North Pacific Magnetotelluric Experiments

• Published in: Journal of geomagnetism and geoelectricity 1980, Vol.32(Supplement1), pp.SI33-SI43
• Main Author: FILLOUX, Jean H.
• Format: Article
• Language: English
• ISSN: 0022-1392; 2185-5765
• DOI: 10.5636/jgg.32.Supplement1_SI33

Electromagnetic signals from two deep sea floor magnetotelluric stations located far from coastal influence in the North Central and North Eastern Pacific are evaluated. The electric field was recorded by means of salt bridge chopper type instruments with a resolution of .01μV/m. Independent magnetic variographs using suspended magnet sensors recorded magnetic variations accurate to .2γ. Both electric and magnetic recorders are characterized by very small long term drift. They are self contained, store data on magnetic cassettes and travel free, between surface and bottom. Magnetotelluric interpretation of records from a first station, located 450 nautical miles to the NNE of Hawaii, at a position which corresponds to magnetic reversal anomaly 31 and to a plate age of 72 m. y. suggests a moderate to low lithospheric conductivity with an average value less than 3×10-3 (ohm m)-1 over the upper 60km, increasing sharply around 100km. A high conductivity tongue, with an average value around 6×10-2 (ohm m)-1 occurs over the interval 180±40km. Meaningful information ceases for a depth beyond 400km. A second and more recent magnetotelluric sounding 700km off central California suggests that a very high and quite shallow conductivity feature implied from earlier work was probably overemphasized. Nevertheless, according to the new data a relatively higher conductivity at a shallower depth seems to differentiate this second station from the first. This result is in harmony with the younger crustal age at the second station (32 m. y.; magnetic reversal 12-13).