Magnetic Structure of the Earth’s Crust in the White Sea Region

The geological structure of the White Sea area and the surrounding land areas has been well studied in the framework of individual case studies. There are a number of local models of the deep structure of the Earth’s crust available. We propose a uniform assessment of deep crustal bodies responsible for long-period (regional) magnetic anomalies and consider their correlation with surface structures. The aim of the study is to build a three-dimensional magnetic model of the Earth’s crust in the White Sea region using aeromagnetic data and modeling technologies of the Integro software package. The model is formed on the basis of a digital map of the anomalous magnetic field reduced to the pole. The sources of magnetic anomalies are considered to be located in the Earth’s crust. The 3D distribution of the relative magnetic susceptibility of rocks was obtained by solving the inverse problem of the magnetic survey. To separate the magnetic sources by frequency and depth, it was necessary to continue the magnetic field of the model upward and to calculate the TDR derivatives, which determine the lateral boundaries of the sources of positive magnetic field anomalies. 2D distributions of magnetic sources of the model for vertical and horizontal sections with depths of 10, 15 and 20 km are analyzed. The correlation between the surface and deep structures of magnetic sources of the Earth’s crust in the region is shown.


Introduction
The practical study of an anomalous magnetic field has shown that it contains a regional constituent, which can be used for the study of the deep structure of the International Journal of Geosciences earth's crust. Methods for identifying mathematically interpreting regional magnetic anomalies (RMA) and understanding the genesis of their sources have been described by many authors [1] [2]. Anomalous magnetic field gradients contain evidence for the positions of magnetic sources. Vertical gradients are sensitive to the depth of occurrence of sources [3]. Obtaining anomalous magnetic field gradients from the sources located in the lower horizons of the earth's crust, remains a complex problem, which has not been completely resolved [4]. Therefore, mathematical modelling remains an efficient and relatively cheap method for the structural study of the earth's crust.
Magnetic models of the earth's crust, based on the results of the areal aeromagnetic survey, establish a relationship between an anomalous magnetic field and rock magnetization. The scope of information obtained using such models is believed to be controlled by igneous rocks' property to preserve information on the Earth's magnetic field in magnetized state upon rock crystallization. Linking geomagnetic field states to time helps reconstruct the past states of the geomagnetic field, lithosphere and climate [5]. Such models are usually constructed for geodynamically active regions [6] [7] [8].
The White Sea basin and adjacent areas are at the conjugation zone of the uplifting Fennoscandian Shield and the Russian Plate is overlain by sedimentary strata. Interest in the region's deep structure and geodynamics is due to its mineralogenic kimberlite magmatism, various useful mineral deposits and oil and gas prospecting [9].
The deep structure of some portions of the Fennoscandian Shield has been repeatedly studied [10] [11] [12] under national and international research projects [13].
The earth's crust of the east-facing slope of the Fennoscandian Shield and the White Sea basin, Russia's inland sea) has mainly been studied by Soviet and Russian scientists [14] [15] [16] [17] [18].
The formation of our knowledge of the deep crustal structure of the White Sea using aeromagnetic data has been described in [19], where an evolutionary model of the white Sea rift system, connecting the structural levels of a magnetically active layer with stages in the region's tectonic activation from the Middle to the Late Riphean to the events that took place during the last glaciations in the Quaternary period.
Our knowledge of the division of the earth's crust in the White Sea region into three basic arbitrary variably deep layers-"sedimentary", granitic-metamorphic" and "granulitic-basic"-differing in density [14] [20], obtained by generalizing seismic data, can be used to estimate the depth of occurrence of the upper and lower margins of RМА sources. Scientists who study RМАs on the Fennoscandian and Ukrainian shields [21] think that the arbitrary "granitic" layer could be connected with local magnetic anomalies.
The construction of magnetic models is contributed to by computer technologies operating with large amounts of data, solving problems under uncertain conditions and presenting the results of studies in various ways [22]. Complex designed to approach various problems in earth sciences [23]. The complex solves direct and inverse geophysical problems automatically, provides the possibility to construct and analyze complex models, to take into account a priori information and to do complexing of methods.
The purpose of the present study is to construct a generalized 3D magnetic model of the earth's crust of the White Sea and adjacent areas using the Integro Complex.

Initial Data and Provisions 3D Magnetic Model Was Constructed Using
 A scheme showing the region's block structure;  1:1,000,000 scale digital maps of an anomalous magnetic field (ΔТ) [24] [25];  A 3D density model of the region's earth crust and a scheme of the depth of occurrence of M-discontinuity [26];  A 1:1,000,000 scale state geological map of Russia and explanatory notes to sheets Q-35, 36, 37 and 38 [27];  1:1,000,000 scale petrophysical and metromagnetic maps of the eastern Fennoscandian Shield [28];  A scheme of temperature distribution in the crustal sequence [29];  Tectonic maps of the White Sea and adjacent areas [17] [30].    Table 1.
The basic aeromagnetic data for the White Sea region, obtained by 1:1,000,000 to 1:200,000 scale survey in 1958-1989, were transformed into a digital matrix with a 500 × 500 m cell of the eastern portion of sheet Q-35 and sheets Q-36 -Q-38 in full format.
The total magnetic intensity matrix was reduced to the pole. The map compiled ( Figure 3) is a superposition of the contributions of variably deep anomalies sources. Regional magnetic field anomalies are believed to control deep   When constructing magnetic models of the earth's crust, selecting magnetic susceptibility values for rocks becomes a problem. In some models, workers use the When solving an inverse magnetic prospecting problem for the White Sea region, which displays a mosaic structure, we oriented ourselves at relative magnetic susceptibility, an arbitrary value used in the model models of the Integro package, rather than the absolute magnetic susceptibility values of rocks [23].
The depth range of magnetic anomaly sources in the model was chosen using Curie's isotherm depth estimate [6] [33] [34]. According to [29]

Data Processing and Modelling Technologies
Stages in the construction and analysis of the 3D model using the Integro International Journal of Geosciences complex comprised preparing a digital map of the region's anomalous magnetic field, its reduction to the pole, solving an inverse magnetic prospecting problem, obtaining the vertical and horizontal sections of the model, recalculating a model anomalous magnetic field upwards, calculating recalculated field derivatives and laterally delineating the sources of positive anomalies in the horizontal sections of the model. Inverse problems in the Integro package were solved on a 3D net using a regularization method [39] [40] and updated spectral algorithms based on Fourier's rapid transformation [40] [41]. The net step of the model along the axes was 1 km. The algorithms used function rapidly and remove marginal effects arising because of lateral limits and lack of data periodicity.
The standard geophysical procedures of the Integro package were performed by reducing the anomalous magnetic field to the pole, recalculating the field up and down and calculating its derivatives [40]. The reduction of the magnetic field to the pole yields the magnetic field of the substance of the same magnetization directed vertically upwards, topography and the various directions of the orientation of rock magnetization. The upward extension of the magnetic field uses a different field variation rate from variably deep sources, yields the distribution patterns of sources differing in spatial frequencies and identifies a field constituent from the sources of horizontal layers with preset depths.
A combination of the derivatives of the model magnetic field recalculated upwards was used as a detector of the lateral boundaries of the sources. The role of a spatial filter was played by a vertical to horizontal field derivative ratio expressed as the arc tangent of an angle denoted as a TDR derivative or a TDR an-  [42]. The TDR derivative is positive above the magnetic source, is close to zero near its boundaries and is negative where no source exists [8]. Similarly [8], the lateral boundaries of positive magnetic anomalies were obtained by placing the transparent windows of the positive TDR derivative on the horizontal sections of the 3D magnetic model.

Discussion
Comparison of the block diagram of the region (see Figure 2) and the map of the anomalous magnetic field (see Figure 3) shows that the boundaries of positive and negative magnetic field anomalies trace the boundaries of lithospheric blocks.
The distribution of magnetic anomaly sources in the volume of the Earth's crust, shown in Figure 5 and Figure 6, confirms that relatively small sources determining local magnetic field anomalies should be assigned to the upper levels of the Earth's crust, and larger sources of regional anomalies, to its middle and lower levels. The magnetization pattern of the Earth's crust is not chaotic. This topology is considered a criterion for diamond potential [43]. The Zimneberezhny ring structure is located on an ancient ledge of the crystalline basement. The diamondiferous kimberlite field is located in the Riphean aulacogen of northwestern run in the conjugation zone of the Kola craton and the Mezen International Journal of Geosciences syneclise. The promising Nenoksa field of the development of olivine melilitites chimney deposits is associated with the Onega Peninsula ring structure [19].
As for the nature of the sources of intense high-frequency magnetic anomalies in the uppermost parts of the Earth's crust, it should be noted that these sources could be intrusions of the basic composition, fluvioglacial deposits of the Late Pleistocene-Holocene-moraines. In the middle and lower levels of the Earth's crust, weakly magnetic rocks predominate. Rocks of high magnetic susceptibility are attributed to the areas of ferruginous fluids intrusion, concentration of ferruginous volcanics, intrusive differentiates and products of their processing.
Such areas are associated with the cores of the most ancient consolidation of the crust, processed cores, suture zones, charnockite-granulite belts [44]. The main carrier of the magnetization of rocks is magnetite.