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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.
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