⋅+ Even today, almost 200 years later, it is impossible to obtain absolute permanent recordings of the geomagnetic field components. The main problem is that the natural changes (variations) are very small in comparison to the total field intensity. It is difficult to guarantee a lasting orientation of the instrument, to observe the Nyquist- theorem, and to stabilize additional fields. Absolute permanent recordings are only possible for the scalar value (total intensity F) of the geomagnetic field vector. Several efforts have been done to develop magnetometers for the recording of components, but the success is small [3, 4, 5, 6, 7, 8, 9]. At the geomagnetic observatories the permanent measurements are so-called variation recordings. Those are relative measurements. The full magnitudes of the magnetic field components are obtained by adding the data of the variation recordings to the so called base line values obtained from absolute measurements, which are frequently carried out. Absolute measurement means, that the complete field vector (two angles, magnitude) is determined for a defined time. The introduction of DI-fluxgate magnetometers in the 1980ies [10] provided a comparatively simple way for the absolute determination the two angles declination D and inclination I of the Earth’s magnetic field for a defined time, and for a defined place. A DI-fluxgate consists of a theodolite and a one component fluxgate magnetometer. The theodolite has to be non magnetic. It is used to determine a very accurate orientation towards the geodetic coordinate system in the horizontal and in the vertical plane. The fluxgate is situated above the telescope axis of the theodolite. For the measurement, the theodolite is oriented in the direction where the magnetometer shows zero. Now the observer has to read and write down the angles, and the determination of declination and inclination can follow under consideration of the azimuth. Errors are eliminated by using all possible positions of the telescope. Such theodolites are expensive (special manufacturing) and difficult to operate with high precision so that good results are generally only obtained if the observer is skilful and experienced. The method suggested by AUSTER[11] is an alternative to the DI-flux. This new instrument consists of a mechanical device to support a three-component fluxgate magnetometer (basket magnetometer). An additional scalar magnetometer is required for both methods to measure the total intensity F for the full description of the Earth field vector. Both the physical principle and the prototype instrument are briefly described. The steps of measurement are outlined. Further, the results of three measurement campaigns are presented. Sources of errors are discussed as well.
Despite the advance of technology, the fully automatic recording of absolute magnetic field vector variation at observatories remains an elusive goal. Primary difficulties are the long term stability of sensor orientation and the stable operation of the sensor system. In standard practice, definitive data are produced through the combination of continuous operation of a variometer and the occasional absolute measurements that are used for calibration of the variometer data. A single, automatic instrument that can continuously acquire absolute vector measurements with 1-second resolution is desired. We introduce a device that will fulfil these requirements. Data are acquired using Serson's method: the ambient magnetic field is modulated by superposed fields. This method has been applied, mostly in connection with Proton magnetometers, for many years. In general it requires that the applied fields have a strength on the order of the Earth's magnetic field. But the sampling rate is limited for most xisting systems. In contrast, our system only requires applied fields of about 5000nT, and the switching rate of polarities is 5Hz. This is possible because we use a fast self oscillating Cs-magnetometer. The self oscillating Cs-magnetometer is calibrated by a Cs-He cell during times without additional fields (tandem-magnetometer).
A calibration facility for search coil magnetometers (SCMs) has been built at Niemegk Geomagnetic Observatory, GeoForschungsZentrum Potsdam. I describe here the commissioning of this facility. The usable frequency range of 10 mHz-100 kHz enables calibration of all standard types of SCM. The homogeneity is 0.1% up to a diameter of 10 cm and up to a length of 2.5 m. Extensive test measurements showed broad agreement with theoretical calculations made before construction. As part of the testing, all SCMs in the GFZ geophysical equipment pool were checked. Several devices showed serious malfunctions, emphasizing the value of such a calibration facility.
In this paper we describe a new design for an optically pumped tandem magnetometer situated at the GeoForschungsZentrum Potsdam. A tandem magnetometer combines the fast response of a self-oscillating vapour magnetometer with the accuracy of a narrow line Mz-type magnetometer. A newly patented method of coupling the two sensors avoids any stray magnetic fields and so allows a compact design of the instrument itself, as well as facilitating its operation in close proximity to other magnetometers. A prototype Cs-K tandem magnetometer for use in magnetic observatories is described. We then show typical results of a long-term comparison with both a second type of optically pumped magnetometer and an Overhauser proton magnetometer. Finally, a resuméis given of four years of continuous operation of this new type of magnetometer with respect to the data quality produced and its operational reliability.
Abstract In this paper a device is presented to measure the geomagnetic field vector absolutely and automatically. In contrast to the standard DI-Flux measurement procedure our automation approach is based on the rotation of a three-component fluxgate magnetometer about precisely monitored axes without using a non-magnetic theodolite. This method is particularly suitable for automation because it only requires exact knowledge of the axes orientations. Apart from this, requirements on mechanical precision are moderate. The design of the facility is presented, and mechanical, optical and magnetic limitations are discussed. First promising results of measuring the Earth’s magnetic field absolutely and automatically with the new device at Niemegk observatory are discussed.
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