Aerial electromagnetic exploration
AERIAL ELECTROMAGNETIC EXPLORATION (EN: aerial prospecting by electric methods; DE: elektrische Luftaufnahme; FR: prospection electrique aerienne; ES: prospeccion aerea por metodos electricos; RU: аэроэлектроразведка) is one of the directions of the aerial geophysical exploration, which is based on the research of the natural or artificially created electromagnetic fields with the help of the equipment, which has been installed on the aeroplane or helicopter. The aerial electromagnetic exploration has been used for the first time for the searches for the useful minerals within Sweden during the start of the 1950-ies, later it has got the distribution within the Canada and USA; it is used within the USSR since the end of the 50-ies.
The excitation of the electromagnetic field during the aerial electromagnetic exploration is conducted with the inductive method, which is not requiring the groundings (see the "Inductive methods for the electromagnetic exploration" article). With the help of the equipment, which has been installed onto the aeroplane, or is towed behind it (Figure), there are measured the primary electromagnetic field, and the induced by it within the Earth's crust secondary magnetic field of the eddy currents, which becomes abnormal within the electrically conductive or highly magnetic rocks (for example, within the ore bodies).
The registration of the characteristics of this field permits to study the situation of the anomaly-forming objects within the Earth's crust and their parameters.
They distinguish the passive and active methods for the aerial electromagnetic exploration according to the method for the excitation of the electromagnetic field. The passive methods for the aerial electromagnetic exploration are based on the study of the alternating electromagnetic fields, which are excited by the lightning discharges [the "afmag" method (from the "AFMAG" English designation, which is the abbreviation for the "audio frequency magnetic")], or which are created by the medium wave or long wave radio stations, namely, the "RadioKiP" method (the Russian designation for the "radio comparation and radio direction finding"). With the help of the equipment, which has been installed onto the aeroplane, there are measured the value of the magnetic component of the field, its orientation or the difference of the phases between the spatial components of the magnetic field. The major advantage of the passive methods is the absence of the devices for the excitation of the field, though the random character of the field requires the complicated techniques for its measurement, and hinders the interpretation of the results.
The active methods for the aerial electromagnetic exploration are based on the measurement of the alternating electromagnetic fields, the sources of which are included into the set of the equipment, and are situated either on the Earth's surface within the area, which is being researched, or on the aircraft. There belongs to the list of the methods with the ground source of the field the so-called method of the long cable, during which the electromagnetic field, which is changing with the frequency of 102-103 hertz, is excited by the alternating current, which is passed through the cable (the length is up to 20 kilometres), which is grounded at the ends. The receiver of the field, namely, the multi-turn coil frame, is towed with the help of the rope-cable at the distance of 30-60 metres behind the aeroplane at the altitude of 40-70 metres from the earth's surface. The amplitude and phase of the magnetic field of the cable are measured along the profiles, which are situated perpendicularly to it. The on-the-ground source of the field, which is changing with the frequency of 102-103 herz (the rectangular elongated coil with the sides of several kilometres), is used also for the "turair" method (from the "two frames airborne" English designation). The magnetic field of this coil is registered with the help of the two measurement frames, which are towed by the helicopter along the profiles, which are situated outside of the coil perpendicularly to its long side. There are determined the ratio of the amplitudes and the shift of the phases of the signals within the measurement frames.
Within several methods of the aerial electromagnetic exploration, which are the aerial variants of the dipole inductive profiling, there serves as the source of the field the electromagnetic dipole (the multi-turn coil frame with the alternating current), which has been fixated onto the same aircraft, which carries also the measurement dipole and the set of the generator-measurement equipment. Within the certain aerial electromagnetic exploration devices of this type, the generating and measuring dipoles are rigidly fixated onto the ends of the wings of the aeroplane, and within the other devices the measuring dipole or the entire installation is towed with the help of the rope-cable behind the aircraft. This method permits to decrease the level of the noise from the electrical equipment of the aircraft, but leads to the additional noise on the account of the mutual movement of the generating and measuring dipoles. Within the aerial variant of the method of the transient processes, the primary field is excited by the periodically occuring impulses of the electrical current within the generating dipole, which has been fixated onto the body of the helicopter. The magnetic field, which is excited by this dipole within the Earth's crust, is registered within the pauses between the impulses of the electrical current with the help of the measuring dipole, which is towed with the help of the rope-cable behind the helicopter.
The modern sets of the equipment for the aerial electromagnetic exploration are provided with the devices for the digital registration of the parameters of the field, which are measured during the flight. This equipment provides the possibility for the computer processing of the results of the measurements. Usually the equipment for the aerial electromagnetic exploration belongs to the integrated aerial geophysical stations, which are including also the magnetic and gamma-ray spectral channels. The aerial electromagnetic exploration is used for the geological mapping and searches for the well conducting ores, which are embedded at the depth of several tens of metres; the method of the transient processes permits to research the great depths. They mainly use the aerial electromagnetic exploration during the small-scale (1:100000-1:200000) mapping works, and during the searches for the ore deposits with the scales of 1:25000-1:50000, less often at the stage of the detailed searches with the scale of 1:10000. The further development of the aerial electromagnetic exploration is associated with the improvement of the equipment, and with the creation of the new methods for the excitation and measurement of the electromagnetic fields.
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