The Borok INTERMAGNET magnetic observatory

Layout and instrumentation

Figure 2

[6] The magnetic observatory is installed in the premises of BGO's Geoelectromagnetic Monitoring Laboratory, which includes several pavilions dedicated to electric and magnetic measurements (Figure 2). One pavilion is used as the variation room, containing the continuously recording magnetometers mounted on stable pillars in a thermally controlled environment. Another pavilion, at about 100 m distance from the first one, is used as the absolute house, containing the equipment for absolute measurements of the geomagnetic field (Figure 3).

Figure 3

Figure 4

[7] The three components of the Earth's magnetic field are measured by an IPGP VM391 triaxial fluxgate magnetometer (Figure 4). This instrument, developed by IPGP since the mid-1990s, is installed in all IPGP magnetic observatories and in several other magnetic observatories throughout the world [Courtillot and Chulliat, 2008]. It satisfies the current INTERMAGNET minimum requirements and is regularly modernized in order to follow the evolution of international standards. One of its main characteristics is that it is homocentric, that is, it measures the three components of the field at the same point. This property is particularly useful in locations where the local gradient is high, such as volcanic islands.

[8] The vector magnetometer samples the magnetic field at 0.2 Hz and one-minute values are produced using the standard INTERMAGNET Gaussian digital filter [St-Louis, 2007]. In the same pavilion, the modulus of the magnetic field is measured every minute by a Geomag SM90R Overhauser-type scalar magnetometer. Vector and scalar one-minute data are acquired by an IPGP ENO2 data logger, which is based on a PC system, and transmitted to the INTERMAGNET Geomagnetic Information Node (GIN) in Paris via email within 72 hours.

[9] An important goal of magnetic observatories is to record the geomagnetic secular variation over decades and even centuries. Yet existing vector magnetometers unavoidably drift on such long time scales, due to the aging of the device. The drift of today's good instruments is small (about 5 nT per year or less), but not regular in time and therefore unpredictable. Its magnitude can reach several nT per year, which is larger than the desired accuracy for secular variation measurements. Additional sources of drift include temperature variations (if the temperature control is not perfect) and slow pillar movements, which are often unavoidable. For these reasons it is necessary to make quite frequent absolute measurements, that is, precise determinations of the declination and inclination using a fluxgate theodolite [Jankowski and Sucksdorff, 1996]. The vector value is then reconstructed using measurements of the modulus by the scalar magnetometer, which drift is assumed negligible.

[10] From April 2004 to September 2007, the fluxgate theodolite used in Borok was a LEMI 203 fluxgate magnetometer mounted on an MG2KP amagnetic theodolite. This apparatus was changed in September 2007 and the observatory is now equipped with a Bartington Mag01H single-axis fluxgate magnetometer mounted on a Zeiss 010A amagnetic theodolite. The azimuth mark was also changed and slightly displaced; the absolute pillar remained the same. Magnetic field differences between the absolute pillar and the variation pillars are regularly measured by an additional scalar magnetometer, in order to detect changes in the local field gradient. Note that the horizontal gradient within the absolute house is less than 2 nT/m, making it a very good absolute house by international standards [Jankowski and Sucksdorff, 1996]. Since the opening of the observatory, absolute measurements have been made twice a week, in agreement with INTERMAGNET recommendations [St-Louis, 2007].