Project _

Geomagnetic survey in the waters around northwest Iki

Implementation Plan (1987)
Planning agency: International Highway Construction Corporation
Implementing agency: Kyokuto Kaihatsu Co., Ltd.

1. Overview

As a continuation of the geomagnetic survey of the Iki Channel and the sea area around Iki, we planned a ``geomagnetic survey of the sea area northwest of Iki'' using a proton magnetometer.
Geological surveys for the Japan-Korea Tunnel Project have been carried out over a wide area along the route, both on land and offshore. In the marine area, starting in 1982, single-channel and digital multichannel sonic surveys using sparkers and water guns as sound sources have been carried out over a total of 8,100 km in the Iki Channel, Tsushima East Channel, and Tsushima West Channel. Furthermore, in 1986, offshore boring was carried out in the Tsushima West Channel, and the overall geological distribution, geological cross-section, geological structure, etc. of this area were revealed.

2. Survey overview

Geomagnetic exploration using proton magnetometer
Survey area: Sea area around northwest of Iki (map omitted) 2.
Survey contents ① Measurement of total magnetic force at sea, ② Observation of fixed points on land (daily variation correction), ③ Sounding, ④ Tide test, ⑤ Positioning 3.
Survey siding (map omitted) Total distance: 530km
Test run: 50km
4. Survey period: April 5th to April 30th, 1985

3. Research method

Two-distance method using a sonic positioning device (tryponder)
to determine ship position The slave station uses the existing reference point and is installed so that the angle of intersection between the two distances falls within the range of 30° to 150°. 2.
acoustic sounding PDR-101.
A fixed line is placed every 2 minutes.
Ship speed: 5kt/h

Total magnetic force measurement A proton magnetometer uses the moment of nuclear magnetism of hydrogen atoms in water to measure the frequency of free precession of protons (protons) within the earth's magnetic field, and provides an absolute measurement of the earth's total magnetic force. It is.
The measurement principle is to enclose an aqueous solution in a coil arranged north, south, east, and west, and to create a forced magnetic field by passing a strong excitation current through this coil, forcing the hydrogen protons to align.When the current is turned off, the earth appears. The protons precess as they try to return to the magnetic field. Its motion is represented by a frequency proportional to the strength of the magnetic field. The formula is
rp=(2.67513±0.00002)×10 4 Sec −1 Γr −1
F= 23.4874 f F=Total magnetic force
This formula shows the strength of the magnetic field.
① The sea area around Tsushima has an average of 44,500 r
② The measurement interval is 30 seconds to 60 seconds
③ The first attempt is made at a ship speed of 5 kt/h
④ Fixed-point observation on land to observe daily changes
Magnetic anomalies represents the magnetic strength of rocks within the earth's crust, and its value is displayed as follows.
△F = F - Fo - Fu
△F: Abnormal value F: Total magnetic force observation value
Fo: Strength of the earth's internal magnetic field (IGRF) Fu: External daily variation correction value
Based on the above principle, the figure below (omitted) shows A proton magnetometer was set up like this, and the sensor was flown about 50 meters behind the research vessel to eliminate the magnetic influence of the ship's hull and take measurements.
The measurement interval shall be 30 seconds/time.

4. Data processing

① Measurement time, sounding, positioning, and geomagnetic values ​​are finally digitized and recorded.
② Calculate geomagnetic anomalies by correcting regional and daily variations in geomagnetism using IGRF and fixed-point observations.
③ Perform model analysis of important points as necessary.
④ Create a batch file of data on gravity, geomagnetism, sonic exploration, bottom sediment, etc.

5. Deliverables

① Report, ② Track map, ③ Survey line map, ④ Magnetic cross section, ⑤ Total magnetic anomaly map, ⑥ Reference map, ⑦ Seafloor topography map

  • Investigation report (overview)

  • construction survey of Japan-Korea tunnel

  • construction survey of Japan-Korea tunnel

  • Overview of the Japan-Korea tunnel

  • project


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