Jeofizik

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Browsing Jeofizik by Author "Cengiz, Özlem."

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    Imaging fluid - rich zones by magnetotelluric method at south Marmara region
    (Thesis (M.S.) - Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2012., 2012.) Cengiz, Özlem.; Tank, Sabri Bülent.
    Magnetotelluric data at sixteen sounding locations along two parallel profiles at south Marmara region were collected to improve the understanding of the crustal electrical conductivity structure. The PW (~40 km) and PE (~35 km) profiles yield data in the frequency range 320-0.0005 Hz. Remote reference technique (Gamble et al., 1979) was used to enhance the data quality. The effects of near surface inhomogenities (galvanic distortions) were removed from the data utilizing Groom and Bailey (1989) decomposition. Following the decomposition, the geoelectric strikes of N102°E and N72°E were calculated for the PW and PE profiles, respectively. The data were rotated using these strike directions. Following this step, they were modeled in two dimensions using the inversion code developed by Ogawa and Uchida (Ogawa and Uchida, 1996), with error floors of 10 per cent for apparent resistivity and 2.86° for phase of both TE (transverse electric) and TM (transverse magnetic) modes. Resulting models suggest that the South Marmara fault possibly corresponds to a lateral resistive boundary between Manyas-Karacabey basin and Bandırma-Karadağ uplift on the PW and Uluabat uplift and Mudanya uplift on the PE profile. The features characterized in geoelectric models also correlate with known faults in the study area. While the conductive zones beneath the northern ends of the profiles at depths greater than 13 km are attributed to partially melting in the crust or the existence of deep crustal fluids below the impermeable layers, the highly resistive zones are associated with low fluid condition and high rigidity.
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    Three-dimensional resistivity modelling and interpretation of geothermal fields in the Gediz graben by magnetotellurics
    (Thesis (Ph.D.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2018., 2018.) Cengiz, Özlem.; Diner, Çağrı.
    The Gediz Graben hosting several geothermal systems is one of the most promising grabens in terms of temperature and production rate of western Anatolia. In order to provide the most comprehensive understanding about the geothermal systems situated in the graben, specifically about the reservoir types, heat sources and structural controls, 253 MT sites were installed at four different areas of the graben to delineate the electrical resistivity distribution at depth. The wide-band MT data were analyzed by phase tensor analysis, and then the data at 31 selected periods in the range from 0.001 s to 1000 s modeled in three-dimensions (3D). The resulting models reveal three different reservoir types, namely (i) a classical geoelectrical distribution of a high temperature geothermal system, with a prominent highly conductive hydrothermal alteration zone sitting above a more resistive deep reservoir zone, (ii) a deep reservoir zone characterized by fractures within metamorphic rocks in the highly resistive basement and (iii) a shallow reservoir (aquifer) corresponding to the hot springs in the shallow sedimentary layer existing in the Gediz Graben. The heat source of the geothermal systems may be attributed to the heat transfer from the interior of the Earth to the upper crust as a consequence of crustal thinning resulted from the extensional tectonics accompanied by magma intrusions into crust in western Anatolia. The 3D models bring out a well-defined interface between the sedimentary cover and underlying metamorphic basement owing to high resistivity contrast between two layers, characterizing the Gediz detachment fault (GDF). The geothermal fields formed along the southern margin of the graben are spatially coincident with the intersecting zone of two fractures, namely the GDF and high angle normal faults, and the circulation of geothermal fluids in reservoirs are dominantly controlled by these fractured zones and major faults. The crustal scale main graben-bounding fault (MGBF) acts as a conduit through which fluids and heat are transported from deeper parts of the crust to near surface. The meteoric waters percolating deep into the crust through the north dipping normal faults are probably heated up by magmatic intrusions, and some of geothermal waters containing meteoric and magmatic fluids rise up to surface through the permeable faults, in particular through the lower bounding sub-horizontal GDF. Furthermore, 3D resistivity models suggest a thick sedimentary layer (2500-3000 m) in the middle part of the graben basin. The thickness of the sedimentary layer decreases gradually on the northern and southern margins of the graben and becomes much thinner towards the eastern end of the graben. 3D resistivity models also delineate an undulating basement topography under the conductive sedimentary fill of the graben.