Aerial Electric Prospecting

a method based on the study of electromagnetic fields developed in the earth’s crust by the action of alternating current sources, using measuring apparatus carried by aircraft. The field of application includes prospecting for ore bodies and “lenses” of fresh water, and mapping of native deposits to depths of several dozens of meters. Aerial electric prospecting makes it possible to rapidly investigate large areas, often in regions that are nearly inaccessible for ground prospecting. The method has been developed in the USSR, Sweden, Canada, and the United States. The techniques generally utilized in the USSR are the infinitely long cable, the rotating magnetic field, and radiokip (radio comparison and direction finding).

In the infinitely long cable technique, an analysis is made of the horizontal component in an AC magnetic field created by a long (to 30–40 km) cable grounded at the ends and laid out parallel to the course of the geological structures. The apparatus for realizing this method consists of generating and measuring units. The generating unit on the ground supplies the cable with current at frequencies of 80–2000 hertz (Hz). By means of the measuring units mounted in a helicopter, the amplitude-phase characteristics of the field’s horizontal component are measured. The aerial electric prospecting profiles are laid out perpendicular to the cable; the flight altitude is approximately 100 m and the flying speed is from 100 to 200 km/hr. In order to minimize electrical interference, the receiving loops are mounted in a gondola which is lowered during flight on a cable 15–20 m long. This technique makes it possible to explore and study electrical conducting areas down to a depth of 100 m. It was used in the Kola Peninsula, Kazakhstan, Yakutia, and other regions of the country.

Compared to the infinitely long cable technique, the rotating magnetic field method has an advantage with respect to apparatus design because there is no generating unit or cable on the ground. The surveying is carried out with two airplanes (type An-2) which follow one another along the same course. Mounted in one airplane are two mutually perpendicular generator loops; the receiving loops are placed in a trailing gondola which has its long axis along the line of flight. When flying over a homogeneous medium, the differential signal in the two loops is zero. Heterogeneities in the electrical conductivity of the medium cause the appearance of a signal which is registered by the measuring apparatus. The parallelism of the corresponding generating and receiving loops decreases the sensitivity of the system to disturbance of the mutual orientation of the loops during a flight. This technique solves the problem of exploration and mappings of electrically conducting zones (tectonic abnormalities and sulfideores), mainly in open regions (without electrically conducting detritus) where there is a high-resistance surrounding medium (not less than hundreds of ohm · m). The prospecting depth is less than in the infinitely long cable method. Measurements are made at 600–2500 Hz at a flying altitude of 70–100 m and a speed of 150 km/hr, with a separation between airplanes of about 250 m. The rotating magnetic field method has been employed in the USSR since 1960 on the Kola Peninsula and in Karelia. Its disadvantage is the use of two airplanes and the complexity of flying together at a low altitude. Therefore, in many cases the induction technique with the apparatus in a single airplane is preferable.

The radiokip technique has been utilized in the USSR since 1959 for subterranean water explorations (for example, in the Kara-Kum Desert) and mapping of frozen zones; it has also been used abroad for searching out buried salt domes. This method also is used to measure the absolute electric field strength of radio broadcasting stations in the frequency range from 0.1 to 1 mHz. The apparatus is carried on board a helicopter or airplane. The radiokip technique reveals subterranean water areas of at least 30 sq km at depths up to 20 m. The technique is improved by shifting to the superlong-wave band (10–30 kHz) and making relative measurements (rather than absolute), which are less subject to the influence of radio wave propagation conditions, the power of the radio stations, and the distances from them.