Measurements are made over two or three decades of frequencies to better determine the source of the IP response. This method has proven extremely valuable for mineral reconnaissance surveys. The early model Zonge GDP released in 1976 was the first commercially available, digitally-controlled, IP/CR receiver.

Relatively few conditions create an IP response. Metallic mineralization — particularly disseminated sulphides — cause increased IP values. Certain dissolved solids in groundwater increase IP response, and clay can increase IP response if the abundance of it is within specific ranges (dependent on clay type).

In structural features, such as faults, contacts generally appear as conductive plates or sheet-like features unless they have been well cemented with little porosity. For example, quartz or calcite-filled veins could be expected to have high resistivity values. In contrast, porous shear zones could be expected to have low resistivity values (relatively conductive). Likewise, alteration products can be extremely conductive (clay-based alteration) or extremely resistive (silicification zones).

The IP effect can be determined by passing an induced current into the ground and measuring the change in voltage with respect to time (time-domain IP/TDIP), or by measuring changes in phase at a given frequency with respect to a reference phase (frequency-domain IP/FDIP).

To produce an IP effect, fluid-filled pores must be present, since the rock is basically an insulator. The IP effect becomes evident when these pore spaces are in contact with metallic-luster minerals, graphite, clays or other alteration products. IP effects make the apparent resistivity of the host rock change with frequency. Generally, the rock resistivity decreases as the measurement frequency increases.

Zonge crew size is typically three to five people with a single pickup truck. No mechanized equipment is used. No earth moving or trenching is needed. Wires, electrode stakes, and survey flagging are removed and the electrode positions are raked when the survey is completed, leaving little or no evidence that the survey occurred.

The transmitter dipole consists of thin, 14-gauge insulated wire laid on the ground. At each end, the wire is connected to the ground with metal stakes, each about ½ inch in diameter, pounded into the ground one to two feet. These stakes are doused with saltwater to provide good electrical contact. The group of stakes pounded into the ground at each end of the wire is called an “electrode.”

Numerous electrodes are set up in a line and connected by wires to the transmitter at a central location. The wire is laid out by walking. Vehicle access along the length of the wire is not necessary. Transmitter equipment is typically kept in a truck with a generator mounted in the back or on a small trailer behind it.

Measurements are made with a man-portable receiver by connecting to different dipoles. The receiver dipoles are also wires laid on the ground, but in this case, they are grounded using small, ceramic “pots” about six inches tall and two inches in diameter, embedded slightly into the soil. The receiver equipment is carried along the survey line by the operator while making measurements at different locations.

The geometry between the transmitter and receiver electrodes varies with different types of electrode arrays. A frequently-used configuration is the dipole-dipole array in which current is introduced into the ground at two adjacent (current) electrodes and the resulting electric field is measured at two adjacent (potential) electrodes. The distance between the current electrodes is generally the same as the distance between the potential electrodes.

As the spacing between the two sets of electrodes is increased, the relative contribution of responses from rocks at depth is increased, providing information on the variation of resistivity and IP effects with depth. The depth of investigation for a given array configuration is a function of the relative resistivity contrasts and the electrode spacing.

Results can be presented in several forms:

• Modeled cross-sections (profiles) of contoured resistivity in ohm-meters and IP data in milliseconds, for each line of data collected
• Plan views at a constant elevation or depth, which aid to reveal trends between data collection lines
• Diagrams showing 2D cross-sections in a three-dimensional context

Resistivity and IP surveys measure “apparent resistivity” and “apparent polarization.” These are a function of the variation in intrinsic resistivity and polarization within a volume of earth, not the actual values at specific plot points. The distribution of intrinsic values for resistivity and IP effect is then interpreted from the measurements using theoretical curves and computer modeling.

Smooth-model inversion is the modern method for converting resistivity and IP measurements to smoothly-varying, model cross-sections. Inversion mathematically “back-calculates” (inverts) from the measured data a likely location, size and depth of the sources of IP and resistivity changes. Resistivity and IP values are iteratively modified until the calculated values match the observed data as closely as possible, subject to certain constraints.