Electrical anisotropy from electromagnetic array data: implications for the conduction mechanism and for distortion at long periods

The superposition of surface and middle crustal electrical conductivity ano malies is considered for the particular case of an electrically anisotropic middle crust. Two simple techniques which facilitate estimation of the two conductances of the anisotropic midcrustal layer art: presented. Two tests fur the electrical anisotropy hypothesis are introduced, evaluating the ma gnetotelluric (MT) phases and geomagnetic transfer functions from electroma gnetic array data. If the midcrustal anisotropic structure: is very inhomog eneous, thm it will act on long period electric fields like a local scatter er, and it will impose static shifts on these long period electric fields. Static shift removal techniques that are adapted to this situation are intr oduced. Field data examples for two frequently occurring anomaly types are presented: (1) regular anisotropy: neighbouring sites have similar MT phase s, conductances and regional strikes; (2) irregular anisotropy, which is ju st one feature of a more heterogeneous midcrustal layer. Then, neighbouring sites have smoothly varying MT phases, conductances and regional strikes, and regional skew occurs in the particular frequency band that belongs to t he depth of the middle crust. The combined interpretation of both the condu ctance and the anisotropy of the conductance yields a quantitative argument against the fluid paradigm: anisotropy can indicate a reduced degree of in terconnection of the conductive component. If a conductance of more than 13 00 S is to be explained by brines which are only partly interconnected, the n either a very high prosity or a very high salinity or an unreasonably lar ge thickness of the conductive layer is required. (C) 2000 Elsevier Science B.V. All rights reserved.