The highest level of the present-day geomagnetic recordings is presented by the absolute controlled magnetic measurements. This quality is permanently fulfilled in geomagnetic observatories (GO) only. The absolute records are based on nuclear magnetometers for the intensity measurement and on DIM (Declination and Inclination Magnetometer) instruments for the direction determination of the ambient magnetic field. Although the density of GOs is not dense enough for every scientific purpose (i.e., modelling the secular variation in detail), repeat station (RS) networks were founded in a lot of countries on the Earth. Regarding to the main task of RS measurements, almost the same requirements must be fulfilled during the fieldwork as in the GOs. Consequently, the environmental and instrumental expectations are pre-defined as well as the procedure of measurement which are generally absolute readings. After the reduction of the absolute measurements to the simultaneous record of the nearest GO is performed, the data can be represented to a relevant yearly mean value.The present article shortly summarises the environmental and the instrumental background of geomagnetic absolute measurements on reoccupations of two different RS in Croatia. In 2010 tree-days-long reoccupations were performed on the selected stations with applying an on-site magnetic variometer.One of the results of data processing pointed out the appearance of a geomagnetic sea-side effect on both of RSs. The phenomenon is the geomagnetic influence of the induced current system in the seawater near to the shore. The anomalous lateral currents are the consequence of the high conductivity contrast between the mainland and the (moving) salinity water and the induction effect of the fluctuating external geomagnetic field.The calculation steps of the geomagnetic elements will be also detailed regarding the recorded samples of a presented dataset as well as the diagnostic values (i.e., offset value and misalignment error of fluxgate probe).

Since the beginning of geomagnetic recording at Tihany Geophysical Observatory, baseline instability has been observed, especially in the case of the old variation house. The regularly observed annual baseline change is independent from the type of magnetometer. Common environmental effects on the instruments (for example temperature effects) are not suitable to characterize this variation. A possible reason of this instability is an unconventional geomagnetic effect on the magnetic properties of the sediment. The water saturation of a lake mud in a geyser cone shows correspondence with the variation of the base values. X-ray powder diffraction measurement detected goethite in the lake mud which, in fact makes possible the appearance of a super-ferromagnetic effect on goethite. In this study we present a possible source and the mechanism of the effect.

(1) Mining and Geological Service of Hungary, Budapest, Hungary (kis.marta@mbfsz.gov.hu), (2) Geodetic and Geophysical Institute, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, Sopron, Hungary (papp.gabor@csfk.mta.hu), (3) University of Miskolc, Department of Geophysics, Research Institute of Applied Earth Sciences, Miskolc, Hungary (gfkrixi@uni-miskolc.hu), (4) University of Vienna, Department of Meteorology and Geophysics, Vienna, Austria (bruno.meurers@univie.ac.at)

The temperature characteristics of the 1-second variometer were studied in the real operation environment. For estimations of the instrument thermal drift the two approaches based on the total field difference and base values analysis were used. The total temperature drift was decomposed to the console and the sensor ones using considerable differences in the temperature variations of these units. The significant non-linearity of the temperature dependencies of the variometer electronic unit was revealed. The temperature corrected total field difference did not exceed +/- 0.5 nT during about one year.

Automatic geomagnetic measuring systems need additional solutions to monitor the variation of the reference frame of the sensor. Reference frame of a vector magnetometer is defined physically by the true direction of sensor's axes. Several methods have already been developed for establishing the correct adjustment of the sensor. Published methods were suitable to align the sensor but continuous monitoring of the reference frame was not ensured especially for declination measurement without an independent reference magnetometer. Introduction of the so called MGEN device to measure optical angle variation between the suspended dIdD sensor and an independent telescope is a new promising improvement. MGEN device was originally designed for astronomical monitoring purposes, but with some modifications it can be used to monitor small movements and rotation of nearby objects like magnetometer sensors. Our paper presents the device and the first long term results of the measurements.

Within the Croatian-Hungarian project for the first time in the Republic of Croatia the repeat stations survey was carried out using an onsite dIdD (delta Declination /delta Inclination) variometer. Since the geomagnetic field vector observations are obtained in the instrument's reference frame, the calibration parameters have to be determined enabling the transformation to the geographic coordinate system XYZ. Three and a half days long occupation of the Sinjsko polje (SINP) repeat station revealed the temporal variability of the geomagnetic field gradients. Although observations were affected by extra magnetic field contributions, the determination of the quiet-time level geomagnetic field differences between the SINP repeat station and THY (Tihany) reference observatory enabled a more reliable data reduction. The comparison of results of different data reduction methods pointed out the improvement of the geomagnetic survey with the onsite variometer.

The first geomagnetic survey using the onsite variometer in Croatia was realized within the bilateral project ‘Joint Croatian-Hungarian Geomagentic Repeat Station survey and Joint Geomagnetic Field model‘ on the repeat stations Krbavsko polje (KRBP), Sinjsko polje (SINP) and Palagruža (PALA) in the period 19-31 July 2010. The results presented in this paper rely on the survey on PALA repeat station performed in the period 27-31 July 2010. In addition to usual equipment used for declination D and inclination I determination i.e. nonmagnetic fluxgate theodolite Zeiss 020A with DMI D&I electronic unit, the magnetic field during the survey was observed and monitored with onsite dIdD variometer. The time variability of the geomagnetic field elements differences between the variometer station (VAR) and repeat station (RS) (i.e. baselines) noticed on KRBP location were more pronounced on SINP and PALA repeat station locations. One of possible explanations of such a temporal changes can be the significant conductivity contrast between the Adriatic sea and the mainland that can result in spatial field variation in small spatial and temporal scales, even in large distances. Moreover, the temporal change of total intensity (F) gradient was detected by two PPM sensors observing on the same vertical (one above the other). Despite of noticed temporal change, the dIdD calibration parameters have been determined enabling the determination of the baselines and subsequent spatial reduction from VAR station to the repeat station. In such conditions, the repeat station was operating as a temporary geomagnetic observatory. The quiet-time difference between the geomagnetic elements on each repeat station and THY reference observatory was determined from 60 minutes long sliding windows provided the STDEV of the difference was < 0.3 nT in X, Y, Z components and F as well. In such a way determined quiet- time level differences have shown significant fluctuations, so they were determined considering several geomagnetic observatories surrounding Croatian territory (CTS, FUR, GCK, PAG, THY). Consequently, the quiet-time differences from 60 minutes sliding windows simultaneously determined with data from all surrounding observatories have provided more reliable results. Absolute set observations collected at the repeat station were reduced to reference observatories using reduction methods based on the assumption that transient (including diurnal) variations of the magnetic field are identical at both repeat station and chosen reference observatory (without and with consideration of the secular variation difference between the repeat station and reference observatory). Definitive geomagnetic element reduced values were given as weighted average (based on STDEV and SCATTER) taking into consideration all surrounding reference observatories. The comparison of different data reduction method results has shown that reduction to a quiet-time level provides better accuracy estimations. In order to provide reliable quiet-time level determinations, it is desirable that the repeat station is surrounded by several reference observatories at acceptable distances and that the survey is carried out during quiet external field conditions. Further investigations should be taken in order to clarify the impact of the high conductivity contrast to the temporal change of the geomagnetic field spatial gradients.

The last Hungarian repeat station survey was completed between October 2010 and February 2011. Declination, inclination and the total field were observed using one-axial DMI fluxgate magnetometer mounted on Zeiss20A theodolite and GSM 19 Overhauser magnetometer. The magnetic elements of the sites were reduced to the epoch of 2010.5 on the basis of the continuous recordings of Tihany Geophysical Observatory. In stations located far from the reference observatory, the observations were carried out in the morning and afternoon in order to decrease the effect of the distant temporal correction. To further increase the accuracy, on-site dIdD variometer has also been installed near the Aggtelek station, in the Baradla cave, during the survey of the easternmost sites. The paper presents the technical details and the results of our last campaign. The improvement of the accuracy of the temporal reduction by the use of the local variometer is also reported.

One of the main challenges on the course of the repeat station surveys is to determine the spatial differences of the geomagnetic elements between the repeat stations and the reference observatory. The difficulty arises from the fact, that the directly obtained differences are affected not only by spatial but also by temporal effects of external origin. The error deriving from the external effects can be efficiently diminished by the installation of an on-site vector variometer. In this case the spatial difference can be computed for night-time period, when the external field is less varying (both spatially and temporally) than during daytime. Installation of the on-site variometer in the field requires the fulfillment of nearly the same conditions as in the observatories, i.e. the control of the reference frame, the scale factors, the offsets, and the temperature effects of the magnetometer. The principle of the fluxgate and DIDD magnetometers is quite different from each other, therefore the two devices provide different possibilities to obtain accurate result. The paper discusses some of the possible instrumental errors and offers a method based on the DIDD technology for the determination of the reference frame of a portable recording station. We analyse real records measured during the joint Hungarian-Croatian repeat station survey.

Recently, a simple method was proposed for the determination of pitch angle between two coil axes by means of a total field magnetometer. The method is applicable when the homogeneous volume in the centre of the coil system is large enough to accommodate the total field sensor. Orthogonality of calibration coil systems used for calibrating vector magnetometers can be attained by this procedure. In addition, the method can be easily automated and applied to the calibration of delta inclination–delta declination (dIdD) magnetometers. The method was tested by several independent research groups, having a variety of test equipment, and located at differing geomagnetic observatories, including: Nurmijarvi, Finland; Hermanus, South Africa; Ottawa, Canada; Tihany, Hungary. This paper summarizes the test results, and discusses the advantages and limitations of the method.

Different adjustment methods have been applied to model the core magnetic field over Croatia and Hungary with the use of data of the 2008.5 repeat station network campaign.The methods have been tested on IGRF model data. PA, CPA, and ASHA (𝜃0=20° ; 𝑘𝑚𝑎𝑥=5) methods resulted in similar mean residuals for the fitted models.Because of its physical justification it is suggested that the obtained ASHA model is superior over the polynomial models. The mean residual of the order of 20-30 nT obtained for the case of the adjustments represents the anomalous part of the stations of the joint Croatian-Hungarian network.