The depth of investigation in any direct current resistivity method of surface geophysical prospecting is defined, following Evjen (1938), as that depth at which a thin horizontal (parallel to ground surface) layer of ground contributes the maximum amount to the total measured signal at the ground surface. Using the equivalence between static and stationary fields, we have found the following values for the absolute depths of investigation in homogeneous ground.

Depth of detection of a target can be defined as that depth below which the target cannot be detected with a given electrode array assuming that the minimum detectable anomaly is 10%. Following this definition, physical modelling was carried out to determine depths of detection of conductive targets of limited lateral extent such as a vertical sheet, a horizontal cylinder and a sphere (infinitely conducting). It is seen that the two‐electrode array has the greatest depth of detection followed by the three‐electrode array, while a Wenner array has the smallest depth of detection, when the array spread is in‐line (i.e. perpendicular to the strike direction). On the other hand, the depth of detection with a Wenner array improves considerably and is almost equal to that of the two‐electrode array when the array spread is broadside (i.e. along the strike direction). With an increase in the depth extent of the vertical sheet from 10 to 20 times its thickness, there is an increase in the depth of detection with all arrays except for the three‐electrode array when the array spread is in‐line, and with the Wenner array when the array spread is broadside.

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Assuming the minimum detectable anomaly to be 10%, depths of detection of a 2D vertical resistive sheet of thickness t are found to be 4.0t, 3.0t, 4.0t and 4.0t with Wenner, two‐electrode, three‐electrode and dipole‐dipole (β‐Wenner) arrays, respectively, when the array spread is in‐line. On the other hand, the depths of detection obtained with a broadside spread of the arrays right over the sheet are much less and are correspondingly 2.5t, 2.0t, 2.5t and 2.5t. An increase in the depth extent W of the sheet from 10t to 20t does not increase its depth of detection with the arrays.  The depths of detection of an infinitely resistive horizontal cylinder of radius R are respectively 1.5R, 1.8R, 2.0R and 2.0R with the above‐listed arrays when the array spread is in‐line. With broadside spread of any of the arrays, the depth of detection is seen to be 2.5R.  In the case of a spherical target of radius R, the detection depths of any of the arrays are found to be small and to vary between 0.8R and 1.1R.  Comparatively, the detection depths of resistive targets are much lower than those of conductive targets of the same size and shape, with any electrode array. Among all the arrays studied, the two‐electrode array performs worst in the detection of resistive targets while it performs best in detecting conducting targets of limited lateral extent. In the case of a spherical target, either resistive or conductive, there is no distinct change in its detection depth with array.

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