Seismic Tomography Results Research Articles

Seismic evidence for failed rifting in the Ligurian Basin, Western Alpine domain

Abstract. The Ligurian Basin is located in the Mediterranean Sea to the north-west of Corsica at the transition from the Western Alpine orogen to the Apennine system and was generated by the south-eastward trench retreat of the Apennines–Calabrian subduction zone. Late-Oligocene-to-Miocene rifting caused continental extension and subsidence, leading to the opening of the basin. Yet it remains unclear if rifting caused continental break-up and seafloor spreading. To reveal its lithospheric architecture, we acquired a 130 km long seismic refraction and wide-angle reflection profile in the Ligurian Basin. The seismic line was recorded in the framework of SPP2017 4D-MB, a Priority Programme of the German Research Foundation (DFG) and the German component of the European AlpArray initiative, and trends in a NE–SW direction at the centre of the Ligurian Basin, roughly parallel to the French coastline. The seismic data were recorded on the newly developed GEOLOG recorder, designed at GEOMAR, and are dominated by sedimentary refractions and show mantle Pn arrivals at offsets of up to 70 km and a very prominent wide-angle Mohorovičić discontinuity (Moho) reflection. The main features share several characteristics (e.g. offset range, continuity) generally associated with continental settings rather than documenting oceanic crust emplaced by seafloor spreading. Seismic tomography results are complemented by gravity data and yield a ∼ 6–8 km thick sedimentary cover and the seismic Moho at 11–13 km depth below the sea surface. Our study reveals that the oceanic domain does not extend as far north as previously assumed. Whether Oligocene–Miocene extension led to extremely thinned continental crust or exhumed subcontinental mantle remains unclear. A low grade of mantle serpentinisation indicates a high rate of syn-rift sedimentation. However, rifting failed before oceanic spreading was initiated, and continental crust thickens towards the NE within the northern Ligurian Basin.

Hydraulic Fracturing Induced Seismicity in the Southern Sichuan Basin Due to Fluid Diffusion Inferred From Seismic and Injection Data Analysis

AbstractIn the southern Sichuan Basin, China, the recent increase in the seismic activity has been suspected to be related to hydraulic fracturing stimulation for producing the shale gas. In this study, we used the monitoring data from a local seismic network within the shale gas blocks to study the earthquakes near the shale gas production wells that have detailed injection data. Comparison of the timing of earthquakes and stimulation schedule of the studied well pads indicates an apparent correlation between the seismic activity and hydraulic fracturing. The results of seismic velocity tomography reveal that the reactivation of preexisting faults due to fluid diffusion is the primary cause of the observed earthquakes. Focal mechanism analysis combined with geomechanical modeling indicates that the increased pore pressures resulted from hydraulic fracturing are sufficient to trigger seismic slip on the faults.

Mechanism of Coal Bursts Induced by Horizontal Section Mining of Steeply Inclined Coal Seams and Application of Microseismic Multiparameter Monitoring in Early Warning

Coal bursts occurring in steeply inclined coal seams (SICSs) are increasingly severe. To solve this problem, a mechanical model for the distribution of static stress on coal‐rock masses along panels and the distribution of dynamic load induced by the breakage of thick and hard roofs with propagation distance was established. The stress characteristics after a superposition of dynamic and static loads on the roof and floor roadways (Rr and Rf) were determined. In addition, precursory information characteristics and index sensitivities of four indices for dynamic loads and the CT index for static loads based on seismic tomography were separately analyzed. The monitoring and warning indices for SICSs and flat seams were compared. The results showed that the static stress of Rr was significantly higher than that of Rf, which provided a basis for the stress‐triggering coal burst behaviors. Three indices for dynamic loads and seismic tomography results exhibited remarkable precursory information and high sensitivity. However, the performance of lack of shock index is poor. The continuous anomaly and the contradiction of indices at Rr and Rf can be considered as precursory information for predicting coal bursts.

Passive Seismic Tomography as a Tool of Rock Burst Hazard Prognosis in Copper Ore Mine in Poland

In Poland, underground mines of copper ores, which belong to KGHM Polish Copper JSC, have been struggling with seismic dynamic events such as tremors and rock bursts since almost the first years of exploitation i.e. the 70s of the twentieth century. Mining activities infringe the original stress balance and make the rock mass accumulate energy and then release it, which triggers seismic hazard even far away from a tremor epicentre. Therefore, prediction of the position, time and energy of tremors plays a significant role in mining operation especially in work safety. For this purpose, many observation methods including seismology, seismic, geotechnical-geological monitoring, are used; unfortunately, most of them help describe the state of the rock mass after but not prior to the occurrence of the seismic phenomena. Only passive seismic tomography is promising since it can be used to forecast, to some extent, the location of places of seismic energy excessive accumulation. In this method, on the basis of seismic events recorded in a given period, zones of high and low seismic wave velocity are determined (calculated), which in the near future may pose areas of increased seismic activity. The phenomenon of the increase of seismic longitudinal wave velocity with the increase of stress in rocks makes the ground of tomography calculations. The prime purpose of the paper is to assess passive seismic tomography as a means for forecasting and evaluation of seismic and rock burst hazard. To accomplish this, the analysis of seismic activity and archival seismic tomography results (seismic wave velocity zones and seismic anomaly zones) with reference to seismic phenomena, their energy, number and location were carried out. On the basis of obtained the results, the effectiveness of seismic hazard forecast with the use of passive seismic tomography was assessed. The research was carried out for one mining division of the Polkowice-Sieroszowice mine and covered ten years. This division was selected due to its high seismic activity and the permanent use of the passive seismic tomography to assess the seismic risk. Linear correlation and determination coefficients between seismic activity characteristics and seismic anomaly as well as longitudinal wave velocity were calculated. It was found that in the study area the effectiveness of passive seismic tomography in forecasting the seismic hazard is relatively satisfying since about half of the tremors were located within zones of high seismic activity (substantial velocity of P-waves and seismic positive anomalies)..

Well-Posed Geoscientific Visualization Through Interactive Color Mapping

Scientific visualization aims to present numerical values, or categorical information, in a way that enables the researcher to make an inference that furthers knowledge. Well-posed visualizations need to consider the characteristics of the data, the display environment, and human capacity. In the geosciences, visualizations are commonly applied to spatially varying continuous information or results. In this contribution we make use of a suite of newly written computer applications which enable spatially varying data to be displayed in a performant graphics environment. We present a comparison of color-mapping using illustrative color spaces (RGB, CIELAB). The interactive applications display the gradient paths through the chosen color spaces. This facilitates the creation of color-maps that accommodate the non-uniformity of human color perception, producing an image where genuine features are seen. We also take account of aspects of a dataset such as parameter uncertainty. For an illustrative case study using a seismic tomography result, we find that the use of RGB color-mapping can introduce non-linearities in the visualization, potentially leading to incorrect inference. Interpolation in CIELAB color space enables the creation of perceptually uniform linear gradients that match the underlying data, along with a simply computable metric for color difference, ΔE. This color space assists accuracy and reproducibility of visualization results. Well-posed scientific visualization requires both visual literacy and visual numeracy on an equal footing with clearly written text. It is anticipated that this current work, with the included color-maps and software, will lead to wider usage of informed color-mapping in the geosciences.

A Multivariate Approach for Mapping Lithospheric Domain Boundaries in East Antarctica

AbstractBeneath the ice of East Antarctica lies a continent that is likely to be as geologically complex as its neighbors in Gondwana. An improved model of the heterogeneous lithosphere is required to progress research on Antarctica's tectonic evolution and support interdisciplinary studies of cryosphere and solid Earth interaction. We make use of multiple data sets, which were updated following the field campaigns and compilations of the International Polar Year of 2007/2008. Seismic tomography results, gravity anomalies, and surface elevation are used in a novel method, which combines spatial multivariate data to map possible boundaries as projected likelihood functions. Six multivariate combinations are tested and compared with sparse geological observations in East Antarctica. The resulting lithospheric domain boundaries contribute to our understanding of the deep continental structure. New boundaries are suggested in the interior, and models agree with likely surface expressions of crustal tectonic boundaries exposed along the coast.

Application of GPR and seismic methods for noninvasive examination of glacial and postglacial sediments in the Psia Trawka glade: the Tatra Mts., Poland

Presented study gives an insight into general proportions of the actual geomorphology, subglacial morphology and thickness of the drift (quaternary sediments) particularly well-pronounced glacial morphology in the Tatras and, on the other hand, the general scarcity of the data in this field. Objectives of the geophysical survey in this study were imaging of the morphology of bedrock surface under the drift (glacial and postglacial) sediments and determination of thickness of the drift and its composition. Two methods were applied: Ground Penetrating Radar (GPR) and seismic refraction profiling. GPR was used to examine drift sediments due to its high resolution and low depth of penetration. Seismic method with lower resolution but higher penetration depth gave an image of boundary between bedrock and drift. In addition, the results of seismic tomography allowed the velocity field imaging which shows changes inside the postglacial deposits. The results of the two methods used in this research suggest that points of depression exist in the subglacial morphology with a depth of about c.a. 40 below the present-day terrain surface and c.a. 25 m below surrounding subglacial surface. This trough has also been estimated to be about 150 m wide. Its considerable depth and steep slopes show that its origin can be related to erosion of subglacial water during the decay of the last (Würm) glaciation of the Sucha Woda and Panszczyca valleys.

Delimiting the Neoproterozoic São Francisco Paleocontinental Block with P-wave traveltime tomography

SummaryThe São Francisco Paleocontinental Block (SFPB) represents part of the Congo-São Francisco Paleocontinent (CSFP), amalgamated around 2 Ga. In the Neoproterozoic, a branched continental rift system evolved to ocean basins around most edges of the SFPB that remained only partially linked to the Congo Paleocontinent by means of the Bahia-Gabon Continental Bridge. After the Brasiliano—Pan-African orogeny, two relatively preserved CSFP sectors formed the São Francisco and Congo cratons, surrounded by Neoproterozoic orogenic belts. Recent results of upper mantle P-wave seismic tomography allowed us to suggest a delimitation in lithospheric depths of the Neoproterozoic SFPB, which comprise the São Francisco Craton, and that this would have been connected with the Congo Paleocontinent along the Araçuaí Belt. It is characterized by high-velocity anomalies and its boundaries with other blocks are marked by low-velocity anomalies at lithospheric depths. We tested the resolution of the tomographic results through synthetic models obtained by a ray tracing scheme using the observed ray configuration. We observe that the lateral resolution is adequate, but the method used was not able to set the depth reached by the SFPB. Our results indicate that the SFPB area in lithospheric depths is larger than the surface area ascribed to the São Francisco craton, and thus, the SFPB basement deeply extends beneath neighboring orogenic regions, suggesting that these Neoproterozoic mobile belts, such as Araçuaí Orogen and the Brasilia Fold Belt, reworked the continental crust. We observe a low-velocity anomaly in the SFPB central region, corresponding to the Pirapora aulacogen. Our results have a good spatial correspondence with the low Bouguer anomalies used to define the SFPB in previous studies. The limits of the SFPB are consistent with deviation of the mantle flow, as suggested by SKS fast polarization.

Upper Crustal Weak Zone in Central Tibet: An Implication From Three‐Dimensional Seismic Velocity and Attenuation Tomography Results

AbstractA series of conjugate strike‐slip faults is the most prominent geologic feature in central Tibet and is considered to accommodate east‐west extension and coeval north‐south contraction. The development mechanism of the conjugate strike‐slip fault system is under debate because of unclear crustal physical properties and compositional variations. P and S wave arrivals from 414 local earthquakes recorded by the temporary Seismic Array Integrated Detection for a Window of Indian Continental Head array and the permanent China National Seismic Network were used for the velocity tomography, with additional P and S wave arrivals from 12 shots of the International Deep Profiling of Tibet and the Himalaya III reflection/refraction profile. The local earthquakes were simultaneously relocated with the updated velocity models. We also inverted for a three‐dimensional upper crustal Qp model with the same earthquake data set. The Vp structure near the surface shows that low‐Vp anomalies generally correspond to sedimentary basins and high‐Vp anomalies are related to exhumed metamorphic blocks in the study area. Relatively low Vp/Vs ratios in the upper crust indicate widely distributed quartz‐rich rocks. The low‐Vp zone from 0‐ to 10‐km depth (resolving depth limit) is spatially correlated with the Bangong‐Nujiang suture, possibly reflecting the compositional difference along the ophiolitic mélange belt accompanied by twin volcanic arcs from a double‐sided subduction. This interpretation is supported by relatively heterogeneous Qp values. This low‐velocity zone also implies relatively uniform stress and continuous deformation in the upper crust of central Tibet. The relatively weak materials in at least the upper crust would result in strain concentration and help the development of the conjugate strike‐slip fault system along the Bangong‐Nujiang suture.

Stress Redistribution Monitoring Using Passive Seismic Tomography at a Deep Nickel Mine

High magnitude seismic events (HMSE), also called major events, triggered at underground mines can severely threaten safety and structures in mines. The aims of this work are to assess stress evolution with the occurrence of them and identify patterns in stress change. Several methods were used to help understand the stress redistribution and rock mass behavior related to two HMSE with moment magnitude greater than 1.0 in a deep nickel mine. Approximately 46,500 seismic events were compiled with 2 HMSE at this mine and used seismic tomography to obtain high-resolution images and temporal-spatial velocity changes before and after these events. This work exhibited surrounding stress evolution and geological structures. Seismic imaging results show that velocity increased in the nearby regions of HMSE before they were triggered. Stress subsequently reduced in the relaxation process after the occurrence of HMSE. Conversely, regions predominantly occupied by low-velocity anomalies increased to higher stress levels after HMSE. Overall, the stress redistributed toward an equilibrium state following HMSE. This study highlights the value of utilizing seismic tomography for estimating stress evolution associated with HMSE. The findings illuminate the applications of seismic imaging around HMSE of hard-rock mines, providing insights into seismic hazard mitigation for deep mining.

Applying full-azimuth depth processing in the Local Angle Domain for Frequency Absorption versus Azimuth) (FAVAz) analysis to predict permeable, oil-saturated fractures

Predicting the permeability of fractured reservoirs is valuable for both reservoir assessment and drilling planning. Characterization of such systems requires advanced amplitude analysis, mainly based on seismic imaging results of the recorded wavefield. A significant amount of work has been done on the subject over the past few decades. Thomsen (1995) showed the effect of seismic amplitude variations for different fractured media. Pisetsky and Fedorov (1998) showed the influence of the size of cracks and the seismic wave length on the change of reflection coefficient. Ruger (1998) used the Zoeppritz equation to formalize the reflection coefficient variation as a function of the wavefront azimuthal direction with respect to the fracture azimuth. An approximation of the latter by Tsvankin and Grechka (1998) became the basis for AVAz inversion analysis. Studies have also been performed on frequency absorption properties of fluid-saturated and dry fractured layers. Goloshubin (2002) observed that reflections are stronger for fluid-saturated fractured layers than for dry fractured layers at low-frequency ranges. Geek (2008) studied high-frequency absorption vs. wavefront azimuth for dry fractures in the acoustic frequency range for P-waves, and showed that the absorption is insensitive to the azimuth for S-waves. Kozlov (2006) made a comprehensive generalization of the physics of frequency absorption effects in the case of dry and fluid-saturated fractures. In his study he showed that in fractured reservoirs, high fluid permeability has an observable effect of absorption of the low-frequency spectral range for reflected P-waves. This observation was also noted in several exploration areas of Western Siberia (Upper and Middle Jurassic deposits, Davydova, 2004). Despite the research detailed above, predicting and delineating oil-saturated fracture systems using low-frequency absorption analysis has not yet become a standard practice in the industry. This is in large part due to the approximation made by standard processing and imaging methods in creating azimuthal data. To make this process more effective, a true amplitude full-azimuth imaging procedure that decomposes the fully recorded seismic wavefield in the local angle domain is required. This procedure is carried out in-situ, in the depth migrated domain, preserving both reflection azimuth and structural azimuth. Such a technology is the ideal solution for extracting the main faults, building the Structural Tectonic Skeleton (STS), exposing the main fracture orientation, and eventually measuring anisotropic frequency absorption effects. This paper presents a study of carbonate reservoirs in an oilfield in the Middle Volga region of Russia, and suggests a workflow based on imaging and processing in the Local Angle Domain to predict prospective areas of oil-saturated permeable fractured reservoirs.

An introduction to seismic exploration of the Micang-Dabashan foothill belt in the Sichuan Basin

Geologic studies indicate that the platform-margin reef-shallow facies in Permo-Triassic marine strata in the Micang-Dabashan foothill belt in the Sichuan Basin are favorable exploration targets for oil and gas exploration. However, the typical dual-complexity problem (complex surface condition and subsurface structure) brings a great challenge for seismic technology targeting of those potential oil and gas reservoirs. To overcome this problem, varieties of advanced seismic acquisition and processing methods have been used to improve the imaging quality of piedmont seismic data since 2000. Some improvements have been achieved: The reflection waves from the far offset and deep layer can be acquired in shot gathers from limestone outcropped areas, and the signal-to-noise ratio (S/N) of reflection and diffraction waves in the stack section has been enhanced significantly so as to reveal amounts of valuable geologic information. The resolution and the S/N of seismic migration imaging for the strong fold zone in marine strata have been improved partially, so that the structure of the step-fault zone and the enveloping of gypsum rock are clearer than those revealed by the old seismic section. Even so, actual drilling data demonstrate that the subsurface structures of the foothill belt are far more complex than those revealed by the current seismic imaging results. Therefore, postdrilling evaluation for the validity of seismic techniques implemented in the Nanjiang and Zhenba piedmont zone has been carried out. The results indicate that the current acquisition scheme and processing workflow cannot completely fulfill the requirements of high-precision velocity modeling and migration imaging of complex structures (such as footwalls of thrust fault and small-scale fault blocks) in the piedmont zone, especially when the rugged surface and the widespread limestone outcrop appear simultaneously. Finally, we have developed some potential needs of seismic theories and techniques in the foothill belt, including seismic wave propagation, acquisition, and processing technology.

Unraveling the Physics of the Yellowstone Magmatic System Using Geodynamic Simulations

The Yellowstone magmatic system is one of the largest magmatic systems on Earth, and thus an ideal location to study magmatic processes. Whereas previous seismic tomography results could only image...

Crustal Structure of the Crimean Mountains along the Sevastopol–Kerch Profile from the Results of DSS and Local Seismic Tomography

The paper discusses the velocity structure of the crust beneath the Crimean Mountains from the results of active and passive seismic experiments. Based on a new interpretation of seismic data from the old Sevastopol–Kerch DSS profile by modern full-wave seismic modeling methods, a velocity model of the crust beneath the Crimean Mountains has been constructed for the first time. This model shows the significant differences in the structure of two crustal blocks: (1) one characterized by higher velocities and located in the western and central Crimean Mountains, and (2) the other characterized by lower velocities and located in the east, in the Feodosiya–Kerch zone, which are subdivided by a basement uplift (Starokrymskoe Uplift). The former block is characterized by a more complex structure, with the Moho traced at depths of 43 and 55 km, forming two Moho discontinuities: the upper one corresponds to the platform stage, and the lower one, formed presumably at the Alpine stage of tectogenesis as a result of underthrusting of the East Black Sea microplate beneath the southern margin of the Scythian Plate in Crimea. At depths of 7–11 km, velocity inversion zone has been identified, indicating horizontal layering of the crust. Local seismic tomography using the data on weak earthquakes (mb ≤ 3) recorded by the Crimean seismological network allowed us to obtain data on the crustal structure beneath the Crimean Mountains at depths of 10–30 km. The crustal structure at these depths is characterized by the presence of several high-velocity crustal bodies in the vicinity of cities Yalta, Alushta, and Sudak, with earthquake hypocenters clustered within these bodies. Comparison of this velocity model of the Crimean Mountains with the seismicity distribution and with the results from reconstruction of paleo- and present-day stress fields from field tectonophysical study and earthquake focal mechanisms allowed the conclusion that the Crimean Mountains were formed as a result of on mature crust at the southern margin of the East European Platform and Scythian Plate, resulting from processes during various phases of Cimmerian and Alpine tectogenesis in the compressional and transpressional geodynamic settings. The collisional process is ongoing at the present-day stage, as supported by high seismicity and uplift of the Crimean Mountains.

Constraining the Seismic Potentiality Analysis for Andaman Arc System, NE Indian Ocean

ABSTRACT Seismic-potential for the Andaman Arc System (AAS) is evaluated using a three-tier approach: (i) the seismic b-values derived using a revised and homogenized earthquake catalog for Mw ≥ 4.7, with uniform Mw, for the period 1964 – 2013 created from ISC Data, (ii) Free-air gravity-anomalies for AAS and their geologic interpretation and (iii) deep crustal structure from seismic reflection surveys and 3D seismic tomography results. Both long-term spatial b-value anomalies for the period 1964-2013 and temporal b-value anomalies for a shorter period (2002 – 2013) have been estimated. The b-value maps are interpreted in terms of the stress regime acting across AAS and stressed asperities inferred along the strike of AAS. Eight distinguishable seismic-zones are identified and their seismic potential is examined by temporal b-value anomalies in producing moderate to large earthquakes. The latter demonstrates ‘low-high’ or ‘high-low’ couplet over years, and that a variation in b-value more than 20% compared to the previous year value is likely to produce an earthquake event with Mw ≥ 6.0. Some support to this interpretation comes from the regional Free-air gravity-anomalies and deep crustal structure interpreted from hypocentral distribution of earthquakes. The high b-values are seemingly associated with magma chambers or low velocity crust; creating asperity zones due to multiple batholithic intrusions at plate boundary. This geologic interpretation is evidenced by 3D P-wave seismic tomography and velocity heterogeneity study for AAS reported by us elsewhere.

Crustal Footprint of the Hainan Plume Beneath Southeast China

AbstractAlthough global and regional seismic tomography results have revealed the presence of a mantle plume beneath Hainan Island, there is little evidence for a hotspot track associated with the Hainan plume. Here a joint inversion of satellite gravity measurements and seismic surface wave dispersion data was performed, and the results show that a linear corridor of high seismic velocity anomalies beneath the crust is located northeast of Hainan Island beneath the Southeast China. Geodynamic modeling further demonstrates that this corridor could have been formed by the interactions between a mantle plume and the continental lithosphere with a weak lower crust. Volcanic age distributions of sporadic oceanic island basalts suggest that this track was likely formed since the late Cretaceous, which helps to constrain the averaged plate velocity of the South China Block, relative to the Hainan plume, to be ~1.8 cm/year to the northeast since 80 Ma. Our result provides a novel explanation for the crustal thickness variations, related Vs anomalies, and observed oceanic island basalts in this area. It also provides an independent way for constraining plate motion history of the continental lithosphere.

Petrogenesis of highly fractionated rhyolites in the southwestern Okinawa Trough: Constraints from whole‐rock geochemistry data and Sr–Nd–Pb–O isotopes

In the southwestern Okinawa Trough, a cluster of approximate 70 submarine volcanoes is located at the cross back‐arc volcanic trail (CBVT), with the eruptive products dominated by rhyolites. New whole‐rock major and trace element and Sr–Nd–Pb–O isotopic data and previously available geochemistry data of volcanic rocks from this region are presented to determine the petrogenesis of these rhyolites. The rhyolites have high SiO2 (>70.01 wt.%), K2O + Na2O (6.71–7.16 wt.%) contents and differentiation index (DI) values (DI > 93), and are metaluminous to weakly peraluminous (A/CNK = 0.99–1.11) and medium‐K calc‐alkaline in composition. The relatively low 104*Ga/Al (2.06–2.17) and FeO*/MgO ratios (4.50–6.62), as well as Zr (91–174 ppm) and Zr + Ce + Nb + Y (175–271) contents suggest that the rhyolites, which contain hornblende, are generally similar to those of typical highly fractionated I‐type granitoids. The rhyolites show arc‐related features such as enrichment in LILEs (Rb, Ba, K), Th, U, Pb, and LREEs ((La/Yb)N = 6.21–6.74) and strong depletion in HFSEs (Nb, Ta, P, Ti) with both nearly identical and more enriched isotopic compositions, including εNd (from −3.2 to −3.5), 87Sr/86Sr ratios (0.705552 to 0.705713), δ18O values (+8.1‰ to +8.6‰), and Pb isotopic compositions (206Pb/204Pb = 18.568–18.589, 207Pb/204Pb = 15.641–15.656, and 208Pb/204Pb = 38.848–38.904). Based on these characteristics, we propose that subduction‐related basalt magma and crust‐derived magma mixed and homogenized within a shallow magma chamber with a homogeneous “base‐level” isotopic signature of >0·7055 (87Sr/86Sr) and > −3 (εNd). This magma would have then experienced the extensive fractional crystallization of minerals, including plagioclase, hornblende, apatite, sphene, and Fe–Ti oxides, to form the CBVT rhyolites. Our geochemical interpretation of the existence of a long‐lived, convecting magma chamber beneath the CBVT is also consistent with seismic tomography results.

Ray Tracing Methods in Seismic Emission Tomography

Highly efficient approximate ray tracing techniques which can be used in seismic emission tomography and in other methods requiring a large number of raypaths are described. The techniques are applicable for the gradient and plane-layered velocity sections of the medium and for the models with a complicated geometry of contrasting boundaries. The empirical results obtained with the use of the discussed ray tracing technologies and seismic emission tomography results, as well as the results of numerical modeling, are presented.

Application K-Means, Fuzzy C-Means and Gustafson-Kessel FCM Methods in Integration of Refraction Seismic Tomography and Electrical Resistivity Data Inversion Results for Evaluation of the Alluvium and Bedrock

Application K-Means, Fuzzy C-Means and Gustafson-Kessel FCM Methods in Integration of Refraction Seismic Tomography and Electrical Resistivity Data Inversion Results for Evaluation of the Alluvium and Bedrock

Deep structure of Kamchatka according to the results of MT sounding and seismic tomography

The key features in the distribution of geoelectric and velocity heterogeneities in the Earth’s crust and the upper mantle of Kamchatka are considered according to the data of deep magnetotelluric sounding and seismotomography. Their possible origin is discussed based on the combined analysis of electric conductivity and seismic velocity anomalies. The geoelectric model contains a crustal conducting layer at a depth of 15–35 km extending along the middle part of Kamchatka. In the Central Kamchatka volcanic belt, the layer is close to the ground surface to a depth of 15–20 km, where its conductivity considerably increases. Horizontal conducting zones with a width of up to 50 km extending into the Pacific Ocean are revealed in the lithosphere of eastern Kamchatka. The large centers of current volcanism are confined to the projections of the horizontal zones. The upper mantle contains an asthenospheric conducting layer that rises from a depth of 150 km in western Kamchatka to a depth of 70–80 km beneath the zone of current volcanism. According to the seismotographic data, the low- and high-seismic-velocity anomalies of P-waves that reflect lateral stratification, which includes the crust, the rigid part of the upper mantle, the asthenospheric layer in a depth range of ~70–130 km, and a high-velocity layer confined to a seismofocal zone, are identified on the vertical and horizontal cross sections of eastern Kamchatka. The cross sections show low-velocity anomalies, which, in the majority of cases, correspond to the high-conductivity anomalies caused by the increased porosity of rocks saturated with liquid fluids. However, there are also differences that are related to the electric conductivity of rocks depending on pore channels filled with liquid fluids making throughways for electric current. The seismic velocity depends, to a great extent, on the total porosity of the rocks, which also includes isolated and dead-end channels that can be filled with liquid fluids that do not contribute to the electric-current transfer. The data on electric conductivity and seismic velocity are used to estimate the porosity of the rocks in the anomalous zones of the Earth’s crust and the upper mantle that are characterized by high electric conductivity and low seismic velocity. This estimate serves as the basis for identifying the zones of partial melting in the lithosphere and the asthenosphere feeding the active volcanoes.