İzmir, Gülbahçe ve Sığacık körfezleri'nin sığ sismik yansıma verileri ile araştırılması
Investigation of gulf of İzmi̇r, Gülbahçe and Sığacık bays by shallow seismic reflection data
Yazar: ZEHRA ALTAN
Danışman: DOÇ. DR. NESLİHAN OCAKOĞLU GÖKAŞAN
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Jeofizik Mühendisliği Ana Bilim Dalı
Derece: Yüksek Lisans, 2014
Konu: Jeofizik Mühendisliği = Geophysics Engineering
Dizin: Jeofizik = Geophysics ; Jeotermal = Geothermal ; Jeotermal alanlar = Geothermal fields ; Sismik niteleyiciler = Seismic attributes ; Sismik yansıma = Seismic reflection

Özet:
Bu çalışmada tek kanallı ve çok kanallı sismik yansıma verileri kullanılarak Batı Anadolu'nun, Ege kıyılarında yer alan Gülbahçe, İzmir ve Sığacık Körfezleri'nin stratigrafik ve yapısal unsurlarının rutin veri işlem yöntemleri ve karmaşık iz nitelik analizleri ile araştırılması hedeflenmiştir. Çalışma alanında karada önemli sıcak su çıkışları mevcuttur. Bu sistemlerin deniz içerisinde devamlılıklarının araştırılması ve bölgenin jeotermal aktivitesinden sorumlu yapısal unsurların sismik kesitlerdeki imzalarının ortaya konması açısından bu tez çalışması kapsamında TUBİTAK-İTÜ-MTA işbirliğinde 2000 yılnda MTA Sismik-1 Araştırma Gemisi ile bölgede toplanan 5 adet tek kanallı ve 1 adet çok kanallı sismik yansıma verileri işlenmiş ve yorumlanmıştır.
Sismik veriler İTÜ Jeofizik Müdendisliği Bölümü 'Nezihi Canıtez Veri İşlem Laboratuvarı'nda Linunx/Unix işletim sistemi altında çalışan Disco/Focus 5.0 ve Echos 2011.1 versiyonlu yazılımlar kullanılarak işlenmiştir. Sismik yansıma verileri ham atış datasından zaman ortamı migrasyonlu kesit eldesine kadar rutin veri-işlem adımlarından geçirilmiştir. Çalışma sahasında olası su veya gaz çıkışlarının sismik kesitlerdeki imzasını inceleme adına rutin veri işlem uygulamaları sırasında yansıma sinyallerinin gerçek genlik değerlerini korunmuş ve verilere otomatik genlik kazancı (AGC) uygulanmamıştır. Daha sonra zaman ortamı göç işlemi uygulanarak elde edilmiş olan sismik yansıma kesitlerinin karmaşık iz nitelik analizleri gerçekleştirilmiştir. Analizler sonucu her bir yansıma profiline ait anlık genlik, anlık frekans, anlık faz ve fazın kosünüsü kesitleri elde edilmiştir.
Elde edilen tüm kesitler stratigrafik ve yapısal anlamda yorumlanmıştır ve birbiri ile ilişkilendirilmiştir. Bu yorumlamalar sonucu yeraltının deniz tabanından itibaren yaklaşık 1 km derinliklerine kadar sismik hız yapısı ortaya çıkarılmıştır.
Bölgede stratigrafik anlamda kaotik yansıma şekillenmesine sahip akustik bir temel ve onun üzerinde yatay tabakalı sediman birikiminin olduğu genç çökeller görülmüştür. Yapısal anlamda ise bölgede pekçok aktif fay haritalanmıştır. Bu faylar genellikle doğrultu atım karakterli olup fay zonları şeklinde Gülbahçe-Sığacık ve İzmir Körfezi içerisinde haritalanmıştır. Bölgede az oranda aktif normal fay da haritalanmıştır. Normal fayların deniz tabanında atımları çok düşük ve açıları oldukça diktir.  Bu faylar İzmir Körfezi'nin iç kısmında İzmir Fayın'nın deniz içerisindeki devamında haritalanmıştır. D-B yönelimli sismik kesitlerde haritalanan yapısal unsurlardan doğrultu atımlı faylar bölgede K-G yönelimlerde deformasyona sebep olurken; K-G doğrultulardaki sismik kesitlerde haritalanan normal faylar ise bölgede D-B doğrultularında haritalanmıştır. 
Karmaşık iz nitelik analizleri yapısal ve stratigrafik yorumlamalarda oldukça önemli bir rol oynamıştır. Örneğin fay olan alanlarda yanal yönde anlık genlik, anlık faz ve anlık frekansta zayıf yansımalar, düşük frekans içeriği ve devamsızlıklar sırasıyla gözlenmiştir. Özellikle Gülbahçe Körfezi'nde su ve gaz çıkışlarının olduğu alanlarda gözlenen olası çamur diapirleri haritalanmıştır. Bu diapirik yapılar dogrultu atımlı fay kontrollü olup, bölgede deniz içerisinde sıcak su çıkışlarının varlığını destekler niteliktedir. Sözkonu yapılarda anlık, genlik, anlık frekans ve anlık faz kesitlerinde sırasıyla zayıf yansıma gücü, frekansta soğrulma ve yanal yönde takip edilememe gözlenmiştir. Yine İzmir Körfezi içerisinde genç çökellerde ve sığdaki bir paleo-delta yapısında görülen yansımasız zonlar ve çok sayıda fay kontrollü olası su veya gaz çıkışları bölgede karada varolan sıcak su ve gaz çıkışlarının deniz içerisinde devamlı olduğunun delillerini ortaya koymaktadır.

Abstract:
Nowadays, seismic reflection method is one of the most useful geophysical methods in exploration of the subsurface in detail. In the past 20 years, the quality of seismic information has been greatly improved. The reflection technique has the potential for being more powerful in terms of its ability to generate interpretable observations over complex geologic structures. Recent applications of the marine seismic methods make it possible to investigate submarine structures and correlate them with onland tectonic features. 
The purpose of this study is the investigation of the stratigraphic and structural features of the Gülbahçe, İzmir and Sığacık Bays to understand the offshore goethermal potentional of the area in the western Anatolian Coast by single channel shallow seismic reflection data. For these aims, two types of process were applied to data. First is conventional process to optain time migrated seismic sections and the second is complex trace attribute analysis to obtain the attribute sections such as reflection strength, instantaneous frequency, instantaneous phase, cosine and sine of phases. 
Turkey has very special geographic position at the crossroads between Europa and Asia and it is located on an active tectonic, orogenetic belt, the Alpine-Himalaya Orogen with young trenches, active tectonic and  volcanism which is the reason for Turkey's substantial geothermal resources. The study area is located in the central Aegean Sea, along the western coats of Anatolia including  Gulf of İzmir, Gülbahçe and Sığacık Bays. Western Anatolia and Aegean Sea are commonly considered as under N-S extension in response to westward motion of Anatolian block which is in collision with Arabian Plate. Onland sources that contain high-temperature geothermal fluid exist mostly in Western Turkey which includes study area because of grabens that are formed in result of  recent tectonic activities. Onshore geology of the study area is widely covered by Mio-Pliyocene and younger units. Additionally, Pre-Miocene basement , which consists of Silurian, Carboniferous, Triassic and Cretaceous rocks, only crops out along high elevated areas such as Karaburun, Seferihisar.
Although the onland geothermal systems are well known in this region, their offshore continuation, structural and stratigraphical features and seismic signatures on the sections are not studied in detail to day. and seismic signatures It is very well known onland geothermal systems. For these aims,  approximately total of 68 km seismic reflection data which were collected in E-W and NE-SE orientations by MTA Sismik-1 research vessel in cooperation with MTA-TÜBİTAK-İTÜ during the summer of 2000, were processed and inpterpreted.
The seismic data were processed in the 'Nezihi Canıtez Data Processing Laboratory' of the Department of Geophysics, İTÜ by using Disco/Focus 5.0 and Echos 2011.1 data processing softwares. A conventional data processing stream was applied to the single channel seismic reflection data as follows: data transcription, editing, choosing one channel, source-receiver array datum correction, sorting, NMO correction, notch filtering, spherical divergence correction, migration, band-pass filtering for single channel lines. For one mult-channel seismic line, multi-channel seismic processsing steps were applied to data which includes velocity analysis, stack, deconvolution and migration steps differ than single channel process. Particulary, automatic gain analysis was not applied to data to keep the true amplitudes. Moreover, to exhibit the special structures in the seismic sections, complex trace attribute analyses were applied to tha data. By this method, the seismic signatures of possible fluid or gas saturated sediments and their outflows were investigated in detail.  Seismic attributes sections and conventionally processed seismic sections were interpreted together to understand  the submarine structure and stratigraphy which may resposible from possible geothermal or gas pathways of this region from sea floor to approx. 1 km depth.  
Stratigraphically, one main erosional surface was interpreted which separates the lower seismic basement unit with chaotic reflection pattern from the upper basin sediments with parallel reflection configuration. The upper seismic unit was interpreted as basin deposits that onlap the basement surface. The upper sediments was marked as Pliocene–Quaternary deposits while the lower basement was interpreted as Miocene rocks due to Ocakoğlu (2004) study. 
Tectonically, many active faults were identified on the sections. According to these intepretation, some faults were interpreted as primary faults and some of them are secondary faults in this region which they align from N-S to NE-SW directions. Primary faults have mostly strike-slip characters and are seen as  fault zones on the seismic sections in the Gülbahçe and Sığa However, secondary faults which have normal characters were rarely mapped at the inner part of the Gulf of İzmir. These faults have very little slip on the sea floor, also they have right-angled. The major of them was follow as the offshore extension of İzmir Fault at the inner part of Gulf of İzmir. It lies in E-W direction. 
After conventional data processing, complex trace attributes analyses were applied to the seismic data to reveal detailed special structures in seismic section which is not possible with conventional seismic data processing. These analyses effect on two quantities which are amplitude and phase. The amplitude attribute is called "reflection strenght". The phase information is both an attribute in its own right and the basis for instantaneous  frequency measurement. Amplitude and phase information are also combined in additional attributes, cosine and sine of phase. All of these attributes, reflection strength, instantaneous frequency, instantaneous phase, cosine and sine of phase were decided to apply to data after considerable number of tests.
Complex trace attribute analysis were applied to the seismic sections in which were observed some elevated structures on the reflection levels in Gülbahçe Bay. Inside these elevated structures, it is seen that the reflection strenght and continuity are very weak and frequency content is apsorbed. These structures were interpreted as possible mud diapirs which represent the early stage of a mud volcano. Diapiric structures are known as domes or anterooms produced by the plastic deformations or flow of fine-grained sediments; they may deform or rupture overlying rocks. Indeed, many mud volcanoes are developed in the crests of such diapirs, but also, many of them are not connected with diapiric structures at all. It is thought that there is a mass transport in the chest of these structures because of the plastic deformation of the tectonic regime of the region. These diapirs formed by the input of the fluidized sediment along the fracture zones, have risen from depth due to density inversion and appear on/under the seafloor. There are two main reasons for these kind of structures: first one is the high sediment accumulation rate, second one is the lateral tectonic compression which makes study region perfect place for these kind of structures. Hence, some possible mud diapirs inside the compressed sediments in the Gülbahçe Bay were interpreted on the seismic sections due to their complex attributes.
In addition to Gülbahçe Bay, conventional data processing steps applied to the seismic data in Gulf of İzmir. Same seismic units are observed in Gulf of İzmir. Thicker basin deposits seperated with an erosional surface from acoustic basement. Complex trace attribute analysis were applied to the seismic sections in Gulf of İzmir  too. Particulary, It was seen very low amplitude values in reflection strength section and very low frequency values in instantaneous frequency section at the shallower part of section which corresponds to offshore continuation of Gediz Delta inside the gulf.  Structurally, it is seen that the gulf of İzmir was highly deformed by strike-slip faults. These fault can be seen as fault zones on the seismic sections. These structural features are well defined on the attribute sections. Particularly, cosine and sine of phases were used to crosscheck these faults on the seismic sections. These attributes have indicated possible potential gas or fluid saturated sediments in the shallow depth and pathways of these contents along fault zones in the Gulf of İzmir. 
It is seen that not only normal faults are responsible from the geothermal activity in the study area but also strike-slip faults are. It is concluded that high deposition rate of organic-rich material from the Gediz River drives biogenic gas generation in the muddy sediments in Gulf of İzmir resulting in acoustic turbidity on the seismic data. 
Although, we have shown evidence for shallow gas related formations in the Gulf of İzmir, some analyses still need to be done to reveal sign of faults on the sea floor and some chemical analyses need to be done to decide whether these activities in the study area are geothermal or gas related. Deeper structures and their possible connection to a deeper overpressured shale formation and their relation to the gas charged sediments in the area should be carefully investigated.