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

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

May 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000pepi..119..237b&link_type=abstract

Physics of the Earth and Planetary Interiors, Volume 119, Issue 3-4, p. 237-257.

Physics

13

Scientific paper

The superposition of surface and middle crustal electrical conductivity anomalies 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 are presented. Two tests for the electrical anisotropy hypothesis are introduced, evaluating the magnetotelluric (MT) phases and geomagnetic transfer functions from electromagnetic array data. If the midcrustal anisotropic structure is very inhomogeneous, then it will act on long period electric fields like a local scatterer, and it will impose static shifts on these long period electric fields. Static shift removal techniques that are adapted to this situation are introduced. Field data examples for two frequently occurring anomaly types are presented: (1) regular anisotropy: neighbouring sites have similar MT phases, conductances and regional strikes; (2) irregular anisotropy, which is just 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 the depth of the middle crust. The combined interpretation of both the conductance and the anisotropy of the conductance yields a quantitative argument against the fluid paradigm: anisotropy can indicate a reduced degree of interconnection of the conductive component. If a conductance of more than 1300 S is to be explained by brines which are only partly interconnected, then either a very high prosity or a very high salinity or an unreasonably large thickness of the conductive layer is required.

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