Results Of Magnetotelluric Sounding Research Articles

Cluster analysis of section properties with the aim of searching areas for drilling exploration geothermal wells

Basing on the results of magnetotelluric sounding, carried out earlier in the geothermal area under examination and developed 2D-models of electrical resistivity, porosity, permeability, temperature, thermal conductivity and specific heat capacity, an integrated cluster analysis of sections of enumerated parameters is fulfilled . 
 Combined cluster analysis enabled to detect two subsurface areas promising for drilling exploratory boreholes. One of them coincides with already developed geothermal reservoir, while another one is located at depths 2-3km in another part of the section and may be of great interest for future exploration.

New magnetotelluric data in the suture zone of Fennoscandia and Sarmatia

Presents the results of magnetotelluric soundings acquired in 2020 along the profile Rzhev– Velikie Luki, in the suture zone of three parts of Precambrian crust of East European platform: Fennoscandia, Sarmatia and Volga–Ural. In the central part of the profile, anomaly zone of increased conductivity in the crust was revealed. The crust conductor is weakly expressed on the components of the telluric tensor due to the screening effect of the conductive sedimentary cover; therefore, the focus in the qualitative analysis of data and inversion was made on magnetovariational parameters.According to the results of two–dimensional inversion carried out in the program of A.E. Kaminsky ZONDMT2D, on the geoelectric model a subvertical zone of low resistivity in the crust is identified, which is probably associated with the paleosubduction zone of the Belarusian Ocean Plate.

FIRST RESULTS OF MAGNETOTELLURIC SOUNDINGS FOR MAPPING THE RIPHEAN SEDIMENTS OF THE BAIKAL FOREDEEP

The article presents the first results of magnetotelluric sounding (MTS) surveys of the Baikal foredeep. This study is part of the project of the Institute of the Earth’s Crust SB RAS, which investigates the geological and geoelectrical structure of the southeastern marginal part of the Siberian platform and updates the model of the Baikal rift zone. An objective is to preliminary assess the mining potential of the Riphean strata in the Baikal foredeep. The relevance of the study centers on the possibility of discovering a new promising oil-gas field in proximity to energy-intensive industrial and agricultural enterprises. The MTS section of the sedimentary cover shows an increased conductivity of its lower part, which is most likely due to highly porous carbonate-terrigenous rocks in the middle – lower part of the ‘Baikal three-member’ structure. The intra-lithospheric conductive layer that hosts most of the earthquake hypocenters of the Baikal rift is investigated.

Studying the Depth Structure of the Kyrgyz Tien Shan by Using the Seismic Tomography and Magnetotelluric Sounding Methods

This paper presents new results of detailed seismic tomography (ST) on the deep structure beneath the Middle Tien Shan to a depth of 60 km. For a better understanding of the detected heterogeneities, the obtained velocity models were compared with the results of magnetotelluric sounding (MTS) along the Kekemeren and Naryn profiles, running parallel to the 74 and 76 meridians, respectively. We found that in the study region the velocity characteristics and geoelectric properties correlate with each other. The high-velocity high-resistivity anomalies correspond to the parts of the Tarim and Kazakhstan-Junggar plates submerged under the Tien Shan. We revealed that the structure of the Middle Tien Shan crust is conditioned by the presence of the Central Tien Shan microcontinent. It manifests itself as two anomalies lying one below the other: the lower low-velocity low-resistivity anomaly, and the upper high-velocity high-resistivity anomaly. The fault zones, limiting the Central Tien Shan microcontinent, appear as low-velocity low-resistivity anomalies. The obtained features indicate the fluid saturation of the fault zones. According to the revealed features of the Central Tien Shan geological structure, it is assumed that the lower-crustal low-velocity layer can play a significant role in the delamination of the mantle part of the submerged plates.

3D model of Svecofennian Accretionary Orogen and Karelia Craton based on geology, reflection seismics, magnetotellurics and density modelling: Geodynamic speculations

A 3D model of deep crustal structure of the Archaean Karelia Craton and late Palaeoproterozoic Svecofennian Accretionary Orogen including the boundary zone is presented. The model is based on the combination of data from geological mapping and reflection seismic studies, along profiles 1-EU, 4B, FIRE-1-2a-2 and FIRE-3-3a, and uses results of magnetotelluric soundings in southern Finland and northern Karelia. A seismogeological model of the crust and crust–mantle boundary is compared with a model of subhorizontal velocity-density layering of the crust. The TTG-type crust of the Palaeoarchaean and Mesoarchaean microcontinents within the Karelia Craton and the Belomorian Province are separated by gently dipping greenstone belts, at least some of which are palaeosutures. The structure of the crust was determined mainly by Palaeoproterozoic tectonism in the intra-continental settings modified by a strong collisional compression at the end of the Palaeoproterozoic. New insights into structure, origin and evolution of the Svecofennian Orogen are provided. The accretionary complex is characterized by inclined tectonic layering: the tectonic sheets, ~15 ​km thick, are composed of volcanic-sedimentary rocks, including electro-conductive graphite-bearing sedimentary rocks, and electro-resistive granitoids, which plunge monotonously and consecutively eastward. Upon reaching the level of the lower crust, the tectonic sheets of the accretionary complex lose their distinct outlines. In the seismic reflection pattern they are replaced by a uniform acoustically translucent medium, where separate sheets can only be traced fragmentarily. The crust–mantle boundary bears a diffuse character: the transition from crust to mantle is recorded by the disappearance of the vaguely drawn boundaries of the tectonic sheets and in the gradual transition of acoustically homogeneous and translucent lower crust into transparent mantle. Under the effect of endogenic heat flow, the accretionary complex underwent high-temperature metamorphism and partial melting. Blurring of the rock contacts, which in the initial state created contrasts of acoustic impedance, was caused by partial melting and mixing of melts. The 3D model is used as a starting point for the evolutionary model of the Svecofennian Accretionary Orogen and for determination of its place in the history of the Palaeoproterozoic Lauro-Russian intracontinental orogeny, which encompassed a predominant part of the territory of Lauroscandia, a palaeocontinent combining North American and East European cratons. The model includes three stages in the evolution of the Lauro-Russian Orogen (~2.5, 2.2–2.1 and 1.95–1.87 ​Ga). The main feature of the Palaeoproterozoic evolution of the accretionary Svecofennian Orogen and Lauroscandia as a whole lay in the causal link with evolution of a superplume, which initiated plate-tectonic events. The Svecofennian–Pre-Labradorian palaeo-ocean originated in the superplume axial zone; the accretionary orogens were formed along both continental margins due to closure of the palaeo-ocean.

The Deep Geoelectrical Section of the Avacha–Koryak Zone of Contemporary Volcanism, Kamchatka

The methods and results of magnetotelluric sounding in the range of 0.0001–1000 s and more with the use small portable magnetotelluric stations are considered. Data on the distribution of electrical conductivity at depths beneath the Avacha–Koryak zone of contemporary volcanism are obtained. The obtained data are interpreted using the modern techniques and methods.

On Electrical Conductivity at Depth in the Shanuch Copper–Nickel and the Aginskoe Gold-Ore Fields, Kamchatka

This paper is concerned with the method of interpretation and with the results of magnetotelluric sounding (MTS) along the line connecting the Shanuch copper–nickel and the Aginskoe gold ore deposit. The interpretation revealed that the upper section divides into two blocks according to the electrical resistivity. The western block has a high resistivity, which is due to the metamorphic and igneous basement rocks that are exposed at the ground surface. The eastern block has a low resistivity, which is due to the presence of a conductive sedimentary–volcanogenic cover involving a wide network of discontinuities. A low resistivity layer was identified at depths of 30–60 km. The deep-seated faults that have been identified by interpretation in the eastern block penetrate through the lithosphere as far as the conductive layer; fluids can move along these faults upward into the upper crust where at lower temperatures they release minerals whose accumulations make up ore deposits of nonferrous and noble metals.

Inversion of magnetotelluric data in fault zones of Gorny Altai based on a three-dimensional model

Results of magnetotelluric sounding (MTS) in Gornyi Altai are interpreted on the basis of a numerical model of MTS curve distortions in a 3D earth. The distortions are modeled using the Trefftz method permitting the application of models of different degrees of similarity to the test medium (depending on available computational resources). The main advantages of this approach are demonstrated. There is no need to choose between different MTS curves (transverse and longitudinal, minimum and maximum, undistorted and distorted). Procedures of normalizing these curves become unnecessary. All recorded curves are fully used as input data for their inversion. Optimization of the model of the medium taking into account the distortions of MTS curves caused by surface and depth inhomogeneities improves the reliability of geoelectric sections.

A geoelectric model for the area of the Tolbachik eruption (named after the 50 year anniverary of the Institute of Volcanology and Seismology)

We consider the methods and results of magnetotelluric sounding in the AMTS and MTS modifications. Audiomagnetotelluric sounding (AMTS) was carried out for the first time in the area of a recent Tolbachik eruption. The results from our analysis of the magnetotelluric parameters show that the geoelectric medium involving a regional fault can be fitted by a 2D inhomogeneous model. The longitudinal and transverse sounding curves were assumed as the leading elements for interpretation. A joint analysis of these curves and of pseudo-sections of impedance phases provides evidence of a geoelectric inhomogeneity in the area where the Naboko Vent is situated. A bimodal inversion of the AMTS curves yielded a geoelectric section that contains a conductive inhomogeneity that is possibly related to a fault that carried fluids up to the ground surface. Along with AMTS, we used MTS curves in a broader range to identify a crustal conductive anomaly at depths of 15–35 km. The data from AMTS, MTS, and other geological and geophysical information were used to develop a conceptual model for the area of study that characterizes a possible origin of the anomalous zones. We obtained approximate estimates of rock porosity in the fault zone that transported magma melts upward into the overlying rocks in the area of the Naboko Vent.

New data on the deep structure of the northern part of the Gulf of Tonkin in the South China Sea based on the results of magnetotelluric soundings

We present a geoelectric model for the northern part of the Gulf of Tonkin in the South China Sea, which was developed for the first time based on the data of magnetotelluric studies carried out in 2012- 2013. The existence of a regionally developed aniso� tropic conducting fractured horizon and a steeply dip� ping electrically conductive fault zones is a distin� guishing property of this model, which has a layer� block structure. The nature of the zones of electric conductivity is related to the accumulation of a water� graphite mixture during longterm propagation of the flow of deep gases, which include H 2 and CH4. This allows us to consider the outcrops of the deep fluid sat� urated fractures as potential oil bearing regions. In the conditions when traditional oil and gas deposits are being exhausted, the search for new objects of accumulation of hydrocarbons has become one of the present pressing problems. In recent years, research into fissured formations in the upper level of the basement, which is a transition complex between the crystal basement and the sedimentary cover, has become widely developed (1). The rocks related to the transition basement are almost not changed by the processes of regional metamorphism. They are dislo� cated more greatly than the overlying columns of the sedimentary cover, transformed by recrystallization processes and catagenesis, and have a different struc� tural position. The oilandgas-bearing properties of such complexes have been established in different regions of the world: East Siberia (2), Venezuela, US, Libya, Egypt, and India (3). In 1988, a unique deposit was discovered in the granites of the Mesozoic base� ment on the shelf of South Vietnam (4). Deep geophysical processes that give us the possi� bility to make a primary estimate of the structural and material characteristics of the basement in the study regions play an important role in the study of these structures. The estimate includes the existence of fis� sured complexes and locations of permeable zones in the lithosphere that provide pathways for migration of deep fluid fluxes, which are sources of saturations of the fissured complexes with hydrocarbons. All this allows us to forecast the oilandgas-bearing potential of the study region. One of the geophysical methods used for the solution of these problems is magnetotel� luric sounding, which makes it possible to study the differentiation of electric resistivity of the tectono� sphere deep section depending on its structural mate� rial composition.

The porosity trend and pore sizes of the rocks in the continental crust of the earth: Evidence from experimental data on permeability

The depth trends of permeability are constructed from the measurements of the tight rocks typical of the basement of the continental crust at temperatures up to 600°C and pressures up to 200 MPa. It is established that the permeability decreases with depth. The statistical processing of the experimental data yielded the generalized dependence logk = −12.6-3.23H 0.223. The method is suggested and, based on the experimental data on permeability, the estimates are obtained for the effective diameters of the pore channels and effective porosity at the PT parameters corresponding to the in situ deep zones of the continental crust. It is found that porosity decreases with depth, while distinct depth dependence of the pore sizes is not observed. The dependence of porosity on the depth is approximated by the relationship logφ = −0.65–0.1H + 0.0019H 2. The porosity is estimated at a few percent for a depth of 10 km with a decline to 0.01–0.1% at 35 km. The estimates of porosity retrieved from the experimental data agree with the theoretical calculations based on the present-day ideas of the structure of the discrete media and with the results of magnetotelluric sounding. Thus, according to three independent estimates, the porosity of the rocks of the continental crust decreases with depth. At the same time, in both the intermediate and lower crust there are intervals where the porosity values suggest the presence of fluid-saturated horizons at these depths.

Geoelectric section of the lithosphere of the Amur-Zeya sedimentary basin deduced from magnetotelluric soundings

The results of magnetotelluric sounding are analyzed along the Korfovo-Astashikha-Novosergeevka profile 200 km long in the south of the Amur-Zeya sedimentary basin. The Korfovo-Astashikha and Korfovo-Novosergeevka profiles were sounded in the AMT and AMT + MTS regimes with a step between the observation points of 1 and 5 km, respectively. The shape of the MTS curves, their variations along the profiles, the shape of the polar plots of the main and additional impedance, and the parameters of the heterogeneity (N) and asymmetry (skew) are characterized. The dimensions of the geological medium is estimated and methods of the interpretation of the magnetotelluric data are chosen. The geoelectric sections are constructed for the depths of 3 and 150 km. The structure and electric properties of the sedimentary cover, the Earth’s crust, and the upper mantle are characterized.

The deep structure of the southern kamchatka volcanic zone from geophysical data

We discuss the interpretation method and results of magnetotelluric soundings in combination with other geological and geophysical data. The interpretation method was developed by studying possible distortions in MTS curves using 3D-numerical modeling of the magnetotelluric field. Deep conductivity was studied by using longitudinal MTS curves below a period of 400 s, which are nearly unaffected by the induction effect due to marine electrical currents. Transverse curves were used to obtain more detail for the geoelectric model. Inversion of average longitudinal MTS curves resulted in a geoelectric section of the lithosphere down to a depth of 60 km. Anomalies of high conductivity in the lithosphere were detected and were found to produce certain effects in gravity and seismic velocities. MTS and seismic tomography data were used to determine the possible origin of the high conductivity anomaly and to estimate rock porosity and the concentration of magma melts.

Geoelectric section of the lithosphere of the Amur-Zeya sedimentary basin deduced from magnetotelluric sounding along the Blagoveshchensk-Birakan profile

The results of magnetotelluric sounding along the 350-km long Blagoveshchensk-Birakan profile are discussed. The profile begins in the Longjiang-Selemdzha orogenic belt and ends in the Jiamusi-Bureya massif, thus intersecting the southern Amur-Zeya sedimentary basin from the northwest to the southeast. Twelve soundings have been performed in the broad range from 1 × 104 to 2 × 10−4 Hz. Geoelectric sections have been constructed for the depths of 2 and 150 km with the determination of the geoelectric parameters of the sedimentary cover within the basin and the identification of the zones of anomalous conductivity in the Earth’s crust and upper mantle.

Geoelectric section of the lithosphere in the central part of the Middle Amur sedimentary basin (Far East) based on the magnetotelluric sounding data

Results of magnetotelluric soundings (MTS) along the profile plotted across the strike of the Middle Amur sedimentary basin are considered. The results of field studies are analyzed and different factors influencing the behavior of sounding curves are discussed. A reference curve and reference geoelectric section selected on the basis of the previous MTS works within the Middle Amur depression allowed us to compensate for the influence of different geoelectric inhomogeneities and construct a starting model for two-dimensional mathematical modeling based on the field data. As a result, a geoelectric model of the lithosphere structure in the Middle Amur sedimentary basin across its strike was compiled.

Depth Distribution of the Electric Conductivity on the Czechoslovak Territory

Results of magnetotelluric soundings have been obtained for a series of five field stations located approximately along the international profile of deep seismic sounding No. VI. Multilayer sounding curves at almost all stations show anisotropic and/or horizontally non-homogeneous situation. Methods for estimating the inhomogeneity measures were applied and attempts have been made to locate the homogeneity axis and corresponding two-dimensional structures. Directional properties of the electric conductivity have been discussed and fitted to the local geology.On respecting the uncertainty and ambiguity of apparent resistivity curves the depths of remarkable discontinuities have been estimated and traced along the profile.