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2 Acta Geophysica Polonica

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Studies on multiple electric discontinuities and dipping anisotropic conditions using multifrequency magnetotellurics

Nagubai D., Saraf P.

Acta Geophysica Polonica

2003

Vol. 51, nr 3

323-335

The Earth is characterized by many irregulars in its physical parameters. Recent deep EM and MT investigations have established that the upper crustal layers are inhomogeneous and anisotropic in their electrical properties. The problem of incorporating inhomogeneities (both in vertical and horizontal directions) is of considerable interest to geophysicists. Hence, an attempt has been made to compare the influence of dipping anisotropy in layers at different depth. Emphasis has also been given to explore the approximate frequency ranges needed to resolve the anisotropy in the subsurface layers at different depths. To achieve this, several numerical studies have been carried out in a three-layer Earth model for different meaningful combinations of thickness of the layers, conductivities, and frequencies of the EM wave parameters Our results show that the negligence of anisotropy nay produce serious errors in delineating subsurface structures. It is also shown that the electrical discontinuities played a major role in mapping anisotropic and dipping subsurface conditions.

Electromagnetic response for medium with elongated magnetically permeable conductors

Nagubai D., Mallick K., Saraf P.

Acta Geophysica Polonica

2002

Vol. 50, nr 3

423-432

Several ore deposits possessing different electrical and/or magnetic properties are found in nature in the form of elongated structures with alteration of mineralization zoning. Such situations can be approximated by a composite two-shell cylindrical model. In the present study, modifications in the response parameters due to changes in conductivity and magnetic permeability of the outer and inner shells have been illustrated for multifrequency EM measurements when the spacing between the shells is large and small. It is shown that usually the changes are reflected only when the spacing between the shells is large. However, in some cases the effect is seen when the spacing between the shells is small. Hence, the studies related to spacing between the cover and the target conductor not only affect the shielding characteristics of the overlying formations but also provide insight into the characteristic inductive interaction between the cover and the target conductors.