Sıfır Ofset, Tek Kanal,sığ Deniz Sismiğinde Geriye Doğru Saçılmış Kırılmaların Yorumlanması

Abstract

Sismik çalışmalarda elde edilen sismik kayıtlardan alınan sonuçlar bazen yeraltının jeolojisini tam olarak yansıtmayıp yanıltıcı olabilmektedirler. Dolayısıyla doğru yorumlama yapıp gerçek sonuca ulaşabilmek için çalışılan bölgenin jeolojisinin çok iyi bilinmesi ve sismik kesitlerdeki yanıltıcı etkenlerin farkına varılması gerekmektedir. Temmuz 1994'de Marmara Denizi'nde MTA Sismik-1 Araştırma Gemisi ile Büyükçekmece'den Silivri açıklarına kadar uzanan Kuzey Marmara Sismik-3 hattında alınan sismik kesitlerde benzer bir durumla karşılaşılmıştır. Kesitin Büyükçekmece açıklarına denk düşen bölümü bölge jeolojisine kesinlikle ters düşen bir yapıyı açığa vurmaktaydı. Bölge jeolojisi; birbiri üzerinde basamaklar oluşturan, doğuya doğru hafif eğimli, karasal, çok iyi çimentolanmış kumtaşlarından oluşmaktadır. Bunların üzerini genç yaşlı denizel çökel örtmektedir. Bir başka deyişle bölgede açısal diskordans sözkonusudur. Ancak sismik kesitin bu teze konu olan aykırı kesiminde hafif eğimli tabakalar birden çok yüksek eğim kazanmakta, sıklaşıp incelmektedir. Bu kısım dikkatle incelendiğinde, çok zayıf da olsa doğuya doğru hafif eğimli kumaşı tabakaları farkedilebilmektedir. Kesitin diğer bölgelerinde ise, olması gerektiği gibi, çok iyi çimentolanmış kumaşı tabakaları görülebilmektedir. Bir önceki paragrafta anlatılan durumda iki farklı olay üstüste binmiş haldedir. Gerçek hayatta böyle bir durumun söz konusu olması mümkün değildir. Gerçek olmayan bu sismik görüntüdeki yüksek genlik ve frekans içeriği, deniz tabanı altındaki gerçek jeolojik yapiyı gizlemektedir. Buna rağmen, sismik kesitin bu sorunlu bölgesinde gerçek jeolojik yapının az da olsa farkedilebilmesi konunun araştırılmasında önemli bir ipucu olmuştur. Bu tez iki önemli noktaya değinmektedir: Bunlar; "Geriye-Doğru- Saçılmış-Kırılmalat" in yanıltıcı etkilerinden kaynaklanan yorumlama hatalarının önlenmesi ve refraktör hızının belirlenmesidir.

Sismik-1, the research vessel owned by the General Directorate of Mineral Researches and Investigation (M.T.A.), acquired high resolution seismic data on the Sea of Marmara in the summer of 1994 Figure(1.1). The seismic sections from the offshores of Büyükçekmece disclosed somewhat peculiar structures which are not consisttent with the geology at the region. This region is characterized by gently eastward dipping cemented sandstone layers which are clearly observed along the profile, except at the opening of the Büyükçekmece Bay where the nature of the seismic images suddenly changes. In this particular area, gently eastward dipping layers rapidly steepen-up while they become gradually thinner. This transition zone is succeeded by a rather long (about 15 kilometers) stack of very steep and very thin uniform layers possessing rectilinear interfaces. They continue down to the first water-bottom multiple where they are suppressed by higher amplitudes of the latter one. These stack of very steep and thin layers are followed by another transition leading to the normal disposition of the gently eastward dipping cemented sandstone layers as before. From geological standpoint, the picture described in the preceding paragraph is not real but just a fictive seismic phenomenon that cannot be associated with any consistent geolological structure. Actually, a careful examination of the seismic section reveals the evidence of the continuation of those gently eastward dipping cemented sandstone layers under the fictive seismic phenomenon. The high amplitude and frequency content of this fictive seismic phenomenon conceal the underlying geological structures. Hopefully, there exists in the seismic section at least one spot that the underlying layers are visible enough to co-exist with this fictive seismic phenomenon. This spot is ah important clue of this investigation. VI Stoi *N MARMARA. DTTNİZt (--^ Büyükçefcmere 51 c vi N_ -~"KMS..1 h^M-H Figure 1 Location map The fictive seismic phenomenon depends on a geological structure called angular discomformiy (Figure 1.3). Gently eastward dipping cemented sandstone layers were truncated at the surface (today's sea bottom) leaving sharp edges toward one direction, but smooth edges in the opposite. Recently they are covered with a young, unconsolidated, low velociiy (say, 1700 m/s), and thin sediment. A seismic wave coming with a critical angle to the interface between the upper young sediment and the lower truncated cemented sandstone layers (greater than 4000 m/s of velocity) initiates a head wave traveling along this interface with the velocity (more than 4000 m/s) of the_cemented sandstone layers. Along this path, in the direction of the sharp edges (which act as diffrators in this direction), when a head wave hits a sharp edge terminating a cemented sandstone layer, it is backward diffracted and travels as a head wave in the opposite direction. We name this seismic phenomenon as backward-diffracted-refractions. Images appearing on the seismic sections due to this phenomenon are fictive, hence should be considered as an interpretation pitfall. In the direction of the smooth edges, however, there would be very weak or none backward-diffracted-refractions at all. Should a comparison is done between the frequency of the backward- diffracted-refractions (misleading steep-thin layers) and the number of gently eastward dipping cemented sandstone layers causing them, it seems obvious that the former out numbers the latter. So how come a limited number of cemented sandstone layer edges cause such a dense backward-diffracted-refractions? The VII answer to this question is simple: Intrabed multiples which occur within the overlying young and thin sediment. Those very short path intrabed multiples (with their multi-ordered replicas of alternating polarity) are numerous enough to fill the gaps. Figure 2 Eastward dippingcemented sandstone layers Having discussed the deceptive effects of backward-diffracted-refractions during the interpretation of a seismic data, one may wonder about its potential benefits. Although zero-offset seismic data image the subsurface with ease and better high frequency content with respect to the multichannel seismic data, velocity information cannot be extracted from them. The only exceptions to this are the diffractions, the backward-diffracted-reflections, and backward-diffracted- refractions, if they are readily present in seismic data. The determination of the velocity is very similar to the conventional techniques known in multichannel seismic data processing. The only difference is the doubling of the offsets appearing in the related equations. Therefore, backward-diffracted-refractions, if identified correctly, provide the means of determining the apparent velocity of a refractor from a zero-offset seismic data. The transition zone, as mentioned above, is the passage from the wavefield of gently eastward dipping cemented sandstone layers to the wavefield of backward-diffracted-refractions. The superposition of these two wavefields VIII displays a gradual passage from one character group (gently dipping, low frequency cemented sandstone layers) to another character group (steeply dipping, high frequency backward-diffracted-refractions). This deceptive picture (an interpretation pitfall) should not be attributed to the actual geological structures. This thesis addresses to two important points: A caution to prevent an interpretation pitfall, i.e. the misleading features of backward-diffracted-refractions, and an aid to determine the refractor velocity. In Chapter 1 the location of the research area has been described. Then the particular section on the seismogram that contains the problems has been presented. Following these descriptions, backward-diffracted-refractions phenomenon have been introduced. In Chapter 2, the backward-diffracted-refractions have been described Figure (2.1). The legs following the primary refraction have been explained as the intrabed multiples occurring within the overlying young sediments. Some special cases /// /A I\M WMMMM (a) \v\\\ iiifiiillJiilflJÎM ijllllliliilhll)} (b) (c) Figure 3 Backward-Diffracted-Refractions