Geomagnetic induction studies of the electrical conductivity of the sublithospheric mantle are currently underway at the University of Washington. This work is based upon the analysis of magnetic time variations in the frequency range 0.01 cpd to 0.2 cpd. The data are obtained from a set of 76 magnetic observatories distributed as uniformly as possible over the earth's surface.
Pioneering work by Banks and others has shown that the P₁⁰ spherical harmonic adequately describes the major part of magnetic field variations in the above range of frequencies. The dominance of the zonal ring current term has been exploited by a number of workers in attempts to determine the global distribution of electrical conductivity. The magnetovariational transfer function, i. e. the vertical over horizontal ratio of the Fourier components of the field variations, is estimated. This function can be related to the magnetotelluric impedance function, and hence can be inverted to yield conductivity structures.
Application of one-dimensional Backus-Gilbert inverse theory to data obtained at Tucson and Honolulu has shown a resolvable difference in conductivity structures at the two sites. This work serves as the basis for the current effort to delineate lateral mantle conductivity variations using the world-wide distribution of magnetic observatory data.
The data for this work consists of 1358 station years. Each station-year contains up to 366 daily mean values for each of the three magnetic components H, D and Z. Much of these data exist as simultaneous timeseries from sites distributed around the globe. The simultaneity of data will allow us to test the validity of the P₁0 assumption and identify and remove noise. A statistical basis for the treatment of spurious data will also be discussed.