Aeromagnetic anomaly images of Vulcano and Southern …...1805 Aeromagnetic anomaly images of Vulcano and Southern Lipari Islands (Aeolian Archipelago, Italy) Fig. 2. Shaded-relief

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ANNALS OF GEOPHYSICS, VOL. 47, N. 6, December 2004

Mailing address: Dr. Riccardo De Ritis, Istituto Nazio-nale di Geofisica e Vulcanologia, Via di Vigna Murata 605,00143 Roma, Italy; e-mail: [email protected]

Key words aeromagnetic data – magnetic anom-alies – geomagnetism – Vulcano

1. Introduction

The Aeolian Arc, in the Southern Tyrrhen-ian Sea, is characterized by active volcanism,which may threaten population safety and ac-tivities. An international activity has beenlaunched, aimed at investigating these highlyhazardous areas by means of integrated multi-disciplinary geophysical studies. The purposeof magnetic surveying is, in fact, to identify theshallow geological components and the crustalcontribution of the Earth’s magnetic field.When devoted to the definition of magneticfields, likely generated by spatial variations inthe magnetization of crustal rocks, magneticanomaly maps are produced to show this con-tribution. By means of the survey described in

the present paper we highlight the lateral varia-tion of magnetizations of the shallow crustalrocks, computing the magnetic anomaly field.

An initial approach was conducted in cooper-ation between the Geological Survey of Austriaand the Istituto Nazionale di Geofisica e Vul-canologia of Rome, acquiring high-resolution air-borne geophysical data over Vulcano Island. Vul-cano is located in the southern sector of the Aeo-lian volcanic district, and lies along a NNW-SSEgraben-like structure, approximately parallel tothe strike-slip «Tindari-Letoianni» Fault (Barberiet al., 1994). The airborne survey, conducted bymeans of a locally hired Squirrel AS-350 helicop-ter, also partially covered the western side and thesubmerged flanks of Lipari Island. The aim of thepresent paper is to image the crustal structure ofthe area by means of magnetic anomaly data, tocharacterize the magnetic sources associated withthe various Volcanic cycles.

In addition to aeromagnetic data laser al-timetry and gamma ray data were also collect-ed. We focus our attention on processing themagnetic data to develop the anomaly patternof the surveyed area and perform mathematicalanalysis and data enhancement.

Aeromagnetic anomaly images of Vulcanoand Southern Lipari Islands (Aeolian Archipelago, Italy)

Robert Supper (1), Riccardo De Ritis (2), Klaus Motschka (1) and Massimo Chiappini (2)(1) Geological Survey of Austria and University of Vienna, Vienna, Austria

(2) Istituto Nazionale di Geofisica e Vulcanologia, Roma, Italy

AbstractNewly acquired high-resolution, low-altitude aeromagnetic data over Vulcano Island and Southwestern Lipari inthe Southern Tyrrhenian Sea resolve the major volcanic features in the area associated with the past and presentactivity. The magnetic character changes in amplitude and frequency from south-east to north-west. The Pri-mordial Vulcano, the Lentia Complex, the Piano Caldera units, the Fossa Caldera deposits, and the currently ac-tive La Fossa cone and Vulcanello represent the main volcanic phases on Vulcano Island. They show a distinctmagnetic anomaly pattern, whereas prior to this survey, no magnetic signatures of these features were found.

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Robert Supper, Riccardo De Ritis, Klaus Motschka and Massimo Chiappini

The whole Tyrrhenian Sea was previously in-vestigated by means of oceanographic campaignsbetween 1965 and 1972 (Allan and Morelli,1971). The cruises acquired bathymetric, gravityand magnetic data. Chiappini et al. (2000) re-processed the magnetic data set and compiled aregional magnetic anomaly map at sea level.

As far as aeromagnetic data are concerned,previous surveys in the Southern TyrrhenianSea were performed by the Italian national oilcompany (Agip) in the late 1970’s. Agip ac-quired total field magnetic data at an altitude of4800 ft (1500 m), and published a grid with anaverage spacing of 2.5 km (Agip Spa, 1981).

2. Acquisition and processing

The airborne survey covered the entire Vul-cano Island and partially the western side andthe submerged flanks of Lipari. An optically-pumped magnetometer acquired the total inten-sity of the Earth’s magnetic field at a varyingbarometric height, which provided a minimumterrain clearance of about 50 m above the high-est peaks. Due to the rugged topographic con-ditions, however, an average clearance wasmaintained around 150 m.

The magnetic sensor was towed about 30 mbelow the helicopter. The ground-based equip-ment consisted of a magnetic base stationrecording the Earth magnetic field variationsfor the entire duration of the survey flights. Themagnetic base station was deployed on the Is-land, and recorded data at 1 Hz.

The aeromagnetic survey was conducted at250 m line spacing with tie lines at an averageof 3000 m apart. Figure 1 shows the distributionof the aeromagnetic profiles, as well as the lo-cation of the magnetic base station. The towed-bird magnetometer acquired data at 10 Hz. GPSreceiver was used for precise survey navigation.Recording of flight altitude was assured by ac-quisition of laser altimeter data at 0.3 Hz. Allmagnetic and pseudorange navigation datawere recorded on a PC-based data acquisitionsystem on board the helicopter.

Data were de-spiked and each measurementprofile was visually inspected. The base stationdata were used to remove the diurnal variation

Fig. 1. Distribution of the aeromagnetic profilesover Vulcano Island and the southwestern flank of Li-pari. The symbol 5 indicates the position of the mag-netic base station. In the inset, the location of the sur-vey area in the Southern Tyrrhenian Sea is indicated.

of the Earth’s magnetic field from the aeromag-netic data. The survey was then reduced to acommon geomagnetic epoch using the follow-ing equation:

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Aeromagnetic anomaly images of Vulcano and Southern Lipari Islands (Aeolian Archipelago, Italy)

Fig. 2. Shaded-relief aeromagnetic anomaly map of Vulcano and Southwestern Lipari.

F F F F. .

s bs s

t

bs

t99 7 99 7= + -^ h

where Fs

t is the magnetic field (nT) acquiredduring the survey at time t at point s; Fbs

t is thevalue observed at the same time t at the basestation; F .

bs

99 7 is the value at the base station at1999.7 and F .

s

99 7 is the value at the same point sreduced to 1999.7.

To compute the magnetic anomaly field, datawere corrected using the Italian Geomagnetic

Reference Field (ItGRF) (Molina and De Santis,1987; De Santis et al., 2003). This model, de-spite having the same spatial gradients as IGRF,is more constrained by the secular variation ob-served at L’Aquila geomagnetic observatory.

Crossover analysis between profiles and tielines showed a few leveling errors, which werestatistically adjusted in a similar fashion as de-scribed in Chiappini et al. (2000). A 60 m cellgrid then was produced as shown in fig. 2,

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where we applied shading algorithms to artifi-cially illuminate the anomalies, emphasizingthose that are perpendicular or oblique to the di-rection of illumination.

3. Aeromagnetic anomalies

The magnetic anomaly pattern of Vulcanoand Southeastern Lipari shows both short and

long wavelength anomalies. While the formerare associated with variations of magnetizationof shallow rocks, the latter may likely reflectthe regional character of tectonic features ormay be caused by bodies deeper than the out-cropping volcanic structure. Superimposing themagnetic anomaly grid to the digital elevationmodel of Lipari and Vulcano Islands and thesurrounding bathymetry, facilitates the identifi-cation of magnetic anomaly features in relation

Fig. 3. Aeromagnetic anomaly field superimposed on the shaded-relief image of the digital elevation modeland surrounding bathymetry.

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to possible shallow sources. This is clearly im-aged in fig. 3. For example, in the off shore sec-tor, northwest of Vulcano, a round positiveanomaly is well resolved (VE in fig. 4). Itseems to be associated with an isolated reliefidentified by Gabbianelli et al. (1991) by meansof seismic profiles, and interpreted as eccentricvolcanic edifices, and clearly identified with thedigital elevation model.

The southwestern flank of Vulcano is char-acterized by a high amplitude, NW-SE trendinglinear positive anomaly, which runs approxi-mately parallel to the coast line. This anomalyalso borders the southwestern margin of the«Primordial Vulcano» as shown in fig. 4 and inthe geological map in fig. 5. This truncatedcomposite cone forms the oldest (120 to 100kyr) part of the Island.

Fig. 4. Major geomorphologic and volcanic features identified by the magnetic anomaly field. VE – off shore vol-canic edifice; LF – sequence of lava flows; R1 – Piano Caldera volcanic rim; FS – sub-circular negative magneticanomaly; LC – La Fossa Cone; R2 – La Fossa Caldera volcanic rim; LT – Lentia Complex; VU – Vulcanello.

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Once the primordial volcano collapsed, asubsequent eruption phase which occurred until20 kyr emplaced the Piano Caldera Units (DeAstis et al., 1997). The volcanic rim (indicatedwith R1 in fig. 4) presents a distinct magneticsignature having its highest amplitude values inthe southwestern part. South-east of R1 a re-markable magnetic anomaly feature, labeled LFin fig. 4 and extending in the offshore sector,could be related to a sequence of lava flows notcompletely outcropping. In the north-westernpart of the Island, a high-amplitude positiveanomaly seems to be associated with the LentiaComplex (LT in fig. 4), which is a remnant of alarger volcanic structure formed between 24and 15 kyr (fig. 5) (Barberi et al., 1994).

Observe in the area of La Fossa a circularpositive anomaly overlapping with the volcanic

rim R2 (fig. 4). La Fossa caldera consists ofvolcanic units erupted between 15 and 8 kyr ap-proximately, being therefore a later volcanicphase (De Astis et al., 1997). Within R2, a low-amplitude, short-wavelength negative anomaly(FS) surrounds a magnetic high, associatedwith the edifice of La Fossa cone (LC in fig. 4).La Fossa cone is an active volcanic edifice ex-tending 391 m high, located approximately in acentral position with respect to the caldera.Formed during the last 6 kyr, the cone producedthe last eruption in 1880-1890, as reported byFrazzetta et al. (1983, 1984).

The northernmost structure of the Island iscalled Vulcanello. Indicated in fig. 4 with VU,Vulcanello is a composite lava platform, formedas a new Island in 183 B.C. By means of ash ac-cumulation due to successive eruptions in the 6th

Fig. 5. Geological map of Vulcano Island, modified from De Astis et al. (1997), showing the six volcanic units.

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and 16th centuries A.D., it was later connected tothe main Island. The lava platform is well re-solved by the aeromagnetic image with a posi-tive long-wavelength pattern.

Moreover the region extending between thesouth western side of Lipari and a narrow stripof sea in front of this Island is characterized byextremely low values of the anomaly magneticfield (−800 nT). A possible reason for suchanomaly values could be related to a negativepart of an induced dipolar field whose positive

counterpart would be the high-amplitude anom-aly situated west of it. Alternatively these valuescould be related to remanent magnetization of abody cooled during a reverse-polarity chron.The latter interpretation seems to be far from re-ality because the Aeolian volcanism took placeentirely in the present Brunes chron. It turns outthat an extension of the survey in the remainingpart of Lipari Island to better understand thepresence of this large negative anomaly area isworthwhile.

Fig. 6. Upward continued magnetic anomaly map (1500 m).

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As previously mentioned, existing aeromag-netic data in the study area belong to an old sur-vey conducted in the late 1970’s by Agip (AgipSpa, 1981). The regional character and the differ-ent acquisition altitude of the Agip map do not re-solve the volcanic features we identified in detail.We attempted to compare our anomaly field withthe Agip data set. Observe in fig. 6 the upwardcontinued map to 1500 m. This image clearlyshows long-wavelength features, as well as a re-markable similarity with the Agip map (Barberiet al., 1994).

4. Conclusions

A high-resolution shaded relief total intensi-ty, magnetic anomaly map was obtained after ahelicopter-borne aeromagnetic survey over Vul-cano Island. The anomaly field is characterizedby a wide range of amplitudes and wavelengths.The main stages of volcanic activity in the areaare investigated by the new data set: the Primor-dial Vulcano, the Piano Caldera units older than20 kyr, the Lentia Complex formed between 24and 15 kyr, the Fossa Caldera deposits and vol-canic units emplaced between 15 and 8 kyr, thecurrently active La Fossa cone, and Vulcanello.

The short-wavelength, high-amplitude anom-alies located in the center of «Piano Caldera» andin the center of «La Fossa Caldera» respectivelylie along the SE-NW direction which is the maindirection of the tectonic features of this region,parallel to the «Tindary Letojanni» Fault whichdevelops more to the south in the northern part ofSicily. This orientation coincides with the direc-tion of the migration of volcanism in the Island.Moving from south east of Vulcano to north westthe volcanism becomes younger.

Acknowledgements

This work was supported by the EuropeanCommission, Contract ENV4-CT97-0697 «Elec-tromagnetic and Potential Field Integrated Tomo-graphies Applied to Volcanic Environments -TOMAVE» and by the Austrian Science Founda-

tion, contract P15515-TEC «Improved Model-ling and Interpretation of Complex GeophysicalData Applied to the Eolian Volcanic Province»(COMVOLC). Additional financial support was giv-en by the Geological Survey of Japan, by IstitutoNazionale di Geofisica e Vulcanologia, Dottoratodi Ricerca in Geofisica – University of Bolognaand the Austrian Exchange Service (ÖAD). Wewould further like to thank Maria Teresa Pareschifor providing the compilation of bathymetric da-ta for the Eolian Islands.

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FRAZZETTA, G., L. LA VOLPE and M.F. SHERIDAN (1983):Evolution of the Fossa Cone, Vulcano, J. Volcanol. Ge-otherm. Res., 17, 329-360.

FRAZZETTA, G., P.Y. GILLOT, L. LA VOLPE and M.F. SHERI-DAN (1984): Volcanic hazards of Fossa of Vulcano: datafrom the last 6000 years, Bull. Volcanol., 47, 105-124.

GABBIANELLI, G., C. ROMAGNOLI, P.L. ROSSI, N. CALANCHI,F. LUCCHINI (1991): Submarine morphology and tec-tonics of Vulcano (Aeolian Islands, SoutheasternTyrrhenian Sea), Acta Vulcanol., 1, 135-141.

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(received February 5, 2004;accepted November 10, 2004)

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Aeromagnetic anomaly images of Vulcano and Southern …...1805 Aeromagnetic anomaly images of Vulcano and Southern Lipari Islands (Aeolian Archipelago, Italy) Fig. 2. Shaded-relief