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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 RTK
GNSS 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, KULA Norinska, 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 surveys 2007 – 2008M. 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
Croatiambrkic@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.
Figure 2: D, I and F maps of repeat station observations
reduced 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/year for D, -0.07 min/year for I, and 29.7 nT/year in F
(Table 3).
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 Zeiss 010B theodolite with nonmagnetic
tripod, along with GEMSys GSM-19G Overhauser 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 D and 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 and I 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
2.1 nT in F.
Table 2: Repeat station observations reduced to THY and
2007.5 epoch.
Croatian Geomagnetic Repeat Stations Network
A brief historical overview of geomagnetism in the
Croatian territory was presented in (Brkić et al. 2003).
The renewal of geomagnetic surveys started in 2004 by
establishing Croatian Geomagnetic Repeat Station
Network (CGRSN) aimed at monitoring the geomagnetic
field and its secular variation determination periodically
(Brkić et al., 2006).
In order to design and establish locations of additional
repeat stations, IAGA (Newitt et al. 1996) and MagNetE
criteria were used. The key in determination of exact
positions of new stations was on site evaluation of all
criteria, the critical one being locally low total intensity
gradients and the absence of civilization noise. In doing
so, GEMSys GSM-19G Overhauser PPM gradiometer was
used.
Establishing two new primary repeat stations – PALAgruza
and 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 Stations Network. The stations
positions are presented as φ and λ coordinates on
Bessel’s ellipsoid.
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 nT greater 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”, Geodetski list 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 Istituto Nazionale di Geofisica. 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.
Xu W.J., Xia G.H., An Z.C., Chen G.X., Zhang F.Y., Wang Y.H., Tian Y.G., Wei Z.G., Ma S.Z., and Chen H.F., 2003. Magnetic Survey and ChinaGRF 2000. Earth Planets Space, 55, 215-217.
StationD
[dms]
Scatt. D
[dms]
I[dms]
Scatt.I
[dms]
F[nT]
Scatt. F
[nT]
POKU 2.4423 0.0058 62.0035 0.0009 47524.7 1.7
MEDJ 2.4051 0.0033 62.5542 0.0006 47843.5 3.0
BARA 3.1948 0.0031 62.3120 0.0004 47829.2 0.4
RACI 3.1407 0.0027 61.3225 0.0015 47541.3 3.5
KONA 2.5110 0.0030 59.1510 0.0018 46862.0 1.4
SINP 2.3319 0.0109 60.2825 0.0024 47012.9 2.0
KRBP 2.3013 0.0046 61.1403 0.0011 47128.5 1.9
PONP 2.1145 0.0034 61.4544 0.0016 47309.3 3.0
Station Lon. [deg] Lat. [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
StationDD /Dt[min/y]
DI /Dt[min/y]
DF /Dt[nT/y]
POKU 5,01 -0,04 29,4
MEDJ 5,56 -0,03 29,4
BARA 5,56 0,06 30,0
RACI 5,47 0,03 29,1
KONA 4,95 -0,14 31,0
SINP 5,20 -0,08 30,4
KRBP 4,69 -0,14 29,4
PONP 5,56 -0,22 29,2
Figure 3: 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 Santis et
al., Chiappini et al., Xu et al.) over Croatia, the Taylor
polynomial was used
(2)
where E(φ,λ,d) was the normal field Enormal, d was a
parameter vector, M was truncation level, j and k were
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 N was
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 ejk were computed, the
standard uncertainty was obtained from:
(4)
where was the sum of the squares of the residuals rsd of
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
kjM
j
jM
kjkeE 00
0 0
),,(
d
min.
N
inormal
epochi EE
1
2
PN
D [deg.] I [deg.] F [nT]
e00 2.76 61.54 47386
e01 0.1852 0.060 66.6
e10 0.0837 0.944 302.0
e02 0.0003 -0.003 -5.5
e11 0.0235 -0.011 -4.2
e20 0.0064 -0.022 11.2
D [min/y] I [sec/y] F [nT/y]
e00 5.25 -4 29.6
e01 0.047 2.73 0.11
e10 0.171 2.57 -0.38
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