Abstract

The Municipal Government of Istanbul plans to construct a second subsea tunnel across the Bosphorus Waterway between the European and Asian sectors of the city, and a submerged tube tunnel across the Golden Horn Waterway which connects to the Bosphorus Waterway. High-resolution 2-D sparker seismic surveys were conducted to delineate geometry of the sub-bottom sedimentary sequence and geometry of the sediment-bedrock interface, and to determine the locations and depths for geotechnical boreholes. In this paper, we discuss aspects of the seismic response of water-bottom sediments. Unlike the usual behavior of water-bottom multiples with alternating polarity observed in the Bosphorus case, we find that the polarity of the water-bottom primary and first-order water-bottom multiples are the same in the Golden Horn case. The polarity behavior of the water-bottom multiples in the Golden Horn is convincing evidence that the low-velocity sub-bottom sediments can be described as a gas-charged suspension — a composition that consists of particles suspended in water with only slight grain-to-grain contact and biogenic gas. The results of the velocity analysis of the Golden Horn seismic data confirm the presence of low-velocity (550–650 m/s) soft water-bottom sediments with an average thickness of 18 m above the bedrock. This makes it imperative to design the foundation pillars for the planned submerged tube-tunnel with sufficient penetration deep into the bedrock.

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