Vol 46 Iss. 2

Until recently, seismic exploration has been little used in the study of crystalline structures of ancient shields. In Karelia in 1958-1959, during the International Geophysical Year, the first regional studies of the ancient shield in the USSR were carried out by the method of deep seismic sounding. In 1960-1961 seismic work in combination with other methods was continued in the north-west of the Kola Peninsula in connection with the study of the deep structure of the Pechenga effusive‑sedimentary series. On the basis of these works, performed by the Western Geophysical Trust, VSEGEI, Leningrad Mining Institute and the organization “Spetsgeofizika”, a number of fundamental issues of the application of seismic exploration in the complex conditions of the Baltic Shield were clarified and interesting geological results were obtained.

The analysis of geophysical studies carried out in recent years within the Pechenga structural zone makes it possible, taking into account the available geological data, to illuminate its deep geological structure. Most researchers (G. T. Makeenko, L. Y. Kharitonov, N. M. Parfenenko) present the structural zone as a synclinorium composed of the Tundra and Pechenga series of the Lower and Middle Proterozoic. The rocks of the Pechenga series, composing the core of the structure, are represented by a sedimentary-volcanogenic complex consisting mainly of four complex effusive covers interspersed with sedimentary rocks. The most distinct among the sedimentary formations is the phyllite horizon, which hosts all the hyperbasite intrusions represented by serpentinites, peridotites and pyroxenites.

A significant part of the area of the Pechenga district is composed of sedimentary‑volcanogenic strata, representing an alternation of low‑thickness packs of sedimentary‑metamorphic rocks and thicker volcanogenic units. Middle Proterozoic metagabbro‑diabase intrusions and late Proterozoic intrusions of basic and ultrabasic rocks occur within the rocks of the Pechenga series. To the north and south of the Pechenga Series occur gneisses and strongly metamorphosed rocks of the Archean.

Separation of crystalline rocks by elastic properties is crucial in determining the applicability of seismic exploration for solving geologic problems in ancient shields. In Northern Karelia, special parametric seismic soundings were carried out in 1958 by the Spetsgeofizika office and the All-Union Research Geological Institute with the participation of the Leningrad Mining Institute. During these observations, made in conjunction with deep seismic sounding (DSS), the same receiving equipment was used: low-frequency seismic receivers of the SP-15 type and seismic stations SS-26-51D, reconstructed for low-frequency reception. Most of the records of elastic waves from explosions are made at the same bandwidth as the DSS (5-15 Hz), a smaller part - at medium-frequency filtering (30-50 Hz).

Seismic exploration by the reflected wave method is successfully used in the study of sedimentary rock structures. In this field, its results are highly accurate and it occupies a leading position among other geophysical methods. Similar studies of the deep structure of crystalline rocks by the reflected wave method could yield equally valuable results to those obtained in the study of sedimentary formations. However, until now, with the exception of a few experimental studies, this method has not been practically applied to crystalline rocks. Therefore, the results of experimental and production work using the reflected wave method, carried out by the Western Geophysical Trust together with the Problem Laboratory of Geophysical Exploration Methods at the Leningrad Mining Institute in the central regions of the Kola Peninsula in 1958–1960, are of considerable interest.

The lawful relationship between tectonic movements and magmatism of the Earth is a definite correlation between different forms of matter movement, established by numerous facts. Modern science owes much of the elucidation of this important geological regularity to Yu. A. Bilibin, V. A. Nikolaev, P. M. Tatarinov, V. I. Serpukhov, A. V. Peyve, V. M. Sinitsin, G. Reed, H. Hess, W. Benson and other researchers.

There are many ways of quantitative geologic interpretation of gravitational and magnetic fields. Let us consider the fitting methods that can be used to determine the elements of occurrence of geologic objects that create complex gravity and magnetic anomalies. In these methods, the curve obtained by field measurements is compared with the curves, which for bodies of regular geometric shape are calculated by appropriate formulas. In the case of irregularly shaped bodies, such curves have to be found using special templates. Finally, modeling devices can be used for this purpose.

One of the tasks often solved using aeromagnetic survey data is to determine the depth of occurrence of magnetized rocks. The experience of using AT anomalies in different geological regions shows that the results of calculations become quite reliable only after statistical generalization. In the simplest case, the values of the depth of occurrence of disturbing masses are averaged over some area, for example, within a single anomalous zone, or over part of the route.

On the territory of the Russian platform, the crystalline basement is overlain by a cover of sedimentary deposits, the thickness of which reaches up to several kilometers. One of the tasks of the aeromagnetic survey (V.N. Zander expedition, 1959-1961) was to determine the buried relief of the basement in connection with the search for oil- and gas-bearing structures in the sedimentary cover. The main method of solving the problem was calculating the depth of the upper edge of magnetic bodies, usually corresponding to the surface of the crystalline basement.

The materials of magnetic observations allow us to consider a number of new questions about the geological structure of the Kola Peninsula and the location of a number of minerals. On the maps of magnetic field plots of the ΔT, a large number of anomalies and anomalous zones, different in size, shape, intensity and character of the field, were identified. The geological nature of many of them is not in doubt and is easily established by comparing the magnetic field maps with detailed geological maps made on the basis of the study of the ground surface and shallow excavations. Such material is insufficient for deciphering anomalies with more complex nature. These anomalies reflect the properties and composition of rocks both partially exposed on the ground surface of the crystalline basement and magnetic masses embedded in the basement rocks, or are caused only by the latter. As usual, they are also divided into regional and local.

The coastal areas of Western Kamchatka north of Cape Omgon (Tigil‑Voyampolka interfluve) have for many years been the object of intensive geological and geophysical study in connection with the search for oil and gas deposits. Since the VNIGRI studies in the early 30's revealed indications of oil‑bearing potential here, geological and geophysical work has acquired a clear oil prospecting direction. To date, considerable data on the stratigraphy and tectonics of the investigated area have been accumulated, which are based mainly on the results of surface or near‑surface geologic observations. The present state of knowledge of the area is determined, first of all, by the studies of geologists of VNIGRI (B. F. Dyakov, I. B. Pleshakov, N. M. Markin) and Kamchatka RayGRU (V. G. Vasilyeva, V. P. Vdovenko, E. P. Klenova, P. A. Koval, V. V. Krylov, K. M. Sevostyanov, and others), who were engaged both in the study of regional geology and tectonics, and in the search for local brachyanticline structures. As a result of many years of research, the position of the Tigil‑Voyampolsky area in the general tectonic structure of West Kamchatka was determined, the scheme of stratigraphy of sediments composing the visible part of the geological section was developed, the prospects of stratigraphic horizons for oil‑bearing potential were evaluated, and a large number of local Paleogene uplifts were revealed. At the same time, each of these questions needs additional study, as a consensus has not yet been reached, mainly due to poorly understood deep geology and tectonics. These questions can be judged only from the data of rotary drilling on the Voyampolka structure and gravity exploration works (A. L. Voshchinsky, V. L. Goldman, Y. S. Dezhanova) in the coastal areas of the West Kamchatka plain in the Kvachina‑Voyampolka interfluve.

The method of airborne electrical prospecting based on the use of a rotating magnetic field (VMGl)-one of the modifications of the induction method-is carried out with the help of two AN-2 type airplanes flying behind each other. One of the airplanes, the generator airplane, carries two mutually perpendicular frames excited by currents of one of the operating frequencies with a phase shift of 90° between them. Due to this, the resulting vector of the alternating magnetic field rotates in a plane perpendicular to the planes of the frames and the flight line. If the magnetic moments of both frames are equal, the electromagnetic field along the aircraft axis will have circular polarization in the plane of rotation of the vector.

Problems related to the study of electromagnetic fields are generally difficult, and in application to geology, due to the diversity of parameters, often cannot be solved theoretically. In this case modeling is the only way to study the regularities of the solution. In order for the results of experiments on models to be transferred to a specific geological case, the design of the experiment and the processing of its results should be carried out under similarity conditions.

Exploration by inductive methods often uses a cable loop, the dimensions of which should be optimal, i.e., those that provide the maximum expected effect. The choice of optimal size is complex and can be solved for certain special cases. The solution is of the greatest interest for the transient method, where the same loop can be used to excite the ore body and register the anomaly. Obviously, the criterion for evaluating the optimum here should be considered the EMF value to be registered.

In the method of infinitely long cable (ILC), a total magnetic field acts on the receiving device located in the vicinity of the ore body, consisting of the primary field H₁ created by the cable and the secondary field H₂ excited by the ore object. As a result, the anomalous secondary field stands out against the background of the primary.

When applying sounding equipment in surveying work, the question naturally arises of its accuracy and, therefore, the possibility of using it to solve specific engineering problems. At the present time - in the period of development and formation of sounding method of measurements in surveying - this issue is of particular relevance.