Yansıma Seyahat Zamanı Tomografisi: İzmir Körfezi için Örnek Bir 2B Çalışma

Year 2019, Volume: 40 Issue: 2, 168 - 189, 15.08.2019

Zehra Altan , Neslihan Ocakoğlu Gualtiero Böhm

https://doi.org/10.17824/yerbilimleri.508154

Abstract

Bu çalışma kapsamında, İzmir Körfezi Foça açıklarında toplanan çok kanallı bir
sismik yansıma profiline, yansıma seyahat zamanı tomografisi yöntemi uygulanarak
sahanın hız-derinlik modeli ilk kez ortaya çıkarılmıştır. Ham atış verileri, gürültülü
izlerin ayıklanması, direk varışların kesilmesi, çentik filtreleme ve küresel genlik
kazanımı gibi ön veri-işlem adımları uygulanarak iyileştirilmiştir. Önceki çalışmalarda
rutin veri-işlem adımları kullanılarak elde edilen sismik göç kesitleri, bu çalışmada
stratigrafik anlamda detaylı olarak yorumlanmış ve belirlenen ara yüzeyler, ön veriişlem adımları uygulanmış atış kayıtları üzerinde işaretlenmiştir. Işın izleme ile
hesaplanan seyahat zamanları, eşzamanlı yinelemeli yeniden yapılandırma (SIRT)
yöntemiyle ters çözüm aşamasında kullanılarak lokal ara hızlar hesaplanmıştır. Her
yinelemede elde edilen bu hızlar kullanılarak, ara yüzeylerin şekli ve derinliği ortaya
çıkarılmıştır. Bu işlemler, seyahat zamanı rezidüelleri (işaretlenen ve hesaplanan
seyahat zamanları arasındaki farklar) minimum olana kadar yinelenmiştir. Üretilen hız
tomogramı, Foça açıklarında deniz tabanından itibaren yaklaşık olarak 1 km derinliğe
kadar inen, beş farklı ara yüzey (H1-H5) ile birbirinden ayrılmış dört farklı sismik
ünitenin varlığını ortaya koymuştur. Bu çökellerin P dalgası ara hızları yaklaşık 1.5-
2.6 km/s’ler arasında değişmektedir. En altta yer alan akustik temel (H5), batıdan doğuya doğru 800 metrelere kadar derinleşerek bir havza geometrisi oluşturmakta;
söz konusu havza doğuda ise 440 metrelere kadar sığlaşarak bir sırt yapısı ortaya
koymaktadır. Havza içerisinde yer alan çökel paketleri havza geometrisi ile uyumlu
olarak batıdan doğuya doğru derinleşerek kalınlaşmaktadır. Bu çalışma, yansıma
seyahat zamanı tomografisi yönteminin, yeraltının derinlik ortamında stratigrafik
yapısının detaylı olarak ortaya çıkarılması ve tabakaların ara hız modellerinin elde
edilmesinde başarılı sonuçlar ürettiğini göstermiştir.

Keywords

Sismik Tomografi, Seyahat zamanı ters çözümü, SIRT algoritması, Minimum Dispersiyon, İzmir Körfezi

Supporting Institution

İstanbul Teknik Üniversitesi Bilimsel Araştırma Projeleri Koordinasyon Birimi

Project Number

Proje no: 39685

Reflection Traveltime Tomography: A 2D case study from Gulf of Izmir (Turkey)

Abstract

In this study, reflection traveltime tomography has been carried out on a multichannel
seismic reflection data in the İzmir Gulf to obtain a velocity-depth model of the study
area for the first time. The time-migrated seismic sections were interpreted
stratigraphically before reflection traveltime tomography application. The raw shot
gathers improved by performing preliminary data-processing steps such as noise
elimination by editing, muting, notch filtering and spherical gain recovery. Migrated
seismic sections produced by using conventional data processing scheme in
previous studies were interpreted stratigraphically in detail. The interfaces defined on
the migrated time sections were picked on the improved common shot gathers.
Travel times calculated by ray tracing were used during travel time inversion adopting
SIRT (Simultaneous Iterative Reconstruction Technique) algorithm to estimate the
local interval velocities. These interval velocities estimated in each iteration were
used for calculating the shape and depth of the interfaces in the study area. The
velocity field is updated by minimizing the travel time residuals. Tomogram revealed
that the velocity model of a sedimentary sequence of four seismic units with a
thickness of about 1 km in the offshore Foça that are separated by five different
interfaces (H1-H5). The interval velocities of these sedimentary sequences vary
between 1.5-2.6 km/s. The acoustic basement (H5) constitutes a basin geometry that
deepens to 800 meters from west to east and gets shallow up to 440 meters in the
east forming a ridge. In addition to this, the sediment units in the basins deepen from
west to east in accordance with the basin geometry. The investigation provided that
the reflection traveltime tomography method is a good tool to obtain stratigraphical properties of layers in depth and to estimate an accurate interval velocity model of
the seismic unit.

Keywords

Seismic tomography, Traveltime Inversion, SIRT algorithm, Minimum dispersion, Gulf of Izmir

Project Number

Proje no: 39685

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