Electrical conductivity of the Fennoscandian upper mantle from electromagnetic mini arrays

Category Geophysic
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Korja, Toivo۱; Smirnov, Maxim۱; Pedersen, Laust B
Holding Date 03 September 2008

Upper mantle conductivity structure in Fennoscandia is studied by small electromagnetic arrays in the EMMA (Electromagnetic MiniArrays) project. The project aims at improving existing multi-site magnetotelluric data processing methods and extending traditional 1D horizontal spatial gradient method for 3D environment. The results of the data processing and inversion will be used for deep studies with control over 3D structure, source effects and temporal variation of the estimated responses. The first EMMA array with twelve instruments was operating from Aug 2005 till June 2006 in North-Karelia, the Archaean part of the Fennoscandian Shield. The second array with 14 instruments was operating from June 2007 to October 2007 in the Kuusamo-Pudasjarvi-Ranua region, also in the Archaean part of the shield. In both cases, the site distance was c. 30 km. Results from the first array show that (1) almost one year recordings makes it possible to extend stable MT responses for periods of about 106 s and hence reach the depths of about 600 km, (2) middle crust in the research area has enhanced electrical conductivity on contrary to previous results from other regions of the Archaean crust, (3) the presence of a conducting region at the depth of c. 100 km, and (4) conductivity below the depth of 200-250 km is higher than that of the laboratory based estimates of dry olivine and we interpret this as the base of lithosphere.


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