Appendix D

Appendix D                  Induced Polarization

 

General Description of Induced Polarization

 

Compared to basic resistivity measurements, IP is a relatively new method, first used in the late 1940s in exploring for disseminated sulfide ores.  IP has also been used in water and clay exploration.  Certain earth materials do not act as perfect ohmic conductors.  A perfect ohmic conductor always has a current flowing exactly "in phase" with the applied voltage.  A perfect ohmic conductor shows no "memory effect" and there will be no current flow when there is no voltage applied (referred to as zero in, zero out).   However, certain earth materials have an IP response that may exhibit all of these phenomena; current may flow in the IP material with no voltage applied, the current due to an applied voltage will not be exactly "in phase" with the applied voltage and the IP materials will exhibit a "memory effect" in that the voltage at any point on the earth will depend of the history of the applied voltages. The IP effect is a useful and interesting earth characteristic.  The MiniRes resistivity meter has an IP function that allows for precise and efficient measurement of the IP response. 

 

There are four main instrument architectures used for measuring the IP response of the earth. 

 

1) The first and the oldest architecture is sometimes called the "overvoltage" method.  In this method, voltage is measured by a receiver dipole.  The ratio of the voltages during the transmitter "ON" to the "OFF" interval is used to estimate the IP effect.  This is a "time domain" technique in that voltage measurements are taken at precise times.  A drawback of this technique is that large and heavy current generators are needed to overcome the noise associated with electric dipole measurements at low frequencies (< 10 Hertz).

 

2) The second method is referred to as the "variable-frequency method".  This is a "frequency-domain" method and requires the measurement of apparent resistivity at two (or more) widely separated frequencies. A perfect ohmic conductor will show a constant apparent resistivity, independent of the frequency, assuming that skin effects are minimal.  IP materials, however, will exhibit a small decrease in resistivity with increasing frequency.  This technique has the advantage of being able to work at higher frequencies than the "overvoltage" method and, therefore, may not require large and heavy transmitters.  The disadvantage is that data must be taken at two widely separated frequencies. This requirement demands a much more complicated instrument because the instrument must remain accurate and stable at two widely separated frequencies.

 

3) The third IP method is referred to the "IP phase method".  Electrical engineers, many years ago, realized that, instead of measuring the frequency response difference at two widely separated frequencies, as in the "variable-frequency method", a simpler single measurement of the PHASE at one frequency may provide equivalent information.  This method overcomes all of the disadvantages of the two earlier techniques.  The PHASE can be measured at a relatively high frequency (5 or 30 Hertz in the case of the MiniRes) and, therefore, large and powerful transmitters are not needed as in the "overvoltage method".  Also, only the phase response at a single frequency is required.  This provides a smaller, lighter and simpler instrument than the "variable-frequency method" or the "overvoltage method" allow.

 

4) The fourth method is the “Spectral IP method”. Phase and magnitude are measured over a range of frequencies from 10-3 to 4 x 103 Hertz.

 

The MiniRes utilizes the "IP phase method".  The "IP phase method" is incorporated with the synchronous demodulation architecture of the MiniRes to allow low noise estimates of the IP phase.  This is done by making separate measurements of the "real" or "in-phase" component of resistivity and also the "imaginary" or "quadrature" component of resistivity.



Induced  Polarization  Comparison

 

 

PHASE DOMAIN (PD)  vs. conventional TIME DOMAIN (TD) IP systems

                                                                                                                                                                                                                                                                                                          

1. Faster! Less Hassle! TD systems require porous-pots (non-polarizable electrodes) for best performance.  This type of electrode has many environmental concerns. The PD MiniRes prefers standard stainless steel electrodes (old rusted rebar may be used as well). This single difference makes the MiniRes much easier to deploy and more cost effective.

2. Low Cost! The PD MiniRes is a fraction of the purchase or rental cost of the TD IP systems.

3. Safe! The PD MiniRes provides high signal-to-noise ratio results with less power, battery weight and electrocution hazard. This survey was acquired with a transmitter current of 10 milliAmps RMS, which is, generally, insufficient to maim or kill animals or humans. TD IP systems, on the other hand, produce hundreds to thousands of milliAmps. These current levels are potentially lethal for both humans and animals.  No potential chemical exposure from porous-pot style electrodes either.

4. Easy to use! The MiniRes has no software or confusing menus to choose - just two buttons to operate - the Resistivity (in phase) and the IP (quadrature) mode buttons.

5. Rugged and accurate with higher resolution! The lighter weight components associated with the 5 Hertz PD architecture allow for a more rugged mechanical design. Accuracy is unsurpassed at better than 0.1 plus or minus 3 LSDs.  IP Phase resolution is 300 microDegrees or 5.2 microRadians.

6. Realistic field deployment! PD IP systems inherently produce a unitless measure of phase angle which is virtually unaffected by errors in array geometry. Thus, a transmitter or receiver electrode may be displaced from its proper position without adversely affecting the PD measurement value. TD systems exhibit a sensitivity to electrode-position-error which is approximately proportional to the error. This allows easier field deployment of a PD system. The electrodes may be offset to avoid sidewalks, roads, rocks, etc. without impacting the resultant measurement.  TD IP is much more adversely affected by near-surface inhomogeneities than the PD MiniRes.

7. Better at mapping solvent contamination? The 5 Hertz PD MiniRes may be more sensitive to clay-organic solvent polymerization IP effects than the lower-frequency TD systems. {Reference: Mapping Organic Contamination by Detection of Clay-Organic Processes - Gary R. Olhoeft)

8. Better at sub-surface imaging! The PD method is not nearly as susceptible to the adverse effects of near-surface inhomogeneities, which can often render the TD IP method useless.                                                                

Olhoeft, G. R., (1992) Geophysical detection of hydrocarbon and organic chemical contamination: Symposium on the Application of Geophysics to Engineering and Environmental Problems, Environmental and Engineering Geophysical Society,  Proceedings, pp. 587-595

Reynolds, John M., (1997) An Introduction to Applied and Environmental Geophysics, Chapter 9, pp. 522-552, John Wiley & Sons

Sumner, J. S., (1976) Principle of Induced Polarization for Geophysical Exploration; Elsevier

Ward, S. H., (1990) Resistivity and Induced Polarization Methods in Geotechnical and Environmental Geophysics, Vol. 1, pp. 147-190; Society of Exploration Geophysics

Zonge, Ken, Jeff Wynn and Scott Urquhart, (2005) Resistivity, Induced Polarization, and Complex Resistivity, Chapter 9, Near-Surface Geophysics, pp. 265-300, Society of Exploration Geophysics  

 


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