The structure of extremely low frequency field reflected by the ionosphere

The amplitudes of variations in the magnetic and electric fields at extremely low frequencies were studied on the Kola Peninsula during a five-day-long experiment under different geophysical conditions. These studies demonstrated that the influence of the ionosphere is distinguishable at frequencies of <10 Hz and the structure of field variations reflected by the ionosphere is similar to that of the transverse wave. It is established that the ratio between semimajor axes of polarization ellipses of the electric and magnetic fields is independent of the state of the ionosphere and is likely determined by the deep structure of the crust beneath the site of the experiment.     

The results of marine electromagnetic sounding with a high-power remote source in the Kola Bay in the Barents Sea

The first Russian six-component seafloor electromagnetic (EM) receivers were tested in an experiment carried out in Kola Bay in the Barents Sea. The signals transmitted by a remote high-power ELF source at several frequencies in the decahertz range were recorded by six receivers deployed on the seafloor along the profile crossing the Kola Bay. Although not all the stations successfully recorded all the six components due to technical failures, the quality of the data overall is quite suitable for interpretation. The interpretation was carried out by the three-dimensional (3D) modeling of an electromagnetic field with neural network inversion. The a priori geoelectrical model of Kola Bay, which was reconstructed by generalizing the previous geological and geophysical data, including the data of the ground magnetotelluric sounding and magnetovariational profiling, provided the EM fields that are far from those measured in the experiment. However, by a step-by-step modification of the initial model, we achieved quite a satisfactory fit. The resulting model provides the basis for introducing the corrections into the previous notions concerning the regional geological and geophysical structure of the region and particularly the features associated with fault tectonics.     

On the solution of the Molodenskii boundary value problem using successive approximations

Gravimetric methods of determining the figure of the Earth can be divided into two groups. The first employs an auxiliary surface—the geoid. The second employs the so-called quasigeoid as an auxiliary surface. Its main advantage is in that it only uses surface measurements and that it does not require knowledge of the structure of the Earth's crust. This method was treated in [1]. The results display an accuracy of the order of the Earth's flattening. The purpose of this paper is to show that this method can be applied theoretically to reach an arbitrary accuracy.     

Trefftz method for the solution of three-dimensional forward and inverse problems of geoelectrics

The Trefftz method is applied to the numerical solution of the three-dimensional (3D) forward problem for an electromagnetic field harmonically oscillating with time in a 3D environment, which ensures the possibility to solve a 3D inverse problem. Here, the known idea of simultaneous joint interpretation of the tangential components of electric and magnetic fields measured on the Earth's surface, which was suggested by A.N. Tikhonov for the development of the electromagnetic sounding method, is employed in the context of the numerical-analytical representation of the forward problem solution in accordance with the Trefftz method. Application of this method allows enables one to generalize the basic plane-stratified vertically 1D model of the medium by allowing for lateral variations in electric conductivity.     

地球物理问题中电磁感应的实验室模拟模型研究

在地球物理问题中,有两种方法通常用于解释地磁和地电异常,这就是数字模型技术和实验室模拟模型法。实验室模拟模型对于分析不易求得数学解的问题非常有用,并且已经广泛用于研究复杂的地球物理问题。本文评述了国外特别是加拿大开展模拟模型研究的情况;描述了平面波、线电流、磁偶极等各种类型的场源模型;给出了该法在研究海岸效应、岛陆通道电流、海浪电磁效应和各向异性导体等方面的应用。模拟模型测量和数字计算的比较结果显示了非常好的一致性,这就进一步证实了模拟模型法对于研究复杂二维和三维感应问题的可靠性。该项研究工作对于了解天然电磁场源的性质、进行地球物理勘探以及研究地球地壳和地幔的电性结构都十分有益。     

Lunisolar Tides Influence on Electrical Conductivity of the Earth’s Crust in the Territory of Kola Peninsula

The results of studying the influence of lunisolar tides on the electrical conductivity of the Earth’s crust in the territory of the Kola Peninsula are presented. Along with the results obtained by the authors, the data of other researchers are also considered. All the studies are based on the analysis of the field produced by the Zevs facility transmitting extremely low frequency (ELF) signals at 82–83 Hz. The measurements were carried out in different years at the Avva-Guba (1998), Lovozero (2009), and Imandra–Varzuga polygon (IVP) monitoring sites (2013) located 180, 90, and 160 km from the transmitter, respectively. The negative correlation between the tides and crustal electrical resistivity is revealed at all the points. This means that tidal rises of the Earth’s surface are accompanied by a decrease in resistivity and vice versa. The overview shows that the higher the resistivity of separate Earth’s crustal blocks the higher the relative amplitudes of the corresponding tidal responses that are observed.     

Review of electromagnetic coupling between the Earth's atmosphere and the space environment

We review our understanding of the electrical properties of the lower and upper atmosphere along with various possible sources of the electromagnetic energy near and far above the Earth's surface. The transport of electromagnetic energy from the atmosphere to the ionosphere and then to the magnetosphere and back to the Earth's surface via ionosphere and lower atmosphere is discussed. The electromagnetic coupling of various regions is also discussed.     

Cauchy integral analogues for the separation and continuation of electromagnetic fields within conducting matter

The main results in the theory of the interpretation of geopotential fields are generalized to the case of arbitrary variable electromagnetic fields by means of elaborating electrodynamic analogues for the integral of the Cauchy type.The generalized Kertz method for separating a variable electromagnetic field into parts related to the sources located in different regions of space is elaborated on the basis of this technique. The generalized Kertz method allows the selection of external and internal, normal and anomalous parts of the geomagnetic field, as well as the separation of geomagnetic anomalies into the surface and deep components caused by conductivity inhomogeneities in the Earth's crust and upper mantle.The theory of analytical continuation of variable electromagnetic fields in a conducting medium is also developed in the present work using the technique of analogues for the integral of the Cauchy type. It is shown that analytical continuation of a field downwards permits the determination of the location and form of deep geoelectric inhomogeneities according to the configuration of the isolines of flux functions for magnetic and electric fields.     

Deep Electrical Conductivity of the Archaean Blocks of Kola Peninsula in the Light of the Results of Murman-2018 Experiment: A Review

Izvestiya, Physics of the Solid Earth - Abstract—In this paper, we review the results of the deep electromagnetic soundings carried out on the Archaean blocks of the Kola Peninsula over the...     

Specific distribution of the noise electromagnetic field level at high latitudes in the vicinity of the first Schumann resonance

The distributions of the natural noise electromagnetic field level in the vicinity of the first Schumann resonance have been studied based on data from Lovozero observatory (the Kola Peninsula). Daily curves of noise level excesses over specified thresholds at different geomagnetic disturbances have been constructed. It has been indicated that the curves are morphologically similar on magnetically quiet days but are highly variable from day to day, although the averaged curves are similar in some characteristics to the unitary variation in the surface quasistatic electric field. The possibilities of applying the known formula for the probability distribution of the VLF atmospheric radio noise amplitude in order to analytically describe the noise level probability distribution have been considered. It has been detected that geomagnetic disturbances pronouncedly affect the distribution parameters, sharply increasing the amount of large-amplitude noise.     

An improvement for calculating the correlation between the geomagnetic and the Earth's gravitational fields

There is a correlation between the geomagnetic and the Earth's gravitational fields to a certain degree. We should consider the zeral harmonic terms to calculate the correlation function. Correlation between the geomagnetic non-dipole field of epoch 1980, which was moved eastwards 165; and the departure part of the Earth's gravitational field from the hydrostatic equilibrium, yields a value of 0.6278. It is significant at the 0.01 level. Calculating the correlations of different epochs, we find that the westward drifting velocity of the geomagnetic non-dipole field is 0.3° year⁻¹. It is not necessary to calculate the correlations between any individual harmonic term from either field.     

Tidal effects in the controlled-source ULF electromagnetic field on the Baltic crystalline shield

The results of the experiment on studying the dynamics of the electromagnetic field (EM) generated by the stationary controlled ULF-band source during 30 days on the Baltic crystalline shield are presented. Diurnal variations in the EM fields and slow variations in the surface impedance with a period of about 14 days are revealed. The diurnal variations in the fields are mainly due to the fluctuations in the ionospheric parameters caused by the changes in the daytime ionization of the ionosphere by solar radiation. By comparing the harmonic component with a period of about 14 days, which was established in the time series of surface impedance, with the slow tidal deformations of the Earth’s crust, we revealed the correlation between the EM variations and tidal processes in the Earth. The estimates for the probable changes induced by tidal deformations in the structure and conductivity of the underlying medium are obtained by modeling.     

Joint inversion of long‐offset and central‐loop transient electromagnetic data: Application to a mud volcano exploration in Perekishkul,Azerbaijan

Mud volcanism is commonly observed in Azerbaijan and the surrounding South Caspian Basin. This natural phenomenon is very similar to magmatic volcanoes but differs in one considerable aspect: Magmatic volcanoes are generally the result of ascending molten rock within the Earth's crust, whereas mud volcanoes are characterised by expelling mixtures of water, mud, and gas. The majority of mud volcanoes have been observed on ocean floors or in deep sedimentary basins, such as those found in Azerbaijan. Furthermore, their occurrences in Azerbaijan are generally closely associated with hydrocarbon reservoirs and are therefore of immense economic and geological interest. The broadside long‐offset transient electromagnetic method and the central‐loop transient electromagnetic method were applied to study the inner structure of such mud volcanoes and to determine the depth of a resistive geological formation that is predicted to contain the majority of the hydrocarbon reservoirs in the survey area. One‐dimensional joint inversion of central‐loop and long‐offset transient electromagnetic data was performed using the inversion schemes of Occam and Marquardt. By using the joint inversion models, a subsurface resistivity structure ranging from the surface to a depth of approximately 7 km was determined. Along a profile running perpendicular to the assumed strike direction, lateral resistivity variations could only be determined in the shallow depth range using the transient electromagnetic data. An attempt to resolve further two‐dimensional/three‐dimensional resistivity structures, representing possible mud migration paths at large depths using the long‐offset transient electromagnetic data, failed. Moreover, the joint inversion models led to ambiguous results regarding the depth and resistivity of the hydrocarbon target formation due to poor resolution at great depths (>5 km). Thus, 1D/2D modelling studies were subsequently performed to investigate the influence of the resistive terminating half‐space on the measured long‐offset transient electromagnetic data. The 1D joint inversion models were utilised as starting models for both the 1D and 2D modelling studies. The results tend to show that a resistive terminating half‐space, implying the presence of the target formation, is the favourable geological setting. Furthermore, the 2D modelling study aimed to fit all measured long‐offset transient electromagnetic Ex transients along the profile simultaneously. Consequently, 3125 2D forward calculations were necessary to determine the best‐fit resistivity model. The results are consistent with the 1D inversion, indicating that the data are best described by a resistive terminating half‐space, although the resistivity and depth cannot be determined clearly.     

Deep seismic soundings on the 1-AP profile in the Barents Sea: Methods and results

Profile 1-AP with a length of 1300 km intersects the Barents Sea from The Kola Peninsula to Franz Josef Land. The combined Common Depth Point (CDP) and Deep Seismic Sounding (DSS) seismic studies were carried out on this profile. The DSS measurements were conducted with the standalone bottom seismic stations with an interval of 5–20 km between them. The stations recorded the signals generated by the large air guns with a step of 250 m. Based on these data, the detailed P-velocity section of the Earth’s crust and uppermost mantle have been constructed for the entire profile and the S-velocity section for its southern part. The use of a variety of methods for constructing the velocity sections enabled us to assess the capabilities of each method from the standpoint of the highest reliability and informativity of the models. The ray tracing method yielded the best results. The 1-PR profile crosses two large basins—the South Barents and North Barents ones, with the thickness of the sediments increasing from 8 to 10 km in the south to 12–15 km in the north. The Earth’s crust pertains to the continental type along the entire profile. Its thickness averages 32 to 36 km and only increases to 43 km at the boundary between the two basins. The distinct change in the wave field at this boundary suggests the presence of a large deep fault in this zone. The high-velocity blocks are revealed in the crust of the South Barents basin, whereas the North Barents crust is characterized by relatively low velocities.     

Localized changes in geomagnetic total intensity values prior to the 1995 Hyogo-ken Nanbu (Kobe) earthquake

Changes in total geomagnetic field intensity, of 2–3 nT, were observed prior to the 1995 Hyogo-ken Nanbu (Kobe) earthquake at the Amagase (AMG) site, located approximately 70 km from the epicenter. We examined whether the observed variations are local signals arising from the Earth's crust, or global variations that are unlikely to originate from the crust. To remove global-scale variations in total geomagnetic intensity data, we employed a regional geomagnetic field model. Using data recorded at five reference sites in Japan, we estimated global-scale variations in total geomagnetic intensity, and removed them from the observed total geomagnetic intensity at the AMG site. The reminder still showed variations during the period prior to the Kobe earthquake. In addition, these pre-seismic variations include two of the largest shifts recorded during the entire observation period at the AMG site, raising the possibility that these variations were indeed related to the earthquake. These variations cannot be interpreted as signals arising from the area close to the seismic source because of the large distance between the epicenter and the site. Therefore, our results raise the possibility that the physical state of the Earth's crust shows marked changes over a wide region in the lead-up to a seismic event.     

The effect of anisotropic diffusivity on rotating magnetoconvection in the Earth's core

A thermal diffusive process in the Earth's core is principally enhanced by small-scale flows that are highly anisotropic because of the Earth's rapid rotation and a strong magnetic field. This means that a thermal eddy diffusivity should not be a scalar but a tensor. The effect of such anisotropic tensor diffusivity, which is to be prescribed, on dynamics in the Earth's core is investigated through numerical simulations of magnetoconvection in a rapidly rotating system. A certain degree of anisotropy has an insignificant effect on the character, like kinetic and magnetic energies, of magnetoconvection in a small region with periodic boundaries in the three directions. However, in a region with top and bottom rigid boundary surfaces, kinetic and magnetic energies of magnetoconvection can be altered by the same degree of anisotropy. This implies that anisotropic tensor diffusivity affects on dynamics in the core, in particular near the boundary surfaces.     

基于全波形反演的探地雷达数据逆时偏移成像

逆时偏移成像(RTM)常用来处理复杂速度模型,包括陡倾角及横向速度变化剧烈的模型.与常规偏移成像方法(如Kirchhoff偏移)相比,逆时偏移成像能提供更好的偏移成像结果,近些年逆时偏移成像越来越广泛地应用到勘探地震中,它逐渐成为石油地震勘探中的一种行业标准.电磁波和弹性波在动力学和运动学上存在相似性,故本文开发了基于麦克斯韦方程组的电磁波逆时偏移成像算法,并将其应用到探地雷达数据处理中.时间域有限差分(FDTD)用于模拟电磁波正向和逆向传播过程,互相关成像条件用于获得最终偏移结果.逆时偏移成像算法中,偏移成像结果受初始模型影响较大,而其中决定电磁波传播速度的介电常数的影响尤为重要.本文基于时间域全波形反演(FWI)算法反演获得了更为精确的地下介电常数模型,并将其反演结果作为逆时偏移成像的初始介电常数模型.为了验证此算法的有效性,首先构建了一个复杂地质结构模型,合成了共偏移距及共炮点探地雷达数据,分别应用常规Kirchhoff偏移算法及逆时偏移成像算法进行偏移处理,成像结果显示由逆时偏移成像算法得到的偏移结果与实际模型具有较高的一致性;此外本文在室内沙槽中进行了相关的物理模拟实验,采集了共偏移距及共炮点探地雷达数据,分别应用Kirchhoff和叠前逆时偏移成像算法进行处理,结果表明叠前逆时偏移成像在实际应用中能获得更好的成像效果.     

长偶极大功率可控源电磁波响应特征研究

地球物理学中关于电磁波勘探研究通常采用的是地球半空间模型.然而,对于几十公里的有限长电缆源（长偶极源）,远距离电磁波场探测必须要考虑电离层的影响,它是一个全空间问题.关于包含电离层、空气层和地球介质（我们称“地-电离层”模式）的电磁波场特征的研究在国外较少,国内几乎是空白.本文采用全空间积分方程法首先对小尺度的可控源电磁波场特征进行了研究,由于此时电离层的影响可忽略,它应该和半空间成熟的CSAMT模拟结果一致,对比结果表明,二者是一致的,验证了全空间模拟方法的可靠性和有效性.随后进行了50 km长电缆电离层和空气层高度都为100 km的“地-电离层”模式大尺度电磁波场模拟,以探讨大尺度可控源电磁波场的特征.给定频率的“地-电离层”模式电磁场的衰减曲线表明长电缆远距离电磁波场由于受电离层的作用存在衰减逐渐变小的过渡场和衰减变小的波导场.为了探讨复杂介质“地-电离层”模式电磁波特征,对“地-电离层”模式的典型地盾和地台多层介质模型进行了数值模拟,得到了偶极源长度50 km、电流200A、收发距离远达1600 km和2500 km的合理的电磁场结果.最后,对一简单含油储层结构模型进行了长偶极、大功率、远距离电磁波场响应计算.储层横向不均匀复杂结构模拟的结果表明,考虑电离层和大气层的“地-电离层”模式大尺度深层复杂介质模拟时,电磁场对深部目标体仍有很好的异常响应.     

Hydrogeological indications of recent movements of the earth's crust

Hydrogeologic indications of recent movements of the Earth's crust in two regions of North Tian-Shan have been studied. Comparison of ground-water level changes and variations of recent vertical movements according to benchmarks located near a well demonstrates their good correlation. The ground-water level varies out of phase with the direction of vertical movements. A correlation between level variation and seismic regime has been observed up to a well, located in a near-fault zone. In the region of the Kochkorskaya Depression, well observations show that against a background of constant increasing of level, sharp decays are observed. The latter can be connected with creep displacements. Hydrogeologic observations, together with periodic levelling enable us to study recent movements of the Earth's crust in detail.     

Waveform Retrieval and Phase Identification for Seismic Data from the CASS Experiment

The little destruction to the deployment site and high repeatability of the Controlled Accurate Seismic Source (CASS) shows its potential for investigating seismic wave velocities in the Earth's crust. However, the difficulty in retrieving impulsive seismic waveforms from the CASS data and identifying the seismic phases substantially prevents its wide applications. For example, identification of the seismic phases and accurate measurement of travel times are essential for resolving the spatial distribution of seismic velocities in the crust. Until now, it still remains a challenging task to estimate the accurate travel times of different seismic phases from the CASS data which features extended wave trains, unlike processing of the waveforms from impulsive events such as earthquakes or explosive sources. In this study, we introduce a time-frequency analysis method to process the CASS data, and try to retrieve the seismic waveforms and identify the major seismic phases traveling through the crust. We adopt the Wigner-Ville Distribution (WVD) approach which has been used in signal detection and parameter estimation for linear frequency modulation (LFM) signals, and proves to feature the best time-frequency convergence capability. The Wigner-Hough transform (WHT) is applied to retrieve the impulsive waveforms from multi-component LFM signals, which comprise seismic phases with different arrival times. We processed the seismic data of the 40-ton CASS in the field experiment around the Xinfengjiang reservoir with the WVD and WHT methods. The results demonstrate that these methods are effective in waveform retrieval and phase identification, especially for high frequency seismic phases such as PmP and SmS with strong amplitudes in large epicenter distance of 80–120 km. Further studies are still needed to improve the accuracy on travel time estimation, so as to further promote applicability of the CASS for and imaging the seismic velocity structure.