Vol 66 Iss. 2

On the Baltic Shield, in foreign and Soviet parts of it, to date, a large amount of work has been carried out by methods of explosive and conventional seismology, allowing to determine the sole of the consolidated crust - the Mohorovičić discontinuity (M) - in its various areas.

To study the deep structure of the Pechenga ore district, the reflected wave seismic (RWS) method has been successfully applied in recent years. Consistent complication of the tasks solved in this case, causes the need to detail the ideas about the velocity section of the studied area, for which data from both borehole and ground seismic observations are used.

The solution of the important question about the accuracy of seismic boundaries construction not only in the Pechenga, but also in other ore areas is complicated by the complex character of the registered wave fields and the change of velocity sections over the area, which is explained by the strong dislocation of folded structures composed of different complexes of high-velocity crystalline and metamorphosed rocks.

Seismic studies within the Pechenga structure are carried out by the reflected wave method along systems of longitudinal profiles combined with spatial observations. The seismic profiles are oriented, as a rule, crosswise to the prevailing strike of the rocks.

A large volume of regional seismic studies has been carried out on the Baltic Shield in order to study the peculiarities of the crustal structure. Observations were carried out in areas of development of various complexes of crystalline metamorphic and magmatic rocks.

Godographs of the observed reflected waves are always complicated by various kinds of interferences. Some of them are associated with certain deviations of real conditions from the assumptions underlying the mathematical model of the hodograph (the presence of relief and LSZ, the effect of refraction of the seismic beam at intermediate boundaries of the medium, etc.).

Located in the northwest of the Kola Peninsula, the Pechenga structure is a large synclinal fold composed of alternating sedimentary and volcanogenic strata.

Mica-bearing pegmatites discovered during geological survey works of the NWTSU, confined to the rocks of the Keivka series of the Lower Proterozoic, allow us to consider the Keivka subzone of the Karelides as promising for muscovite.

The Wind Belt structural zone is a series of higher-order structures and is a southeastern continuation of the East Karelian syncline zone. From the northeast, the Wind Belt zone is bounded by the Belomorsky block of gneisses and gneisso-granites, and to the southwest of it are distributed granitoids of the Vodlozersky block.

The ore occurrence is located on the shore of the White Sea. Structurally, it is located within a narrow graben-like synclinal fold, confined to the western wing of the Primorsky anticlinorium and composed of Archean metamorphic rocks: garnet, feldspar and monomineral amphibolites interbedded with biotite-amphibole, biotite and zoisite-biotite-amphibole gneisses.

The plan of development of the national economy of the USSR in the North Onega region provides for the construction of a mining and processing plant. In this connection it is supposed to strengthen prospecting works for bauxite. The need to reduce the cost and improve the efficiency of prospecting inevitably leads to the use of a common set of geophysical methods.

One of the most important tasks of geological and geophysical services of the North-West RSFSR is the search for copper-nickel ores on the Baltic Shield and, in particular, in Karelia. The Burakovsko-Aganozersky massif, located on the eastern shore of Lake Onega, is one of the largest objects of such prospecting.

A significant achievement in the development of AC electrical exploration is the development of low-frequency inductive methods with amplitude-phase measurements. The use of optimal frequencies for solving a particular geological problem and measurement, along with amplitude, time (phase) characteristic of the field certainly contributes to improving the efficiency of AC electrical exploration.

Inductive electrical exploration is widely used in ore areas of the Karelian-Kola region. It is characterized by a significant number of modifications and types of equipment, with the help of which are measured values that determine the intensity H of the alternating magnetic field.

Laboratory methods of studying the composition of liquid substances have developed earlier than solid substances. However, to study the composition of matter in remote measurements (e.g., in a borehole), mainly physical methods that do not utilize the properties of the liquid phase are used.

To calculate the elements of occurrence of magnetized bodies, the field Z caused by them can be approximated by the field of a set of bodies of the simplest form. In a number of cases it is expedient to choose as an elementary body a low-power layer.

When conducting magnetic surveys, anomalies are often revealed, the nature of which quite clearly reflects the influence of the lower boundary of magnetized bodies of formation form. The use of anomalous magnetic roll to estimate the possible dip distribution of such bodies and to determine the dip angle may be important in prospecting and exploration of many ore deposits, solving various structural problems.

At present, there are a large variety of electrical exploration methods for prospecting for mineral deposits, both AC and DC. All of them, with the exception of methods of natural zero, “radio kip” and telluric currents, are based on the use of special current generators and receiving measuring equipment, which registers the secondary electromagnetic field or potential difference created by these current generators, between some points on the surface of the earth or in mine workings.

In geologic mapping of ore districts on a large scale (1:50,000, 1:25,000 and larger), detailed gravity survey is widely used, which makes it possible to reveal many features of the geologic structure of bedrock overlain by loose sediments.

The southern slope of the Baltic Shield has been drilled by many wells, mostly shallow. In order to compare the thermal state of crystalline rocks in different regions of the Baltic Shield, temperature measurements in deep boreholes that are not distorted by geomorphologic, hydrogeologic and other influences are necessary.