The Induced Polarization (IP) tool consists of an assembly of electrodes, usually including a current electrode and two potential (measurement) electrodes. A square wave current with an 'off' time between positive and negative parts of the waveform is transmitted (waveforms may be from 1 second to 8 seconds duration). Potential measurements made at selected times in the waveform can be related to the IP effect (chargeability of the rocks), the resistivity (R) of the rocks, and to self-potentials (SP) generated in the rocks. The transmitter is a constant current source located at the surface. A detailed explanation of the IP probe will be given below.

In time domain IP measurements, the ratio of the secondary voltage measured during the current off-time to the primary voltage measured during the current on-time is related to the electrical polarizability of the rock and is called chargeability. A high chargeability response is an indication of the presence of metallic sulphides and oxides or cation-rich clays such as illite and montmorillonite (Mwenifumbo, 1989). One of the major alteration processes within a number of base metal and gold mining camps is pyritization and this is a target for most IP logging in gold exploration.

The electrical resistivity of rocks depends on several factors including the presence of conductive minerals such as base metal sulphides or oxides and graphite in the rock. Most rocks without these minerals are usually poor conductors and their resistivities are governed primarily by their porosity, degree of fracturing, salinity of the pore water, the degree of saturation of the pore spaces, and to a lesser extent by the intrinsic minerals that constitute the rock. Some alteration processes such as silicification and carbonatization tend to reduce the porosity and hence increase the resistivity of the rock. In igneous and metamorphic rocks, the resistivity log is useful mainly in mapping conductive minerals and fracture zones. In sedimentary rocks, the resistivity log is frequently used in lithologic mapping because changes in lithology are often associated with changes in porosity.

SP anomalies are mainly an indication of the presence of graphite and/or high concentrations of base metal sulphides including pyrite. Large self potentials observed within and around sulphide and graphite bodies are mainly caused by electrochemical processes (Sato and Mooney, 1960, Hovdan and Bolviken, 1984). Low resistivity anomalies correlating with SP and IP anomalies are, therefore, good indications of the presence of conductive minerals. SP anomalies can be also generated by fluid flow in porous media (electrokinetic or streaming potentials - Bogoslovsky and Ogil'vy, 1970, 1972) and heat flow (thermal electric coupling - Corwin and Hoover, 1979).

The transmitter on surface is a constant current source capable of supplying up to 250 mA. There are 4 selectable pulse times for the current waveforms: 0.25s, 0.5s, 1s and 2s (i.e. full waveforms of 1 second to 8 seconds duration). The long pulse times would be used when logging at very low speeds in order to avoid errors that could be introduced by smearing measurements over large depth intervals. The volume of rock sampled is related to the electrode spacings. The full waveform is recorded (digitized at 4ms intervals) on magnetic media. Logging speed ranges from 1 to 6 m/minute according to the chosen pulse length (waveform duration). The sample interval is dependant on the logging speed and waveform period. Typically, a 1 second period with a logging speed of 6 m/minute results in sampling every 10 cm along the borehole. This tool must be run in uncased, water-filled holes.

The standard IP parameter is the chargeability determined during the middle of the 'off' time of the decaying waveform. The apparent chargeabilities can be measured with 3 types of electrode arrays: 40-cm normal array, lateral array (pole-dipole array) and the 10-cm Dakhnov micronormal array. The downhole current and potential electrodes are gold-plated brass cylinders, 40 mm in diameter (Mwenifumbo, 1990).

The resistivity measurements are derived from the waveforms received during the constant current 'on' time of the square waveform, after the initial IP charging effects are over. Resistivity measurements are made with the same arrays as are used in the IP measurements. Single point resistance measurements can also be made using a single downhole current/potential electrode (Pb) and a return/reference electrode on the surface.

The self potential is determined during the late 'off' time of the IP decay waveform. SP measurements are carried out either in the gradient mode with the same arrays as are used in the IP measurements, or in the potential mode with a single Pb or Cu/CuSo4 electrode downhole and a reference electrode on the surface. SP can be measured simultaneously with the IP/Resistivity measurements or in a separate logging run with current off. The latter is the preferred approach.