MGB – 2008 M. Brkic, M. Pavasovic, D. Sugar, M. Rezo, M. Pilic, E. Jungwirth

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CROATIAN GEOMAGNETIC NETWORKS

Mario Brkic, Marko Pavasovic, Danijel Sugar, Milan Rezo, Marko Pilic

University of Zagreb, Faculty of Geodesy Institute of Geomatics

Kaciceva 26, HR10000 Zagreb, Croatia, mbrkic@geof.hr, mpavasovic@geof.hr, dsugar@geof.hr, mrezo@geof.hr, mpilic@geof.hr

Enio Jungwirth

Ministry of Defense of the Republic of Croatia Institute for Research and Development of Defense Systems

Ilica 256b, HR10000 Zagreb, Croatia, ejungwirth@morh.hr

ABSTRACT

Geomagnetic networks in Croatia consist of the repeat stations network, and the part of network for field mapping nowadays. The Croatian Geomagnetic Repeat Stations Network was established in 2004. Herewith, the setup of two additional repeat stations in 2008 is described. Lessons learned during the repeat station surveys of 2004, 2007, and 2008 are revealed. The design of the dense Croatian Geomagnetic Network for Field Mapping, that is to consist of approximately 90 stations with the average distance from one station to another of 25 km, is presented. The setup and the survey of the first third of the network, covering the southern Dalmatia, including islands, was completed in summer 2008. The instruments and procedures used, as well as the experiences gained are described. In addition, the attempts to determine the total intensity of geomagnetic field on the Adriatic volcanic islands of Jabuka and Brusnik are presented. Keywords: geomagnetic repeat station network, geomagnetic network for field mapping, geomagnetic surveys 1 CROATIAN GEOMAGNETIC REPEAT STATIONS NETWORK (CGRSN)

A brief historical overview of geomagnetism in the Croatian territory was presented in Brkić et al. (2003). In Croatia the renewal of geomagnetism started in 2004 by establishing the first national repeat stations network aimed at monitoring the geomagnetic field and its secular variation determination periodically (Brkić et al., 2006).

The network was designed and established in accordance to IAGA criteria (Newitt et al. 1996), MagNetE recommendations, available historical geomagnetic data, geological and topographic maps as well as GIS materials. All eight primary repeat stations of the Croatian Geomagnetic Repeat Stations Network (Figure 1 and Table 1), namely POKUpsko, MEDJimurje, BARAnja, RACInovci, KONAvle, SINjsko Polje, KRBavsko Polje and PONte Porton, have permanent marks made of hard limestone block. For each repeat station two to four azimuth reference marks were set up more than a few hundreds meters away from the repeat station, and the auxiliary point was set up roughly 7 m away from the repeat station.

The repeat stations as well as reference mark coordinates were determined by GPS relative static positioning with Trimble 4000 SSI receiver to the accuracy better then two cm, enabling reliable calculation of the true north azimuths.

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Figure 1: Croatian Geomagnetic Repeat Stations Network. The average distance between stations is 178 km

Table 1: Primary Repeat Stations of the Croatian Geomagnetic Network. The stations positions are presented as φ and λ coordinates on Bessel’s ellipsoid

St. Name Longitude

[deg] Latitude

[deg] Height

[m]

POKU 45.4732 15.9834 105

MEDJ 46.4838 16.3318 199

BARA 45.8363 18.7870 86

RACI 44.8563 18.9696 81

KONA 42.5322 18.3402 47

SINP 43.6495 16.6886 296

KRBP 44.6698 15.6299 648

PONP 45.3560 13.7347 5

PALA 42.3929 16.2622 34

LOSI 44.5649 14.3848 33

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In order to propose locations of additional repeat stations the NGDC-720 crustal field model (URL1) was used (Figure 2).

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Figure 2: NGDC-720 Z-component at 0 m

However, the key in determination of exact positions of new stations was on site evaluation of all IAGA and MagNetE criteria, the critical one being locally low total intensity gradients and the absence of civilization noise. In doing so, GEMSys GSM-19G Overhauser PPM instrument and four methods of gradiometery were used (Brkić et al., 2006). Besides the set up, gradiometery was used in site maintenance as an indication of possible site’s contamination. Establishing two new primary repeat stations - PALAgruza (Figure 3a) and LOSInj in the summer of 2008, completed the expansion of CGRSN. Dense field mapping on Palagruza was performed as well (Figure 3b).

Figure 3a and 3b: Repeat station survey on PALAgruza (left) and dense field mapping (right)

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The absolute instruments used in 2004, 2007 and 2008 CGRSN surveys were Bartington D/I MAG01H fluxgate with MAG Probe A and Zeiss 010B theodolite with nonmagnetic tripod, along with GEMSys GSM-19G Overhauser PPM. As expected, 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 D, inclination I and total intensity F observations, performed by the null method, were recorded in the early mornings and evenings, usually during two 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 and I set of observations. It was expected two to five years period for reoccupation of the whole network.

Data reduction was performed using near reference observatories (Tihany, Fuerstenfeldbruck, L’Aquila) data. A simple mathematical reduction model as shown in Eq. (1) was adopted,

00 )()( EtEEtE −=− (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. As a result, annual means of declination, inclination and total field intensity were obtained. 2 CROATIAN GEOMAGNETIC NETWORK FOR FIELD MAPPING (CGNFM)

Croatian Geomagnetic Network for Field Mapping was similarly designed as the repeat stations network. The main 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. Regarding to geological aspects, CGNFM stations were located in three different geotectonical units.

The north sector represented the south part of Pannonian Basin that is Drava-Sava Depression, the middle sector (Gorski kotar, Banovina) was a part of Inner Dinarides with pyrite nests and the south sector represented the Outer Dinarides belt that was Karst region and islands. Differences between sectors were found in thickness and genesis of quaternary sediments and edaphic stratum.

The mayor barrier to precise measurements were: Terra Rossa, which covers lower parts of carstic regions, irregular shape and space distribution bauxite ore deposits (''Open'' - type or ''Pocket'' - type or ''Blind'' - type) and karst terrain with many fractures filled by clayey material enriched by ferruginous oxides.

The design shown on Figure 4 was preliminary 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

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gradiometry method was replaced by a simpler, modified cross gradiometry method. Points position determination was made by RTK GNSS device within 2 cm position accuracy.

Figure 4: Croatian Geomagnetic Repeat Stations Network - red circles, Croatian Geomagnetic Network for Field Mapping - blue circles (designed)

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 5). The Reoccupation period for the whole network was suggested to be eleven years.

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Figure 5: Croatian Geomagnetic Network for Field Mapping 2008 (southern part)

During the campaign of CGNFM2008 islands Jabuka (Figure 6a and 6b) and Brusnik (Figure 7a and 7b), near the Croatian Territorial Sea Border, north - west from island Vis, were visited in order to take the F and gradiometry measurements. Islands Jabuka and Brusnik are famous beacuse of their volcanic origin and location in the heart of Adriatic anomaly (Figure 1).

Figure 6a and 6b: Surprisingly, the mission to take F and gradiometry measurements at Jabuka failed

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Figure 7a and 7b: On the other hand, the F measurements on Brusnik were approx. 300 nT

greater then expected from IGRF and NGDC-720 models, and the F gradients approx. 13 nT/m

3 CONCLUSION

Motivated by needs for up to date geomagnetic information on official and military topographic and navigational maps, the recent years were dedicated to fundamental work of set up and surveys of geomagnetic networks. Established in 2004 and extended in 2008, the Croatian Geomagnetic Repeat Stations Network reached its final form. Though, and additional repeat station in the territory of Bosnia and Herzegovina was highly required in order to improve the network’s purpose, the determination of secular variation. Secondly, two thirds or approximately 60 sites of dense Croatian Geomagnetic Network for Field Mapping were to be set up and surveyed in the following two years, thus providing terrestrial geomagnetic field available for the Republic of Croatia. Acknowledgements

The geomagnetic networks of Croatia were funded by projects ‘Basic geomagnetic network

of the Republic of Croatia – for the purposes of official cartography’ for the State Geodetic

Administration, and ‘Renewal of geomagnetic information’ for the Institute for Research and

Development of Defense Systems of the Ministry of Defense of the Republic of Croatia, as

well as 'Geopotential i geodynamics of the Adriatic (Geo++Adria)’ for Ministry of Science,

Education and Sports. The authors wish to express their gratitude to these institutions.

REFERENCES

[1] M. Brkić, T. Bašić, G. Verbanac, Geomagnetism in Croatia – a Historical Overview, Geodetski list 57 (80) 3, Zagreb 2003, pp. 183-194.

[2] M. Brkić, D. Šugar, M. Rezo, D. Markovinović, T. Bašić, 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, pp. 137 - 144.

[3] L. R. Newitt, C. E. Barton, J. Bitterly, Guide For Magnetic Repeat Station Surveys,

IAGA, Boulder, USA, 1996. [4] URL1: http://www.ngdc.noaa.gov/geomag/EMM/emm.shtml#coef (28.10.2008)