On the electrical conductivity of the mid-mantle-I. Calculation of equivalent scalar magnetotelluric response functions

The problem of global conductivity sounding has been re-posed in order to investigate the possible existence of large-scale lateral heterogeneities in mid-mantle conductivity. A response function (Z/H) is robustly estimated and scaled into an equivalent scalar magnetotelluric impedance by assuming the geomagnetic field spatial variations are adequately described by a P01 spherical harmonic. Data found to be inconsistent with this representation in either the time, frequency or spatial domain are excluded from the analysis. Making use of this relationship, robust statistical techniques and careful data handling procedures have been applied to a new global magnetic database containing well in excess of 106 daily mean values (1358 observatory-years spanning the period 1883–1980). A set of spatially distributed scalar frequency domain impedance functions for a variety of tectonic provinces result from this analysis. Response functions consistent with both the P10 assumption and local 1-D earth structure were found for 22 of the 79 observatories examined. The set of acceptable response functions is presented, along with a summary of data quality for the observatories represented in the database.