JP3753830B2 - Marine geomagnetic measurement method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a marine geomagnetic measurement method for marine geological survey, and more particularly to a marine geomagnetic measurement method in a coastal area and a shallow sea area.
[0002]
[Prior art]
In general marine geomagnetic measurements, the proton magnetometer sensor is measured while towing at a distance of about three times the length of the hull to avoid the influence of hull magnetism. For example, if the hull length is 30 m, the sensor will be hung about 100 m behind the stern. In this case, in order to maintain a sufficient towing force, a speed of at least 5 knots must be maintained.
[0003]
[Problems to be solved by the invention]
With such a conventional method, it is difficult to tow the sensor stably in the bay mouth or coastal and shallow water areas where there are many navigating ships, or when the geomagnetic measurement is performed at high tidal currents. It is virtually impossible to perform a measurement while detecting the position of the sensor.
[0004]
An object of the present invention is to provide a marine geomagnetic measurement method capable of performing geomagnetic measurement stably and accurately in a bay mouth, a coast, a shallow sea area, or the like.
[0005]
[Means for Solving the Problems]
In order to achieve this object, the marine geomagnetism measurement method according to the present invention provides a first magnetometer through a holder made of a nonmagnetic material at a position protruding from a ship hull formed of a nonmagnetic material by a predetermined length. The sensor is fixed,
A second sensor similar to the first sensor is installed on the gimbal disposed in the front center of the ship in order to measure the amount of change in geomagnetism due to the shaking of the ship,
The ship has a plurality of input units for the geomagnetic measurement data by the first sensor, the geomagnetic fluctuation by the second sensor, and the measurement values of the ship's ship position, bow direction, and water depth at the ship position. The measurement device is arranged so that it can be recorded simultaneously in real time using the recording system ,
According to the navigation of the ship, the geomagnetic measurement data at each measurement position on the trajectory of the navigation is recorded at the time interval necessary for the measurement device together with the respective measurement values ,
Correction for removing magnetic noise included in the geomagnetic measurement data by the first sensor using the recorded data of the geomagnetic fluctuation value and the respective measurement values by the second sensor is performed. It is configured as follows.
[0006]
DETAILED DESCRIPTION OF THE INVENTION
For the implementation of the present invention, the sensor of a magnetometer, such as a portable terrestrial “overhauser proton magnetometer”, is made nonmagnetic so as to reduce as much as possible the influence of an induced magnetic field from an engine made of a magnetic material such as iron. For example, a small ship (FRP ship) made of material protrudes from the tip of the ship and is integrated with the ship. At the same time, a similar magnetometer fixed on the gimbal is installed in the front center of the hull, At one second intervals, geomagnetic measurement and difference measurement of geomagnetism due to ship motion are measured.
The ship position, bow direction and water depth measurements required for analyzing the geomagnetic measurement results, and the two geomagnetic measurements at the tip and front of the hull are output separately and independently as digital values from each measuring instrument. However, these data are recorded as a sequence of the same time data in real time with a single recorder with a built-in personal computer.
Thereby, the geomagnetic measurement data at each measurement position on the trajectory of the ship's navigation and each measurement value necessary for the analysis can be extracted at a necessary time interval, that is, a necessary measurement point density.
[0007]
【Example】
FIG. 1 is a schematic diagram showing a state where a survey ship is located on the sea to be surveyed in order to carry out the method of the present invention, and FIG. 2 is a connection system diagram showing a configuration example of a measurement system.
A portable overhauser proton magnetometer sensor 1 is attached to the tip of a non-magnetic pole 3 in order to avoid the magnetism of the hull 2 as much as possible, for example, projecting forward in the bow. The sensor 1 may be mounted overhanging on the stern or the ship side.
A similar magnetometer sensor 5 fixed on the gimbal is installed at the front center of the hull in order to measure the geomagnetic turbulence component due to the hull shaking.
In order to obtain ship position data, a ship position measuring device 6 such as a GPS measuring device is disposed together with a positioning antenna 7. In order to obtain the heading data, a heading measurement magnetic compass 8 is provided. In order to obtain water depth data, an acoustic sounding instrument 9 is arranged together with an acoustic sounding instrument transducer 10. Reference numerals 11 and 12 denote arithmetic devices such as an overhauser proton magnetometer measurement arithmetic device for processing data from the sensors 1 and 5, respectively. Measurement data from each of the measuring devices 11, 12, 6, 8, and 9 is recorded in a data recording device 13 such as a personal computer.
[0008]
The matters related to the implementation of the method of the present invention will be described below.
(1) Accuracy required for investigation The accuracy of a portable proton magnetometer for land use is within 0.1 nT (nano tesla). For reference, the geomagnetic field near Japan is approximately 40,000 to 50,000 nT.
Further, the accuracy required for the survey is about several nT.
[0009]
(2) Causes of magnetic noise When non-towed geomagnetic measurement is performed, the most disturbing factor is the magnetic field of the ship's hull, that is, an induced magnetic field generated by an object made of iron such as an engine.
It is known both theoretically and experimentally that the force of a general magnetic field is inversely proportional to the cube of the distance from an object. Therefore, if the hull is made of a non-magnetic material, the magnetic noise source is almost limited to the engine, so if the magnetometer sensor is sufficiently away from the engine, the effect can be suppressed. Since there is a limit to the distance, the noise cannot be completely removed. In the experience so far, noise of about several tens of nT is recognized even in the arrangement as shown in FIG. From the above considerations, the sensor 1 is not limited to the tip, but may be arranged so as to protrude from the stern or in some cases from the ship side. It is judged that it is convenient to arrange.
[0010]
(3) Shape of magnetic noise The magnitude of the noise caused by the induced magnetic field varies depending on the orientation viewed from the noise source. That is, a negative abnormality occurs on the north side of the noise source (here, the engine), and a positive abnormality occurs on the south side. If the ship shake is small and the relative position of the noise source and the magnetometer sensor does not change, the magnetic noise affecting the magnetometer sensor is constant (only the magnetic DC component) and depends only on the orientation.
However, in reality, since the ship is shaken, the relative position of the two changes with the period of the shake, and the magnetic noise changes accordingly. Therefore, this noise is periodic and almost coincides with the oscillation period.
[0011]
(4) How to remove magnetic noise a) Part 1
Since the magnetic noise depends on the direction, the ship is turned into a figure 8 at an appropriate location in the survey area, and the change in the magnetic field due to the heading is measured.
Next, an average value of the figure 8 data is obtained, and a value obtained by subtracting the average value from a value for each azimuth is set as a correction depending on the azimuth and applied to an actual measurement value.
The actual measurement results in this case are shown in FIG. It is divided into four stages, but from the top (1) observed value, (2) corrected value of magnetic field change due to heading, (3) depth value, (4) heading value Yes. The observation ship makes two round trips in the north-south direction, but the direction is changed from south to north on the left side of the figure, and the same is repeated once again on the right side of the figure. The change in the magnetic field due to the change in the heading is 100 nT or more at the time of observation, but is 10 nT or less after the correction (2) shown in the second stage from the top.
As shown in FIG. 4 (a), this correction calculates the difference between the observed value on the ship and the land fixed point at the same time, and functionalizes the value as shown in FIG. 4 (b). The amount of correction for
b) Part 2
In a normal survey, the geomagnetic wavelength to be measured is 100 m or more. Compared to this, the wavelength (period) of the magnetic noise caused by the shaking of the ship is several m to 10 m at most, and is removed using a high cut filter.
c) Part 3
When a relatively short wavelength measurement is required, the above-described high cut filter cannot be used. In this case, the fluctuation is corrected using the measurement value of the magnetometer sensor fixed on the gimbal in the front part of the hull.
That is, a low-cut filter is applied to the measurement result of the magnetometer sensor at the front part of the bow to extract only the short wavelength component.
Next, the horizontal axis represents the magnetic force value on the gimbal, and the vertical axis represents the above result to create a correlation diagram. A linear regression equation is obtained from the correlation diagram.
Since the magnetic force value on the gimbal is close to that of the engine, the fluctuation amount almost represents the influence of hull shaking. Accordingly, the slope of the straight line obtained from the correlation diagram indicates the magnification of the magnetic turbulence of the hull fluctuation due to the difference in the distance from both engines.
When the magnification is obtained, the original data (data without the filter) is returned, and the shake correction is performed by subtracting the value obtained by multiplying the magnetic value on the gimbal from the measured value of the magnetometer sensor at the front of the bow.
[0012]
(5) Device for improving measurement efficiency Two magnetic measurement values, and further, ship position data, heading data, and water depth data are output as digital values from each instrument from a connector called RS232C. In normal cases, these outputs are directly connected to the input part of the RS232C of the personal computer, but only one input part is attached to one personal computer. Therefore, in order to record five types of data, five personal computers are required and are recorded independently.
The above-described method has many inconveniences such as high cost, and the individually recorded data must be re-edited as a sequence of the same time data, which takes time.
In order to solve this problem, a recording system having a plurality of RS232C input units is created so that independent data can be recorded as an array of simultaneous data. The recorder is composed of a child CPU control unit that controls to receive data at each input unit and a parent CPU that controls the results of the entire child CPU, and a personal computer is used as the parent CPU.
[0013]
【The invention's effect】
As described above in detail, according to the present invention, the magnetic sensor can be circulated in the measurement area while maintaining a certain distance from the sea surface in a stable posture at the bay mouth or the coastal and shallow sea area, etc. As a result, it is possible to suppress the influence of inductive noise from the sea, so that stable marine magnetic measurement can be performed. Furthermore, since the ship position data, heading data, and water depth data are recorded in parallel with the geomagnetic measurement data at the same time, it is easy to analyze and organize the subsequent measurement data. Moreover, it is possible to efficiently remove the magnetic noise associated with the swaying of the ship, and a highly reliable measurement result can be obtained, so that the practical value is extremely high.
[Brief description of the drawings]
FIG. 1 is a schematic side view showing an implementation state of a marine magnetic measurement method according to the present invention.
FIG. 2 is a connection diagram showing an arrangement example of measuring instruments used in the marine magnetic measurement method according to the present invention.
FIG. 3 is a waveform example showing a specific example of an observed value according to the method of the present invention.
FIG. 4 is a measurement characteristic diagram (a) showing a relationship between an azimuth and a relative magnetic force in an observation example according to the method of the present invention, and the same characteristic diagram (b) as a function.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Sensor of overhauser proton magnetometer 2 Hull 3 Nonmagnetic pole 4 Gimbal device 5 Magnetometer sensor 6 GPS measuring device 7 Positioning antenna 8 Magnetic compass for bow direction measurement 9 Sound depth measuring device 10 Sound depth measuring device 11 and 12 Oberhauser proton magnetometer measurement calculation device 13 Data recording device

Claims (1)

The first sensor of the magnetometer is fixed via a nonmagnetic material holder at a position protruding a predetermined length from the ship hull formed of the nonmagnetic material,
A second sensor similar to the first sensor is installed on the gimbal arranged in the front center on the ship to measure the change in geomagnetism due to the shaking of the ship,
The ship has a plurality of input units for the geomagnetic measurement data by the first sensor, the geomagnetic fluctuation by the second sensor, and the measurement values of the ship's ship position, bow direction, and water depth at the ship position. The measurement device is arranged so that it can be recorded simultaneously in real time using the recording system ,
According to the navigation of the ship, the geomagnetic measurement data at each measurement position on the trajectory of the navigation is recorded at the time interval necessary for the measurement device together with the respective measurement values ,
Correction for removing magnetic noise included in the geomagnetic measurement data by the first sensor using the recorded data of the geomagnetic fluctuation value and the respective measurement values by the second sensor is performed. Marine geomagnetic measurement method.

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