A three-dimensional crustal geoelectric model of the Ukrainian Shield

A 3-D crustal geoelectric model of the Ukrainian Shield (USh) is constructed from magnetic variation data. High electrical conductivity anomalies with resistivities of 1–100 Ω m are located at depths of up to 30 km from the basement surface. A high conductivity layer with ρ = 25 Ω m and with its upper boundary at a depth of 70 km is supposed to exist in the upper mantle of the southwestern USh.     

Three-dimensional geoelectrical model of metallogenic zones in the Kuznetsk-Alatau folded area

The three-dimensional (3D) geoelectric model of the Kuznetsk-Alatau folded area is reconstructed by magnetotelluric inversion using 3D fitting. It is established that the zones of ore mineralization within the Batenevsky massif are confined to the subvertical faults characterized by the electric resistivity of 100–300 Ω m. Blocks with ρ ≈ 10−100 Ω m are identified at a depth below 10 km in the western part of the model. The blocks are located close to the areas marked by the increased thermal flux, reduced seismic velocities, and elevated Moho boundary. This is probably associated with the presence of the rift zone in this area.

     

Deep electromagnetic sounding of the lithosphere in the eastern Baltic (fennoscandian) shield with high-power controlled sources and industrial power transmission lines (FENICS experiment)

The paper addresses the technique and the first results of a unique experiment on the deep tensor frequency electromagnetic sounding, the Fennoscandian Electrical conductivity from results of sounding with Natural and Controlled Sources (FENICS). In the experiment, Energy-1 and Energy-2 generators with power of up to 200 kW and two mutually orthogonal industrial 109- and 120-km-long power transmission lines were used. The sounding frequency range was 0.1–200 Hz. The signals were measured in the Kola-Karelian region, in Finland, on Svalbard, and in Ukraine at distances up to 2150 km from the source. The parameters of electric conductivity in the lithosphere are studied down to depths on the order of 50–70 km. A strong lateral homogeneity (the one-dimensionality) of a geoelectric section of the Earth’s crust is revealed below depths of 10–15 km. At the same time, a region with reduced transverse crustal resistivity spread over about 80 000 square kilometers is identified within the depth interval from 20 to 40 km. On the southeast the contour of the anomaly borders the zone of deepening of the Moho boundary down to 60 km in Central Finland. The results are compared with the AMT-MT sounding data and a geodynamic interpretation of the obtained information is carried out.     

The effect of screening of the magnetotelluric field by high resistivity layers: Three-dimensional numerical modeling constraints

A 3-D numerical modeling approach is applied to the effect of screening of the magnetotelluric (MT) field by high resistance (≥10⁶ Ω m²) layers, which up to now has mainly been studied in terms of 2-D heterogeneous models of geological sections [Berdichevsky and Yakovlev, 1990]. Three-layer models above a poorly conducting basement (ρ=10⁴ Ω m) are used. The resistance of the screening layer was generally taken equal to 2×10⁶ Ω m² and its thickness was varied from 200 to 2000 m. The resistivity of the host medium was set at 10 Ω m. Heterogeneities of a low and a high resistivity (ρ=10^?3 and 10⁴ Ω m, respectively) ranging in horizontal size from 4 to 40 km and having a height of 1–2.8 km (a protrusion on the basement) were examined. Based on calculations of these models, 2-D and 3-D screening effects were compared. The 3-D modeling determined values of model parameters at which the screening properties are preserved both in the case of a screen of limited horizontal dimensions and in the presence of a rupture in the screen comparable in horizontal size with the 3-D heterogeneity. As follows from the modeling results, the screening effect of the high resistivity model layers seriously complicates the use of MT soundings for the identification of a local heterogeneity of both a low and a high resistivity (ρ=10^?3 and 10⁴ Ω m, respectively) if its lateral size is smaller than the 4-km thickness of the studied three-layer section in question and its height is 1–1.5 km. The regular patterns of the screening effect revealed in this work are of interest in electromagnetic sounding applications.     

长白山天池火山岩浆系统分析

针对外媒报道长白山天池火山在近2年内有可能喷发的言论,在长白山天池火山区布测了一条长度约为103 km的二维大地电磁测深观测剖面,对火山区深部电性结构进行探测研究.由于研究区内不明来源的电磁干扰非常强,对数据采用了远参考处理、Robust处理、Rhoplus分析、张量阻抗分解和基于一维层状介质电阻率与相位互算方法等先进处理技术,获取到一批在强干扰区质量较为可靠的电磁数据,利用数据计算分析了长白山天池火山区二维构造走向和感应矢量特征,采用NLCG二维反演技术对资料进行了二维反演解释,并将反演结果与前人探测结果进行了对比分析.探测结果表明:在天池火山口下方存在明显的直立型岩浆通道,岩浆通道在下方约5~8 km位置形成关闭;在火山口下方往北方向附近,在埋深位置约7 km深处存在一个明显的低阻异常体,电阻率小于10 Ωm,且与岩浆通道对接,推测其可能是地表浅部发育的岩浆囊;在长白山山门附近C07-C09号测点之间和C04-C05号测点之间,在埋深约7~17 km深处发现近直立型低阻带,低阻带与下方低阻体直接相连,推测低阻带内赋存有活动的岩浆;随着埋深的增加,从天池火山口南部约20 km位置往北方向,在埋深13~30 km之间壳内广泛发育明显的低阻异常体,推测其可能是活动的岩浆囊.反演结果与前人探测结果整体电性特征相似,但又局部不同.     

Results of electromagnetic sounding with JASC submarine cable

We present the results of long-term deep geoelectric studies using the JASC (Japan Sea Cable) submarine communication cable in the region of the Sea of Japan. In the 2D inversion of the amplitude and phase’s apparent resistivity curves and the frequency dependences of the tipper, we invoked the geological and geophysical information about the region and on-shore electromagnetic observations to fit the model to the observations. The resulting geoelectrical cross section of the region of the Sea of Japan along the JASC cable obtained in this way agrees well with the experimental data. The upper part of the section contains a conductive block beneath the bottom of the Central basin of the Sea of Japan at a depth of 10–40 km, a fault submerging below the continent in the marginal part and a deep fault in the continental region. In the lower portions of the cross section, the high-resistivity block interrupts the continuity of the horizontal conductive layers beneath the Yamato Uplift, and the conductive bottom part of the geoelectric cross section submerges under the continent. In the continental segment of the cross section, there is a large block with reduced electric resistivity, which is located between the conductive layers at a depth interval of 200–560 km. We analyze the characteristic features of the geoelectric cross section and the deep section imaged by seismic tomography in the region of the Sea of Japan.     

A comparative study of results obtained in magnetotelluric deep soundings in Villarrica active volcano zone (Chile) with gravity investigations, distribution of earthquake foci, heat flow empirical relationships, isotopic geochemistry Sr/Sr and SB systematics

The first magnetotelluric deep soundings in Chile were carried out during 1986 in the Villarrica active volcano zone (39°25′S, 71°57′W). In the TM mode of polarization, the curves show a distorted segment with dispersion. A static distortion at long periods is observed in curves in the TE mode of polarization; the segment was shifted vertically to fit the geomagnetic global model values at daily periods. This modified curve was used for 1D modelling to determine the electrical structure in the study area. The upper level of the intermediate conducting layer of resistivity 20–60 ω m is found to be at 35–50 km depth. A higher resistivity layer (600 ω m), starting at 100 km depth, may be resolved in the intermediate conducting layer. A sharp decrease in the resistivity is shown by the model at 500 km.

Large heterogeneities at the level of the conducting layer encountered in the 1D modelling, and increased resistivity of the ultimate layer, may account for distortion observed at long periods. Two-dimensional test models show that the conducting layer in the area of Villarrica volcano may be an anomalous heated layer surrounded by rocks of higher resistivity of about 2 × 10³ ω m. These features correspond to the interaction with a subsiding oceanic lithosphere resulting in a complex thermal structure and perturbed resistivity distribution in transition zones of the Pacific type such as Chile, and to the existence of a megafault and a system of fractures in the sounding area. These facts make it difficult to determine the conductance of the electrical asthenosphere.

The parameters of the model structure correlate well with geophysical and geochemical results obtained in the area by other workers. Gravity studies indicate a maximum crustal thickness of about 37 km, which implies a non-full compensation according to the Airy hypothesis. The morphology of the Wadati-Benioff zone clearly shows a sharp decrease of earthquake foci at 50 km depth, and a reinitiation of seismic activity from a depth of 100 km down to 160 km. Kaufman and Keller and Levi and Lysak empirical relationships between heat flow and thickness of the lithosphere are somewhat consistent when the parameters of the model structure are assumed. The strontium isotope ratios are indicative of minimal crustal contamination of mantle-derived magmas, thus allowing a relatively thin crust under the area. Furthermore, the SB index of partial melting of mantle peridotite may suggest the contribution of uprising material from the deep asthenosphere through fractures extending to depth.     

Resistivity and self-potential changes associated with volcanic activity: The July 8, 2000 Miyake-jima eruption (Japan)

The Miyake-jima volcano abruptly erupted on July 8, 2000 after 17 years of quiet and gave birth to a crater, 1 km in diameter and 250 m deep. This expected unrest was monitored during the years 1995–2000 by electromagnetic methods including DC resistivity measurements and self-potential (SP) surveys. Beneath the 2500 yr old Hatcho-Taira summit caldera audio-magnetotelluric soundings made in 1997–98 identified a conductive medium, 200–500 m thick (within the 50 Ω m isoline) located at a few hundred metres depth. It was associated with the active steady-state hydrothermal system centred close to the 1940 cone and extending southward. A DC resistivity meter set in a Schlumberger array with 600, 1000 and 1400 m long injection lines evidenced strong resistivity changes between September 1999 and July 3, 2000 in the vicinity of the newly formed crater. The apparent resistivity has reached about three times its initial values on the 1400 m long line and has lowered to about 20% on the 600 m line. Just prior to the July 8, 2000 eruption SP mapping made inside the summit Hatcho-Taira caldera revealed negative anomalies where positive ones had occurred during the previous tens of years. The largest negative anomaly, −225 mV in amplitude, mainly took place above the 1940 cone which collapsed in the crater formation. A permanent 1 km long SP line across the caldera suggests accelerating changes during the 3 months preceding the eruption. On a larger scale, the comparison between 1995 and 2000 surveys has shown a global increase of the hydrothermal activity beneath the volcano. Its source could have been 250 m to the south of the crater. These observations suggest that the hydrothermal system was slowly disturbed in the months preceding the eruption while drastic changes have occurred during the 2 weeks before the summit collapse when tectonic and volcanic swarms have appeared.     

S-wave shadows in the Krafla Caldera in NE-Iceland,evidence for a magma chamber in the crust

During the present tectonic activity in the volcanic rift zone in NE-Iceland it has become apparent that the attenuation of seismic waves is highly variable in the central region of the Krafla volcano. Earthquakes associated with the inflation of the volcano have been used to delineate two regions of high attenuation of S-waves within the caldera. These areas are located near the center of inflation have horizontal dimensions of 1–2 km and are interpreted as the expression of a magma chamber. The top of the chamber is constrained by hypocentral locations and ray paths to be at about 3 km depth. Small pockets of magma may exist at shallower levels. The bottom of the chamber is not well constrained, but appears to be above 7 km depth. Generally S-waves propagate without any anomalous aftenuation through laver 3 (v_p=0.5 km sec^?1) across the volcanic rift zone in NE-Iceland. The rift zone therefore does not appear to be underlain by an estensive magma chamber at crustal levels. The Krafla magma chamber is a localized feature of the Krafla central volcano.     

Analysis of Deformation Characteristic and Uplift Mechanism in Longgang Volcanoes,Jilin

吉林龙岗火山区是第四纪活动火山,具有潜在喷发危险。通过GNSS和水准资料分析区域三维地壳运动,得出2011—2019年相对欧亚板块水平运动以东南向为主,速度＜10 mm/a。敦化-密山断裂以东受日本2011年“3·11地震”影响强烈,现以拉张运动为主;近年来水准资料揭示火山区垂直运动以隆升为主。2014—2019年的InSAR资料显示隆升集中在靖宇一带。结合MT剖面反演得到的深部电性结构,龙岗火山区西侧高阻体分布在深度18 km以上,金龙顶子火山下方最浅。中部为早期喷发形成的火山,下方高阻体分布在深度40 km以上,东侧抚松一带分布在深度约20 km以上,地壳范围内的高阻结构表明岩浆已固结。高阻结构层可分辨出断层两侧电阻的差异性。高阻体下方存在大规模低阻结构,推测为中下地壳岩浆系统。金龙顶子火山深度10 km以下的低阻结构可能为岩浆通道,并与中下地壳岩浆系统相连。东侧区域岩浆平均深度约30 km,相对较浅。龙岗火山区幔源物质的上涌及间断性的向上运移引起地壳隆升及地震活动。     

An inversion of geomagnetic deep sounding data using simulated annealing

A maiden attempt on the use of the global optimization technique of Simulated Annealing (S.A.) inversion to model the conductivity structure derived from the geomagnetic deep sounding data of NW India is reported here. The location of the proposed model is now in agreement with the theory, since the conductive bodies are centered exactly below the center of the response function which was not feasible by the earlier linearized inversion. The central body is located at a depth of 19 km from the surface, suggesting thickness of 15 km and resistivity of 14 Ω m. The resistivity contrast of this ensemble of conductive bodies with the background is varying by a factor of 100 to 385. Existence of mid-crustal conductor (the conductivity estimate is marginally different) is clearly indicated which was also not detected in the earlier study. A likely explanation could be due to the presence of graphitic carbon at lower crustal depths, however, the role of electrolytic fluid present in the interconnected pore-spaces of rocks may be another explanation.     

Three-dimensional geoelectrical model of metallogenic zones in the Kuznetsk-Alatau folded area

The three-dimensional (3D) geoelectric model of the Kuznetsk-Alatau folded area is reconstructed by magnetotelluric inversion using 3D fitting. It is established that the zones of ore mineralization within the Batenevsky massif are confined to the subvertical faults characterized by the electric resistivity of 100?C300 ?? m. Blocks with ?? ?? 10?100 ?? m are identified at a depth below 10 km in the western part of the model. The blocks are located close to the areas marked by the increased thermal flux, reduced seismic velocities, and elevated Moho boundary. This is probably associated with the presence of the rift zone in this area.     

Preliminary models of upper mantle P and S wave velocity structure in the western South America region

Upper mantle P and S wave velocities in the western South America region are obtained at depths of foci from an analysis of travel time data of deep earthquakes. The inferred velocity models for the Chile–Peru–Ecuador region reveal an increase of P velocity from 8.04 km/s at 40 km to 8.28 km/s at 250 km depth, while the S velocity remains almost constant at 4.62 km/s from 40 to 210 km depth. A velocity discontinuity (probably corresponding to the L discontinuity in the continental upper mantle) at 220–250 km depth for P and 200–220 km depth for S waves, with a 3–4% velocity increase, is inferred from the velocity–depth data. Below this discontinuity, P velocity increases from 8.54 km/s at 250 km to 8.62 km/s at 320 km depth and S velocity increases from 4.81 km/s at 210 km to 4.99 km/s at 290 km depth. Travel time data from deep earthquakes at depths greater than 500 km in the Bolivia–Peru region, reveal P velocities of about 9.65 km/s from 500 to 570 km depth. P velocity–depth data further reveal a velocity discontinuity, either as a sharp boundary at 570 km depth with 8–10% velocity increase or as a broad transition zone with velocity rapidly increasing from 560 to 610 km depth. P velocity increases to 10.75 km/s at 650 km depth. A comparison with the latest global average depth estimates of the 660 km discontinuity reveals that this discontinuity is at a relatively shallow depth in the study region. Further, a velocity discontinuity at about 400 km depth with a 10% velocity increase seems to be consistent with travel time observations from deep earthquakes in this region.     

长白山天池火山区大地电磁测深初步观测

在长白山火山区沿着两条剖面进行了１５个测点的大地电磁测深观测，对地磁感应矢量和地下电性分布维数特征进行了分析，并使用阻抗张量分解和二维自动反演等新技术对资料进行了解释。结果表明，火山区及其周围地区的地下电性结构具有二维特征，但在火山口及其附近地下存在局部三维异常体，异常体埋深约１２ｋｍ，初步推测可能是火山岩浆囊     

On the origin of El Chichón volcano and subduction of Tehuantepec Ridge: A geodynamical perspective

The origin of El Chichón volcano is poorly understood, and we attempt in this study to demonstrate that the Tehuantepec Ridge (TR), a major tectonic discontinuity on the Cocos plate, plays a key role in determining the location of the volcano by enhancing the slab dehydration budget beneath it. Using marine magnetic anomalies we show that the upper mantle beneath TR undergoes strong serpentinization, carrying significant amounts of water into subduction. Another key aspect of the magnetic anomaly over southern Mexico is a long-wavelength (∼ 150 km) high amplitude (∼ 500 nT) magnetic anomaly located between the trench and the coast. Using a 2D joint magnetic-gravity forward model, constrained by the subduction P–T structure, slab geometry and seismicity, we find a highly magnetic and low-density source located at 40–80 km depth that we interpret as a partially serpentinized mantle wedge formed by fluids expelled from the subducting Cocos plate. Using phase diagrams for sediments, basalt and peridotite, and the thermal structure of the subduction zone beneath El Chichón we find that ∼ 40% of sediments and basalt dehydrate at depths corresponding with the location of the serpentinized mantle wedge, whereas the serpentinized root beneath TR strongly dehydrates (∼90%) at depths of 180-200 km comparable with the slab depths beneath El Chichón (200-220 km). We conclude that this strong deserpentinization pulse of mantle lithosphere beneath TR at great depths is responsible for the unusual location, singularity and, probably, the geochemically distinct signature (adakitic-like) of El Chichón volcano.     

Distortions of EM transients in coincident loops at short time‐delays

Transient electromagnetic measurements with short time‐delays of transients are used for solving different problems within the upper part of a geoelectric section. However, it is necessary to take into consideration distortions connected with self‐transients within the transmitter–receiver system. From the practical point of view, it is important to estimate the minimum time‐delay after which these distortions may be neglected. We present such an estimation which uses a simple approximation method for a single‐loop (or coincident‐loop) configuration. For common values of the loop size (10 m × 10 m to 40 m × 40 m) and of the resistivity of a homogeneous half‐space (1–100 Ωm), the minimum time‐delay beyond which we can use a standard interpretation is in the range of 2–10 µs. This is equivalent to a minimum depth of investigation in the range of 1–30 m.     

The global asthenosphere

Based on the analysis of the thermal history of the mantle, we hypothesize the existence of the partially molten layer at a depth of approximately 700–1100 km under the entire Earth's surface. We present the seismological data that are consistent with this hypothesis and the geoelectric results revealing the conductive layer at similar depths in Central and East Europe.     

Resolution of airborne VLF data

The interpretation of airborne VLF data represents an important aspect of geophysical mapping of the upper few hundred meters of the Earth's crust, especially in areas with crystalline rocks. We have examined the ability of the single frequency VLF method to provide quantitative subsurface resistivity information using two generic models and standard airborne parameters with a flight altitude of 70 m and a frequency of 16 kHz. The models are long thin conductor (10 m thick, 10 Ω m resistivity and 1 km long) and a wider buried conductive dike (100 Ω m resistivity and 500 m wide). Using standard regularized inversion it turned out that for both models the conductivity of the conductors are underestimated and the vertical resolution is rather poor. The lateral positions of the minimum of the resistivity distributions coincide well with the true positions of the shallow conductors. For deeper conductors the position of the minimum resistivity moves from the edges of the conductor into the conductor. The depth to the minimum of the resistivity anomalies correlates well with the true depth to the top of the conductors although the latter is always smaller than the former.Interpretation of field airborne data collected at 70 m flight height resolved both small scale and large scale near surface conductors (conductance ∼1 S). Deeper conductors show up in the VLF data as very long wavelength anomalies that are particularly powerful in delineating the lateral boundaries of the conductors. Many of the VLF anomalies in the Stockholm area are dominated by these deep conductor responses with some near surface conductors superimposed. The deep conductors often follow topographic lows coinciding with metasediments. We interpret the frequent absence of near surface responses at 70 m flight height as a result of weak coupling between the primary VLF wave and the small scale (in all three dimensions) near-surface conductors.Radio magnetotelluric (RMT) ground measurements were carried out along a short profile coinciding with part of an airborne profile. Using data at 9 frequencies (14–250 kHz) small scale conductors in the upper few tens of meters, not identified from the airborne data, could be well resolved. Large scale deeper conductors could be identified by both methods at nearly the same positions.     

DEEP “ARCH-BRIDGE” MAGMATIC SYSTEM OF THE AERSHAN VOLCANIC GROUP AND ITS STABILITY ANALYSIS

The Aershan volcanic group has been active since the Pleistocene to the modern and has potential to erupt, so it is of great significance to strengthen the study on the Aershan volcano group and evaluate its activity. The magmatism is characterized by low resistivity in electrical properties. The electrical structure obtained by magnetotelluric sounding can be used to study the magmatic occurrence and volcanic activity. It is an effective method to detect the deep structure of volcanic area. Based on the magnetotelluric sounding data of the Aershan volcanic group, the two-dimensional nonlinear conjugate gradient inversion is obtained after the normalization of the data and the two-dimensional electrical structure of the Aershan volcanic group is obtained. It is found that there is a large-scale "arch-bridge" low-resistivity anomaly (resistivity less than 320Ω·m) and there are obvious high-conductivity anomalies (resistivity less than 40Ω·m) respectively on the west of the town of Ershi and the east of Chaihe town, the former is relatively small in sizes, buried at the depth of 40~60km, and the size of the latter is larger, buried at the depth of 60~90km, or even deeper. Combined with geological and geochemical data, it is inferred that the "arch bridge" anomaly is the channel of the basaltic magma transport from the epithermal basins on its both sides. The two high-conductivity anomalies it contains are probably the uncondensed or gathering magma chamber, so the Aershan magma system consists of "arch bridge" channel and asthenosphere-derived basaltic magma, the volcanic group has a unified magmatic system. Further analysis shows that the melting percentage of the "arch bridge" channel material is not less than 0.5%, and the lithosphere structure tends to be stable. The melting degree of the two magma chambers it contains is 2.5%~11.5%, and the grain boundary may all be wetted by the melt, rock flow intensity is relatively low, lithosphere structural stability is poor. In addition, the regional seismic distribution and the formation of hot springs also have a certain correlation to the Aershan volcanic group magma system. There are indications that the Aershan volcano group is in dormancy, rather than an extinct volcano, there is the possibility of eruption, so it should be closely monitored.     

Results of the monitoring of geomagnetic field variations in the Kamchatka region

The transfer function between the vertical and horizontal components of geomagnetic field variations is studied and the frequency responses of its parameters are presented. The relation to geoelectric heterogeneities of the medium is analyzed. The coast effect is considered and a deep curve of the apparent electrical resistivity constructed on its basis is used for estimating the depth to the asthenospheric conducting layer. The behavior of the induction vectors in the frequency-time domain is studied. Specific features of the behavior of the real and imaginary parts of the induction vector related to geoelectric heterogeneities of the medium are determined. Monitoring results are compared with time moments of earthquakes of K = 13?14 at epicentral distances of up to 150 km.