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

Yıl 2019, , 168 - 189, 15.08.2019

Zehra Altan , Neslihan Ocakoğlu Gualtiero Böhm

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

Öz

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.

Anahtar Kelimeler

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

Destekleyen Kurum

İstanbul Teknik Üniversitesi Bilimsel Araştırma Projeleri Koordinasyon Birimi

Proje Numarası

Proje no: 39685

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

Öz

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.

Anahtar Kelimeler

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

Proje Numarası

Proje no: 39685

Kaynakça

• Accaino, F., Böhm, G., Brancolini, G., 2005. Analysis of Antarctic glaciations by seismic reflection and refraction tomography. Marine Geology, 216, 145-154. Aki, K.; Lee, W.H.K., 1976. Determination of three-dimensional velocity anomalies under a seismic array using P arrival times from local earthquakes: 1.A homogeneous initial model. J. Geophys.Res., 81, 4381–4399,123.
• Altan, Z. & Ocakoğlu, N., 2016. Shallow seismic study of the geothermal areas in the Gülbahçe Bay and İzmir Gulf (Aegean Sea, Western Turkey). Mar. Geop. Research., 37 (4), 297-311.
• Anderson, D.L., Dziewonski, A.M., 1984. Seismic tomography. Scientific American, 251, 60-68.
• Battaglia, J., Zollo,A., Virieux, J., Dello Iacono, D., 2008. Merging active and passive data sets in traveltime tomography: the case study of Campi Flegrei caldera (Southern Italy). Geophys. Pros., 56, 555–573.
• Bishop, T.N., Bube, K.P., Cutler, R.T., Langan, R.T., Love, P.L., Resnick, J.R., Shuey, R.T., Spindler, D.A., Wyld, H.W., 1985. Tomographic determination of velocity and depth in laterally varying media. Geophysics, 50, 903–923.
• Bois, P., LaPorte, M., LaVergne, M., Thomas, G., 1972. Well-to-well seismic measurements. Geophysics, 37, 741-480.
• Bording, R.P., Gersztenkorn, A., Lines, L.R., Scales, J.A., Treitel, S.T., 1987. Applications of seismic traveltime tomography. Geophys. J.R. astr. S., 90, 285-303.
• Bourbiē, T., Coussy, O., Zinszner, B., 1987. Acoustics of porous media. Institut Français du Pētrole Publications, Paris (syf. 240).
• Böhm, G., Rossi, G., Vesnaver, A., 1999. Minimum time ray tracing for 3D irregular grids. J. Seism. Explor., 8, 117–131.
• Böhm, G., Accaino, F., Rossi, G., Tinivella, U., 2006. Tomographic joint inversion of first arrivals in a real case from Saudi Arabia. Geop. Prospecting, 54, 721-730.
• Böhm, G., Ocakoğlu, N., Picaotti, S., De Santis, L., 2009. West Antarctic Ice Sheet evolution: New insights from a seismic tomographic 3D depth model in the Eastern Ross Sea (Antarctica). Marine Geology, 266, 109-128.
• Boehm, G., Francese, R., Giorgi, M., 2010. Bedrock detection from an integrated procedure of refraction analysis and tomographic inversion of the first arrivals. The 16th European Meeting of Environmental and Engineering Geophysics of the Near Surface Geoscience Division of EAGE, Zurich, Switzerland, Abstracts, p41.
• Bracewell, R.N., 1956. Strip integration in radio astronomy. Australian J.of Physics, 9, 198-217.
• Bube, K.P. and R. Burridge, 1983. The one-dimensional inverse problem of reflection seismology. SIAM Review, 25, 497-559.
• Carrion, P., Boehm, G., Marchetti, A., Pettenati, F. ve Vesnaver, A., 1993a. Reconstruction of lateral gradients from reflection tomography. J. Explor. Seism, 2, 55-67.
• Çiftçi, N. B., Temel, R. O. ve Terzioğlu, N. M., 2004. Neogene stratigraphy and hydrocarbon system of the region surrounding the Gulf of Edremit, NW Anatolia, Turkey. Bulletin of Turkish Association of Petroleum Geologists, 16, 81–104.
• Cormack, A.M., 1963, Representation of a function by its line integrals with some radiological applications. J. Appl. Phys., 34, 2722-2727.
• Cormack, A.M., 1964. Representation of a function by its line integrals with some radiological applications. II. J. Appl. Phys., 35, 2908-2913.
• DeRosier, D.J., Klug, A., 1968. Reconstruction of Three Dimensional Structures from Electron Micrographs. Nature, 217, 130-134.
• Dines, K. A. ve Lytle, R. J., 1979. Computerized geophysical tomography. Proceedings of the IEEE, 67(7): 1065-1073.
• Dziewonski, A.M., Hager, B.H., O’Connell, R.J., 1977. Large-scale heterogeneities in the lower mantle. J. Geophysical Res., 82(2), 239-255.
• Gilbert, P.F.C., 1972. An iterative method for three-dimensional reconstruction of an object from projections. J. Theor. Biol., 36, 105-117.
• Gürgey, K., Simoneit, B. R. T., Bati, Z., Karamanderesi, I. H., ve Varol, B., 2007. Origin of petroliferous bitumen from the Buyuk Menderes-Gediz geothermal graben system, Denizli-Saraykoy, western Turkey. Applied Geochemistry, 22, 1393–1415.
• Hobro, J. W. D., Singh, S.C., Minshull, T.A., 2003. Three-Dimensional tomographic inversion of combined reflection and refraction seismic traveltime data. Geophysical Journal International, 152 (7), 79-93.
• Hounsfield, G.N., 1972a. A method of and apparatus for examination of a body by radiation such as X-ray or gamma radiation. Patent Specification, 1283915.
• Hounsfield, G.N., 1972b. Computerized traverse axial scanning (tomography) Part I: Description of system. British J. Radiology, 46, 1016-1022.
• IOC, IHO ve BODC, 2003. "Centenary Edition of the GEBCO Digital Atlas", published on CDROM on behalf of the Intergovernmental Oceanographic Commission and the International Hydrographic Organization as part of the General Bathymetric Chart of the Oceans; British Oceanographic Data Centre, Liverpool.
• Lauterbur, P. C., 1973. Image formation by induced local interactions: Examples employing nuclear magnetic resonance. Nature, 242, 190-191.
• Mason, I.N., 1981. Algebraic Reconstruction of a two-dimensional velocity inhomogeneity in the High Hazles seam of Thoresby Colliery. Geophysics, 46, 298-308.
• NASA’s Earth Observing System Data and Information System (EOSDIS), Copernicus Sentinel data [2018]. Retrieved from ASF DAAC [1.10.2018], processed by ESA.
• Ocakoğlu, N., 2004. İzmir körfezi ve Alaçatı-Doğanbey-Kuşadası açıkları aktif tektoniğinin sismik yansıma verileri ile incelenmesi. Doktora Tezi, İstanbul Teknik Üniversitesi, İstanbul, Türkiye.
• Ocakoğlu, N., Demirbağ, E., Kuşçu, İ., 2005. Neotectonic structures in İzmir Gulf and surrounding regions (western Turkey): Evidences of strike-slip faulting with compression in the Aegean extensional regime. Marine Geology, 219, 155-171.
• Operto S., Ravaut C., Improta L., Virieux J., Herrero A. ve Dell’Aversana P., 2004. Quantitative imaging of complex structures from dense wide-aperture seismic data by multiscale traveltime and waveform inversions: a case study. Geophysical Prospecting, 52, 625-651.
• QGIS Development Team (2018). QGIS Geographic Information System. Open Source Geospatial Foundation Project. http://qgis.osgeo.org
• Radon, J., 1917. Über die Bestimmung von Funktionen durch Integralwerte langsgewisser Manigfaltigkeiten. Ber.sachs. Akad der Wiss., 69, 262-277.
• Rossi, G., Böhm, G., Madrussani, G., 2011. Tomographic inversion of ocean bottom seismograph (OBS) data: Problems and solutions applied to the NW Svalbard Hydratech data set. Computers & Geosciences, 37, 1535-1544.
• Vesnaver, A., 1996. The contribution of reflected, refracted and transmitted waves to seismic tomography: a tutorial1. First Break, 14 (5), 159-168.
• Vesnaver, A., Böhm, G., Madrussani, G., Rossi, G., Granser, H., 2000. Depth imaging and velocity calibration by 3D adaptive tomography. First Break, 18 (7), 303-312.
• Vesnaver, A., Lovisa, L., Böhm, G., 2010. Joint 3D processing of active and passive seismic data. Geophys. Prospect., 58, 831–844.
• Worthington, M. H., 1984. An introduction to geophysical tomography. First Break, 22, 20-26.
• Zhang, H., Sarkar, S., Toksöz, N., Kuleli, H., Al-Kindy, F., 2009. Passive seismic tomography using induced seismicity at a petroleum field in Oman. Geophysics, 74(6), WCB57-WCB69.