Marmara Denizi sismik yansıma verilerinin değerlendirilmesi (profil M97-014)
Başlık çevirisi mevcut değil.
- Tez No: 75365
- Danışmanlar: YRD. DOÇ. DR. AYSUN GÜNEY
- Tez Türü: Yüksek Lisans
- Konular: Jeofizik Mühendisliği, Geophysics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 1998
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Jeofizik Mühendisliği Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 57
Özet
ÖZET Bu çalışma Marmara Denizi içinde Kuzey Anadolu Fay Zonu'nun (KAFZ) uzantısı olduğu düşünülen doğrultu atımlı fay segmentlerinin sismik imzalarını yansıma kesitlerinde araştırmak için yapılmıştır. Yansıma verileri Maden Tetkik ve Arama Genel Müdürlüğü (MTA), Türkiye Bilimsel ve Teknik Araştırma Kurumu (TÜBİTAK), İstanbul Teknik Üniversitesi (İTÜ) ve Cambridge Üniversitesi' nin işbirliği ile Ulusal Deniz Jeolojisi ve Jeofiziği Programı kapsamında proje bazında toplanmıştır. Bu çalışma kapsamında Marmara Denizi'nin ortasını kuzey-güney doğrultusunda geçen, 64 km uzunluklu M97-014 profiline ait yansıma sismiği verileri işlenmiş, yorumlanmış, sismik göç işlemi yapılmış kesit üzerinde fayların imzalan araştırılmıştır. M97-014 profiline ait yansıma sismiği verileri çok kanallı (96 kanal) ve 8 s kayıt süreli verilerdir. Verilerin örnekleme aralığı 2 ms ve kayıt formatı SEG-D formatındadır. Standard veri işleme yöntemlerinin uygulanmasında DISCO/FOCUS adlı veri işleme paketi kullanılmıştır. M97-014 profiline ait sismik göç işlemi uygulanmış kesitte kuzeyde eğim atımlı normal fayların hareket etmesi ile halen aktif, güneyde ise içi sedimanlar ile dolu pasif iki havza görülmektedir. Kuzeydeki havza içinde birbirlerinden bir sırt ile ayrılmış iki küçük havza gelişmektedir. Sırtın kuzeyinde yeralan küçük havzanın sıkışma bileşenli doğrultu atımlı fay zonu ile meydana gelmiş çek-ayır (pull-apart) tipi bir havza olduğu sonucuna varılmıştır. Sırtın güneyinde yeralan küçük havza ise eğim atımlı faylar ile gelişmektedir. vu
Özet (Çeviri)
SUMMARY THE EVALUATION OF THE SEISMIC REFLECTION DATA IN THE MARMARA SEA (PROFILE M97-014) The Marmara Sea is situated in the western end of the North Anatolian Fault Zone (NAFZ). The region is under the influence of both strike-slip pattern of the NAFZ and the extensional regime of southwestern Anatolia. Therefore, the Marmara Sea has a complex tectonic behavior (Barka and Kadinsky-Cade, 1988). The NAFZ is a broad belt of crushed rocks a few kilometers wide rather than a single line. It is subdivided into several fault segments, which are seperated mostly by releasing and restaining stepovers. A detailed investigation of these fault segments has been published by Barka and Kadinsky-Cade (1988). They defined fault segments as fault sections, which are separated by sharp geometric discontinuities (stepovers 2: 1 km, bends ^ 5°). The major earthquakes along the NAFZ since 1939 have shown that the fault geometry plays an important role. Most strong earthquakes are occurred next to a fault belt (Straub, 1996). The Marmara region has a big importance because of Istanbul metropolitan and its urbans. Large number of housing, industruial and commercial sites, natural gas pipelines and sewage disposal systems have been constructed with increasing speed. The region has been taking migration and investment at a large scale. The Marmara region has also high seismicity and possesses a great potential of earthquake hazard. Investigation of marine geophysics and geology is therefore important as far as the earthquake safety is concerned. Previous published seismic reflection studies in this region are limited in number and based on shallow seismic reflection data with single trace (Smith et al., 1995; Wong et al., 1995). On the other hand, results of commercial multichannel seismic studies done by Turkish Petroleum Company (TPAO) indicate that the multichannel deep seismic reflection data are necessary to explain the complex tectonic behavior of the Marmara Sea. The purpose of this thesis is to investigate the seismic signature of the strike-slip faults in the middle of the Marmara Sea. The acquisition of seismic reflection data used in this study has been done in cooperation with the General Directorate of Mineral Research and Exploration (MTA), the Scientific and Technical Research Council of Turkey (TÜBİTAK), Istanbul Technical University (İTÜ) and Cambridge University. The study is an extension of the Marine Geology and Geophysics Program. The seismic reflection profile (M97-014) elongated in the north-south direction was chosen since it cuts the most tectonics features in the Marmara Sea. The length of the seismic profile is 64 km. The number of channels is 96. The maximum record length is 8 s. The data was recorded by SEG-D format with the sampling interval 2 ms. The shots were vmrepeated in each 50 m (20 ms). The offset is 40 m and the number of CDP is 12. The specifications of the source and receiver are given in Table 3.2 and 3.3. Before we process the multichannel seismic data of the profil M97-014, we examined the single trace of the channel 96 for all shots along the seismic profile. On the single trace section of the profile, we identified clearly the signals with high frequency in the shallow part of the section and the reflections from the sea bottom. However, we could not get a clear picture of the general tectonic features. Therefore, we decided to process the multichannel seismic data by using the standard processing methods. The seismic reflection data of the profile M97-014 were processed by the standard routines of the seismic data processing package, DISCO/FOCUS 4.0. We applied the following DISCO modules to seismic data: EDIT, AGC, GAIN, SORT, VELDEF, NMO, MUTE, STACK, MIGRATE. EDIT The traces with low singal/noise ratio are not included in the later processing steps by using EDIT module. AGC This module is applied to seismic data to bring up weak signals. AGC is time- variant. This is a kind of balancing each trace in a group of traces to get the same desired rms amplitude level (Yılmaz, 1991). GAIN The compensation of the wave front divergence is called as the geometric spreading correction. GAIN module compensates the wave front divergence. Therefore, it is applied to seismic data in early processing steps. SORT The seismic data are transformed from shot-receiver to midpoint-offset coordinates by this module. SORT requires field geometry information. In this study, we assumed that the streamer is exactly in the direction of the seismic profile. Therefore, we used a simple geometry description for the module SORT. VELDEF This module is used to analyses the velocity information. The computed velocities in this step are used to correct for NMO (Normal Move Out) so that the reflections are aligned in the traces of a CMP (Common Mid Point) gather before stacking (Yılmaz, 1991). The application of this module is very important since the quality of the stack of the signal in the further step depends on the accuracy of the velocity analyses. In the application of this module, we tried not to select the reflection hyperbolas of sea bottom multiples so that the energy of them can be decreased during the stacking of IXthe signals. The processing results by using this method indicated that the seismic energy of multiples could not be removed completely from the seismic section. Since the application of this module is based on the selection of the reflection hyperbola correctly, we faced the difficulties in the case, which the reflection hyperbola can not be seen easily, such as the interference of the direct arrivals with first reflections for the shots in the shallow sea water. NMO The difference between the two-way time at a given offset and two-way zero offset time is called NMO. The travel times are corrected to remove the influence of offset by using the velocity information obtained from VELDEF. MUTE The signals described as noise in seismic processing, such as direct wave, refracted waves, surface waves and the signals created as a result of NMO stretching are cut by this module. STACK The traces for each common mid point are summed along the offset axis. The result is a stacked section. The purpose of this application is to get as much as information from a reflection point. The random noise in the seismic section is decreased and signal/noise ratio is increased by this method. MIGRATE Migration is a process that maps dipping events on a stacked section to their true subsurface locations (Yılmaz, 1991). The migration method used in this study is based on Claerbout's method for propagating a wave field using finite difference approximations to the wave equation implementing a time frame referenced to a vertically upwards traveling wave (Claerbout and Johnson, 1971; Claerbout and Doherty, 1972). We applied the module MIGRATE to post stack seismic data. After we applied the migration method to the seismic data, we observed the side effect of the migration on the seismic sections as an upward hyperbolas, which are dominant in the deeper part of the seismic section. This showed us that the velocity analyses could be done in detail in the deeper part of the seismic section to get a better picture. We used the migrated section instead of the stacked section in the interpretation step since the diffraction hyperbolas created by the fault planes are dominant in the upper part of the stacked seismic section The migrated section of the seismic profile, M97-014 is plotted by using small and large scale as Model 1 and Model 2 respectively (Appendix A and Figure 4. 1). On the migrated seismic section, we firstly identified the sea bottom, the sea-bottom multiples, partly the geometry of the bottom of the basins, and we observed the parallel, dipping and wavy reflection patterns inside the basins. In the interpretation step, we faced two important difficulties. The first was the determination of the bottom of the deeper basin because of the remains of high seismic energy of the sea-bottom multiples. Thesecond was the interpretation of the wavy reflection pattern observed at the north edge and inside the big basin on the seismic section. Therefore, we examined the same section both in detail (Model 1) and in general (Model 2). Model 1 Appendix A indicates that two basins, HI, and H2, exist along seismic profile. We interpreted that the basin on the north of the profile, HI, developed by control of the faults inside the basin and the fault at the edges of the basin. The geometry of the sea floor along the seismic section indicates that these faults are active. In contrary, the faults developing the basin on the south of the seismic profile, H2, are passive since they are covered by sediments. In detailed examination of the basin HI, we observed that it includes two basins; HI a and Hlb and a ridge separating these two basins. The shallower basin, Hlb, is situated on the south slope of the basin HI. We observed the parallel reflections inside the deeper basin, HI a, dipping and widening reflections inside the basin Hlb. We also observed the fault zone, F2, with the compressive component and the fault zone, F3, with the tension component inside the basin HI a. These findings indicate that the factors controlling the formation and development of the small basins are the faults inside and the edges of the small basins too. From Model 1, we proposed two points: 1- The region including the basin HI has a flower structure. The indicators of this are; existance of the fault zones, F2 and F3 with the strike-slip components inside the basin, and existance of the wavy reflection pattern at the north slope and inside the basin 2- The fault zone, F2, is one of the branches of the North Anatolian Fault Zone and the basin, HI a, was developed by this fault zone in the form of a pull-apart basin. Model 2 In the large scale plot of the migrated seismic section of the profile M97-014 (Figure 4.1), the certain tectonic features are observed clearly than those in Model 1 (Appendix A). In Model 2, we examined whole migrated section in abroad scale rather than in detail as we did in Model 1. There is a large basin on the north of the seismic section. The south slope of this basin was deformed by a strike-slip fault. We believe that this fault could be one of the ridges in the Marmara Sea. We also observed a small hanging basin on the south slope of the basin. Inside the big basin in the north of the profile, we observed folded and discontinued reflection patterns in the rocks under the young sediments. In a feature study, the accuracy of the proposed models, Model 1 and Model 2, for the profile M97-014 should be tested after the application of the following steps: 1- Applying the correction for the geometry of the streamer, 2- Applying the correction for the source and receiver statics, 3- Applying the correction for the sea-floor statics, 4- Suppresing of high seismic energy of the multiple reflections by using different methods from the method which we applied in this study, XI5- Moving the reflections in their actual positions by applying migration methods in detail, 6- Combining the results obtained from the seismic data of the profile M97-014 with the results obtained from the other seismic profiles in the Marmara Sea. Those applications will provide us a more accurate seismic section for a geologic interpretation, especially for the wavy reflection patterns observed in the north of the seismic section. We will also be able to see certain geologic structures in deeper part of the seismic section in case we are successful in suppressing the seismic energy of the sea bottom multiples on the seismic section. xn
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