Bhattacharya and Sen (1981) are the first to study the depths of investigation of various collinear electrode arrays for a homogeneous anisotropic half-space. In their study they substituted the aniosotropic medium by an appropriate homogeneous medium of resistivity ρm. However, they committed a serious error at assuming their basic equation-13 as the “expression of potential at a point (X, y, Z) due to a point source of current strength I placed over a semi-infinite homogeneous isotropic medium of resistivity ρm”. The error is that X and Z in equation (13) are not independent coordinates. As a consequence, all their expressions for NDICs for different electrode arrays become incorrect. Consequently, their results and conclusions also become invalid. The mistake was detected only recently by the senior author of this paper (A. Apparao) in the course of his writing a book on 'Developments in geoelectrical methods'. Since the subject of anisotropy is very interesting, there arises an urgent need to derive the correct expressions of the depth investigation characteristics for different electrode arrays. We present in this paper the expressions for normalized depth investigation characteristics (NDIC) for homogeneous and anisotropic half spaces for different electrode systems, including dipolar systems.

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