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ANNALS OF GEOPHYSICS, VOL. 49, N. 2/3, April/June 2006

Key words geomagnetic field – marine magneto-meters – seafloor observations – skin depth

1. Introduction

The Earth’s magnetism has its origin main-ly within the fluid outer core by a dynamomechanism that produces an essentially dipolargeomagnetic field (95% of the total field),whose axis is slightly inclined with respect tothe rotation axis by about 10°. In detail around98% of the total field measured at the Earth’ssurface is of internal origin and 1% of the totalfield is due to crustal magnetisation characteris-ing the different tectonic plates. The measuredgeomagnetic field also includes external fieldswhose sources are in the ionosphere and mag-netosphere that, together with their crustal in-duced counterparts, contribute about 1% of thetotal field. Finally, the oceans also make a small

contribution through the motions of seawater inthe geomagnetic field. The geomagnetic fieldchanges with time. Among its variations we re-call, listed from longer to shorter characteristicperiods: a) the secular variation, which is theslow (annual, decadal, and secular) trend of thefield; b) solar diurnal variation, which is thedaily oscillation due to the solar ionisation ofthe high atmosphere; c) bays, substorms andstorms (range: from a few minutes to 1-2 days),the latter representing the most rapid variationswith significant amplitude recorded by geo-magnetic observatories on the Earth’s surface.

Since land covers only around 1/3 of theEarth’s surface, to better understand the mor-phology of the geomagnetic field it is necessaryto extend magnetic observations to the sea-floors. Measuring the geomagnetic field be-neath the sea has the characteristic that the ex-ternal rapidly varying fields are partiallyscreened by seawater (average conductivity of3-4 S/m depending on salinity), while slowlyvarying fields, which are of internal origin, arepractically unperturbed. The simultaneous mag-netic variations recorded on the seafloor and atdifferent locations on land are also related tothe presence of electrical conductive layers or

Deep seafloor magnetic observations under GEOSTAR project

Angelo De Santis, Domenico Di Mauro, Lili Cafarella, Roberto D’Anna, Luis R. Gaya-Piqué,Paolo Palangio, Giovanni Romeo and Roberta Tozzi

Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy

AbstractPerforming good quality magnetic observations is not an easy task; making them in the extreme marine environ-ment is even more challenging. The European funded GEOSTAR project succeeded in reaching this difficultgoal. After the shallow seawater test experiment performed in the Adriatic Sea in 1998, the main aims of theGEOSTAR project were achieved two years later during the six-month deep seafloor mission in the TyrrhenianSea at around 2 km depth. Details and results of the shallow seawater mission in the Adriatic Sea were publishedin previous articles. This paper is concerned with the deep seafloor mission in the Tyrrhenian Sea close to Usti-ca Island and presents some results related to the geomagnetic recordings.

Mailing address: Dr. Angelo De Santis, Istituto Nazio-nale di Geofisica e Vulcanologia, Via di Vigna Murata 605,00143 Roma, Italy; e-mail: desantisag@ingv.it

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of lateral electrical discontinuities in the ocean-ic crust and upper mantle on a global scale andto the presence of local patterns in restricted ar-eas.

The under-sea environment presents evidentconstraints on any kind of measurements. Thepressure increases by 1 atm for every 10 mdepth increment; hence at a depth of 2 km wehave around 200 atm. This high pressure re-quires that the instrumentation must be con-tained in strong, non-collapsible pressure cases.Corrosion is another adverse factor affectingmany components of instruments or sensors,therefore proper protection must be adopted.The most important aspect for seafloor magnet-ic measurements is the seawater electrical con-ductivity. Salinity and pressure variations donot really influence this quantity significantlybut temperature does. Seawater temperature de-creases with depth: from 20-30°C (typical tem-peratures at middle temperate latitudes) at thesurface to few degrees at the seafloor (e.g.,around 3°C at −3 km) with very high stability.Correspondingly, the seawater electrical con-ductivity decreases with depth (from 6 S/m atthe surface to 3.5 S/m at around −1 km and to3.3 at around −2 km). Below a depth of 2 kmthe conductivity has an almost constant value of3.3 S/m; for the oceans this latter value can beconsidered as the average conductivity. Smallerseas such as the Mediterranean can presenthigher conductivity values that, in any case, arenot higher than 4 S/m. The relatively high con-ductivity of seawater screens the propagation ofelectromagnetic (EM) variations. This can beeasily understood by introducing the so-calledskin depth δ (expressed in meters), of an EMvariation of period T in a medium of conductiv-ity σ as the propagation distance in a homoge-neous conductor over which EM waves are at-tenuated by a factor of e−1, i.e.:

δ=503 (T/σ)1/2

with T in seconds and σ in S/m (Filloux, 1987).From the Fourier power spectrum of naturalmagnetic variations in open oceans, it can bededuced that there is no significant attenuationfor periods longer than 30 min. At periodsshorter than 20 min, seafloor magnetic varia-

tions undergo considerable attenuation. If oneneeds a very high accuracy in studying magnet-ic variations between 1 day and 1 h, water mo-tions must be taken into account. However formany general purposes they can be neglectedsince they are one or two orders of magnitudesmaller than natural variations; otherwise theyshould be monitored and then removed.

In summer 1998, the benthic station of theEuropean Project GEOSTAR (GEophysical andOceanographic STation for Abyssal Research; aproject in the framework of the MArine Scienceand Technology Program, MAST III) was de-ployed for the first time in the Adriatic Sea, run-ning for around 1 month and the magnetometersprovided a good set of magnetic data (e.g., DeSantis et al., 1999). The GEOSTAR deep-seamission was then performed from September 25,2000 till March 16, 2001 close to Ustica Island(38°32l24mN, 12°46l30mE; Sicily, Italy).

Details on the GEOSTAR project and its de-ployment and observational system can be foundin some recent literature (e.g., Beranzoli et al.,1998, 2000, 2003; Gasparoni et al., 1998, 2002;Marvaldi et al., 1998, 2002; De Santis et al.,1999; Favali et al., 2002), therefore here we pres-ent only a brief description of the magnetic ex-periment of the deep-sea mission and some pre-liminary analysis of the magnetic data.

The next section introduces the aims of theproject and some information about magneticmeasurements within the GEOSTAR project.The next section is dedicated to the calibrationand estimation of the orientation of the magne-tometers. Two sections then describe the mag-netic data together with some spectral analysis;another section shows some results obtainedfrom the method of geomagnetic deep sounding.Finally the conclusions assess the most impor-tant results and findings.

2. Seafloor magnetic measurements within GEOSTAR

In terms of the magnetic experiment, theGEOSTAR project had the following objectives:

– Contribution towards achieving an evendistribution of observatories around the worldin order to improve the reliability of global

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models (such as the International GeomagneticReference Field, IGRF), and regional (Mediter-ranean) models of the magnetic field.

– Study of geomagnetic temporal varia-tions: a) long period (e.g., secular variation); b)short period (studies of solar diurnal variation,geomagnetic storms and substorms at mid lati-tudes). Concerning long period variations, par-ticular attention should be paid to the study ofsome peculiar aspects of the secular variationsuch as, for instance, the so-called jerks. Jerksare sudden changes in the rate of the secularvariation that occur on a time scale of 1-2 years(e.g. Courtillot and Le Mouël, 1984). Althoughthey are among the most investigated subjectsof geomagnetism, their origin and their natureare still a matter of debate: seafloor magneticfield measurements might contribute to the elu-cidation of this intriguing phenomenon whencompared with simultaneous ground data.

– Study of Earth’s conductivity structure:contemporary magnetovariational investiga-tions on the seafloor and at the surface allow, inprinciple, the detection of conductive layers inthe oceanic crust and upper mantle; this kind ofinformation is very important since electricalconductivity is closely related to temperature,phase and composition of materials.

– Coincidence with Oersted (1999-Present)and CHAMP (2000-present) satellite missions.Since the study of the radial variation of themagnetic field has always been one of the mostinteresting aspects of geomagnetism, it is veryrelevant to have contemporary magnetic meas-urements taken at three such different altitudes,i.e. from 700 to 300 km (satellite altitude),around 0 km (i.e. at the Earth’s surface), a fewkm under the sea level (seafloor observations).

Two magnetometers were installed on theGEOSTAR benthic station for the Tyrrheniandeep-sea mission: a scalar Overhauser GSM-19L proton magnetometer by GEM System Inc.(Canada) and a three-axis suspended magne-tometer, designed and built in the Laboratoriesof Istituto Nazionale di Geofisica e Vulcanolo-gia (Rome and L’Aquila, Italy). Both sensorswere protected from high pressure by two suit-able benthospheres.

The omnidirectional sensor of the scalarmagnetometer provided the total intensity ofthe geomagnetic field with a nominal resolutionof 0.1 nT, an absolute accuracy of 1 nT and apower consumption of 1 W at a sampling rate of1 value per minute.

The fluxgate three-component sensor wassuspended to ensure its verticality. Its resolu-

Fig. 1. October 5, 2000 – L’Aquila geomagnetic observatory (thick line) and GEOSTAR (thin line) X, Y, Zcomponents. X, Y, Z are north, east and vertically downward magnetic components, respectively. An appropriatebaseline must be added to each component.

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tion was 0.1 nT, with an absolute accuracy of 5-10 nT and a power consumption of 2 W at asampling rate of 6 values per minute. Its analogoutputs were digitised by a 16 bits A/D convert-er, achieving a quantisation step size of 1.4 nTin order to cover all possible ranges of the geo-magnetic field.

The GEOSTAR deep-sea mission of 2000-2001 provided 4123 h of geomagnetic data. Asudden failure in the electronic device of thescalar magnetometer occurred at the very begin-ning of the mission and caused a reduction ofthe planned sampling rate of 1 value per minuteand the achievement of a rate of only 1 valueevery twelve minutes.

In figs. 1 and 2 we show a comparison be-tween GEOSTAR and L’Aquila observatory

(42°23lN, 13°19lE, 0.7 km altitude) vector mag-netic components and total intensity of the mag-netic field for October 5, 2000 (characterised bymoderate magnetic activity) and March 15, 2001,respectively.

In order to have a reference station closer thanL’Aquila and in close proximity to the GEO-

Fig. 3. Location of GEOSTAR site, Ustica andGibilmanna (Sicily, Italy).

Fig. 2. March 15, 2001 – L’Aquila geomagnetic observatory (continuous line) and GEOSTAR (crosses) totalfield intensity, F.

Fig. 4. Electronic equipment (top) and the three-ax-is fluxgate sensor (bottom) for the land stations.

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STAR site, a land station was installed at UsticaIsland (38°42lN, 13°11lE, 0.2 km altitude) andanother at Gibilmanna (37°59lN, 14°01lE, 1 kmaltitude) (see fig. 3). Each station was based on avector fluxgate sensor, a control unit and a datalogger (fig. 4), using a solar panel for power sup-ply. Moreover, at the beginning and at the end ofthe mission, absolute (static) measurements ofthe magnetic declination and inclination at Gibil-manna and at Ustica Island were performed. Suchmeasurements were necessary to calibrate andvalidate the recordings made by the variographs.

3. Calibration and orientation of GEOSTAR frame

Two important considerations must be takeninto account when analysing the GEOSTAR da-taset: firstly the calibration and compensation formagnetic materials in the other instruments, andin the supporting structure of the module; second-ly the orientation offset of the structure during themission with respect to the true magnetic north.

For convenience, a dedicated calibrationwas carried out in the Italian magnetic observa-tory of L’Aquila (Central Italy) some days afterthe recovery of the benthic station (fig. 5). A

comparison between the simultaneous record-ings of the magnetic field components fromL’Aquila and from the magnetometers mountedat the end of the booms on the submarine mod-ule, gives a quantification of the artificial offsetdue to GEOSTAR.

The compensation model used can be ex-pressed as follows:

Bref= Bgeos− Bperm− δ⋅Bgeos

where Bref is the magnetic field as recorded bythe reference observatory, Bgeos is the magneticfield as observed by GEOSTAR, Bperm is thepermanent field due to the station frame’s met-al parts, and δ is a 3×3 matrix of constant coef-ficients characterising the induced field (seenby GEOSTAR). δ and Bperm are unknowns withnine and three parameters, respectively. For in-stance, for the x direction, we have

(3.1)

where Bx, By and Bz are the Cartesian compo-nents of the geomagnetic field in the differentcoordinate references.

- +Bx- By$+ δBx Bx= $Bx

Bz

ref geos perm geos geos

geos

xx xy

xz $+

δ

δ

6

@

Fig. 5. GEOSTAR submarine module (frame) at L’Aquila during calibration and comparison with magnetic da-ta from observatory. Magnetometers are mounted at the extremity of the oppositely extended booms.

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First we noticed that the original Bygeos andBxgeos had to be exchanged; then, the sign of Bzgeos had to be reversed. From now on we willdenote the Cartesian components observed byGEOSTAR as those obtained after these changes.

We see that eq. (3.1) has two separate con-tributions, the permanent and induced parts, re-spectively. Inverting eq. (3.1) for the parametersδ when considering different orientations of thestation with respect with true north, we foundthat the induced part is practically negligible;for what concerns the permanent part, we esti-mated the following values: Bxperm= 1075.4 nT,

Byperm= 448.2 nT, and Bzperm= 1077.6 nT. Apartfrom its intermittent difficulties, the scalar mag-netometer provided values that need a correc-tion of −295 nT.

An estimated orientation of the whole sta-tion frame during the final release at a depth of2000 m under the sea surface was known fromthe compass included in the 3D-ACM (three-component Acoustic Current Meter), whichgave an angle of +242º (counter-clockwise fromnorth). An in-depth evaluation of the GEO-STAR frame orientation was made comparingdata recorded by GEOSTAR with data from thetwo land stations of Ustica Island and Gibilman-na and from L’Aquila Observatory. The compar-ison was made using least squares regression todiscriminate between small or large angle of ro-tation, as shown in fig. 6a,b respectively.

To check the method we compared the hor-izontal X, Y components as recorded at L’Aqui-la and Castello Tesino (46°03lN, 11°39lE, 1.2km altitude) observatories for a day of moder-ate magnetic activity (February 15, 2001) andthen we calculated the rotation offset betweenL’Aquila Observatory-GEOSTAR and Usticastation-GEOSTAR. The compensation anglebetween the corresponding orthogonal sensorframes had a median value of –1.9º, which is asmall deviation, as expected for reference sys-tems of observatories.

Fig. 7. Results of the orientation analysis between L’Aquila Observatory and GEOSTAR.

Fig. 6a,b. Two possible orientations of the GEO-STAR frame.

a b

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For the same day we calculated the median(and the mean) values for the couples L’Aquila-GEOSTAR (fig. 7) and Ustica-GEOSTAR, ob-taining respectively −120.64º (mean −120.98º)and −119.97º (mean −123.73º) which confirm,within a few degrees, the values provided by thecompass in the 3D-ACM.

4. Data description

Data validation for the whole GEOSTARdataset was achieved by reducing any manipu-lation of the original recordings as much as pos-sible. Each component was reconstructed ap-plying the rotation and calibration correctionsas described in the previous section to the vec-tor magnetometer recordings. The six valuesper minute were averaged to produce 1-min val-ues after applying a spike remover filter whichacted on 6 to 15 points for each day. A list of allmissing data segments (for both vector andscalar recordings) is given in table I.

The scalar magnetometer provided a total of20615 available data. The leakage of data cor-responds to the ratio of available data over the

Table I. Missing data segments of the GEOSTARdeep-sea mission.

Data Starting time Ending time

27/09/2000 13:00 13:2530/09/2000 13:00 14:2503/10/2000 03:00 04:2508/10/2000 12:00 13:2509/10/2000 08:00 08:2520/10/2000 20:00 21:2526/10/2000 05:00 05:2504/11/2000 07:00 07:2516/11/2000 02:00 03:2502/12/2000 00:00 01:2502/12/2000 20:00 20:2503/12/2000 00:00 00:4920/12/2000 22:00 22:2506/01/2001 01:00 01:2525/01/2001 16:00 16:2528/01/2001 16:00 16:2522/02/2001 09:00 10:2526/02/2001 05:00 05:2528/02/2001 13:00 13:2503/03/2001 01:00 02:25

Fig. 8. Six months of seafloor recordings. Cartesian components and derived F are displayed. Daily variationsand some magnetically active periods are clear in all curves.

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Fig. 9. Comparison between derived and measured F and their difference. Parameters of linear fits on theirsmoothed curves are also reported.

Fig. 10. Total geomagnetic field F: comparison between GEOSTAR and L’Aquila Observatory. Dataset timewindows: September 25, 2000 at 11:00 – March 16, 2001 at 00:59 – tick mark at 23:59 of the day.

total duration (in minutes) of the mission. Forthe scalar data we have about 91.6% of missingdata. The vector magnetometer provided a totalof 1484282 available data but between 6 and15 spikes are present for each 24-h segment of

recording. In this case we have about 0.2% ofmissing data. The total intensity F of the mag-netic field was then derived as the square rootof the sum of the squares of the three re-orient-ed and calibrated components.

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The three geomagnetic components togetherwith the F values obtained in this way are dis-played in fig. 8. The total intensity directly meas-ured by the scalar magnetometer mounted onGEOSTAR is displayed in fig. 9 (third horizon-tal panel) as single points, since the instrumenta-tion collected only 1 data point every 12 min.

The difference between derived and meas-ured F values is shown as scattered points in thelower part of the upper panel of fig. 9. From thispanel an adjustment of the system in the first 10days is clear, after which the value of differenceis quite stable with an average value of 294.88nT (sd=1.15 nT). A greater spread of points inthe initial three months was due to strong mag-netic activity occurring at that time. This re-flects the instability conditions during strongmagnetic field activities. Besides the goodagreement between the derived and measured Fdaily smoothed curves in fig. 9, we observe thatthe daily variations completely vanish in thelinear fit applied to the smoothed curves. Thisshows the good agreement between the six-

month increases in the two datasets: that de-rived at the GEOSTAR site and that measuredat L’Aquila Observatory (fig. 10) with a similarvalue of secular variation of around 42 nT/year.

5. Data analysis

In order to faithfully compare data recordedat L’Aquila Observatory with those recorded byGEOSTAR in terms of the energy and dynam-ics of the corresponding time series, we chosethree days characterised by different geomag-netic activity and evaluated their power spectra.

The K activity index computed from L’AquilaObservatory geomagnetic recordings allows us tomake an appropriate selection of data segments tobe used for both power spectra analysis and elec-tromagnetic induction studies (fig. 11). For ourpurposes we considered the daily sum of the cor-responding eight K-indices, calling this value thedaily total K-index. Then we selected three dayswith different levels of magnetic activity.

Fig. 11. Comparison between total magnetic F and L’Aquila Observatory daily total K-index.

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Fig. 12a-d. Spectral analysis of GEOSTAR (a, c) and L’Aquila (b, d) total intensity F for October 5, 2000 (a, b)and January 01, 2001 (c, d). From left: temporal plot, spectrum with frequency and period at the abscissa. Thecorresponding daily total K-index was 43 (a, b) and 3 (c, d).

a

b

c

d

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The selected days are October 5, 2000 (dai-ly total K-index=43; strong magnetic activity),January 1, 2001 (daily total K-index=3; lowmagnetic activity), and January 22, 2001 (dailytotal K-index=21; moderate magnetic activity).The analysis of each daily segment of data (1440data points) provides suitable spectral informa-tion over the frequency range from 0 to 0.5 cy-cle/min. The cases examined show in particular adifference between the L’Aquila and GEOSTARpower spectra, which becomes less and less evi-dent with decreasing total K-index (fig. 12 a-f),i.e. with decreasing geomagnetic activity. Thisdifference consists in a larger content of high fre-quency energy at L’Aquila than at the GEO-STAR site in the case of high geomagnetic activ-ity, on the contrary the low frequency energycontent remains of the same order of magnitudefor any activity level. This behaviour can be ex-plained by the screening effect of the seawater,especially on short period components and it ismore marked when the total K-index has large

values as in the case of October 5, 2000 when itwas 43. In this case the high frequency portion ofthe power spectrum of GEOSTAR data asymp-totically approaches values that are at least twoorders of magnitude lower than the analogousspectrum obtained for the signal simultaneouslyrecorded at L’Aquila. On the contrary low fre-quency variations affect measurements taken atL’Aquila and those taken at the GEOSTAR sitepractically in the same way. The seawater screen-ing effect is also confirmed in the following de-scription of how the induction mechanism domi-nates at longer periods.

6. Preliminary analysis of EM inductionand electrical conductivity

A combination of the vertical and horizontalcomponents of the fluxgate magnetometer data-set allows the estimation of the so-called Par-kinson arrows (or induction arrows) of the Ge-

Fig. 12e,f. Spectral analysis of GEOSTAR (e) and L’Aquila (f) total intensity F for January 22, 2001. Fromleft: temporal plot, spectrum with frequency and period at the abscissa. The corresponding daily total K-indexwas 21.

e

f

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omagnetic Depth Sounding (GDS) technique(e.g., Armadillo et al., 2001). They provide bothquantitative and qualitative indications of thepresence of conductivity contrasts near thepoint of measurement from the complex ratios(i.e. the so-called transfer functions) of theFourier transforms of the horizontal and thevertical magnetic components at different peri-ods (the longer the periods the deeper the corre-sponding sounded layer). In the graphic repre-sentation, the real parts of such arrows are di-rected towards regions with higher conductivi-ty. This technique can be successfully appliedalso in the band limited seafloor environment.We applied the GDS technique to the time vari-ations of the GEOSTAR magnetic data for apreliminary selection of 12 events (duration:512 min) from 6 magnetically undisturbed days(not shown here). In general, most of the arrowsbecome larger as the period increases. This con-firms that at the GEOSTAR site (i.e. at a depth of2000 m) the bulk of electromagnetic inductiongenerated by variations in the geomagnetic fieldcontains mainly long period components. More-over, most of the arrows point toward north ornorth-east, indicating that a higher electrical con-ductivity contrast is located along these direc-tions with respect to Ustica Island. This anomalycould be possibly due to the asthenospheric up-weeling of the Tyrrhenian Basin. We expect thatthis aspect will be better investigated by ana-lysing the whole dataset.

7. Conclusions

The magnetic experiment of the GEOSTARproject succeeded in providing reliable magnet-ic data for most of the period of the deep watermission. Techniques were applied in order tocalibrate and orientate the GEOSTAR magne-tometers. Comparison with ground stations(Gibilmanna and Ustica sites) and observatories(L’Aquila and Castello Tesino) showed a goodagreement, confirming the quality of the data.

Power spectral analysis was performed onselected data segments (characterized by thedaily total K-index). The results showed that theenergy in the power spectra at short periods islarger at L’Aquila than at the GEOSTAR site

especially when the magnetic activity is high.On the contrary, the energy involved at longerperiods is of the same order of magnitude at theGEOSTAR site and at L’Aquila for any level ofgeomagnetic activity.

Finally, induction studies showed that ahigher electrical conductivity contrast is locatednorth-north-east of the Ustica Island.

In the future, more detailed analyses will beneeded to better evaluate the data, together witha careful comparison with the correspondingsatellite data from the Oersted and Champ mis-sions (e.g., Reigber et al., 2005).

Acknowledgements

The GEOSTAR project was funded by theEC under the Marine Science and TechnologyProgramme (contract n. MAS3-CT95-0007).

We thank all the people who were involveddirectly or indirectly in the GEOSTAR project.Among them, Giuseppe («Pino» to his friends)Smriglio merits special mention for his greatpatience, strength and humanity expressed toeverybody during this time. His premature de-parture has left a great emptiness in all of us.

We thank David Barraclough, Mioara Man-dea and Antonio Meloni for providing impor-tant comments for improving the original ver-sion of the paper.

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