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