Geomagnetic survey of the sea area northwest of Iki

Project

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

1. Overview

As a continuation of geomagnetic surveys in the Iki Strait and the surrounding sea area, a "Geomagnetic Survey of the Sea Area Northwest of Iki" using a proton magnetometer was planned. Geological surveys, primarily focused on the geology, for the Japan-Korea Tunnel Project have been conducted extensively along the route, both on land and at sea.
For the sea area, since 1982, single-channel and digital multi-channel acoustic surveys using sparkers and water guns as sound sources have been conducted over a total of 8,100 km in the Iki Strait, Tsushima East Strait, and Tsushima West Strait. In addition, offshore boring was conducted in the Tsushima West Strait in 1986, revealing the general outline of the overall geological distribution, geological cross-section, and geological structure of this sea area.

2. Survey Overview

Geomagnetic Survey using Proton Magnetometer
1. Survey Area: Sea area northwest of Iki Island (map omitted)
2. Survey Contents: ① Measurement of total magnetic field at sea, ② Fixed-point observation on land (diurnal variation correction), ③ Depth sounding, ④ Tide gauge, ⑤ Positioning
3. Survey Line (map omitted)
: Total distance: 530 km
, Test run: 50 km
4. Survey Period: April 5, 1987 - April 30, 1987

3. Survey Methodology

1. Two-distance method using a transponder for ship position determination . The slave station uses an existing reference point, and is set up so that the intersection angle of the two distances falls within the range of 30° to 150°. 2. Continuous depth sounding using a PDR-101 acoustic sounder, with a fixed line drawn every 2 minutes at a ship speed of 5 knots/h.

3. Total Magnetic Force Measurement : A proton magnetometer uses the moment of the nuclear magnetism of hydrogen atoms in water to measure the frequency of the free precession of protons within the Earth's magnetic field, thereby making an absolute measurement of the Earth's total magnetic force. The measurement principle involves sealing an aqueous solution in coils arranged in the east, west, north, and south directions, and passing a strong excitation current through these coils to create a forced magnetic field, forcing the hydrogen protons into alignment. When the current is cut off, the protons precess as they try to return to the Earth's magnetic field. This motion is represented by a frequency proportional to the strength of the magnetic field. The formula is 2πf = rpF, where rp = (2.67513 ± 0.00002) × 10⁴ ^Sec⁻¹ Γr⁻¹ ^F = ^23.4874 f F = Total Magnetic Force . The strength of the magnetic field can be shown from this formula. ① The average magnetic field in the sea area around Tsushima is 44,500 r. ② The measurement interval will be 30 to 60 seconds. ③ The initial attempt will be made at a ship speed of 5 kt/h. ④ Fixed-point observations will be conducted on land to observe the diurnal variation . A magnetic anomaly represents the strength of the magnetism of rocks within the Earth's crust, and its value is expressed 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: External diurnal variation correction value Based on the above principle, a proton magnetometer will be set up as shown in the figure below (omitted), and the sensor will be moved about 50m behind the research vessel to eliminate the magnetic influence of the ship's hull and perform the measurement. The measurement interval will be 30 seconds/measurement.

4. Data Processing

① The measurement time, depth, position, and geomagnetic values are finally digitized and recorded.
② Regional and diurnal variations in the geomagnetic field are corrected using IGRF and fixed-point observations, and geomagnetic anomaly values are calculated.
③ Model analysis is performed for important locations as needed.
④ Gravity, geomagnetic, seismic survey, and sediment data are compiled into a single file.