For simplicity of description a direct-current field has been assumed; actually alternating current is more commonly used for reasons which will appear later. The methods just described, in which current is introduced directly into the ground are known as galvanic methods. In-stead, the ground may be energized by setting up induced currents in it.

What with the variety of methods that can be used to energize the ground and the variety of methods of measuring the effects, a large range of combinations is available to the geophysicist. Those most commonly used in mining may be classified as follows:

I. According to method of energizing the ground.

A.  Self-potential methods.

In this case no artificial power is applied; the feature that is measured is the natural E.M.F. generated by electrochemical reaction in the ground. A sulphide body, if chemical or mineralogical conditions at its opposite ends are dissimilar, reacts with the ground-water to form a gigantic electric battery which sets up electric currents of its own. The currents are usually detected at the surface by measuring the potential drop between pairs of copper electrodes each of which is placed in a porous pot containing saturated copper sulphate solution. The pots are placed in good contact with the ground 25 to 100 feet apart. Important anomalies between 100 and 500 millivolts may be distinguished from normal back-ground fluctuations up to 50 millivolts.

B.  Galvanic methods.

(1)    Direct current methods. Current is supplied by a battery, or more commonly a  direct-current  generator,  and introduced into  the  ground through a  pair of point  electrodes   (as  already  described)   or  through "line electrodes" consisting of a pair of bare wires laid out parallel to each other on opposite sides of the area to be surveyed, grounded at frequent intervals by electrodes sunk into the earth.    Direct current meth¬ods have the advantage of simplicity, but the electrolytic action resulting in polarization of  the  electrodes   introduces   complications.  One way of overcoming this is to use nonpolarizing electrodes (porous cups), as in self-potential measurements.

(2)       Alternating current -methods. Current is supplied by an A.C. generator and introduced into the ground through either "point" or "line" electrodes.
As compared with direct currents, alternating currents introduce certain mathematical complications, due to surface effects, inductance and electrostatic capacity. For this reason, however, if properly interpreted, they permit the observation of electrical properties which direct currents fail to reveal. They have practical advantages in that they can be easily amplified and. recorded 011 rugged meters or if a frequency within the audible range is used, they can be detected by earphones. The fre¬quency that is selected depends partly on the depth of penetration that is desired. High frequencies (over 10 kilocycles) penetrate only to short distances below the surface and for most mining purposes have not proved very satisfactory. Low and intermediate frequencies (5 to 100 cycles) are used in potential methods and particularly where deep penetration is desired.

C.     Induction methods. In contrast to galvanic methods in which the source of power is connected directly to the earth, induction methods employ an insulated circuit. The flow of current through an insulated cable sets up a magnetic field and this, in turn, induces currents in the earth in the same way that the primary circuit of an induction coil or transformer sets up a magnetic field and induces current in a secondary circuit. Either the magnetic field or the induced current may be measured.

The primary circuit is a loop of insulated cable which may be hori¬zontal—laid on the ground in a circular or rectangular shape—or verti¬cal. Theoretically, a vertical loop would be better for detecting steeply dipping orebodies, but such an arrangement is difficult to set up.

II.     According to method of detecting the electrical properties. The current (or lack of current) in the ground can be detected either by direct measurement or by induction.
For direct measurement, probing electrodes are used. Usually two probing electrodes are connected through some device for detecting or measuring the current. It may consist of a galvanometer, earphones (in the case of alternating current within the audible range), voltmeter, potentiometer, or milliammeter. When the ground is energized by in¬duction, the surveying instrument may be equipped with a compensator to cut out the direct effect of the primary circuit, leaving only the in¬duced current to be measured.

In the equipotential method of surveying, the probing electrodes are used to trace out a series of points. One probing electrode, inserted into the ground, may remain temporarily fixed while the other is moved from point to point until a place is found where no current flows between the two electrodes. This means that the two points are on the same equipotential line. Then a third point on the same line is sought, and so on, until the line has been traced out. Alternatively, readings may be taken along parallel picket lines and the equipotential contours drawn later.

By resistivity methods, the resistivity (reciprocal of the conductivity) of component portions of the area are determined by measuring the po¬tential difference and the current intensity between each pair of points. The resistivity is calculated according to Ohm's law, / = E/R (or the corresponding modification of it for alternating currents).
In potential-drop-ratio methods a traverse is run across the area and the drop in potential for each interval is compared with the drop in the next interval.

In electromagnetic methods, the current in the ground is not measured directly, but the magnetic field is surveyed by means of portable coils in which it sets vip induced currents. By rotating a detecting coil one can measure not merely the horizontal but also the vertical component of the magnetic field.