55. Make up word combinations from the given words.

survey equipotential lines or potential profiles

produce inductive methods and refraction methods

supply formation boundaries

apply alternating current

induce high radio frequency

radiate potential difference

furnish frequency bands

observe point electrodes

compare emission characteristics

detect potential drop

56. Read the text, do the exercises. Electrical Methods

Mineral deposits and geologic structures may be mapped by their reaction to electrical and electromagnetic fields. These are produced by either direct or alternating current, except where ore bodies spontaneously furnish their own electrical field (self-potential methods). Electrical energy may be supplied to the ground by contact or by induction. Three main groups of electrical methods may be distinguished: (1) self-potential, (2) surface-potential, and (3) electromagnetic methods. Frequently the first two groups are combined into one group of potential methods; the electromagnetic methods are usually subdivided into galvanic-electromagnetic and inductive-electromagnetic.

Four frequency bands may be used in connection with alternating current electrical prospecting: (1) low frequencies of from 5 to about 100 cycles; (2) the audio-frequency range of from 200 to 1000 cycles; (3) high frequencies of from 10 to 80 kilocycles; and (4) radio frequency of from 100 kilocycles to several megacycles. The low frequency range is applied in most potential methods; the audio-frequency range is used in some potential and most electromagnetic methods; the high-frequency range in the high-frequency electromagnetic methods; and radio-frequency in the radio methods of electromagnetic prospecting. The application of high radio frequencies is limited owing to their lack of depth penetration; of greatest importance are the audio frequencies and the low frequencies. In a number of respects, electrical methods are similar to seismic methods; comparable to refraction methods are resistivity and the potential-drop-ratio methods; inductive methods as applied to the mapping of horizontal beds are comparable to reflection methods but lack their resolving power.

Self-potential method. The self-potential method is the only electrical method in which a natural field is observed; its causes are spontaneous electrochemical phenomena. These phenomena occur on ore bodies and on metallic minerals and placers; they are produced by corrosion of pipe lines and on formation boundaries in wells by differences in the conductivity of drilling fluid and formation waters. Ore bodies whose ends are composed of materials of different solution pressure and are in contact with solutions of different ion concentration, act as wet cells and produce an electrical field which can be detected by surveying equipotential lines or potential profiles. For the mapping of equipotential lines, a high-resistance milliammeter is connected to two unpolarizable electrodes are used. One is kept stationary and the other is moved until the current vanishes. At that point the electrodes are on an equipotential line.

Equipotential-line and potential-profile methods.

Equipotential lines of the current

When a source of electrical energy is grounded at two points, an electrical field is produced. Distortions of this field result from the presence of bodies of different conductivity; good conductors will attract the lines of flux, and vice versa. As it is difficult to survey these lines of flux, lines of equal potential, that is, lines along which no current flows, are mapped instead. In practice power is supplied to two grounded electrodes from an alternating current generator.

Resistivity methods.

Equipotential-line methods, while useful for the mapping of vertical or steeply dipping geologic bodies, are not suited to the investigation of horizontally stratified ground. Conversely, resistivity methods are applicable to depth determinations of horizontal strata and the mapping of dipping formations.

In resistivity procedures not only the potential difference between two points but also the current in the primary circuit is observed. The ratio of potential difference and current, multiplied by a factor depending on electrode spacing, gives the resistivity of the ground.

Potential-drop-ratio methods. The essential feature of the resistivity methods is a determination of the potential difference between two points at the surface and a measurement of the current in the external circuit. In potential-drop-ratio methods current measurements in the external circuit are not made and the potential drops in two successive ground intervals (represented by three stakes arranged in a straight line, radiating from one of the power electrodes) are compared. The potential-drop-ratio method is best suited for the location of vertical formation boundaries (faults, dikes, veins, and the like).

Electromagnetic-galvanic methods. Electromagnetic methods of electrical prospecting differ from potential methods in that the electromagnetic field of ground currents and not their surface potential (electric field) is measured. They fall into two major groups: (1) electromagnetic-“galvanic” methods in which the primary energy is supplied by contact as in the potential methods; (2) electromagnetic-“inductive” methods in which the ground is energized by inductive coupling (with insulated loops). To supply electrical energy to the ground by contact, line electrodes are laid out at right angles to the strike, point electrodes parallel with the strike.

Electromagnetic-inductive methods. In inductive procedures power is supplied to the ground by insulated loops which will cause induction currents to flow in subsurface conductive bodies. An advantage of inductive methods is the ease with which power may be transferred into the ground when the surface formations are poor conductors. Since currents induced in the subsurface conductors are dependent on frequency, interpretative advantages may be gained by regulating the frequency.

Radio methods. Since radio methods employ frequencies still higher than the high-frequency-inductive methods, they are subject to the same limitations. In one group of radio methods the effect of subsurface conductors on the emission characteristics of a transmitter is observed. In a second group a receiving arrangement is employed in addition to the transmitter, and the variation of field intensity with location is measured. In the category of radio methods belong the so-called “treasure-finders.” These are portable instruments for the location of shallow metallic objects, pipe lines, and the like.

(C.A. Heiland. Geophysical Exploration. New York, 1940)