Compatibility of induction methods for mantle soundings

摘要

Formulations that form the basis of experimental impedances in induction soundingsresult from the impedance boundary conditions or from the simplified theoretical models.The formulations are essentially different for the magnetotelluric and magnetovariationsounding methods. In order to increase reliability of mantle investigations, studies of themantle's electrical properties are often carried out by the joint inversion of impedancesobtained by both sounding methods. A forward modeling approach is used to verify theaccuracy of merging the long-period impedances obtained by the magnetotelluric andmagnetovariation methods. The spherical modeling of the responses above 2-D and 3-Dmantle inhomogeneities has shown that the different induction methods can give mutuallyinconsistent results and the combination of their responses can be problematic in practice.For this reason much attention is given to the generalized horizontal spatial gradientsounding method which results in impedance functions that in space and frequencydomains closely resemble the magnetotelluric impedances. In this study some interestingproperties of the induction arrows above a spherical inhomogeneity, excited by aninhomogeneous external field, are estimated for long periods. A final comprehensivemodel, assuming a shell of realistic conductance at the Earth's surface, is evidence that thegeneralized horizontal spatial gradient method is promising for the study of mantleinhomogeneities and can be reliably used in combination with the magnetotelluric methodin a specific way.