Vol 66 Iss. 2

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

To study the deep structure of the Pechenga ore district, seismic exploration by the reflected-wave method (RWM) has been successfully applied in recent years. The consistent complication of the tasks being solved causes a need to detail the ideas about the velocity section of the studied area, for which data from both downhole and surface seismic observations are used.

The solution of the important question of the accuracy of constructing seismic boundaries not only in the Pechenga, but also in other ore areasis complicated by the complex character of the recorded wave fields and the change of velocity sections across 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 areal observations. The seismic profiles are oriented, as a rule, across the prevailing strike of the rocks.

A large volume of regional seismic studies has been carried out on the Baltic Shield with the aim of studying the peculiarities of the structure of the Earth's crust. Observations were carried out in areas where various complexes of crystalline metamorphic and magmatic rocks are developed.

Travel-time curves of the observed reflected waves are always complicated by various kinds of noise. Some of them are associated with certain deviations of real conditions from the assumptions underlying the mathematical model of the travel-time curve (the presence of relief and the low-velocity zone, the effect of refraction of the seismic ray at intermediate boundaries of the medium, etc.).

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

The mica-bearing pegmatites identified during geological survey work of the NWTSU, confined to the rocks of the Keev series of the Lower Proterozoic, make it possible to consider the Keev subzone of the Karelides as promising for muscovite.

The Vetreny Poyas structural zone is a series of higher-order structures and regionally is a southeastern continuation of the East Karelian synclinorium zone. To the northeast, the Vetreny Poyas zone is bounded by the Belomorian block of gneisses and gneiss-granites, and to the southwest of it granitoids of the Vodlozero block are developed.

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 limb 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 for the 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 planned to intensify prospecting work for bauxite. The need to reduce the cost and increase the efficiency of prospecting inevitably leads to the use of an integrated set of geophysical methods.

One of the most important tasks of geological and geophysical services of the Northwest 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 A.C. electrical exploration is the development of low-frequency inductive methods with amplitude-phase measurements. The use of optimal frequencies for solving a specific geological problem and the measurement, along with the amplitude, of the time (phase) characteristic of the field certainly contributes to improving the efficiency of A.C. electrical exploration.

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

Laboratory methods for studying the composition of liquid substances took shape earlier than those for solid substances. However, for studying the composition of matter in remote measurements (e.g., in a borehole), mainly physical methods that do not use the properties of the liquid phase are used.

For calculating the attitude elements of magnetized bodies, the field Z caused by them can be approximated by the field of a set of bodies of the simplest shape. In a number of cases it is expedient to choose as an elementary body a thin bed.

During magnetic surveys, anomalies are often detected, the nature of which rather clearly reflects the influence of the lower boundary of magnetized bodies of bedded form. The use of the anomalous magnetic field to estimate the possible down-dip extent of such bodies and to determine the dip angle may be of important significance in prospecting and exploration of many ore deposits and in solving various structural problems.

At present, there is a large number of various electrical exploration methods for prospecting for mineral deposits, both on A.C. and D.C. All of them, with the exception of natural-field methods, “radiokip” and telluric currents, are based on the use of special current generators and receiving measuring equipment that records the secondary electromagnetic field or the potential difference created by these current generators between any points on the earth's surface or in mine workings.

During geological mapping of ore districts at large scales (1:50,000, 1:25,000 and larger), detailed gravity surveying is widely used, which makes it possible to identify many features of the geological structure of bedrock overlain by unconsolidated sediments.

The southern slope of the Baltic Shield has been drilled with many boreholes, mostly shallow. 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.