... ...
The electric fields include information about conductivity heterogeneity very local to the measurement site. This effect persists to very long periods and can be aliased if site spacing is too wide.
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From page 127... ...
Complete field processing of MT time series in essentially real time now assures that data of adequate quality are being collected. New deployment and processing techniques using remote-reference sensors can mitigate the biases associated with some forms of noise, particularly of cultural origin.
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From page 128... ...
Furthermore, the complete lack of digitally recording magnetometers for MV work in the United States also means that MT data cannot be collected at periods above 1,000 seconds, although these data are needed to probe the upper mantle. The recent collapse of industrial contractors capable of providing data of the quality required has made it difficult, although perhaps not impossible, to use an industrial alternative for projects amenable to shorter-period data.
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From page 129... ...
Some of the results of motional induction experiments need to be communicated to the wider geomagnetic community. For example, the spectrum of motionally induced horizontal electric fields rises rapidly at periods longer than a few days and is very likely going to determine the precision with which weak signals from Earth's core can be measured.
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From page 130... ...
In addition, de facto joint funding is shared between NSF and DOE, because imaging advances supported by the Geosciences Program within the Office of Basic Energy Sciences and instrumental development supported by the Geothermal Technology Division are crucial to the MT profiling projects funded by the Continental Dynamics Program. Support has also been provided by the U.S.
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From page 131... ...
· To improve our knowledge of mantle conductivity and to understand the constraints it provides on composition, physical state, and dynamics of the Earth's interior require a multifaceted approach. It should include new data, such as ultra-low-frequency, long-baseline MT measurements; improved observatory coverage; better understanding of the effects of source morphology on interpreted conductivity structure; more sophisticated time series processing and inversion methods; and improved laboratory measurements of mantle minerals under controlled thermodynamic conditions.
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From page 132... ...
To monitor long-term variability of the geoelectric field for both MT ant} oceanic stuclies in the deep ocean requires long grounded dipoles. Use of abandoned submarine cables appears to be promising in this context, but it would require close cooperation between scientists and telephone companies.
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From page 133... ...
APPENDIX A: TOPICAL WOWING GROUP REPORTS 133 · To calibrate apparatus in which new materials are being measured, it is essential that the conductivities of known materials at high temperatures be determined. San CarIos olivine (Fo-90)
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