Marmara Denizi Kumburgaz Havzası'nda çok kanallı sismik yansıma verilerinin incelenmesi
Investigation of Marmara Sea-Kumburgaz basin using multi-channel seismic reflection data
Yazar: BUĞU DORAN
Danışman: PROF. DR. HÜLYA KURT
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Jeofizik Mühendisliği Ana Bilim Dalı / Jeofizik Mühendisliği Bilim Dalı
Derece: Yüksek Lisans, 2019
Konu: Jeofizik Mühendisliği = Geophysics Engineering

Özet:
Türkiye'nin kuzeyinde yaklaşık doğu-batı doğrultuda uzanan sağ yanal atımlı Kuzey Anadolu Fayı'nın (KAF) Marmara Denizi içindeki geometrisi ve karmaşık aktif tektonik özellikleri, İstanbul gibi büyük bir metropole yakın olması nedeniyle önem teşkil etmektedir. Özellikle 1999 Gölcük depremi (M=7.4) sonrasında bilimsel ilgi odağı haline gelen Marmara Denizi'nde birçok jeolojik ve özellikle çok kanallı sismik yansıma olmak üzere jeofizik verileri toplanmış ve incelenmiştir.
Bu tez çalışması kapsamında, Marmara Denizi'nde 2008 ve 2010 yıllarında Dokuz Eylül Üniversitesi'ne ait K.Piri Reis Araştırma Gemisi ile TAMAM (Turkish American Multichannel Project) ve TAMAM-2 (PirMarmara) araştırma projeleri kapsamında toplanan çok kanallı sismik yansıma verilerinden, Kumburgaz Havzası'na ait olanları işlenmiş ve yorumlanmıştır. Bu amaçla havzayı K-G doğrultuda kesen 7, D-B doğrultuda kesen 3 adet sismik hat olmak üzere toplamda yaklaşık 411.6 km uzunluğa sahip bir veri seti kullanılmıştır. Sismik veri toplama parametreleri olarak TAMAM projesi için, kanal sayısı 72, kayıt uzunluğu 4 s, alıcı grup aralığı 6.25 m ve atış aralığı 18.75 m seçilmiştir. TAMAM-2 projesi için, kanal sayısı 240, kayıt uzunluğu 5 s, alıcı grup aralığı 6.25 m ve atış aralığı 18.75 m seçilmiştir. Veriler, band geçişli süzgeçleme, F-K süzgeci, hız analizi, CDP sıralama, yığma ve migrasyon gibi geleneksel veri işlem adımları uygulanarak işlenmiştir. Veri işlem sonucunda oluşturulan zaman ortamı sismik migrasyon kesitleri, havzaya ait detaylı batimetrik haritaların ışığında yapısal ağırlıklı olarak yorumlanmıştır.
Kumburgaz Havzası'na ait sismik kesitlerde KAF'ın kuzey kolu deniz tabanından yaklaşık 1750 ms'değerine kadar düşey bir süreksizlik olarak gözlenmektedir. Kullanılan tüm sismik kesitlerden yorumlanarak harita düzlemine aktarılan KAF'ın, havzanın kuzey sınırında genel olarak D-B doğrultuda, büklümler yaparak uzandığı gözlenmektedir. KAF'ın harita düzlemindeki bu geometrisinin bir sonucu olarak, fayın güneye büklüm yaptığı yerlerde sırt yapıları, kuzeye doğru büklüm yaptığı alanlarda ise çukur yapılarının oluştuğu gözlenmektedir. Havzada deposantır doğu uçta yer alır. Sismik kesitlerde takip edilen tabaka kalınlıklarının bu noktaya göre artış göstermesi, faylanmanın devam etmesi ile çökel kalınlığının artmasına işaret etmektedir. Doğrultu atımlı fayların havza içindeki hareketinin bir sonucu olarak, havzanın güney batısında normal fayların olduğu gözlenmiştir. K-1, K-2 ve K-3 olarak isimlendirilen BKB-DGD doğrultulu ve kuzeye dalımlı bu fay yapılar, sağ yanal atımlı fay alanına ait gerilme elipsoidinde gözlenen ikincil yapılar olarak yorumlanmaktadır.
Kumburgaz Havzası'nın güneyinde yer alan Orta Sırt üzerinde güneye dalımlı normal faylar olarak berlirlenen OS-1 ve OS-2 fayları, sismik kesitlerde deniz tabanından yaklaşık 1500 ms derinliklere kadar takip edilebilmiştir. Bu fayların normal bileşeni olduğu sismik kesitlerden ve batimetriden gözlenebilmektedir. Fayların güney ve kuzeyinde yer alan birimlerin birbirleri ile farklılık göstermesi bunların doğrultu atım bileşenine sahip olabileceğine işaret etmektedir. Sırtta gözlenen bu faylarla ilişkili olabilecek gazlı yapıların varlığı sismik kesitlerde gözlenen diğer önemli unsurlardır. Bu amaçla oluşturulan yansımanın gücü ve anlık frekans kesitleri incelenmiş, gaz içerdiği düşünülen bölgenin çevre birimlere göre yüksek genlik ve düşük frekansa sahip olduğu görülmüştür.

Abstract:
The complex active tectonism and the geometry of right-lateral strike-slip North Anatolian Fault (NAF) is considered important due to its location close to a metropol city like Istanbul. Marmara Sea has been a topic for recent studies, especially after the Gölcük earthquake (M=7.4), we can say Marmara Sea has been a center of scientific attraction. The deformation caused by the NAF on land side of the area can be perfectly observed and can be easily studied, but it is hard to understand what kind of deformation has been formed in Marmara Sea as the marine technology is recently developed. To understand the deformation caused by the northern branch of the NAF, geological and geophysical (especially multichannel seismic reflection) data have been collected and interpreted in the Marmara Sea.
Kumburgaz Basin has a 30 km length and 10 km of width, with an approximate depth of 820 m. The basin never been interpreted as one of the main basins formed in the Marmara Sea. The In most studies, it is considered as a secondary basin and usually has been studied as a part of Central Basin, as the depth is significantly shallower (the average depth of the other 3 basins are approximately 1200 m). Yet, Kumburgaz Basin is a good example to understand the deformations caused by a strike slip fault.
The dataset from Kumburgaz Basin that used in this study have been collected in the frame of TAMAM Project (Turkish American Multichannel Project) in 2008 and TAMAM-2 Project (PirMarmara) in 2010 with R/V K. Piri Reis of Dokuz Eylul University, with the contribution of Istanbul Technical University, Dokuz Eylul University Institute of Marine Sciences and Technology and Lamont-Doherty Observatory of Columbia University has been processed and interpreted. During both projects, approximately 3000 km 2D data were collected in different azimuths, but for this study, only 10 of these lines were processed. 7 of these seismic reflection lines of these data sets are elongated in N-S direction and 3 of them elongated in W-E direction in the Kumburgaz Basin, forming totally 411.6 km data length. The data acquisition parameters are given as 72 number of channels, 4 s recording length, 6.25 m receiver interval and 18.75 m source interval for the TAMAM project and 240 number of channels, 5 s recording length, 6.25 m receiver interval and 18.75 m source interval for the TAMAM-2 project. As the length of the streamer cable is longer on the second project, the fault value is higher. The recording length is also longer compared to the first project, which means we were able to see the deeper part of the same area. But we can say there was no difference between the layers we were able to track after 3500 ms in both projects due to the high attenuation in rich sediments. The conventional data processing steps were applied to the data like band pass and F-K filtering, velocity analysis, CDP sorting, stack and migration and finally, time migrated seismic sections have been generated. To have better results in migration sections, Kirchoff pre-stack time migration was preferred.
These sections were interpreted as structurally with the help of the high-resolution bathymetry map of the area. This map is a result of a study of Ifremer, where they used AUV (autonomous underwater vehicle) to have a detailed map of the basins in Marmara (approximately 20m of resolution). The processing steps were completed with Promax software, and the interpretation steps were done with Kingdom Suite, and the maps were prepared on CorelDraw.
Only 4 of the seismic lines were showed as an example to see the NAF, other active/passive faults and the canyon formations. 3 of these lines were elongated in N-S direction, so it was easy to follow NAF and related normal faults. One of these lines was elongated in E-W direction, parallel to NAF, so we can perfectly see the increasing thickness from west to east, showing that the depocenter of the Kumburgaz Basin is in the easternmost part of the basin, in other words, the thickness of the sediments are increasing towards the depocenter due to the active faulting of the area.
The northern branch of NAF can be traced as a vertical discontinuity in the migrated seismic sections to the 1750 ms depth from the sea bottom. With all the seismic section interpretations, the fault map of the area has been produced. From the fault map, NAF has generally E-W direction, and has some bendings. As a result of this geometry, the bending to the south of NAF results in ridge structures, and the bending to the north results in basin structures. Several normal faults were observed on the south-west part of the basin, as a result of strike slip faulting of NAF, named as K-1, K-2, and K-3. The faults in the area are more than these 3 faults, but we only mapped the faults we can both see in more than one seismic section and can be followed on the high-resolution bathymetry map. These faults have a slip, too, but we can say this amount of slip is so small that we can ignore and consider these 3 faults as normal. These normal faults have north-dip with WNW-ESE direction, are interpreted as minor structures according to the strain ellipsoid of right lateral fault area. Strain ellipsoid perfectly explains the deformation that caused by the right-lateral strike slip fault. The fault map and the strain ellipsoid were put together to see the deformations like normal faults and thrust faults. The Riedel shears seen on that figure were not observed on the study area. Basically, we can say that the strain causes a downfall on the area (we can consider Kumburgaz Basin was formed in that way), and the stress causes ridges on the edges of the area, as Mid Marmara Ridge, the border ridge between Çınarcık Basin and Kumburgaz Basin can be considered in that category.
The normal faults which have south dip and located in the southern part of the Kumburgaz Basin on the Mid-Ridge, were named as OS-1 and OS-2 and were determined till 1500 ms depth on the seismic sections. The dip of these faults were observed both in the seismic sections and the bathymetry map. As the units on both sides of these faults are not matching each other, these faults were considered as also oblique faults due to the lateral movement. The gas charged areas related to these faults are another fact observed in this study. It would be better to say that the horizons we were not able to track can be a result of gas charged areas or can be due to strike-slip fault form. So, to distinguish the difference, the reflection strength and instantaneous frequency sections were analysed and interpreted. The areas considered as gas charged parts of the basin have high reflection compared to neighbour layers, and low frequency values compared to the surrounding sediments. The recent studies related to gas charged areas around the NAF in Marmara Sea were a key for us to support our theory about the gas occurred due the normal faults. These normal faults are both the reason for the gas travel from deeper parts of the basin and causing a trap and keeping the gas under the ridges. The studies show a high amount of gas spots around NAF through the Marmara Sea. In this case, the stations they sampled gas is matching with the locations we marked as gas charged areas. We can say that these areas have significant amount of gas, and we would have better and certain results after doing a 3D seismic reflection data in that specific area.