Anisotropy of Magnetic Susceptibility in Variable Low-Fields: A Review

Chapter
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Part of the IAGA Special Sopron Book Series book series (IAGA, volume 1)

Abstract

Theory of the Anisotropy of Magnetic Susceptibility (AMS) assumes field-independent rock susceptibility in the low fields used by common AMS meters. This is valid for rocks whose AMS is carried by diamagnetic and paramagnetic minerals and also by pure magnetite, while rocks with pyrrhotite, hematite or titanomagnetite may show significant variation of susceptibility in common measuring fields. Consequently, the use of the contemporary AMS theory is in principle incorrect in these cases. Fortunately, it has been shown by practical measurements and mathematical modelling of the measuring process that the variations of the principal directions and of the AMS ellipsoid shape with field are very weak, which is important in most geological applications. The degree of AMS, however, may show conspicuous variation with field and, if one wants to make precise quantitative fabric interpretation, it is desirable to work with the AMS of the field-independent component. Three methods exist for simultaneous determination of the field-independent and field-dependent AMS components, all based on standard AMS measurement in variable fields within the Rayleigh Law range. The field-dependence of the AMS can be used in solving some geological problems. For example, in volcanic and dyke rocks with inverse magnetic fabric, one can decide whether this inversion has geological (special flow regime of lava) or physical (SD vs. MD grains) causes. In rocks consisting of two magnetic fractions, one with field-independent susceptibility (magnetite, paramagnetic minerals) and the other possessing the field-dependent susceptibility (titanomagnetite, hematite, pyrrhotite), one can separate the AMS of the latter fraction and in favourable cases also of the former fraction.

Keywords

Susceptibility Tensor Orientation Tensor Magnetic Lineation Magnetic Foliation Initial Susceptibility 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.
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Notes

Acknowledgements

Dr. Martin Chadima is thanked for providing the field-dependent AMS of the dike rock from the locality of CS27. The research was partly supported financially by the Ministry of Education and Youth of the Czech Republic (Scientific Program MSM0021620855).

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Copyright information

© Springer Science+Business Media B.V. 2011

Authors and Affiliations

  1. 1.AGICO Inc.BrnoCzech Republic
  2. 2.Institute of Petrology and Structural Geology, Charles UniversityPrahaCzech Republic

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