magnetic prospecting[mag′ned·ik ′prä‚spek·tiŋ]
a geophysical prospecting method based on the difference in the magnetic properties of rocks. Magnetic prospecting is used in all stages of geological studies and includes measurements of the intensity of the geomagnetic field or its elements, compilation of magnetic charts, and geological interpretation of the results of measurements based on determinations of the magnetic characteristics of the rocks.
Magnetic prospecting studies the magnetic anomalies produced by geological bodies that have been magnetized by present-day (induced magnetization) and ancient (residual magnetization) geomagnetic fields. The magnetization of rocks is determined by the presence of ferromagnetic minerals (magnetite, pyrrhotite). Especially intensive magnetic anomalies are produced by igneous rocks of basic and ultrabasic composition, magnetitic iron ores, and other rocks. Measurements in magnetic prospecting are made on the ground, from airplanes or helicopters (aeromagnetic surveying), from moving ships (hydro-magnetic surveying or marine magnetic prospecting), in underground workings (underground magnetic prospecting), and in drill holes (well magnetic prospecting). Various magnetometers are used for measurements. The relative values (spatial increments) of the vertical component of the geomagnetic field intensity ΔZ (ground surveys) are measured most often, and the values of the horizontal component ΔH are measured less frequently. In aeromagnetic and hydromagnetic surveying the vectoral modulus of the total intensity of the geomagnetic field Tor its increment ΔT is measured. In magnetic prospecting magnetic variations must be taken into account. Ground surveys generally are conducted on the basis of rectilinear profiles, the ratio of distances between profiles and observation points ranging from 10:1 to 1:1. In aeromagnetic and hydromagnetic surveying the measurements are made continuously or at discrete intervals along a network of rectilinear or curvilinear (in mountainous terrain) profiles.
The depth and disposition of magnetized bodies in the earth’s crust that serve as sources of the anomalous magnetic field are determined through magnetic prospecting data. Magnetic prospecting is used independently and in conjunction with other geophysical and geological methods to study the regional deep structure of the earth’s crust, including determination of the depth of occurrence of the basement complex (in prospecting for oil and gas). It is also used for geological mapping of surveys of magnetic varieties of iron ores and ore- and nonore-bearing deposits associated with basic and ultrabasic rocks (such as nickel, chromium, titanium, and diamonds); for mapping of nonferrous, rare, and precious metals whose ores contain accessory magnetic minerals (such as lead, tin, placer gold, and platinum); for mapping of skarn ore deposits, generally enriched with magnetite (such as iron, tungsten, molybdenum, and copper); for mapping of deposits of piezooptic minerals (piezoquartz, Iceland spar, optical fluorite) associated with magnetitic mineralization, fracture zones, and intrusions of ultrabasic rocks; and for mapping of aluminum ores that are represented by magnetic varieties of bauxites.
In prospecting for iron ores magnetic prospecting, used in conjunction with measurements of the magnetic susceptibility of rocks in underground workings and drill holes, makes it possible to pinpoint the position of iron ore bodies and estimate the percentage of magnetic iron in ores.
Magnetic prospecting originated in the 18th century when D. Tilas in Sweden invented a device used to prospect for magnetic ores, the Swedish surveyor’s compass. In Russia the first magnetic observations by compass in searching for iron ores were made in the mid-18th century in the Urals, where the Magnitnaia Mountain was discovered. Highly magnetic ores were sought with a dip compass in the United States and Canada in the 1820’s. In 1899 magnetic prospecting, which led to the delineation of a number of iron ore deposits, was carried out in the Urals on the initiative of D. I. Mendeleev. The iron ore deposits of the Kursk Magnetic Anomaly were discovered by magnetic prospecting. The work of the Soviet geologist A. D. Arkhangel skii led to the use of magnetic surveys in 1922 to study deep geological structure, the basement platforms overlain by strata of sedimentary rocks. In 1936 the Soviet geophysicist A. A. Logachev built, with A. T. Maiboroda, the world’s first aeromagnetometer and worked out the methods of aeromagnetic surveying. In the 1950’s and 1960’s the equipment and methods used for magnetic prospecting in drill holes were developed in Finland, Sweden, and the USSR.
REFERENCESLogachev, A. A. Magnitorazvedka, 3rd ed. Leningrad, 1968.
Fedynskii, V. V. Razvedochnaia geofizika, 2nd ed. Moscow, 1967.
Magnitorazvedka. (Spravochnik geofizika, vol. 6) Moscow, 1969.
V. E. NIKITSKII