Introduction

Induced polarization (IP) has been used since the mid 1950s in mineral exploration. It is a rather complex phenomenon, but easy to measure. When a voltage applied between two electrodes is abruptly interrupted, the electrodes used to monitor the voltage do not register an instantaneous drop to zero, but rather records a fast initial decay followed by a slower decay. If the current is switched on again, the voltage will first increase at a very high rate and then build up slowly. This phenomenon is known as induced polarization. The technique is mostly concerned with measuring the electrical surface polarization of metallic minerals.

Disseminated sulphides have very good induced polarization responses. In theory, massive sulphides should have lower responses, but in practice they have very good responses. This is due to the mineralization halo generally surrounding massive sulphides. Clay minerals may also produce significant IP responses, and the IP technique is often used to distinguish between clay and for example water saturated media which have similar resistivities, but different chargeabilities.

The data is acquired in a similar manner to resistivity, and resistivity is in fact measured by default during an IP survey. The same electrode configurations used for resistivity are also used for IP investigations. Various electrode layouts can be used (pole-dipole, dipole-dipole, etc.); varying the distance between the electrodes results in soundings to different depths, which may be used to map the variability of resistivity and chargeability with depth.

Operation

Electrode arrays are placed usually in a straight line over an area to be surveyed. Two current electrodes are used to supply current which flows into the surrounding subsurface. Two additional non-polarizable electrodes are placed at a specified spacing from the current electrodes. The spacing can remain fixed and the array progresses along a given line and additional parallel lines surveyed or the spacing between electrode pairs may be incremented in order to obtain measurements having increasing depth. The information gathered is compiled into apparent IP and Resistivity psuedosections and can be inverted using inversion software to obtain actual IP and Resistivity responses of the earth.

3D Model of zone of high chargeability (IP) to map disseminated sulphide mineralization.

Topographical challenges of geophysical surveys