Magnetotelluric measurements over the Alpha Ridge

Details Magnetotelluric measurements over the Alpha Ridge Magnetotelluric measurements over the Alpha Ridge

Mar 1987

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987pepi...45..101n&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 45, Issue 2, p. 101-118.

Physics

3

Scientific paper

Magnetic and telluric field variations were recorded on the Arctic Ocean sea ice over the Alpha Ridge in April and May, 1983. The magnetotelluric measurements were made near the base camp of Operation CESAR (Canadian Expedition to Study the Alpha Ridge) to investigate the deep structure and electrical properties of the lithosphere and upper mantle beneath the ridge. Thirty days of usable data were obtained within the period range 120-20 000 s. During the experiment the base station drifted across a deep (2000 m) median valley or graben lying parallel to the ridge axis and then over a steep rise on its northern flank. One data set was obtained over the graben, and the remainder of the measurements were made as the station travelled back and forth over the rise. The directional properties of both the geomagnetic induction arrows and the magnetotelluric impedance tensors were consistent with changes in water depth along the drift path. One-dimensional inversions of E-polarization MT data revealed a resistive lithosphere about 70-85 km thick with a conductive (10 μm) upper mantle (asthenosphere?) below. Sea-water depths estimated in the inversion process agreed remarkably well with the known bathymetry, indicating that the ocean floor contained less than 100 m of highly conducting sediments along most of the drift path. Two-dimensional models required simulation of the topography of the entire section of the Alpha Ridge between the Canada and Makarov Basins in order that a satisfactory fit be obtained to the H-polarization data. Two-dimensional forward modelling indicates that the anisotropic character of the impedance measurements can entirely be attributed to the bathymetry of the ridge. The data provided no indication of structural inhomogeneity within the oceanic crust and lithosphere, consistent with results from companion seismic refraction and gravity surveys.
The thickness of the lower conducting region was not resolved because reliable long period data (T > 20 000 s) were not available for analysis. Impedance estimates at these periods were poorly determined and subject to rapidly rising tensor skew. They were also subject to bias errors caused by water movements, in particular the M2 tide.

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