On the electrical conductivity of the mid‐mantle: II. Delineation of heterogeneity by application of extremal inverse solutions

Abstract

Long‐period equivalent scalar magnetotelluric impedance functions from 22 magnetic observatories distributed over the Earth's surface are inverted to test the hypothesis that there exists significant lateral heterogeneity in the electrical conductivity structure of the mid‐mantle. Limited spaces of extremal models are examined, and strict bounds are placed on the range of permissible models. Joint D+ inversion of combined response functions from observatory pairs proves the requirement for global heterogeneity, while exploration of H+ layered‐earth model space suggests systematic regional heterogeneity, indicating that enhanced upper mantle conductivity is found within the Pacific basin and Baikal Rift zone and is absent elsewhere. The results of these extremal inverse solutions are compared with the Backus–Gilbert‐like Schmucker inversion and a heuristic inversion. Copyright © 1990, Wiley Blackwell. All rights reserved