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ANNALS OF GEOPHYSICS, VOL. 47, N. 5, October 2004

Key words geomagnetism – aeromagnetic anomaly –magnetic methods – aeromagnetic cartography

1. Introduction

Potential field imaging has a great importancein mapping geologic structures. At a reasonablecost it is possible to cover large areas in a shorttime, and being not invasive, this technique doesnot alter the equilibrium of the investigated area.Magnetic field data obviously are particularly ef-

fective in mapping structures characterized bymajor magnetic susceptibilities, such as magmat-ic or infra-sedimentary ophiolitic bodies.

Moreover large scale maps are particularlyeffective when applied to the study of the tecton-ic setting because they permit extraction of infor-mation about the main lineaments and structuraltrends. On the contrary, if the database is charac-terized by an intensive sampling, it is possible tomap smaller areas with dense data to extract in-formation about high frequency components ofthe field connected with shallower sources, suchas volcanic complexes or outcropping susceptivelithologies. The data that compose the new aero-magnetic anomaly map have been assembled in-to a digital database in such a way as to permitdifferentiated analyses finalized to compile dif-ferent scale maps, according to user needs.

The revised aeromagnetic anomaly map of Italy

Fabio Caratori Tontini (1) (2), Paolo Stefanelli (1) (2), Italiano Giori (3),Osvaldo Faggioni (1) (2) and Cosmo Carmisciano (1) (2)

(1) Istituto Nazionale di Geofisica e Vulcanologia, Sede di Portovenere, Fezzano (SP), Italy(2) Consorzio Universitario della Spezia (Cunispe), Istituto di Geofisica Marina, La Spezia, Italy

(3) Eni Spa – Exploration & Production Division, S. Donato Milanese (MI), Italy

AbstractThis paper presents the revised aeromagnetic anomaly map of Italy and its surrounding seas, projected at refer-ence altitude of 2500 m and geomagnetic epoch 1979.0. The magnetic data set used for the map compilation iscomposed of the total intensity field data acquired partly during the aeromagnetic surveys performed by the Ital-ian National Oil Company (Agip – Direzione Esplorazione Idrocarburi) between 1971 and 1980, and during thenew surveys committed by the Geophysical Corporate Services of Eni Spa – Exploration & Production Divisionin the years 2001-2002. In both campaigns the recorded data were very dense and uniformly distributed over theexamined area. A detailed re-processing of this data set and a re-organization into a new digital database werecarried out. The re-processing was done using modern adequate techniques, obtaining a remarkable exploitationof the data information content. The result is a colour shaded relief map that shows on a large scale many of thestructural lineaments of the Italian area. The inclusion of a larger number of data and the subtraction of an ap-propriate magnetic reference field are the main reasons of an enhancement in the anomaly definition. This newmap replaces the previous Agip version, and aims to become the reference aeromagnetic cartography of the Ital-ian area. We think this work will be useful both for researchers interested in large scale tectonic studies, and foranyone interested in the investigation of smaller scale structures, such as volcanic complexes or infra-sedimen-tary magnetic bodies, as well as for mining research.

Mailing address: Dr. Fabio Caratori Tontini, IstitutoNazionale di Geofisica e Vulcanologia, Sede di Portovene-re, Villa Pezzino, Via Pezzino Basso 2, 19020 Fezzano(SP), Italy; e-mail: caratori@ingv.it

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Fabio Caratori Tontini, Paolo Stefanelli, Italiano Giori, Osvaldo Faggioni and Cosmo Carmisciano

The present magnetic cartography on theItalian area is based on two main works: theaeromagnetic anomaly map of Italy (Agip andSGN, 1994) compiled by Agip, whose datahave been partly used in this work, and theshaded relief magnetic anomaly map at sealevel, by Chiappini et al. (2000), compiledwith the off-shore data obtained by the Osser-vatorio Geofisico Sperimentale (OGS) of Tri-este and the ground data from the repeat sta-tion network and 2nd order station network bythe Istituto Nazionale di Geofisica e Vul-canologia (INGV). The latter map shows a sig-nificant result obtained merging ship-borneand ground data sets acquired with differentmethodologies, instrumentations and date ofsurveys.

The previous Agip map realized for specif-ic mining targets shows some peculiarities inthe data processing. These characteristics arepartly due to old techniques used at that time,but mostly depend on the selection of a specif-ic geomagnetic reference model especially de-voted to the study of mining problems. Infact,the main field was subtracted using an inter-polating surface composed of the envelope oflocal planes, obtained from horizontal gradientstudies (Cassano, 1984). The residual map,obtained after this procedure, shows the phe-nomenon of a linear trend aligned NNW-SSE,with an average slope of 0.5 nT/km. This mapwas thus appropriate for industrial applica-tions, showing some problems when used forstructural studies, where the trend effects canmask or even deteriorate the large wavelengthanomalies. However the aeromagnetic data,that are acquired at a determined height overthe topographic surface, naturally filter out ge-ologic noise coming from small shallow mag-netic bodies and for this reason they are veryuseful in the analysis of deep large scale struc-tures.

The Consorzio Universitario della Spezia(Cunispe), whose researchers are currently inthe Istituto Nazionale di Geofisica e Vulcanolo-gia, in cooperation with Eni Spa – Exploration& Production Division, has updated the oldaeromagnetic anomaly map by re-processingthe original data set. These data in fact show avery high potentiality, because of the care ap-

plied during their acquisition, their remarkabledensity and the homogeneous and uniform dis-tribution over the covered areas. The occasionfor this global revision, also derived from thepossibility to insert in the map some «patch» ar-eas surveyed by Eni in 2001-2002 to fill somegaps in the previous coverage.

The integration was done through the appli-cation of procedures able to make all availablemeasurements comparable. The different sur-vey epochs and the different flight altitudes ofeach of the separate micro-surveys were inte-grated to converge into a systematic nationalcoverage. The final map, where the old ficti-tious trend is now removed, shows remarkableprogress with respect to the original aeromag-netic Agip map, also for a better localizationand definition of the magnetic anomalies gener-ated by the shallower sources.

In the following sections we will describethe data organization and the processingmethodology in detail. Particular attention willbe taken to explain the integration procedurethat has a capital importance in merging the sin-gle micro-surveys, separate in space and time,that compose the global map.

2. Data characteristics and organization

The data that have been processed to com-pile the new aeromagnetic anomaly map comemainly from 36 original micro-surveys execut-ed by Agip (Direzione Esplorazione Idrocar-buri) from 1971 to 1980 over the entire nation-al territory and the nearby seas. These datahave been integrated with 5 recently surveyedareas acquired in 2001-2002 in the context of ajoint project between Eni and Cunispe (EniSpa – Exploration & Production Division Pro-ject 4700000254/MR1). The geographical lo-calization of these more recently studied areasis Ancona off-shore, Molise on/off-shore,North-East Calabria on/off-shore, Sicily on-shore and Southern Sicily off-shore. Figure 1shows the total line-path of the global survey.The flight lines, mainly oriented in SW-NE di-rection, are intersected by orthogonal controltie lines. The total length of the surveyed pro-files amount to 326823 km, and the surface

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The revised aeromagnetic anomaly map of Italy

covered by the global survey measures approx-imately 783576 km2.

The 36 micro-surveys executed by Agip arecharacterized by an average line spacing of 5-10 km and an average tie line spacing of 10-15km. The sampling step ranges from 0.05 to0.25 km. There is an exception that is repre-sented by a small area located between Cro-tone latitude and the Messina Straits, wherethe data furnished by Agip were organized in-to a 2 km cell grid. The typical wavelength ofthe magnetic field in this zone is significantlygreater than 4 km so that this difference in thedata distribution does not affect the globalproduct. The new micro-surveys performed byEni are more accurate because the averagespacing between the lines is 2-5 km and 5-10km for the tie lines, with an average samplingstep of 0.05 km.

The total intensity magnetic field in boththe campaigns was measured by means of anoptically pumped caesium magnetometer withhigh resolution (0.01 nT). At the center of eachcontinental micro-survey, or in the nearbycoast in marine acquisitions, a base station wasinstalled to record the geomagnetic variationduring the aeromagnetic flights (Parkinson andJones, 1979). The geographical localizationwas evaluated using Loran C receivers andphotographic cameras for the profiles location.In the case of the recent micro-surveys by Enithe position was directly acquired by the GPS

system. The flight altitudes were at differentlevels in the range from 4800 feet over the ma-rine areas, to 13000 feet in the Alpine region,depending on the topographic pattern of thesurveyed area.

Data from each single micro-survey werelabelled using a numerical code that permitstheir identification, according to the originaldenomination of the ASCII file containing thedata. This data organization, that was also im-plemented in the digital integrated database,has the great advantage of making consulta-tion easy and efficient also for a detailedanalysis focused on a particular area. The user,either for structural studies or mining/petrole-um research, can then select only those sur-veys effectively belonging to the examinedarea, with a consequent faster execution timeand with the possibility to extract grids withsmall edge cells and to print very detailedmaps of the selected areas. Depending on spe-cific geomagnetic styles, in addition to differ-ent geographical and geological characteris-tics, the surveys are grouped in four mainmagnetic provinces as shown in table I, wheresome information is also presented. Figure 2shows the position and the geometry of thepolygons enclosing each of the 41 areas de-fined by any separate micro-survey. The jointcomparison between fig. 1 and fig. 2 revealsthe great accuracy in the coverage of the entireItalian territory.

Table I. Survey codes for the magnetic provinces and associated statistics. Recently areas surveyed by Eni arelabelled by the codes 1-5.

Province Survey code Records Latitude Longitudemin max min max

Alps-Po Valley 58-59-60-67- 512 622 43°46′58″ 47°03′07″ 06°36′07″ 13°53′27″70-87-171

Apennine- 1-5-69-71-75- 300 901 41°09′52″ 45°01′23″ 11°07′31″ 17°14′41″Adriatic 79-86-63.1-63.2

Tyrrhenian 56-57-66-68-72- 678 863 37°26′21″ 45°14′49″ 07°57′53″ 16°01′48″74-80-81-83-85

Siculan- 2-3-4-13-61-62- 1339 449 35°13′55″ 42°33′49″ 11°30′03″ 19°59′51″Calabrian- 64-65-73-76-77-

Ionian 78-82-84-121

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Fabio Caratori Tontini, Paolo Stefanelli, Italiano Giori, Osvaldo Faggioni and Cosmo Carmisciano

Fig. 1. Line path orientation and density of the aeromagnetic compilation.

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The revised aeromagnetic anomaly map of Italy

Fig. 2. Micro-surveys geometry and distribution of the aeromagnetic acquisition.

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Fabio Caratori Tontini, Paolo Stefanelli, Italiano Giori, Osvaldo Faggioni and Cosmo Carmisciano

3. Data processing and integrationmethodology

First of all, we performed a careful standardquality control of the database. Each single linewas analysed to avoid residual spikes or doublerecords, so that the entries of the database werefully employable. Data processing was then per-formed in different ways, depending on the levelof rawness of the data acquired by Eni with re-spect to the original data partially processed byAgip. The recent micro-surveys in fact, were or-ganized into an ASCII file divided into differentchannels. Moreover the coordinates channels, thetotal intensity magnetic field, and the associatedmagnetograms from a coherent base station wereavailable to perform the time variation subtrac-tion (Faggioni and Caratori Tontini, 2002). Theapplication of the statistical leveling to avoid in-tersection errors between the lines and the tie-lines, and the micro-leveling (Minty, 1991; Fer-raccioli et al., 1998) to reduce the effects of line-path stretching led to a correct observed fieldwhere the time variation is filtered.

The Agip data format instead posed someproblems for the organization of the workingprocedure. The original channels were not al-ways available. The only channel common toeach of the 36 micro-surveys was the residualfield, i.e. the magnetic field obtained after sub-traction of the time variation, and after the resid-uation of the reference field. This channel wasused as the starting point for the integration pro-cedure. The data were assumed to be effectivelytime reduced by Agip, but they were affected bythe subtraction of that particular reference field,composed of simple bi-dimensional interpolat-ing surfaces (planes) with characteristics typicalof each single micro-survey, with a loss of gen-erality. The subsequent step to converge towardsunification was the removal of the effects of thereference field surface built with the local hori-zontal gradients found in Cassano (1984). Theplanes relevant to each micro-survey were addedto the anomaly field to obtain the assumed cor-rect observed field. In some cases the statisticalleveling and microleveling was performed toproduce a data channel without fictitious anom-alies at the crossing points or elongated anom-alies aligned as the line-paths. Each of the 41

micro-surveys had a particular coordinate sys-tem. To achieve an efficient merging we project-ed every data set into a common cartographicsystem: Transverse Mercator Projection withLatitude 0°, Longitude 14°, False East 1500000m and False North 0 m, using the WGS84 Da-tum. The subtraction of the reference fieldIGRF79 led to a correct global anomaly field.

All data were projected to the common alti-tude of 2500 m. This altitude represents an aver-age value between the various original altitudes,typical of each micro-survey, that is a good bal-ance between the high frequency noise filteringand the signal power natural suppression. Theprocedure was achieved through the Bottom Re-duction Method (BTM) procedure developed byCunispe (Faggioni et al., 2001), permitting thecomparison among various micro-surveys andavoiding the generation of high frequency spuri-ous signals when the data are downward contin-ued. The data are projected over the topograph-ic surface according to the following formula:

, , , .

,

F F d

KF

F21, ,

,

,

,

max

c

L H L H

L H

L H

L H

$

$ $

/ +{ m { m { m{ m

∆ ∆

∆∆_ _ _

_

i i i

i(3.1)

where ϕ and λ are the geographical coordi-nates; c ( , )F { m∆ ,L H is the low (high) frequencyBTM projected intensity of the geomagneticanomaly field; ( , )F { m∆ ,L H is the low (high)frequency intensity of the geomagnetic anom-aly; d(ϕ, λ) is the depth of the topographic sur-face; KL,H is the low (high) frequency verticalgradient; Fmax∆ ,L H is the low (high) frequencymaximum intensity of the anomaly field.

The operator of eq. (3.1) spreads the mag-netic data values measured at a certain altitudeover a reference topographic surface (in thiscase a plane surface at 2500 m), according todifferent vertical gradients defined by the K pa-rameter. The convolution of the high and lowfrequency projected field gives the BTM cor-rected anomaly field.

The last integration was done reducing thenoise in the bordering areas, statistically level-ling lines coming from a region over the lines ofthe bordering region and vice versa (Minty et al.,

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The revised aeromagnetic anomaly map of Italy

Fig. 3. Colour shaded relief map of the aeromagnetic anomaly field.

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Fabio Caratori Tontini, Paolo Stefanelli, Italiano Giori, Osvaldo Faggioni and Cosmo Carmisciano

2003). This step was particularly time-consum-ing because the number of overlapping areas isconsiderable. When present, residual trends weresuppressed by polynomial subtraction in a leastsquares sense (Bullard, 1967). The final productis an A0 colour shaded relief map with scale1:1500000, contour interval 10 nT, and illumi-nation with inclination 45° and declination 45°,with a grid cell size of 2 km. This map is also re-ported in this paper as fig. 3, where only thecolour shaded relief map is drawn with no con-tour lines. The data range measures 1500 nT, andthe palette was chosen to satisfy graphic require-ments that give both a global vision of the anom-alies pattern according to their various typicalwavelengths, and to show their main geologic-structural trend signatures.

It is quite difficult to assess a statistical errorestimate for each single datum. The cross-overerror analysis would permit this quantification inprinciple at least in the subset of the crossingpoints, that are approximately 30 000. A graphicrepresentation of this error distribution is prob-lematic, however a detailed statistical analysishas demonstrated that it is almost uniformly dis-tributed over the whole data range. The absoluteerror is generated mostly by the measurementprocedure (height and lag error, magnetic meas-urement statistic fluctuation, positioning, …) andby residual time variation effects, but scarcelyever exceeding 5 nT. The gridding procedure ob-viously introduces further approximations caus-ed by the interpolation (minimum curvaturemethod), depending on its typical parameters (in-terpolation radius, grid cell size, low-pass de-sampling factor, …). The final associated error isthus subjective and difficult to evaluate, but it be-longs to the graphic production stage and it doesnot corrupt the digital database itself.

4. Conclusions

The updated aeromagnetic anomaly map isproposed as a new starting point for structural re-gional studies, for an enhanced consistency of themagnetic data with respect to the previous ver-sion, due to the application of more appropriateand modern processing techniques. The associat-ed database has been structured to help the user

in the choice of the minimum amount of dataneeded to perform the analysis, with a conse-quent reduction of the time execution and the ma-chine computation, and can be upgraded withoutdifficulty incorporating newly surveyed data. Themap, born to be used by Eni Spa – Explorationand Production Division for petroleum and min-eral exploration, reflects the actual Italian tecton-ic setting, stressing its main lineaments. We em-phasize the usefulness of this tool also for anyonewho is interested in studying the evolution of thisarea to develop new interpretations. The maskingeffects of the geomagnetic linear trend that wasevident in the previous version has been removedwith a consequent increase in the anomaly defi-nition minimizing previous existing long wave-length errors. The aeromagnetic map, if com-pared to the map at sea level of Chiappini et al.(2000), obviously shows a lower signal powerbecause of the greater altitude, that however isbalanced by the very dense number of records(>2.8 × 107). Joining together the informationcoming from the two maps, we expect that it willbe possible to have a very powerful tool for in-terpreting structural setting of the Italian region.

Acknowledgements

The authors are grateful to Eni Spa – Explora-tion & Production Division for granting permis-sion to publish the data. Ivo Loretti of Eni Spa –Exploration & Production Division is appreciatedfor his valuable contribution during the acquisi-tion of the new micro-surveys. Dr. Antonio Mel-oni and Dr. Massimo Chiappini of IstitutoNazionale di Geofisica e Vulcanologia are partic-ularly appreciated for their encouragement aboutpublishing this work and for their helpful revi-sion. We thank also Dr. Angelo De Santis of Isti-tuto Nazionale di Geofisica e Vulcanologia for hisuseful criticism essential to improve this work.

Note: copies of the A0 aeromagnetic anomaly map can beobtained free of charge, after Eni Spa – Exploration & Pro-duction Division Management approval, sending a request toEni reference author at the following address: Dr. ItalianoGiori, Via Emilia 1, 20097 San Donato Milanese (MI), Italy;e-mail: italiano.giori@agip.it

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The revised aeromagnetic anomaly map of Italy

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(received August 22, 2003;accepted August 26, 2004)