Relevance of magnetic properties of soil in the magnetic observatories to geomagnetic observation
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2012
Received: 14 May 2012
Accepted: 20 September 2012
Published: 7 May 2013
Annual geomagnetic variations with a maximum amplitude of 5 nT, and in phase with ground temperature variations at a depth of 1–2 m, were observed in the baseline values of fluxgate magnetometers installed at three JMA magnetic observatories. A possible origin of the annual variations is a change in magnetization of the soil due to changes in ground temperature. In order to examine the effect of temperature changes on soil magnetization, we measured the magnetic properties of soil samples collected at the JMA observatories. Magnetization of soil samples in a magnetic field of 0.05 mT ranged within 0.05 × 10−3−1.6 × 10−3 A m2/kg and the temperature dependence of magnetization ranged within 0.3 × 10−6−14 × 10−6 A m2/kg °C, except for a sample having an extraordinarily strong magnetization. Based on the measured magnetization, and their temperature dependence, of samples from Memambetsu, which shows the largest values among the samples from the three observatories, we determined the distribution of the geomagnetic field and its annual variation produced by soil magnetization. The maximum amplitude of annual variation in the geomagnetic field is 7 nT, which is consistent with the observed annual variation of the baseline value of the magnetometers.
Observing conditions, such as a change in temperature and the tilt of the magnetometers, are the most plausible causes of annual variations, but cannot fully explain the observed annual variations. The annual variations shown in Fig. 2 are already corrected using tiltmeters installed in the sensors of the magnetometers, and are out of phase with the temperature variation of the magnetometers. Furthermore, differences in total geomagnetic intensities, obtained with three proton magnetometers installed at different locations within MMB, show annual variations with amplitudes of ~1 nT (Nishimura et al., 2012). As observations with proton magnetometers are not likely to be largely affected by observing conditions, the observed annual variations can be considered to be actual geomagnetic variations.
A possible origin of annual variations is a change in the magnetization of the soil due to changes in the ground temperature. In volcanic areas, the conclusion that geomagnetic annual variations are caused by a change in the magnetization of surface rock produced by a change in ground temperature (Utada et al., 2000), is widely accepted. This idea is also applicable to non-volcanic areas in Japan, because surface soil in Japan often contains materials of a volcanic origin. For example, many volcanic tephra layers are widespread within sediments in Hokkaido (Machida and Arai, 2003) and some of them show strong magnetic susceptibility (Kawamura et al., 2007). The change in soil magnetization produced by a change in the ground temperature has also been suggested as a possible cause of the change in geomagnetic observational data at observatories in Japan (e.g., Ogawa and Koyama, 2009; Yamazaki et al., 2012). However, the non-availability of soil magnetic properties has prevented a quantitative evaluation of geomagnetic changes resulting from a temperature change of the soil.
In this study, we measured the magnetic properties of soil samples collected from the three JMA observatories, and we have examined the effect of temperature change on the soil magnetization to the geomagnetic annual variations.
2. Samples and Methods
Summary of the collected soil samples from the JMA observatories. Color taken from Oyama and Takehara (1967).
MMB North of Variation House #2
MMB North of Total Forth (79F)
MMB Continuous Observation House
mixture of transparent, grayish white and black
KAK South of Comparison & Calibration House
KAK East of Underground Variation House
KNY North of Underground Variation House
KNY North of Continuous Observation House
mixture of reddish brown and reddish black
The change of magnetization in response to temperature change was measured. The measurement was performed with a Quantum Design Magnetic Property Measurement System (MPMS-XL) at the Geological Survey of Japan, the National Institute of Advanced Industrial Science and Technology. A small subsample (~100 mg) of each sample was analyzed. In order to reduce the residual magnetic field in the instrument, the superconducting magnet was quenched before the measurement runs. The applied static magnetic field on a subsample was set to 50000 nT (0.05 mT) in order to impart induced and remanent magnetization. Each subsample was then heated from 2°C to 27°C, at a rate of 1°C/min, and the change of magnetization was monitored at intervals of 1°C.
Low-field magnetic susceptibility was measured with a Bartington MS2B magnetic susceptibility meter at an operating frequency of 465 Hz, a peak applied field of 0.25 mT and a temperature of 26°C. Subsamples filled into 6.86-cm3 paleomagnetic cubes were used for the low-field magnetic susceptibility measurement. Bulk density was calculated from the weight and volume of the subsamples. Low-field magnetic susceptibility and bulk density of the M5 sample were not measured, because the amount of sample available was insufficient.
Summary of the magnetic properties of the soil samples.
Magnetization (12°C) (10−3Am2/kg)
Temperature dependence (10−6Am 2/kg°C)
Low-field magnetic susceptibility (10−5SI)
Bulk density (103 kg/m3)
1.378 ± 0.002
−3.057 ± 0.186
1.455 ± 0.006
−13.965 ± 0.737
0.747 ± 0.000
− 1.737 ± 0.041
1.195 ± 0.003
−2.932 ± 0.374
4.294 ± 0.098
−56.646 ± 11.940
0.057 ± 0.000
−0.476 ± 0.015
0.115 ± 0.000
−0.701 ± 0.023
1.579 ± 0.002
−7.807 ± 0.296
1.628 ± 0.001
−4.371 ± 0.084
0.639 ± 0.000
−0.570 ± 0.057
0.840 ± 0.000
−0.329 ± 0.059
0.621 ± 0.001
− 1.384 ± 0.099
0.885 ± 0.000
−0.900 ± 0.039
Samples from KNY have the smallest decrease rate (0.3 × 10−6−1.4 × 10−6 A m2/kg°C) of the three observatories, though the magnetization (0.6 × 10−3−0.9 × 10−3 A m2/kg) is not so small. Spatial differences in magnetization and decrease rate were notable between samples from two pits at KAK (K1, K2 and K3, K4). Samples from MMB showed the largest magnetization (0.7 × 10−3−1.5 × 10−3 A m2/kg) and the largest decrease rate (1.7 ×10−6−14× 10−6 A m2/kg °C), even excluding the M5 sample (4.3 × 10−3A m2/kg; 57 × 10−6 A m2/kg °C).
The bulk density of the subsamples filled in the pale-omagnetic cubes ranges between 0.87−1.63 × 103 kg/m3 (Table 2). The variation in bulk density might be affected by compaction and the void space of the subsamples in the cubes. For convenience, we adopted 1 × 103 kg/m3 as the bulk density in order to convert the unit of magnetization.
Low-field magnetic susceptibility ranged from 39.9− 848 × 10−5 SI units (Table 2). Four samples from MMB, and two samples from KAK (K3 and K4), which have a large magnetization, show high magnetic susceptibility (569− 848 × 10−5 SI). The samples from KNY show a moderately-high magnetic susceptibility (216−689 × 10−5 SI). Two samples from KAK (K1 and K2), which have the smallest magnetization, show the lowest magnetic susceptibility (39.9-73.4 x 10-5 SI).
The magnetic susceptibility range of 39.9−848 × 10−5 SI corresponds to an induced magnetization in the range 0.016−0.34 A/m in a AC magnetic field of 50000 nT, and is almost a third of the magnetization measured with the MPMS. A possible cause of the difference between the MPMS-measured magnetization and the calculated induced magnetization from the magnetic susceptibility might be a residual magnetic field in the sample position in the MPMS. Although efforts were made to minimize the residual field by using magnet reset, a residual field of 0.1 mT might be possible in an MPMS, which is usually operated with a strong magnetic field up to 5 T. Another possible cause of the difference in magnetization is the viscous remanent behavior of magnetic minerals in the soil. Viscous remanent magnetization may be acquired during a measurement run in an MPMS, which takes typically an hour, but not in low-field magnetic susceptibility measured in an alternating magnetic field of 465 Hz. Despite the problem of the magnetic field accuracy, in this paper we adopt the magnetic parameters measured with the MPMS, because it provides the best temperature control within the ground temperature range.
4. Determination of Geomagnetic Annual Variations
We determined geomagnetic annual variations based on the annual variations of the ground temperature and the measured values of magnetization and its temperature dependence. Samples from MMB showed the largest temperature dependence of magnetization of the three observatories, illustrating that an annual variation exists at MMB. We determined the geomagnetic annual variations around the second variation measurement house and the absolute measurement house of MMB (96FM and ABS in Fig. 3), where absolute measurements are performed and a fluxgate magnetometer is installed, respectively.
The temperature dependence of magnetization is largest for the MMB samples, and smallest for the KNY samples. It is consistent with the amplitude of the annual variations, in the baseline values of the fluxgate magnetometers, which was largest at MMB and smallest at KNY. The small temperature dependence of magnetization in the KNY samples can also explain the insignificant annual fluctuations in the spatial distribution of the total geomagnetic intensity reported by Yamazaki et al. (2012).
The positive magnetic anomaly up to ~15 nT in the calculated spatial distribution of the total geomagnetic intensity (Fig. 7(a)) is similar to the local features around the second variation measurement house described above.
The ranges of the calculated annual variation amplitudes for each component are comparable to the observed annual variation amplitudes of the baseline values. At MMB, both H and Z components show high baseline values, (i.e. low observed values of the fluxgate magnetometer) during summer and autumn when the ground temperature is high. The observed annual variations of both H and Z in antiphase to the ground temperature can be explained by a calculation locating a strongly-magnetized body beneath the north of the fluxgate magnetometer. The calculated phase of the geomagnetic annual variation is common to the H, Z and D components, and consistent with the depth of the strong magnetization. The different phase of the observed annual variations of H and Z may be explained by multiple magnetization anomalies at different depths. The calculated annual variation of D reaches up to ~5 nT above the eastern and western edges of the strongly-magnetized body, but is smaller than 1 nT between them. The insignificant observed annual variation of D can be explained by assuming that the fluxgate magnetometer is located above the central region of strong magnetization.
The annual variations in the geomagnetic field caused by changes in soil magnetization in response to temperature changes was determined from measuring the magnetization, and its temperature dependence, of soil samples collected from three JMA magnetic observatories. The amplitude of the determined annual variations at the MMB observatory was up to ~7 nT, which corresponded approximately with the observed annual variations in the baseline values of the fluxgate magnetometer.
We thank Toshitsugu Yamazaki and Hi-rokuni Oda for the use of their facilities. We also thank two anonymous reviewers for their constructive comments.
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