Akademik Veri Yönetim Sistemi
74. Jeoloji Kurultayı, Ankara, Türkiye, 11 - 15 Nisan 2022, ss.194
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.