Crustal Investigations by the Magnetotelluric Tensor Impedance Method

  1. John G. Heacock
  1. D. R. Word,
  2. H. W. Smith and
  3. F. X. Bostick Jr.

Published Online: 15 MAR 2013

DOI: 10.1029/GM014p0145

The Structure and Physical Properties of the Earth's Crust

The Structure and Physical Properties of the Earth's Crust

How to Cite

Word, D. R., Smith, H. W. and Bostick, F. X. (1971) Crustal Investigations by the Magnetotelluric Tensor Impedance Method, in The Structure and Physical Properties of the Earth's Crust (ed J. G. Heacock), American Geophysical Union, Washington D. C.. doi: 10.1029/GM014p0145

Author Information

  1. Electrical Engineering Department, the University of Texas at Austin, Austin, Texas 78712

Publication History

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

ISBN Information

Print ISBN: 9780875900148

Online ISBN: 9781118664049



  • Apparent resistivity;
  • Central Texas;
  • Llano uplift;
  • Magnetotelluric (MT) sounding method;
  • Ouachita system;
  • Paleozoic and Precambrian surface;
  • Rotation loci


A magnetotelluric (MT) sounding method that involves a tensor impedance relationship between the surface electric and magnetic micropulsation fields of the earth has been developed. The role of the tensor impedance, including the vertical magnetic field relationship, is discussed. MT soundings made at 7 sites along a sixty-mile traverse east of Austin, Texas, in the frequency band 10−4 to 10 Hz are analyze. The traverse bears northwest-southeast and crosses the Ouachita fold belt system on the fhinks of the Llano uplift. The sounding covers the approximate depth range of 0.1 to 100 km. An earth model is derived from the sounding and includes a vertical cross section of the estimated conductivity distribution and estimates of the strike directions of the various formations represented by the cross section. The model is compared with available geological information, and an attempt is made to explain some of the observed three-dimensional properties of the impedance function. The model is in good agreement with expected geology to depths for which surface and borehole data can be reasonably extrapolated. The sounding responds to materials ranging from Cretaceous sediments through the upper mantle and provides reasonable definition of a high-resistivity region which appears to be the Precambrian granite basement. Along the traverse, the top of this basement occurs at depths ranging from 3 to 10 km. In the upper mantle, a resistivity decrease to about 1 Ω-m or less is found to occur at depths of about 60 to 100 km. An apparent northwesterly rise in the conductive substrate is found. This and other evidence seems to suggest an uplift in the conductive mantle in association with the Llano uplift.