10:45 AM - 11:10 AM
[SEM14-07] Accuracy and precision evaluation of the new method for the absolute
magnetic measurements
★Invited Papers
Keywords:Geomagnetic field, Observation, Measurement, Accuracy, Secular variation, Residual method
The absolute magnetic measurements, which consists of measurements of the total magnetic force (F), the declination (D) and the inclination (I) at a given location and time, are performed for the purpose of precisely acquiring absolute values of the three components of the geomagnetic vector. A scalar magnetometer, such as proton magnetometer, is used for measuring F, and an instrument called `DI-meter' for D and I. The DI-meter is operated manually, for which a certain skill is required for the observers to ensure the accuracy of the measurements (0.1 arcmin or less).
In order to observe the geomagnetic secular variation accurately, it is desirable for the measurements to be repeated as frequently as possible, while it is generally unpractical for observers. The measurements usually take place at intervals of days to months at magnetic observatories, and annually or even less often at repeat stations.
Currently, the DI-meters commonly used at world's major magnetic observatories are those equipped with a uniaxial fluxgate magnetometer mounted on a non-magnetic theodolite, THEO, produced by Zeiss. D and I are determined from a certain number of angular measurements of specific directions perpendicular to either of them, or the “null-directions” (Jankowski & Sucksdorff 1996). The "null method" of the absolute measurements, which has been exclusively used in Japan, requires some manual skills for an operation to strictly align the magnetometer axis to the null-direction.
The proficiency required makes it hard particularly for inexperienced observers to hold the accuracy in circumstances, for instance, where the measurements are conducted outdoors or allowable time is limited. Meanwhile, for its handiness, an alternative method called "residual method" has become more common through the past years (Worthington & Matzka 2017). In the residual method, the null direction is derived indirectly by calculating it from angular measurements of the magnetometer axis roughly oriented in the null direction. Without the delicate manipulation, observers may enjoy multiple advantages: reduction in physical and psychological stresses, quickness in learning, and alleviation of the magnetic cleanliness for observers, etc.
We report the results of an evaluation test the accuracy of the absolute magnetic measurements with the residual method. The test was carried out for 10 months from April 2020 to January 2021.
In the Calibration House of Kakioka Magnetic Observatory, D and I were measured with the residual method using a DI-meter (THEO 010B), to investigate their differences (referred to as instrumental error) from the corresponding values obtained by DI-72, the reference instrument of Kakioka for measuring D and I. The measurements were carried out by seven members of the observatory staff abundantly experienced in the null method.
It was confirmed that the instrumental error was <0.1 arcmin, which compares to that for the null method with the same DI meter. The observation error was also about the same as that for the null method. The observation duration was generally shorter for the residual method.
The duration would be shortened even more effectively under severe situations, such as outdoors, or in cases with limited time, all thanks to the absence of the fine manipulation, and hence of the psychological burden. An experiment of the residual method is now underway for the absolute measurements at Syowa Station, Antarctica, in order to investigate the effect of increasing the measurement frequency on stabilization of the baseline values. Furthermore, the residual method is planned to be introduced in the operation of the magnetic station Chichijima, where the absolute measurements are made outdoors.
Prof. Hisayoshi Shimizu (ERI, U. Tokyo) provided a great deal of support for one of the authors in this study to receive training of the residual method at Niemegk magnetic observatory in Germany.
In order to observe the geomagnetic secular variation accurately, it is desirable for the measurements to be repeated as frequently as possible, while it is generally unpractical for observers. The measurements usually take place at intervals of days to months at magnetic observatories, and annually or even less often at repeat stations.
Currently, the DI-meters commonly used at world's major magnetic observatories are those equipped with a uniaxial fluxgate magnetometer mounted on a non-magnetic theodolite, THEO, produced by Zeiss. D and I are determined from a certain number of angular measurements of specific directions perpendicular to either of them, or the “null-directions” (Jankowski & Sucksdorff 1996). The "null method" of the absolute measurements, which has been exclusively used in Japan, requires some manual skills for an operation to strictly align the magnetometer axis to the null-direction.
The proficiency required makes it hard particularly for inexperienced observers to hold the accuracy in circumstances, for instance, where the measurements are conducted outdoors or allowable time is limited. Meanwhile, for its handiness, an alternative method called "residual method" has become more common through the past years (Worthington & Matzka 2017). In the residual method, the null direction is derived indirectly by calculating it from angular measurements of the magnetometer axis roughly oriented in the null direction. Without the delicate manipulation, observers may enjoy multiple advantages: reduction in physical and psychological stresses, quickness in learning, and alleviation of the magnetic cleanliness for observers, etc.
We report the results of an evaluation test the accuracy of the absolute magnetic measurements with the residual method. The test was carried out for 10 months from April 2020 to January 2021.
In the Calibration House of Kakioka Magnetic Observatory, D and I were measured with the residual method using a DI-meter (THEO 010B), to investigate their differences (referred to as instrumental error) from the corresponding values obtained by DI-72, the reference instrument of Kakioka for measuring D and I. The measurements were carried out by seven members of the observatory staff abundantly experienced in the null method.
It was confirmed that the instrumental error was <0.1 arcmin, which compares to that for the null method with the same DI meter. The observation error was also about the same as that for the null method. The observation duration was generally shorter for the residual method.
The duration would be shortened even more effectively under severe situations, such as outdoors, or in cases with limited time, all thanks to the absence of the fine manipulation, and hence of the psychological burden. An experiment of the residual method is now underway for the absolute measurements at Syowa Station, Antarctica, in order to investigate the effect of increasing the measurement frequency on stabilization of the baseline values. Furthermore, the residual method is planned to be introduced in the operation of the magnetic station Chichijima, where the absolute measurements are made outdoors.
Prof. Hisayoshi Shimizu (ERI, U. Tokyo) provided a great deal of support for one of the authors in this study to receive training of the residual method at Niemegk magnetic observatory in Germany.