Geomagnetic survey of the sea area northwest of Iki

Project

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

1. Overview

As a continuation of geomagnetic surveys in the Iki Channel and the surrounding sea area, a "geomagnetic survey of the sea area northwest of Iki" using a proton magnetometer was planned. Geological surveys for the Japan-Korea Tunnel Project have been conducted over a wide area along the route, both on land and at sea.
In the offshore area, single-channel and digital multi-channel sonic surveys using sparkers and water guns as sound sources have been conducted over a total distance of 8,100 km in the Iki Channel, Tsushima East Channel, and Tsushima West Channel since 1982. Furthermore, offshore drilling was conducted in the Tsushima West Channel in 1986, revealing an outline of the overall geological distribution, geological cross section, and geological structure of the area.

2. Survey Overview

Geomagnetic survey using a proton magnetometer
1. Survey area: Sea area northwest of Iki (map omitted)
2. Survey contents: ① Measurement of total marine magnetic field, ② Fixed-point observation on land (correction for diurnal variation), ③ Depth measurement, ④ Tide observation, ⑤ Positioning
3. Survey siding (map omitted)
Total distance: 530 km
Test run: 50 km
4. Survey period: April 5th to April 30th, 1987

3. Survey Methodology

1. Ship position determination:
Two-distance method using an acoustic positioning device (Trisponder). The slave station uses an existing reference point, and is installed so that the intersection angle of the two distances falls within the range of 30° to 150°.

Continuous sounding using the PDR-101
acoustic sounding Fixed lines are placed every two minutes
Ship speed: 5 kt/h

3. Total Magnetic Force Measurement:
A proton magnetometer uses the nuclear magnetic moment of hydrogen atoms in water to measure the frequency of the free precession of protons in the Earth's magnetic field, thereby making an absolute measurement of the Earth's total magnetic force. The measurement principle involves enclosing an aqueous solution in coils aligned north-south, east-west, and west. A strong excitation current is passed through the coils, creating a forced magnetic field that forces the hydrogen protons to align. When the current is turned off, the protons precess as they return to the Earth's magnetic field. This motion is expressed by a frequency proportional to the strength of the magnetic field.

The formula is
2πf = rpF,
where rp = (2.67513 ± 0.00002) × 10 ⁴ Sec ^-1, Γr ^-1,
F = 23.4874 f, and F = total magnetic force
. This formula indicates the strength of the magnetic field.
① The sea area around Tsushima has an average magnetic field of 44,500 r.
② Measurement intervals should be 30 to 60 seconds.
③ Initially, the ship's speed should be 5 kt/h.
④ Fixed-point observations should be made on land to observe daily variations
. Magnetic anomalies represent the magnetic strength of rocks in the Earth's crust, and their values are displayed as follows:
△F = F - Fo - Fu
△F: Anomaly value F: Total magnetic field observation value
Fo: Strength of the Earth's internal magnetic field (IGRF) Fu: Corrected value for external daily variation
Based on the above principles, set up a proton magnetometer as shown in the diagram below (omitted), and move the sensor about 50 m behind the research vessel to eliminate the magnetic influence of the vessel's hull and perform measurements.
The measurement interval should be 30 seconds per measurement.

4. Data Processing

1. Measurement time, depth measurement, positioning, and geomagnetic values are finally digitized and recorded.
2. Regional and daily variations in geomagnetic field are corrected using IGRF and fixed-point observations, and geomagnetic anomaly values are calculated.
3. Model analysis is performed at important locations as necessary.
4. Data on gravity, geomagnetic field, sonic surveys, bottom sediments, etc. are compiled into a single file.