Submarine topography and geological survey of the Tsushima West Channel

Project

Implementation Plan (1987)
Planning Agency: International Highway Construction Corporation
Implementing Agency: Far East Development Co., Ltd.

1. Overview

* Seabed geological survey area along the Japan-Korea Tunnel route

This survey planned to include single-channel sonar surveys using sparkers and seafloor topography and geology surveys using side-scan sonar. The most promising basic route for the Japan-Korea tunnel is considered to be from the Higashimatsuura Peninsula in Saga Prefecture, Kyushu, through Iki Island, into the southern part of Tsushima Island (Shimoshima), and then from there, from the vicinity of Gozaki on the west side to Geoje Island on the Korean side.

Seafloor topography and geology surveys of the sea area along this route have been conducted since 1983, beginning with multi-channel sonar surveys using sparkers, followed by single-channel sonar surveys and bathymetric surveys using sparkers, multi-channel sonar surveys using water guns, dredges, and ocean boring, resulting in a wealth of seafloor topography and geological maps. This time, we will conduct a seafloor topography and geology survey of the area north of the Tsushima West Channel to provide basic data for route selection.

2. Survey Contents

① Seafloor topography survey using acoustic bathymetry
② Investigation of seafloor geology and geological structure using sparker and single-channel sonar
③ Investigation of seafloor topography and sediment using side-scan sonar
④ Tidal level measurement using tide gauge

3. Survey quantity

① Acoustic depth sounding: 1,160 km
② Sparker single-channel sonar: 810 km
③ Side-scan sonar imaging: 350 km
④ Tide gauge: Measured every 10 minutes during operation

4. Research Organization

The survey period was from May 1 to July 31, 1987.

5. Research Methodology (and Data Processing)

1. Ship position measurement
will be performed using the two-distance intersection method with a radio positioning device (triconder). A known reference point will be used as the slave station. Four slave stations will be used, and two stations suitable for ship position measurement will be selected as needed so that the intersection angle of the two distances falls within the range of 30° to 150°. Data will be printed using an X-Y converter and input to magnetic tape.
To improve work efficiency, the virtual origin of the exchange coordinate system used in this case
was set to System 1: X=152490.0 Y=-12191.3 θ=320.0°.

*Sonic exploration using sparkers as the sound source

2.
Continuous depth sounding will be performed using an acoustic depth sounder (PDR-101 type). Fixed lines will be drawn every two minutes to correlate with the ship's track.
Bar checks will be performed once a day or each time the recording paper or pen is changed. Depth will be measured twice, once lowered and once raised, every 5m up to 50m. Depths
greater than 50m will be calculated from a revised curve obtained from data such as seawater temperature, pressure, and salinity. Draft adjustments will be made according to changes in draft.

3.
A single-channel sonar survey will be conducted using a sparker as the sound source. The energy source for this survey can be used from 200 to 8000 joules. Prior to the survey, test runs will be conducted to determine the ship speed, magnitude of seismic energy, towing length of the spark array and hydrohorn, depth, filter bandwidth, etc., to set initial conditions suitable for the survey.

4. Seafloor
acoustic imaging survey using site scan sonar. 100kHz/500kHz sound waves are emitted in a fan shape from inside a towed tower (two-fish) towed from the stern towards the seabed. The reflected waves from the seabed are received, converted into electrical signals, and then corrected for oblique distance, amplitude, and ship speed before being output to recording paper. This data is also recorded.

*Outline of the research vessel's equipment

5.
During the tide gauge survey period, tide gauge records from Sasuna, Kamiagata-cho, Tsushima will be used.
If the survey is conducted in a location without a tide gauge station, a suitable observation site will be determined, and the tide height will be measured every 10 minutes to the nearest centimeter using a direct scale. Leveling surveys will be conducted between the hydrographic department's basic leveling marker and the observation site to calculate the basic level plane. This will be used as the depth correction value for acoustic sounding.

6. Deliverables