Distribution of Acoustic Masking Zones Along the Kapıdağ Fault in the Sea of Marmara


Sayılar B. C., Yıldız E. C., Kurt H., Okay Günaydin S.

74. Jeoloji Kurultayı, Ankara, Turkey, 11 - 15 April 2022, pp.194

  • Publication Type: Conference Paper / Summary Text
  • City: Ankara
  • Country: Turkey
  • Page Numbers: pp.194
  • Dokuz Eylül University Affiliated: Yes

Abstract

The distribution of shallow gas deposits provides important information about their origin and the geological factors controlling them. In the world, investigation of sediments which contain shallow gas is important in terms of geological and economic production. Significant shallow gas anomalies are observed along the seismic lines that cut the middle branch of the North Anatolian Fault Zone distributed in the southern Marmara shelf. It is also important to investigate these anomalies in relation to active faulting. While the northern shelf is 10 km wide in the sea of Marmara, the southern shelf covers a wider area of 40 km. The bathymetry of Southern Marmara shelf, is generally flat, with a maximum depth of around 100 m. The study area exists between the Kapıdağ peninsula and the Marmara island where the water depths vary between 50-70 m. This area also covers the area where the Kapıdağ fault is actively observed. Acoustic masking zones, one of the indicators of shallow gas, were mapped in relation with the North Anatolian Fault and its branches by using seismic sparker and chirp seismic data collected in the southern Marmara shelf. Gas accumulations even at a depth of several meters were clearly observed with this high frequency and very high resolution data. The penetration of the chirp data enabled the visualization of sediments up to 15-45 m from the seafloor, while the penetration of sparker seismic data allowed imaging of sediments up to 40-75 m from the seafloor. The presence of free gas in marine sediments causes typical acoustic anomalies that create high acoustic reflection anomalies, due to the acoustic impedance difference between the gas-containing sediments and the non-gas-containing sediments. Such anomalies make the seismic data completely invisible in the form of acoustic masking. In this study, according to their widths, acoustic masking zones, gas columns and gas chimneys were observed respectively. The depths from seabed, widths and distances to the fault of these structures were calculated and plotted. Approximately 1-1.2 km away from the fault zone, anomalies in the form of gas chimneys with widths of 100-600 m were observed. 2000- 3000 m wide acoustic maskings coincide with the fault trace on the sea floor and north of it. The depths of the high amplitude reflections, which form the upper limits of the anomalies, vary between 3.75-30 m. The upper border of the acoustic masking zone is located close to the seafloor or below the horizon just below the seafloor. Acoustic columns are distributed along the fault zone. Away from the fault zone, the width of the acoustic masking zone narrows and observed as acoustic columns and gas chimneys. Acoustic maskings on the fault is thought to be associated with deep structures. The bright spots and strong reflection anomalies occurring in the fault zones strengthens the possibility that the gas was transported along the fault zone through the permeable layers.