D. Šugar, A. Csontos, M. Brkić Geomagnetska izmjera na sekularnoj točki Sinjsko polje i redukcija mjerenja upotrebom terenskog dIdD variometra Tehnički vjesnik 22, 3(2015), 615-622 615 ISSN 1330-3651 (Print), ISSN 1848-6339 (Online) DOI: 10.17559/TV-20140417135956 GEOMAGNETIC SURVEY ON SINJSKO POLJE REPEAT STATION AND DATA REDUCTION USING THE ONSITE dIdD VARIOMETER Danijel Šugar, András Csontos, Mario Brkić Original scientific paper 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. Keywords: calibration parameters; data reduction; dIdD; geomagnetic survey; onsite variometer; repeat station Geomagnetska izmjera na sekularnoj točki Sinjsko polje i redukcija mjerenja upotrebom terenskog dIdD variometra Izvorni znanstveni članak U sklopu hrvatsko-mađarskog projekta po prvi puta u Republici Hrvatskoj geomagnetska izmjera izvedena je upotrebom terenskog dIdD (delta Inklinacija / delta Deklinacija) variometra. S obzirom da su opažanja vektora geomagnetskog polja dobivena u referentnom okviru instrumenta, potrebno je odrediti kalibracijske parametre čime se omogućuje transformacija u geografski koordinatni sustav XYZ. Tri i pol dana trajanja izmjere na sekularnoj točki Sinjsko polje (SINP) otkrilo je vremensku promjenjivost gradijenata geomagnetskog polja. Iako su opažanja bila pod utjecajem doprinosa dodatnog magnetskog polja, određivanje mirne razine razlika geomagnetskog polja između sekularne točke SINP i referentnog opservatorija THY (Tihany) omogućilo je pouzdaniju redukciju. Usporedba rezultata različitih metoda redukcija ukazala je na poboljšanje geomagnetske izmjere korištenjem terenskog variometra. Ključne riječi: dIdD; geomagnetska izmjera; geomagnetska sekularna točka; kalibracijski parametri; redukcija opažanja; terenski variometar 1 Introduction In the Republic of Croatia the geomagnetic field has been periodically observed and monitored on the stations of the Croatian Geomagnetic Repeat Stations Network (CGRSN) (Fig. 1). Figure 1 Croatian Geomagnetic Repeat Stations Network The network was designed according to IAGA (International Association of Geomagnetism and Aeronomy) criteria [1] as well as recommendations of the MagNetE (Magnetic Network in Europe) working group [2]. In the vicinity of the repeat station (RS), usually in North-East or East point of the outer grid for gradient determination, the auxiliary station (AUX) was monumented. During declination D and inclination I observations on the repeat station, the total field intensity F was observed on the AUX station. After the network setup completion, the average distance between repeat stations was approximately 178 km [3]. The whole CGRSN was surveyed every year in the period between 2007 and 2010 [4]. All these surveys enabled the determination of the geomagnetic elements secular variation over the Croatian territory, see e.g. [5], [6], [3] and [7]. Declination D and inclination I were observed by the null-method using the nonmagnetic theodolite Zeiss Theo 010B with fluxgate probe and electronic unit Bartington Mag-010H, the total field intensity F was observed with GEM System GSM-19G Overhauser PPM (Proton Precession Magnetometer). Coordinates of the repeat stations as well as the azimuth reference marks were determined by the GNSS (Global Navigation Satellite System) methods and subsequently the ellipsoidal azimuths were calculated. GNSS observations according to unique methodology were carried out on the repeat stations and azimuth reference marks, simultaneously with the geomagnetic survey in 2010. After the vector baselines processing and geodetic network adjustment, the coordinates were obtained in CTRS96 (Croatian Terrestrial Reference System 1996) and azimuth values determined in reference to the GRS80 (Geodetic Reference System 1980) ellipsoid [8]. Before each survey, geomagnetic instruments were checked and compared with the observatory standard. Since 2007 the nonmagnetic theodolite Zeiss Theo 010B was regularly checked in the Laboratory for Measurements and Measuring Technology of the Faculty