geomagnetic measurements at the predmeja reference station

MGB – 2008 J. Kocen, D. Paliska, D. Zivkovic, S. J. Mihajlovic, P. Kosovac, R. Cop

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GEOMAGNETIC MEASUREMENTS AT THE PREDMEJA

REFERENCE STATION

Jernej Kocen, Dejan Paliska

Higher Education Centre Sezana Laboratory for Geomagnetism and Aeronomy

Kraska 2, 6210 Sezana, Slovenia [email protected], [email protected]

Dusan Zivkovic, Spomenko J. Mihajlovic

Geomagnetic Institute Grocka Put za Umcare 3, 11306 Grocka, Serbia

[email protected]

Pavel Kosovac, Rudi Cop

Higher Education Centre Sezana Laboratory for Geomagnetism and Aeronomy

Kraska 2, 6210 Sezana, Slovenia [email protected], [email protected]

ABSTRACT For the purpose of organizing and carrying out the first three-component measurements on site and the geomagnetic-field’s vector change records on site in Slovenia, locations appropriate for such measurements/records have been selected on the basis of the detailed GIS monitoring of Slovenia and the characteristic analysis of the geological units in Slovenia. The positions of these locations and their distance from the artificial sources of changes & interferences in the Earth’s magnetic field were additionally analysed in order to select proper locations. The researchers at the Higher Education Centre, Sezana (VSS-Visokošolsko središče Sežana), Geomagnetic & Aeronomy Laboratory, in co-operation with the two researchers from the Grocka Geomagnetic Observatory (the Republic of Serbia) have carried out geomagnetic measurements & records at the 6000 Predmeja Reference Station on site and at the two repeat stations 6001 Kazlje (Kras) and 6002 Polje (Šentviška Planota). At the aforementioned stations a joint team of the researchers has carried out several types of geomagnetic measurements and testing: the testing of the geomagnetic-field’s total-intensity gradient homogeneity, observations of the Sun’s azimuth, the absolute, basic and three-component measurements of the geomagnetic field, and records of the geomagnetic-field’s component diurnal-variations. Keywords: geomagnetic measurements on site, Republic of Slovenia, absolute value, diurnal variation records 1 INTRODUCTION

Three locations were selected for the purpose of organizing and carrying out the first geomagnetic measurements and tests on site in Slovenia. The selection was made on the basis of detailed monitoring based on the GIS (geographical information system) of Slovenia and characteristic analysis of geological units in Slovenia. The positions of these locations and their distance from artificial sources of change and interference in the Earth’s magnetic field were additionally analyzed in order to select the right location. Such interference is caused by the following sources: electric railways of direct current, roads, motorways, power plants, electric current transfer and distribution systems, TV transmitters, mobile telephony networks, airport radars and others.

The team of the Higher Education Centre Sezana – Laboratory for Geomagnetism and Aeronomy (Slovenia) in co-operation with two researchers from the Grocka Geomagnetic


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Observatory (Serbia) has carried out geomagnetic measurements and recording at the Predmeja Reference Station on site. A joint team of researchers performed the following geomagnetic measurements: the testing of the gradient homogeneity of the total geomagnetic field’s intensity, observations of the azimuth of the Sun, the absolute and basic measurements of the geomagnetic field and recording of diurnal variations in the geomagnetic-field’s components.

2 GEOMAGNETIC MEASUREMENTS AND RECORDS

Predmeja was selected as a location because it meets all the preconditions set for the selection of a reference station location for carrying out triple-component measurements and recordings of diurnal variations in the geomagnetic field’s components. Using regional and local roads is very convenient and enables easy access to the location and easy approach to the Reference Station.

In the preselected micro locations at Predmeja Reference Station, in a magnetically homogenous area, the digital triple-component magnetometer was installed for recording variations in the magnetic field. The resolution of the diurnal variations which are recorded by this instrument is 1 sec/data. In the addition to the triple-component magnetometer, a proton procession magnetometer was installed for recording the diurnal variations in the total geomagnetic field’s intensity with a resolution of 1 sec/data. The overview of the magnetometers which were used during the performance of the magnetic measurements at Predmeja Reference Station over the period 24–30 September 2008 is presented in Table 1.

Simultaneously with recording of the diurnal variation of the geomagnetic field’s components at the base-point, the measurements of the absolute values of the geomagnetic field’s components near the variation station are being carried out. Standard DIM declination-inclination magnetometers and the proton magnetometer are used for these kinds of magnetic measurements. Table 1: Overview of instruments and equipment

Magnetometer GMF components

Records Recording speed

Recording resolution

Values

Fluxgate

magnetometer MAGSON 11 (Variometer)

X,Y,Z

Digital, continual and

automatic

1 sec.; 1 min.

0.1 nT Relative and/or

absolute

GSM-19.fg and/or

GSM-19.gw (Scalar)

F Digital, continual and

automatic

1 sec.; 1 min.

0.01 nT Base and/or

gradiometer

DIM Bartington

Mag - 01H D,I Absolute

measurements 1 min. 0.1 nT

3 MAGNETOMETER INSTALLATION

The coordinates of the position of installation for the variometer MAGSON 11 are X=89204.806; Y=413591.727; h=879.738 m (Figure 1). The coordinates of the position of installation for the proton magnetometer GSM-19 are X=89203.946; Y=413580.789;


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h=877.148 m (Figure 2). The coordinates of the above mentioned places are presented in the Gauss-Kriger coordinate system.

The light soil at the location of the instalations is 10–15 cm thick, and the foundation of the terrain is of stone blocks and calcareous rocks. For this reason the mounting bracket for the sensor is fixed on a stone block and stabilised with excavated earth and gravel. The sensor of the triple-component magnetometer MAGSON 11 is installed on a bracket (Figure 1).

Figure 1: Installation location of the triple-component magnetometer MAGSON 11 sensor

A wooden pillar is embedded at a distance of 10 m from the sensor of the flux-gate

magnetometer and the sensor of the Overhauser magnetometer is affixed to it. (Table 1; Figure 2). At a distance of approximately 20–25 m from the magnetometer sensor, the sensor cable with connectors is installed in a waterproof tent and connected with the acquisition unit of triple-component magnetometer and with a console of the proton magnetometer. Uninterrupted power supply for the magnetometers is provided through batteries located in the tent.

On 25 September 2008, at 11:45 UTC (Universal Time Coordinated) the installation process of magnetometers was completed and at 12:00 UTC the first recording was made and thus the Predmeja Reference Station began the continual recording of the diurnal variations in the X, Y, Z and F components of the geomagnetic field.

4 RECORDING OF THE DIURNAL VARIATIONS

At Predmeja Reference Station the recording of the diurnal variations in components of the geomagnetic field were carried out by a sample rate of 1 data per second. Figure 3 presents the recording of the diurnal variations in the components of the geomagnetic field over the period 25–30 September 2008.

In 10 to 15 hours from the start time the magnetometer sensors had to be fully stabilised mechanically and thermally in relation to the conditions of the external environment. The sensor and magnetometer drift that occurs during the recording of diurnal variations had to be reduced (Figure 3). It was also necessary to stabilise the temperature in the electronics box of the triple-component magnetometer.


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After the interval of mechanical and thermal stabilisation of the magnetometers the group of diurnal variations in the geomagnetic field was recorded. The morphology of the diurnal variations in the geomagnetic field’s components, which are recorded at Predmeja Reference Station, may be compared with the diurnal variations recorded at the geomagnetic mid-latitude observatories.

Figure 2: Installation location of the Overhauser proton magnetometer GSM-19 sensor

The components presented in Figure 3 are: - dX/dt; dY/dt; dZ/dt: data of the variations in the geomagnetic field recorded by the

triple-component fluxgate magnetometer - relative amount; - dFP/dt: data on the total geomagnetic field’s intensity recorded by the Overhauser proton

magnetometer; - dFM/dt: changes in the total geomagnetic field’s intensity calculated from the data

recorded by the triple-component magnetometer; - FP – FM: difference between the data in the variations in the geomagnetic field’s total

intensity; - Te, Ts: temperatures of the electronics box and the sensor of the triple-component

fluxgate magnetometer.

5 ABSOLUTE AND BASE MEASUREMENTS

The location of the secondary station was selected on the basis of tests on the homogeneity of the total geomagnetic field’s intensity gradient at the micro-location of Predmeja Reference Station. There, measurements were taken of the absolute and base values of the geomagnetic field’s components. The coordinates of the secondary station (base-station) are X=89285.443; Y=413558.373; h=895.107 m.

The secondary station was stabilized and a granite block with subterranean centre was buried. Around the base-station the conditions for carrying out all groups of geomagnetic observations and measurements are optimal. We performed: the determination of azimuth on the basis of solar observations, the observation of the lines and angles of several azimuths of


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reference markers which are deployed at different distances along the circle and geomagnetic absolute measurements.

Figure 3: Diurnal variations in the components of the geomagnetic field

over the period 25–30 September 2008 at Predmeja Reference Station

5.1 Azimuth determination on the basis of solar observations

At the base-station we used solar observations to determine the azimuths of reference markers. We used the hour-angle method of solar observation during the process of azimuth determination. This method used readings of the angle value from the horizontal circle of a theodolite as well as readings of UT universal time (Greenwich Mean Time, GMT). The angles on the horizontal circle are read when one side of the solar disk (left limb) coincides with vertical bar in telescope. The time signal, which indicates the moment of the coincidence of the solar limb with the vertical ending, has to be precisely read. The accuracy of readings of the time-signal must be 1 second or less, while the accuracy of the reading of angles on the horizontal solar limb is defined by the properties of the theodolite. The astronomic azimuth determination on the basis of solar observations was carried out by a series of observations in the morning, when the Sun was on the eastern horizon, and in the afternoon, when the Sun was on the western horizon.

At the base-station in Predmeja, solar observations were performed using a THEO 10B theodolite in the first series of measurements. In the second series of measurements a Wild T1 theodolite with filters for solar observations was used. For accurate readings of UT time we used communication with GPS through a proton magnetometer GSM-19 (Figure 4). 5.2 Absolute and base measurements of values of the components

The absolute and base measurements of the geomagnetic field’s components were carried out over the period 26–29 September 2008. We used a measuring method which consists of measuring the D, I, F components of the geomagnetic field with a DIM (declination-inclination magnetometer) and a proton procession magnetometer.

For the absolute measurements of the angle elements of the geomagnetic field, D – declination, I – inclination and for the measurements of the total intensity of the geomagnetic field vector, we used the following procedure:

- Recording the total intensity of the geomagnetic field vector over a period of 10 minutes before starting the absolute measurements;

- Solar observation was carried out if the sky conditions allowed astronomic observation;


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- Observation of the angles of the azimuths of reference markers in the first position; - Observation of declination angles on the horizontal circle in four positions. The

declination angles was read by a fluxgate DIM when we adjusted an amplitude for the total intensity of ∆F = ± 0.00 nT (zero read method) at the time (hh:mm).

- Observation of the angles of the azimuths of reference markers in the second position; - Calculation of the mean value of the declination angle for four observations; - Focusing a telescope on the magnetic meridian at the base of the mean value of the

declination angle; - Observation of inclination angles on the vertical circle in four positions. The inclination

angles were read by a fluxgate DIM when we adjusted an amplitude for the total intensity of ∆F = ± 0.00 nT (zero read method) at the time (hh:mm).

- Calculation of the mean value of the inclination angle for four observations.

Figure 4: Measurements at Predmeja Reference Station REFERENCES [1] K.A. Wienert, Notes on geomagnetic observatory and survey practice, Earth sciences,

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International Association of Geomagnetism and Aeronomy, 1997. [4] Intermagnet Technical Reference Manual. Version 4.2. Edited by: Benoît St-Louis.

Denver (CO, US): U.S. Geological Survey; Denver Federal Center, 2004. Available also from Internet in PDF format: http://www.intermagnet.org/im_manual.pdf.

[5] Geomagnetics for Aeronautical Safety: A Case Study in and around the Balkans. NATO Science for Peace and Security Series C: Environmental Security. 1 edition. Dordrecht (NL): Springer, 2006.


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[6] S. J. Mihajlovic, Morfologija varijacija geomagnetskog polja registrovanih na

Geomagnetskoj opservatroriji Grocka u periodu 1958 - 1990 godine. Monograph in Serb language. Beograd: Geomagnetski institut Grocka, 1996.

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geomagnetic measurements at the predmeja reference station

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