a group of methods that are used in geophysical exploration and are based on the study of natural or artificially generated electric or electromagnetic fields in the earth’s crust. The physical basis of the electrical methods is the differences between rocks and ores in such properties as resistivity, dielectric constant, and magnetic susceptibility.
Electrical methods were first used to search for useful minerals in the late 19th century by C. Barus (USA) and E. I. Ragozin (Russia). In 1912, C. Schlumberger (France) developed and used methods based on the study of stationary electric fields. Between 1919 and 1922, K. Lundberg and C. Sundberg (Sweden) laid the foundations for electrical methods that are used to study alternating electromagnetic fields. In the USSR, electrical methods were first used in 1924 by A. A. Petrovskii to study natural electric fields generated as a result of electrochemical processes that occur at the interface between ores and surrounding rocks.
According to the nature of the electric or electromagnetic fields being studied, electrical methods are divided into several groups.
Resistivity methods are based on the study of stationary electric fields that are produced in the earth’s crust by a pair of embedded electrodes connected to the terminals of a DC source. The electric field is studied by means of a measuring circuit consisting of a second pair of embedded electrodes and an instrument for measuring the potential difference between the second pair of electrodes. The results of the measurements are expressed in terms of apparent resistivity, which gives an idea of the geological structure of the area being studied.
Self-potential methods are used to study electric fields that are generated around ore deposits, mineral zones, and other geologic objects as a consequence of the spontaneous polarization of the objects. Such polarization may be caused by natural electrochemical processes in which an ore body participates—for example, oxidation or reduction—or by electrochemical processes that are brought about artificially by means of a transmitted current. From the distribution of the electric potentials, the presence and location of polarized objects are determined. The main area of application of self-potential methods is prospecting for ore deposits.
Magnetotelluric methods are used to study the alternating component of the earth’s natural electromagnetic field. Owing to the skin effect, the depth to which the natural electromagnetic field penetrates the earth depends on the frequency of the field. Therefore, the behavior of low field frequencies (hundredths or thousandths of a hertz) reflects the structure of the earth’s crust at depths of several kilometers, while the behavior of high field frequencies (tens or hundreds of hertz) reflects the structure of the crust at depths of several tens of meters. The analysis of the frequency dependence of the electric and magnetic field components makes it possible to study the geological structure of the territory being explored.
Electromagnetic sounding methods make it possible to study a geologic section in the vertical direction. In geometric sounding, measurements are made at the same point of a profile by varying the separation between electrodes. In parametric sounding, the frequencies of the electromagnetic field are varied. Electromagnetic sounding is used mainly to study dipping geologic structures, including structures that are favorable for the accumulation of oil and gas.
In inductive methods, a field is induced by, for example, ungrounded AC circuits (see).
Radio-wave methods are based on the study of radio-wave absorption as radio waves propagate in rocks. The main radio-wave method is radio transmission, wherein a radio transmitter is placed in a borehole or a mining excavation and the electromagnetic field intensity is measured in a nearby borehole or mining excavation. Ore deposits that are good conductors and are located in the space between the boreholes or excavations absorb a large portion of the electromagnetic field and produce a radio shadow in the area where the field is measured. The existence and outlines of ore bodies are determined from the location and size of the shadow. The geological structure of the parts of a geologic section that lie near the surface—that is, at depths of 20–30 m—is studied by using the fields of radio broadcast stations. Such fields propagate along the earth’s surface and induce secondary currents in conducting objects.
According to the nature of the geological problems to be solved, electrical methods may be used in searching for ores, in studies of the structures of the earth’s crust, or in engineering geology. Specific areas of application include archaeology and glaciology. Electrical methods may be used on the ground, in the air, in boreholes and mines, or at sea.
The equipment used in electrical exploration comprises current sources, electromagnetic field sources, and measuring equipment. The current source may be a dry battery, a generator, or a storage battery. The field source may be a line grounded at the ends or an ungrounded AC or DC circuit. The measuring equipment consists of the following: an input transducer, or field sensor; a system of intermediate signal transducers, which converts signals for recording and for the filtering of interference; and output devices, which measure the signals. Equipment intended for the study of a geologic section at a depth of not more than 1–2 km is manufactured as lightweight portable units. For studies at greater depths, electrical exploration stations are used.
In the preliminary processing of the results of field observations, such parameters as apparent resistivities and DC potentials are calculated and then presented in the form of plots, charts, and tables. In the further geological interpretation of the results, the observed field is compared with results of theoretical models of the geologic section; information pertaining to the electromagnetic properties of rocks and results obtained by other methods are also used.
Electrical exploration makes it possible to accelerate and to reduce the costs of geological exploration by reducing the amount of expensive shaft-sinking and drilling operations. The further development of electrical exploration is associated with the development of new techniques, the study of the earth’s crust at greater depths, and increases in the reliability of the results obtained.
REFERENCESZaborovskii, A. I. Elektrorazvedka. Moscow, 1963.
Iakubovskii, Iu. V. Elektrorazvedka. Moscow, 1973.
Iakubovskii, Iu. V., and L. L. Liakhov. Elektrorazvedka, 3rd ed. Moscow, 1974.
IU. V. IAKUBOVSKII