The depth of investigation of gradient arrays using point, finite, and infinite line sources over homogeneous isotropic ground has been studied. The depth of investigation depends for all cases on the radial distance of the plotting point (i.e., the midpoint of the line joining the potential probes) and separation between the current electrodes. For point electrode systems, it also depends upon azimuth of the plotting point, whereas for finite line electrode systems, it depends upon the azimuth and the length of the line electrode.The depth of investigation at the center of the array is maximum (0.145 L) for infinite line electrodes, decreases to 0.135 L for finite line electrodes of length L, and becomes least (0.125 L) for the point source, where L is the separation between the current electrodes.The depth of investigation for finite line electrodes of unit length L is practically independent of the y-coordinate which is mathematically the case for infinite line electrode systems. The depth of investigation is least toward the current electrode and increases as one moves away from the current electrode; it ultimately reaches the maximum value of 0.135 L at the center of the system. The depth of investigation for point electrode systems is also minimum near the current electrode and increases with the increase of azimuth for any radial distance.