Abstract: An apparatus includes a logging tool having electrodes for introducing a current into a downhole formation and for detecting a voltage produced in the formation by the current. The current is provided by a transmitter circuit, and the voltage is received and processed by a receiver circuit including a computer programmed to provide induced polarization and resistivity measurements based on the detected voltage. The electrodes are equidistantly spaced but are adjustable (either on a single logging tool or as between different logging tools) so that various logging runs can be made to obtain different measurements whereby the intrinsic complex resistivity (induced polarization) of formation layers can be suitably resolved. The apparatus also includes a contact resistance measurement circuit for making measurements used by another computer means to determine the contact resistance of each of the electrodes.