Induced Polarization Methods

Types of IP Measurements

There are three basic methods of measuring induced polarization:

· The overvoltage method (Time Domain IP)

· The frequency domain method (Frequency Domain IP)

· The phase shift method (Phase Domain IP)

These methods are described in Appendix D. The MiniRes uses the phase shift method. This method requires the minimum of power and, by thoughtful design, the minimum of instrumentation. It is the newest of the three methods.

With the phase shift method, an alternating current is injected into the ground through the current electrodes. If there is no induced polarization, the signal received at the potential electrodes is in perfect phase with the injected signal. However, the more induced polarization in the earth, the more the received signal is out of phase with the injected signal.

The early MiniRes (serial numbers 1 through 32) inject a 30 Hertz current. More recent MiniRes (serial numbers above 100) inject a 5 Hertz current. As an option, the Ultra MiniRes is available with 2.5 Hertz injected signal.

IP Data Acquisition

Much more attention to detail is required for acquiring high accuracy IP data than is required for simple resistivity data. It is recommended that high quality cables be used. Leaky cables on moist ground will ruin the data. It is also recommended that spools of cable NOT be used. Cable segments should go straight from the MiniRes to the associated electrode. A spool of cable will have a tendency to introduce an unpredictable and varying phase response. If the "cable spread" is moved along a line then every effort should be made to keep the cable layout as identical as possible at each setup.

Whereas, the normal resistivity measurement is relatively immune to high electrode contact resistances, the IP method requires a good low-resistiviity electrode contact. Where the ground is very dry, it may help to wet down the area around each receiver electrode. As long as the "LINE OPEN" LED on the transmitter does not come on, it is not necessary to wet the transmitter electrodes.

The "real" component is simply the meter reading that is normally measured by pushing the red "POWER" pushbutton. This value should be written down in a field notebook.

The "imaginary" or "quadrature" component is measured by pushing and holding the "IP" blue pushbutton switch. This value should also be written down in the field notebook. The scale for the "imaginary" component is the same as the scale for the real component.

Calculating the IP Phase Value

The "PHASE" is simply obtained by taking the arctangent of the ratio of the imaginary to real components of the resistivity.

IP PHASE = arctan ( imaginary / real )

For example, if the resistivity reading was 19.00 and the IP reading was 0.100 then:

IP PHASE = arctan (0.100/19.00) = arctan (.00526) = 5.26 milliradians

For small values of the arctan (x) the value in radians is approximately the same, (x).

Some people report the phase in degrees instead of milliradians. To calculate the phase in degrees:

IP PHASE (degrees) = 57.3 * IP / R = 57.3 * 0.100 /19.00 = 0.302 degrees

A simple calculator may be used to compute the phase in units of milliradians or degrees from the real and imaginary components stored in the field notebook. Typically, the phase response will be small, even for a strong IP area. A large copper porphyry deposit has been described having a maximum IP phase response of about 6 degrees (0.105 radian or 105 milliradians). Most non-sulfide, non-clay regions will exhibit an IP phase much smaller than 6 degrees.

Note that no geometry factors are associated with the phase formula. It makes no difference if the measurement array setup is Wenner or Schlumberger. The IP phase is always calculated the same way.

Plotting the Data

As mentioned above, instrument layout and ground conditions may exhibit a phase response that combines with the native IP response of the ground being measured. Therefore, it may be helpful to plot the difference in phase about the average of all the phases on a profile. A strong increase in phase should indicate an associated strong increase in IP effect.

It is the phase that should be plotted; NOT the IP. See the following illustration. Little sense can be made of the resistivity or IP reading, but the PHASE clearly shows a strong anomaly.

References

At the end of Appendix D are some general references on the IP method.

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