Accuracy in geomagnetic measurements of Japanese magnetic observatories

104

Pt~vsi~wof the Earth and Planetary Interiors, 59 (1990) 104 111

Elsevier Science Publishers B.V., Amsterdam

Printed in The Netherland.,

Accuracy in geomagnetic measurements of Japanese magnetic observatories M. Kuwashima Kakioka Maqnetic Obser~atocv, 595 Kakioka Yasato-machi Niihari-gun, lbaraki 315-01 (Japan)

(Received December 5, 1987; revision accepted June 27, 1989)

Kuwashima, M.. 1990. Accuracy in geomagnetic measurements of Japanese magnetic observatories. Phys. Earth Planet. Inter., 59:104 111. A highly reliable three-axis fluxgate magnetometer has been developed, which has been used for variation measurements since 1984 at the Japanese observatories. The purpose of the present report is to demonstrate the high reliability of this fluxgate magnetometer developed in Japan.

1. Outline of the fluxgate magnetometer As is well known, observatory work consists of two parts, one is absolute observation a n d the other is variation observation. For the variation observation, the classical variometer has been used for a long time. Recently, the fluxgate magnetometer has been developed as a magnetic observatory i n s t r u m e n t system. However, there have been some problems with the fluxgate magnetometer being accepted as a magnetic observatory i n s t r u m e n t in place of the classical variometer. O n e of the p r o b l e m s is instability caused by the a m b i e n t temperature change. The observational data from the fluxgate m a g n e t o m e t e r are correlated with the variation of the a m b i e n t temperature of the room where the i n s t r u m e n t s are located. For m a n y fluxgate magnetometers, the temperature coefficient is, for the E D A fluxgate magnetometer for example, as large as 1.0 n T ° C ~. The t e m p e r a t u r e coefficient must be within 0.1 n T ° C ~ to adopt the fluxgate m a g n e t o m e t e r as a useful magnetic observatory instrument. We have redesigned the fluxgate m a g n e t o m e t e r in order to improve the p r o b l e m s of stability and to be able to measure the geomagnetic field with a 0031-9201/90/$03.50

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1990 Elsevier Science Publishers B.V.

high reliability. The fluxgate m a g n e t o m e t e r has been m a n u f a c t u r e d at S h i m a d z u C o r p o r a t i o n in Japan. Figure 1 shows a block d i a g r a m of the fluxgate m a g n e t o m e t e r m a n u f a c t u r e d by Shimadzu Corporation. The fluxgate m a g n e t o m e t e r consists of three parts, sensor, controller, a n d cables of 100 250 m length c o n n e c t i n g the other two parts. The sensor part consists of three bi-axial elements which are fixed orthogonally to each other. The o r t h o g o n a l ity is designed to be better than 0.1 ° so that interference from other c o m p o n e n t s is kept within 0.2% of the measured values. The accuracy of the o r t h o g o n a l i t y has been c o n f i r m e d e x p e r i m e n t a l l y to be 0.052 o. The controller part consists of an oscillator, filtering circuit, amplifier, c o m p e n s a t i o n field circuit, etc. The high-permeability element of the sensor is excited by the f u n d a m e n t a l frequency f to produce a second h a r m o n i c frequency 2 f , with an a m p l i t u d e which is p r o p o r t i o n a l to the intensity of the a m b i e n t magnetic field to be measured. The 2 [ signal picked up is filtered, amplified, rectified to a d.c. signal, and then fed back to the sensor to produce a negative feedback magnetic field. The c o m p e n s a t i o n circuit supplies a con-

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stant negative field by means of a c o n s t a n t current d e t e r m i n e d at the initial adjustment. The stability of the fluxgate m a g n e t o m e t e r d e p e n d s u p o n the stability of this c o n s t a n t current, i.e., the current for field c o m p e n s a t i o n must be kept c o n s t a n t for a long time to m a k e a stable magnetic observation. T h e newly designed fluxgate m a g n e t o m e t e r keeps the fluctuation of the current within 2 x 10 ~' so that one measures the g e o m a g n e t i c field of 5 × 104 nT with a stability of 0.1 nT. To stabilize the c o m p e n s a t i o n field circuit, a b a n d - c a p (voltage reference) is used instead of a zener d i o d e in the stabilized d.c. circuit. The specifications of the b a n d - c a p are 3 ttV ° C 1 in the t e m p e r a t u r e drift a n d 3 ttVp_p in noise level. W e expect 0.1 nT ° C 1 in t e m p e r a t u r e drift a n d 0.1 nTp p in the noise level for the newly designed fluxgate magnetometer. Those expected abilities have been confirmed e x p e r i m e n t a l l y as discussed in a later section.

2. Experimental results Figure 2a a n d 2b show e x a m p l e s of the experimental results for the fluxgate m a g n e t o m e t e r . Figure 2a shows the results for the S h i m a d z u fluxgate

m a g n e t o m e t e r (type MB160B) which has been used at J a p a n e s e observatories. F i g u r e 2b shows results for the E D A ( F M 1 0 0 B ) m a g n e t o m e t e r . T h e exp e r i m e n t used the following procedures. (1) The sensor part was set in the r o o m where a m b i e n t t e m p e r a t u r e was kept a l m o s t constant. (2) The controller p a r t was set in the r o o m where a m b i e n t t e m p e r a t u r e was c h a n g e d from - I O ° C to + 1 0 ° C . (3) The e x p e r i m e n t a l results are s u m m a r i z e d using the K a k i o k a A u t o m a t i c S t a n d a r d M a g n e t o m e t e r ( K A S M M E R ) d a t a as a s t a n d a r d . In Fig. 2a, the S h i m a d z u fluxgate m a g n e t o m e t e r , there is not a n y clear t e m p e r a t u r e effect, a l t h o u g h the t e m p e r a t u r e change had a range of 2 0 ° C . However, we could find a clearly t e m p e r a t u r e - d e p e n d e n t variation in the results for the E D A fluxgate m a g n e t o m e t e r as shown in Fig. 2b. T e m p e r a t u r e - d e p e n d e n t variation could be found more clearly in the H- a n d Z - c o m p o n e n t s . This result suggests that the t e m p e r a t u r e effect is caused by the c o m p e n s a t i o n field circuit because this circuit is used only for the H- and Z - c o m p o n e n t s . The results for the t e m p e r a t u r e effects shown in Fig. 2a a n d 2b are s u m m a r i z e d in T a b l e 1. T e m p e r a t u r e effects are very small for the S h i m a d z u fluxgate m a g n e t o m e t e r , in which t e m p e r a t u r e

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ACCURACY IN GEOMAGNETIC MEASUREMENTS OF JAPANESE MAGNETIC OBSERVATORIES

cients are 0.54 nT °C 1, 1.08 nT ° C 1 and - 0 . 0 3 4 0 ' ° C -1 with H-, Z - a n d D-components. Temperature effects of each part of the Shimadzu fluxgate magnetometer are summarized in Table 2. The sensor part shows a slightly larger temperature effect. However, the small diurnal and annual variations of the ambient temperature of the sensor house have little effect on the output values of the fluxgate magnetometer.

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using 60 1-s values from 00 min 30 s to 01 min 29 s. Minute values are also obtained from the proton and the vector proton magnetometers for the F-, H- and Z-components. Those minute values are recorded on a digital tape cassette. The data recorded on the digital cassette are reproduced on a floppy disk every day in order to confirm quality of data by the procedure shown in Fig. 3. First, a variation check is employed by testing the minute to minute change in field values. If any one of the changes exceeds a given limit (5 nT is usual), a special flag is attached to that data, and appropriate data will be replaced after detailed examination. After this variation check, three kinds of comparison checks are employed. The first comparison check is employed between the fluxgate magnetometer and the vector proton magnetometer with the H- and Z-components. If any one of the differences exceeds a given limit (0.3 nT is usual), a special flag is attached to the data. The second comparison check is a trigonometric check between H- and Z-components from the fluxgate magnetometer and the F-component from the proton magnetometer. A limit of difference of 0.3 nT is usual in this check too. The third comparison check is employed between the fluxgate magnetometer and the variometer for the D-component. In this comparison check, the digital data from the fluxgate magnetometer are converted to analogue format using the X - Y plotter of the computer system, and then the digital magnetogram is compared with the normal-run magnetogram from the classical variometer. For the data which have been flagged, detailed examinations are carried out, then appropriate data are replaced. Correction of the minute values is carried out every month in order to make the data absolute. The correction is carried out using the baseline values at every hour which are determined from the absolute observation carried out once or twice a week using the magnetic theodolite D, I and the proton magnetometer F. The correction to the absolute values is carried out as follows. (1) V H = H + B L H (baseline value of the Hcomponent) (2) V Z = Z + B L Z (baseline value of the Zcomponent)

OBSERVATORIES

109

(3) VD = D + B L D (baseline value of the Dcomponent) After the checks shown in Fig. 3, 1-min values are compiled as authorized data to be reported to the World Data Center.

4. Stability of the fluxgate magnetometer The stability of the fluxgate magnetometer may be examined by behaviour of the baseline values. Figure 4a shows the behaviour of the baseline values with the fluxgate magnetometer used as a supporting system for the optical pumping magnetometer of the K A S M M E R system at Kakioka for the period 1984 1985. In the figure, temperature (TEP) of the sensor room is also shown. As shown in Fig. 4a, there is an annual variation of 20 ° C in the ambient temperature of the sensor part of the fluxgate magnetometer. The baseline values of the H- and Z-components ( B L H and B L Z ) show clear changes in association with the temperature variation. Such an undesirable variation of the baseline values could be easily corrected using the temperature coefficients determined previously in Table 2. Figure 4b shows the temperature-corrected baseline values as well as the original ones. We could keep the stability of the baseline values within 2 nT year-1 by performing the temperature

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A C C U R A C Y IN G E O M A G N E T I C

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correction. There are still small fluctuations of the c o r r e c t e d values in Fig. 4b. Those fluctuations might be caused by a slight tilt of the sensor pier. A t M e m a m b e t s u a n d K a n o y a observatories, the sensor r o o m is c o n t r o l l e d to keep the a m b i e n t t e m p e r a t u r e nearly constant. F i g u r e 5 shows the p l a n of the sensor r o o m at K a n o y a observatory. T h e sensor r o o m is built u n d e r g r o u n d to reduce the u n d e s i r a b l e effects due to daily and a n n u a l v a r i a t i o n of the a m b i e n t room temperature. The sensor is set a b o u t 6 m under ground. The sensor house was built as the v a r i o m e t e r house in 1974. T h e r o o m t e m p e r a t u r e is kept a r o u n d 1 5 ° C with an a n n u a l change within 2 o C. The relative h u m i d ity of the r o o m is held at a b o u t 40% by a small a i r - c o n d i t i o n e r which is c o n n e c t e d to the r o o m by a d u c t a b o u t 20 m long. Figure 6 shows the baseline values at K a n o y a for the p e r i o d from 1984-1985. Because of the stability of the r o o m t e m p e r a t u r e (TEP), baseline values of the fluxgate m a g n e t i o m e t e r ( B L H , BLZ, BLD) have been kept very stable. The annual c h a n g e of the baseline values is within 2 nT a year. A slight drift is not i m p o r t a n t , because such a slight drift can easily be corrected by the absolute observation. Ultimately, accuracy of the observation de-

1I I

pends u p o n the stability of the a b s o l u t e observation. Figure 7 shows the s t a n d a r d d e v i a t i o n of the absolute o b s e r v a t i o n s at K a n o y a . Both the results for the fluxgate m a g n e t o m e t e r ( F M ) a n d that of the v a r i o m e t e r (VM) are r e p r e s e n t e d in the figure. Figure 7 indicates a highly reliable a b s o l u t e observation with small values of the s t a n d a r d deviation, which are less than 0.3 nT with the H- and Z - c o m p o n e n t s , and less than 0.03' with the Dc o m p o n e n t for b o t h the fluxgate m a g n e t o m e t e r a n d the variometer. In conclusion, we believe that the fluxgate magn e t o m e t e r system used at the J a p a n e s e o b s e r v a t o ries has sufficient reliability to be used as a magnetic o b s e r v a t o r y instrument.

References Kuwashima, M. and Sano, Y., 1984. Improved Kakioka automatic standard magnetometer (KASMMER). Geophys. Survey, 6: 357-365. Sano, Y., 1975. Stability and accuracy of standard magnetic observation by KASMMER system. Mem. Kakioka Mag. Obs., 16:121 141. Yanagihara, K., 1973. New standard magnetic observatory system of Kakioka (KASMMER). Geophys. Mag., 36: 217-281.