# Part D: Induced Polarization, a Review

## Abstract

Investigations into the causes of induced polarization have shown that the largest effects are due to the presence of electronic conducting minerals in the pore system of the rocks. Theoretical studies into the charge transfer reactions involved when current is sent through such mineralized rocks have led to some simple rules concerning the time or frequency structure of the induced polarization effects. There are a few indications that at very low frequencies or at long time scales, deviations from these rules occur that may provide further parameters for evaluating induced-polarization effects.

The greatest difficulties in making field measurements arise from electromagnetic coupling and telluric noise problems. The electromagnetic coupling produces effects that are superimposed on the induced polarization effectsthis sets an upper limit on the frequencies that can be used for the measurement. In most applications one should use frequencies that are less than 10 cps. The telluric noise is more of a problem at the low end of the spectrumit sets a limit ori the periods that can be used. In typical situations, one must work with periods of less than 60 sec. Filtering techniques are very useful in cutting down the telluric noise effect.

The problems of interpretation are examined briefly and some simple cases are discussed.

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*georef*

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### Figures & Tables

### Contents

# Mining Geophysics Volume II, Theory

The relative merits of any geophysical method in a given situation can be predicted by careful study of the expected message-to-noise1 ratio. For example, let us draw or deduce from the subsequent text, the anomaly formulas due to a spherical inhomogeneity in the subsurface and the symbols in each formula are explained in the text. The gravity, magnetic, resistivity, and induced-polarization surveys all are volume dependent, whereas the electromagnetic method is dependent only upon the area of the inhomogeneity, normal to the inducing field. Thus, a thin disk can give nearly the same electromagnetic anomaly as a sphere of the same radius.

If we can make a reasonable estimate of the physical property contrast anticipated to exist between ore and host, we can then predict the anomaly magnitude expected from the sphere, when buried at any given depth, via the geometric factor. Note that from this viewpoint, given the maximum or saturation value of unity for the physical property factor, the magnetic and resistivity methods theoretically give the same percent anomaly due to a sphere. The physical property function *M–iN* for the electromagnetic method has a maximum value of one half for a sphere while the change with frequency of the electrical resistivity contrast.

Thus, except for a factor of two, the magnetic, resistivity, electromagnetic, and induced-polarization methods should give the same maximum anomaly. Note that the geometry of the anomalous fields for each of these methods is an induced dipole with a resultant fall-off of peak anomaly proportional to the inverse cube of the depth to the center of the sphere below the measuring plane. In contrast, the gravity method exhibits an inverse second power fall-off due to an induced monopole. The density contrast between ore and host sometimes exhibits a maximum value of two. Thus f r om a maximum message viewpoint, one would be inclined to rate the methods in the order given above. However, we need to counter this bias by considering expected values of the physical property factor and the noise for any given geologic situation.

Let us look, then, at iron ore, massive sulfides, and disseminated sulfides, items treated i n Volume I. We should expect the following physical property ranges: A very wide range of properties is evident and hence the prediction of an anomaly magnitude looks hopeless.