A Review of the Evidence on Low-Resistivity Layers in the Earth's Crust

  1. John G. Heacock
  1. H. Porath

Published Online: 15 MAR 2013

DOI: 10.1029/GM014p0127

The Structure and Physical Properties of the Earth's Crust

The Structure and Physical Properties of the Earth's Crust

How to Cite

Porath, H. (1971) A Review of the Evidence on Low-Resistivity Layers in the Earth's Crust, in The Structure and Physical Properties of the Earth's Crust (ed J. G. Heacock), American Geophysical Union, Washington D. C.. doi: 10.1029/GM014p0127

Author Information

  1. Geosciences Division, University of Texas at Dallas, Dallas, Texas 75230

Publication History

  1. Published Online: 15 MAR 2013
  2. Published Print: 1 JAN 1971

ISBN Information

Print ISBN: 9780875900148

Online ISBN: 9781118664049

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Keywords:

  • Apparent-resistivity curves;
  • Conductive and low-resistivity layers;
  • Deep Schlumberger sounding;
  • Direct-current electrical deep sounding studies;
  • Geomagnetic deep sounding;
  • Magnetic variation anomalies;
  • Magnetotelluric studies;
  • Northwestern United States and southwest Canada

Summary

It has been suggested in a number of publications that data from geomagnetic deep sounding, magnetotelluric, and electrical resistivity studies of several regions of the earth indicate the presence of conductive layers in the earth's crystalline crust. This review shows that, in all instances studied, an alternate solution can be presented or that the geological setting of a region in which measurements were made does not allow a unique interpretation of observations. Anomalous magnetic-variation fields observed in geomagnetic deep sounding studies in the western United States are shown to be related to lateral variations in the electrical conductivity of the upper mantle and do not require a conductive lower crust. In geological provinces with an extensive, conductive, sedimentary sequence, magnetic variation anomalies result from the concentration and channeling by sedimentary structures of large-scale current systems induced either in the oceans or in the continental crust. Apparentresistivity curves derived from magnetotelluric studies are affected by the anomalous currents in the vicinity of sedimentary conductive structures. It is shown that these lateral changes in the resistivity of the upper crust rather than an increase of conductivity at depth can cause the observed features in the apparent resistivity curves. This conclusion is supported by examples of apparent-resistivity curves in regions of known, intense, anomalous currents and by resistivity curves computed for stations near two-dimensional structures which under certain assumptions can model resistivity changes in the upper crust. Deep sounding, using some form of direct-current electrode method appears to be the approach best suited for detailed investigations of the electrical structure of the earth's crust, since the source field has a known configuration and is limited compared to the global dimensions of fields used in magnetotelluric studies.