Geoelectric studies in earthquake hazard assessment – the case of the Kozlodui nuclear power plant, Bulgaria (original) (raw)
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Geoelectric studies of the Kozloduy nuclear power plant region, Bulgaria
Geofizicheskiy Zhurnal, 2022
The task of the work was geoelectrical studies using variations of the magnetotelluric (MT) field of the Kozloduy nuclear power plant (KNPP) region and the integration of its results with other geological and geophysical knowledge. This paper presents the determined interpretation parameters of the MT field. The KNPP is located on the right bank of the Danube River in close proximity to the river. This fact, together with the location of electrified railways determined the unique network of locations of observation points for MT field variations. Based on the analysis of Earthquake Catalogs of Bulgaria and international seismicity databases, a map of the seismicity of nuclear power plant areas was built. Over the past 50 years, about 750 earthquakes (mainly south of KNPP) have been recorded at a distance of 40—80 km from the KNPP. Two magnetotelluric stations GEOMAG-02 were used at measurement sites, but equipment for recording electrical channels was available only for one station ...
Exploring Seismicity in Bulgaria Using Geomagnetic and Gravity Data
Comptes rendus de l'Académie bulgare des sciences: sciences mathématiques et naturelles
The present study aims at demonstrating the capability of gravity and magnetic anomalous data for revealing deep structures in the Earth crust of Bulgaria. Interpretation of gravity and magnetic data is well known [ 1 ] and often applied [ 2, 3 ] to delineate various geological structures such as faults, flex-ures, thrusts, borders of dislocated blocks, etc., which create a significant rock density contrast in horizontal planes. Delineated gravity and magnetic anomalies with their characteristics (amp-litude, width, length and coordinates) are compared with the spatial distribu-tion of seismicity and map of the active faults on the territory of Bulgaria. As a result, integrated geophysical data and geological information is incorporated to prove the reliability of potential filed data application for the purposes of seismic hazard assessment.
Geoelectric variations related to earthquakes observed during a 3-year independent investigation
Geophysical Research Letters, 1996
We present observations of the geoelectric field prior to some earthquakes. The data were collected during a three year (1992-1994) independent experimental investigation of VAN at the University of Patras Seismological Center. The recorded signals were: a) Gradual Variations of the Electric Field (GVEF), b) Periodic Variation of the Electric Field (PVEF), and c) Seismic Electric Signals (SES).
Studia Geophysica et Geodaetica, 2013
A magnetotelluric survey was performed at the Çatalca Region, west of Istanbul, Turkey with the aim of investigating geoelectrical properties of the upper crust near the Çatalca Fault and its vicinity. Broadband magnetotelluric data were collected at nine sites along a single southwest-northeast profile to image the electrical resistivity structure from surface to the 5 km depth. The dimensionality of the data was examined through tensor decompositions and highly two-dimensional behavior of the data is shown. Following the tensor decompositions, two-dimensional inversions were carried out where E-polarization, B-polarization and tipper data were utilized to construct electrical resistivity models. The results of the inversions suggest: a) the Çatalca Fault extends from surface to 5 km depth as a conductive zone dipping to southwest; b) the thickness of the sedimentary cover is increasing from SW to NE to 700 m with low resistivity values between 1100 Ωm; c) the crystalline basement below the sedimentary unit is very resistive and varies between 2000100000 Ωm; d) a SW-dipping resistivity boundary in the northeastern part of our profile may represent the West Black Sea Fault.
Geoelectromagnetic and geothermic investigations in the Ihlara Valley geothermal field
Journal of volcanology and …, 1997
The Ihlara Valley is situated within a volcanic arc that is formed by the collision of the eastern Mediterranean plate system with the Anatolian plate. In this study we will present data from a reservoir monitoring project over the Ihlara-Ziga geothermal field, located 22 km east of Aksaray, in central Anatolia. Although identified geothermal resources in the Ihlara Valley are modest, substantial undiscovered fields have been inferred primarily from the volcanic and tectonic setting but also from the high regional heat flow (150-200 mW m-z) on the Ktraehir Massif. In 1988 and 1990, geoelectromagnetic surveys were undertaken by MTA-Ankara to confirm the presence of a relatively shallow (= 0.5-I km), hydrothermally caused conductive layer or zone. CSAMT and Schlumberger resistivity data show good correspondence with each other, and 2-D geoelectric models are also in harmony with geologic data and gravity anomalies. The depth of the resistive basement, which is interpreted as Paleozoic limestone, is 200-250 m in the western part and increases eastward (= 600-750 m>, This may imply N-S-oriented normal faulting within the survey area. The parameters of the top layer are a resistivity of 25 to 9.5 ohm m and a thickness of between 100 and 250 m. The thickness of the conductive tuffs between the top layer and the basement, whose resistivity is about 4-5 ohmm, also increases eastward (from 100 to 450 ml. The apparent resistivity maps for the frequencies between 32 and 2 Hz reveal a localized low resistivity anomaly to the east of Belisnma. 0 1997 Elsevier Science B.V.
Earth, Planets and Space, 2018
Seismic swarm areas below the southeast flank of Ontake volcano, central Japan, provide an important opportunity to study interactions between seismicity, volcanic processes and crustal fluid. On June 25, 2017, an M5.6 earthquake occurred in the Ontake swarm area where geochemical and geophysical studies suggest that pore fluid pathways from the lower crust and mantle affect fault rupture. To clarify the electrical resistivity distribution (that reflects pore fluids, altered sediments and temperature), audio-frequency and broadband magnetotelluric data were measured at 35 sites around the aftershock area of this earthquake. A 3D resistivity inversion model based on these observed magnetotelluric data shows the following key features: (1) two conductive zones (C-1 and C-2) underlie springs where isotope studies indicate fluids of mantle or lower crustal origin and (2) aftershock hypocentres locate in a resistive area between these two aseismic conductive zones (C-1 and C-2). The relationship between seismicity and conductivity suggests that the C-1 and C-2 conductors can be interpreted as interconnected pore fluid, high temperature and/ or sediment under aseismic elastic conditions. In addition, the fault rupture of the M5.6 earthquake was located near the boundary between the central resistive and conductive C-2 zone, indicating stress accumulation associated with heterogeneity of rock, temperature and/or pore fluid distribution. If these features are observed generally in seismic areas, surveys of resistivity structure could contribute to estimating the magnitude of potential earthquakes and evaluation of risk.
2008
A geophysical survey was carried out in the city of San Miguel Uspantan, Quiche, Guatemala, located along the seismically active Polochic fault which forms, with the Motagua Fault, the plate boundary between the N- American and Caribbean plates. During its history, the city of San Miguel Uspantan and its close region had experienced severely damaging earthquakes induced by the Polochic fault. The earthquakes generated by the fault during the last two centuries, although of moderate intensity, produced important damages. The importance of the destructions and the fact that even earthquakes of small magnitude can cause such important destructions indicate a possible site effect. Within the city, very little is known about the affected ground and the location and character of the investigated active fault as its surface expression was removed by human activity. For this reason, 2D Electrical Resistivity Tomographies (ERT) have been carried out in order to contribute to the geological knowledge of the recent tectonic structures by identifying and locating the active fault that traverses the city and by characterizing the thickness, nature, and physical proprieties of the ground on which the city was constructed. Firstly, geomorphologic observations and ERT profiles showed that the city was constructed on a Quaternary deposit of 20 m thickness with thixotropic properties that can be responsible for an amplification of seismic waves during an earthquake. Secondly, ERT profiles confirmed the presence of the fault within the city limits. The fault is expressed as a pronounced, near-vertical anomaly characterized by a sharp lateral resistivity contrast that not only allows interpreting the location of the fault zone to within a few meters, but also confirms that the fault was active until recent time. Finally, this study has significant implications for seismic hazard analysis in this tectonically active and populated region. The results of this work can help plan urban development and diminish the population exposure to natural hazards.
Geoelectrical investigations by means of resistivity methods in karst areas in Romania
Environmental Geology, 2008
The applicability of resistivity methods to ground water investigations is well recognized. As watersaturated rock formations have a lower electrical resistivity than dry ones, an electrical resistivity survey should result in low resistivity anomalies. Normally, such anomalies are interpreted to indicate areas of potentially significant ground water flows. In karst areas, however, interpretation may not be as straightforward: for example, large electrically conductive domains can represent water bearing zones, whose fluid-permeability may be poor; alternatively, fast flow conduits, which may be unsaturated, occur as slender objects, and not as clear anomalous features. In order to deal with such extreme heterogeneities, resistivity investigations require some specific adjustments. One example is the so-called ''mise à la masse'' method. In Romania, it was used to trace the cold karst water inflows that detrimentally affected the commercial exploitation of a thermal spring, Hercules at Baile Herculane. Conventional geoelectrical approaches--such as using resistivity highs to detect air-filled cave passages, are proved to be less efficient in the considered karst investigations.
Information about subsurface structures is crucial to support work related to civil construction, one of which is the identification of basement rock for a preliminary research of laying the foundation of a building. Hard layers play an important role in maintaining the stability of the foundation from the effects of natural disasters (earthquakes) and land shifts caused by the exploitation of soil carrying capacity. One of the methods that can be used for identifying hard layers is geoelectrical resistivity method. This method processing uses Marquardt's inversion and damping factor functioning to stabilize the inversion process. The inversion process will be carried out when the value of RMSE between the synthetic resistivity data correlation of the initial estimation (starting model) and the field resistivity measurement data is over 10%. Error values of each line (L.1, L.2, and L.3) are 9.3%, 7.2 and 6.1%. Hard layers are identified as layers with resistivity values above 100 Ωm, namely tuffaceous sandstone, tuffaceous gravel, inserted breccia and limestone.
Earth, Planets and Space, 2014
The 2008 Iwate-Miyagi Nairiku earthquake (M 7.2) was a shallow inland earthquake that occurred in the volcanic front of the northeastern Japan arc. To understand why the earthquake occurred beneath an active volcanic area, in which ductile crust generally impedes fault rupture, we conducted magnetotelluric surveys at 14 stations around the epicentral area 2 months after the earthquake. Based on 56 sets of magnetotelluric impedances measured by the present and previous surveys, we estimated the three-dimensional (3-D) electrical resistivity distribution. The inverted 3-D resistivity model showed a shallow conductive zone beneath the Kitakami Lowland and a few conductive patches beneath active volcanic areas. The shallow conductive zone is interpreted as Tertiary sedimentary rocks. The deeper conductive patches probably relate to volcanic activities and possibly indicate high-temperature anomalies. Aftershocks were distributed mainly in the resistive zone, interpreted as a brittle zone, and not in these conductive areas, interpreted as ductile zones. The size of the brittle zone seems large enough for a fault rupture area capable of generating an M 7-class earthquake, despite the areas distributed among the ductile zones. This interpretation implies that 3-D elastic heterogeneity, due to regional geology and volcanic activities, controls the size of the fault rupture zone. Additionally, the elastic heterogeneities could result in local stress concentration around the earthquake area and cause faulting.