Croatian Geomagnetic Network for Field Mapping

1. Croatiangeomagnetic surveys 2007 – 2008 M. Brkić†, D. Šugar†, M. Pavasović†, M. Rezo† and E. Vujić‡ University of Zagreb †Faculty of Geodesy, Institute for geomatics, Kačićeva 26 ‡Faculty of Science, Geophysical Department, Horvatovac 95 HR10000 Zagreb Croatia mbrkic@geof.hr, dsugar@geof.hr, mpavasovic@geof.hr, mrezo@geof.hr, eugvujic@gfz.hr Abstract. The establishment of Croatian Geomagnetic Repeat Stations Network was completed, and surveys were carried out in 2007 and 2008. The reduced geomagnetic field of 2007.5, its annual variation as well as the normal field was presented. The establishment and the survey of the first third of the dense Croatian Geomagnetic Network for Field Mapping, covering the southern Dalmatia, including islands, was completed in summer 2008. Keywords: geomagnetic repeat station network, geomagnetic network for field mapping, geomagnetic surveys, normal field. Acknowledgements. The establishment and surveys of geomagnetic networks were funded by the State Geodetic Administration, the Institute for Research and Development of Defense Systems of the Ministry of Defense of the Republic of Croatia, and Ministry of Science, Education and Sports. The authors wish to express special gratitude to dr. Ivan Hrvoić and GEMSys, for lending the GSMP-35G and aid during the campaigns. Croatian Geomagnetic Network for Field Mapping Croatian Geomagnetic Network for Field Mapping (CGNFM) was similarly designed as the repeat stations network. The basic purpose of this network was dense mapping of the geomagnetic field in order to update geomagnetic declination on the official and military topographic as well as navigational maps. The original network design was altered as the exact choice of locations was led by field evaluation of criteria, local pedological features, actual mine situation, as well as distribution of GPS and trigonometric points used in RTK GPS determination of points’ positions.For example, Terra Rossa because of its composition and a considerable influence on local gradiometry at few designed sites required relocation during the set up and survey of CGNFM sites. In addition to DI observation point, one azimuth orientation point and auxiliary point were set up at each site as well. The instruments and methods applied in CGRSN set up and survey were used in CGNFM set up and survey as well. The outer grid gradiometry method was replaced by a simpler, modified cross gradiometry method. Points position determination was made by RTKGNSS device within 2 cm position accuracy.Observation and auxiliary points were permanently marked with PA6(polyamide) peg. The southern part of CGNFM consisted of 29 locations established during the summer of 2008 (Figure 4). Locations were as follows: VITAljina, CILIpi, GROMaca, STON, KULANorinska, SAPLunara, GOVEđari, LUMBarda, BLATo, LASTovo, STAri Grad, HVAR, VRGOrac, IMOTski, OMIS,RAMLJane, VIS, MILNa, Novo SELo, TROGir, ZIRJe, ZATOn, Sv. FILIp i Jakov, Veli RAT, CUSC, ZEMunik Donji, Kastel ZEGarski, OKLAj, and GARJak. The mean distance between these stations was 22,7 km. Figure 4: Croatian Geomagnetic Network for Field Mapping (southern part, 2008). Croatian Geomagnetic Repeat Stations Network A brief historical overview of geomagnetism in the Croatian territory was presented in (Brkićet al. 2003). The renewal of geomagnetic surveysstarted in 2004 by establishing Croatian Geomagnetic Repeat Station Network (CGRSN) aimed at monitoring the geomagnetic field and its secular variation determination periodically (Brkićet al., 2006). In order to designand establish locations of additional repeat stations, IAGA (Newitt et al. 1996) and MagNetE criteriawere used. The key in determination of exact positions of new stationswas on site evaluation of all criteria, the critical one being locally low total intensity gradients and the absence of civilization noise.In doing so, GEMSysGSM-19GOverhauser PPM gradiometer was used. Establishing two new primary repeat stations – PALAgruzaand LOSInj in the summer of 2008, completed the expansion of CGRSN (Figure 1).The average distance between repeat stations is approx. 178 km today (Table 1). Figure 1: Croatian Geomagnetic Repeat Stations Network. Table 1: Primary Repeat Stations of the Croatian Geomagnetic Repeat StationsNetwork.The stations positions are presented as φ and λ coordinates on Bessel’s ellipsoid. Repeat Station Survey of 2007 The absolute instruments used in 2007 and 2008 CGRSN surveys were Bartington D/I MAG01H fluxgate with MAG Probe A and Zeiss010Btheodolite with nonmagnetic tripod, along with GEMSys GSM-19GOverhauser PPM. The comparison of the instruments at geomagnetic observatories was required before and after the survey. The space weather was continuously checked for geomagnetic activity Kp index during the campaign. The surveys were performed without the on-site variometer. In the azimuth determination, the GPS coordinates azimuth reference marks determined by observation and adjustment were transformed into local geodetic datum.After the transformation, north grid azimuths were calculated and final true north azimuths were obtained by adding the meridian convergence values. For each repeat station four or more sets of geomagnetic declination Dand inclination I observations, performed by the null method, were recorded in the early mornings and evenings, during two or more days.Total intensity was continuously recorded at the near auxiliary point at the same time. The azimuths to the reference points were checked before and after each D andI set of observations. Data reduction was performed using Tihany (THY) observatory data. A simple mathematical reduction model assuming equal secular variation at the observatory and station was used, E – Eo =E(t) -Eo(t), (1) where E(t), and Eo(t) referred to elements measured at time t, and E, as well as Eo referred to the annual mean values at the repeat station and the observatory, respectively. Total intensity was corrected for height as well. As a result, annual means of declination, inclination and total field intensity were obtained. The final values of the geomagnetic elements as well as their scatter refer to the epoch of 2007.5 (Table 2 and Figure 2). The scatter of each repeat station shows acceptable measurements’ quality, as well as suitability of the simple reduction model. The overall error of the whole network estimated as the average scatter amounts to 0.7’ in D, 0.2’ in I, and 2.1nT in F. Table 2:Repeat station observationsreduced to THY and 2007.5 epoch. In addition, islands Jabuka and Brusnik(Figure 5), famous because of their volcanic origin and location in the heart of Adriatic anomaly, were visited in order to take the F and gradiometry measurements. Surprisingly, PPM was useless and the mission to Jabuka failed. However, the F measurements on Brusnik succeeded and were approx. 300 nTgreater then expected from IGRF and NGDC-720 models, and the F gradients were approx.13 nT/m. Figure 5: Jabuka and Brusnik. Conclusion Established in 2004 and extended in 2008, the Croatian Geomagnetic Repeat Stations Network reached its final form.Though, an additional repeat station in the territory of neighboring Bosnia and Herzegovina was required in order to improve geomagnetic field solution. Regarding dense Croatian Geomagnetic Network for Field Mapping, two thirds or approximately 60 sites were to be set up and surveyed in this and the next year. Jabuka island remained to be investigated with GSMP-35G Potassium Gradiometer. References Brkić M., Bašić T. & Verbanac G.: “Geomagnetism in Croatia – a Historical Overview”, Geodetskilist 57 (80) 3, Zagreb 2003, pp. 183-194. Brkić M., Šugar D., Rezo M., Markovinović D., Bašić T.: “Croatian Geomagnetic Repeat Stations Network”, Geomagnetics for Aeronautical Safety: A Case Study in and around the Balkans", NATO Security through Science Series, Proceedings of the NATO Advanced Research Workshop on New Data for the Magnetic Field in the former Yugoslav Republic of Macedonia for Enhanced Flying and Airport Safety, Ohrid, 18-22 May 2005, Springer, 2006, pp. 137 - 144. Newitt, L. R., Barton, C. E., i Bitterly, J.: Guide For Magnetic Repeat Station Surveys, IAGA, Boulder, USA, 1996. De Santis A., Chiappini M., Dominici G. and Meloni A., 1997. Regional geomagnetic field modelling: the contribution of the IstitutoNazionalediGeofisica. Annal. Geophys., 40 (5), 1161-1169. Chiappini M., De Santis A., Dominici G. and Torta J. M., 1999. A Normal Reference Field for the Ionian Sea Area. Phys. Chem. Earth (A) 24, No. 5, 433-438. XuW.J., Xia G.H., An Z.C., Chen G.X., Zhang F.Y., Wang Y.H., TianY.G., Wei Z.G., Ma S.Z., and Chen H.F., 2003. Magnetic Survey and ChinaGRF 2000. Earth Planets Space, 55, 215-217. Figure3: D, I and F maps of CGNRF 2007.5. Table 3:Annual variation between 2004.5 and 2007.5. In order to describe the normal field (see e.g. De Santiset al.,Chiappiniet al.,Xuet al.) over Croatia, the Taylor polynomial was used (2) whereE(φ,λ,d)was the normal fieldEnormal, dwas a parameter vector, Mwas truncation level, j and kwere integers, and the reference values were: 0 = 44.9 and 0 = 16.8. The adjustment method to survey data was weighted least squares (WE-fit). For this fit the multilinear regression of the form (3) was used to obtain unknown coefficients ejk. Here Nwas the number of repeat stations that participate in regression, and Eiepoch were the geomagnetic element values at i-th station. After the normal field coefficients ejkwere computed, the standard uncertainty was obtained from: (4) where  was the sum of the squares of the residuals rsdof the normal field (rsd = Eiepoch – Enormal), and P the number of unknown parameters (P=6 for D, I and F; or P=3 for their secular variations). The weight was equal to 0 for the sites where the rsd was more than 2, and equal to 1 otherwise. The procedure was iterated until all of the rsd were below 2. Croatian Geomagnetic Normal Reference Field (CGNRF) for 2007.5, shown on Figure 3, was represented by second degree Taylor polynomial (2), with the coefficients Similarly, the normal field annual variation during the period 2004.5 – 2007.5 was represented by the following coefficients Figure2: D, I and F maps of repeat station observationsreduced to THY and 2007.5 epoch. In sequel, annual variation was calculated for the first time from 2004.5 and 2007.5 reduced survey data.The average annual variation over Croatian territory was 5.25 min/yearforD, -0.07 min/yearforI, and 29.7nT/year in F(Table 3).