thermal survey of mount etna volcano from space

GEOPHYSICAL RESEARCH LETTERS, VOL.19,NO.7,PAGES 725-728, APRIL3, !992

THERMAL SURVEY OFMOUNT ETNAVOLCANO FROMSPACE A. Bonnevilleand P. Gouze

C.N.R.S. - Centre G6ologique etG•ophysique, Universit6 Montpellier II, France Abstract.Surveys of ground thermal anomaliesand the

ground thermalanomalies mustbe studied in •t th'-eriiaai

monitoring of theirevolutionareof greatimportance in the

infraredwindow (8-14 grn) [Bonnevilleef al., 1985; BonnevilleandKerr, !987].

study of volcanoes.Thermalmonitoring techniques could beusedin conjunction with classicalmonitoringtools(i.e. seismological and deformationnetworks),to give better

There have beenfew studiesof low-ter•pe•ulre phenomena because theonlythermalIR sensors av'ailable to thecivil sector havea low spatialresolution (e.g.1-!0 km

predictions of the onsetof a volcanicevent.In orderto detectand emphasize small anomalies in the surface

forweather satellites). Sincethelaunching of Landsat 5, we

temperature of the ground,we have developeda new methodbasedon the joint useof two satelliteradiometers: theNOAA-AdvancedVery High ResolutionRadiometer (AVHRR)for the qualityof its thermalbandcalibrationand

theLandsat ThematicMapper(TM) for its high spatial resolution. This method is applied to the Mount Ema volcano, Sicily, and revealsseveralthermalanomalies.One anomalyis already known, and is associatedwith the permanently activecratersof the summitzone.The second is a largerarea where an eruptionoccurredone week after

thedataacquisition(10/23/86). The generaltrend of the thermalanomalies leads to recognition of a large, semi-circular intrusivezonecorresponding to the bordersof

havehad at our disposalhighresolutionthermalinfrared

data(imagesizeof 185x185km with a pixel si• of 120x120 m) whichis bettersuited for studying vol•canoes. However,the small amplitudeof the observedthermal anomalies requires highly accurate radiometric

temperatures, whichmustbe determined fromnight:time

datain orderto minimize thedirectsolai'heating. To achievethis g9al, well-calibrated insmmientafion and atmospheric corrections fiaustbe made.Theselattereffects couldbe of the sameorderof magnitude. asthe expec.t•

anoma•i.'es dueto thelargealtitticle variations genera•l• y

presented by volc•oes.in view of thesediffibulties, we

havedev61oped a method thatsimultarieo/as!•, employs two satelliteradiometers: the NOAA-Advanced Very High

the we!l-known Valle del Bore.

Resolution Radiometer (AVHRR), which •ak•sadi.antage

Introduction

of highprecision thermalsensors, andtheL•dsat Thematic

Temperatureis one of the most logical physical parameters to monitor on an active volcano becauseof the

relationship betweenthe volumeof hot intrusivemagma andthe groundsurfacetemperature [e.g. Francis,1979].

However, the large thermalgradientsthat prevailat the soil-airboundary preventa largethermalsignature from appearing at thegroundsurface, exceptin thecaseof strong groundwater convectionbetweenthe magmaticintrusion andsurface, or,of course, duringaneruption. Wherestrong groundwater convectionoccurs,thermal anomaliescan be

Mapper(TM) .witha highspatial resolutiofi..We present the

results fromtheprocessing of datagathe.red overMount Etna (Figure 2) on October,23 1986, one month after a summiteruptionandoneweekbeforea flankeruption. Activityof Mount Etna

MountEtnavolcano hasbeencharacterized in recent historicaltimesby persistent, activity•r0m the summit ,

craters, mosfiy consistingof mild strombolianand

hydromagmafic outgassing, and by flank eruptions,

detected atthesurface andareoftenexpressed byfumarolic emissions (Figure1). Thetimeevolutionof theseanomalies could bea goodindicationof magninmovement towardthe

frequenfiy occurring on its upper-to-middleslopes. FollowingArmientiet al. [1989], the eruptiveactivity observedbetween1971 and 1987 can be describedusing

groundsurface.

the classification of Ritmmnn[1965]. Amongdie flank eruptions, threetypesoferuptiveeventscanbefound:

Thermalmappingcanbe achieved by remotesensing fromanaircraftor froma satellite. Thewavelength of the maximum energyrecordedat the radiometer on-boardthe remote sensing platformdependson the temperature of the

(1) Subterminal effusions, Which consist O[ shallow (1) Subterminal effusions, which consist of shallow

magmatic injections originating fromiheuppermost partof

emitting surface. Thisimpliesthatwe haveto usedifferent themainfeeding conduit. Thelavais hotamifluidand spectral windowsfor studyingphenomenaoccurringat flows quiefiy from shallow fissures .that. ex.tel•d d0mslope. different temperatures. For example,high temperature (2) Lateraleruptions,which originatefro•i radial dike phenomena like lava lakes or lava flows can be studiedin injections thatpropagate upwardto thesurface andproduce

shortwavelenght infraredwindows(1.2-2.5gin) [e.g.

both lava effusionand outgassingactivity. Transitionfrom

Francis andRothery, 1987; Rothery eta!.,1988;Pierietal., 1990] whereas lowtemperature phenomena suchassmall

Copyright 1992 bytheAmerican Geophysical Union.

subterminal to lateralstages oftenoccursdurin•the same eruption. (3) Eccentriceruptions,Whichare of deeporigin and not relatedto themainfeedingconduit.

Fortheperiodof timein whithwe areinterested, Mount

Paper number 92GL00580

Ema had experienceda lateral type empfi0n dinSrig Septemberof 1986, and a transitionalevent between

00t)4-8534/92/92GL-005 80503.00 725

726

BonnevilleandGouze:ThermalSurveyof Mt Etna

,,,

onlyonethermalband(TM6), prevents usfromusingsuch processing.However, this is possible with NOAA meteorological satellites whichhavea radiometer, AVHRR, .:i•crat©r

I"v.flow••',•'•,•

furnerolic

(. field

with two thermalbands(4 and 5). AVHRR data,taken90 minutesbeforethe TM image, at 20:30 local time, showno

cloudsanda smallvapourplumefrom the volcano.First,

VOLCANO ' // •1•,/';/-intrulion • •'0 J Fig. 1. Sketchof thermalexchanges on a volcano.0 is the temperature, A0 is the differencebetweenair temperature and surfacegroundtemperature. To a fn'stapproximation andin normalconditions A0--0;butwith a largeheatsupply (magmaticextrusionor naturalconvectionin a porousor fracturedmedium), A0 becomesstronglypositive.This temperatureanomalycan be detectedby a satelliteinfrared radiometer.

these data are correctedfor geometricaldistortionsand re-sampled,usingthe nearestneighbortechnique,in order to coincidewith the LandsatTM imagepixels.We thenuse a linearcombinationof band4 (10.3 - 11.3 I.tm) andband5

(11.5 -12.5 I.tm) according to the Split WindowAlgorithm to correctfor atmosphericabsorptions[Deschamps and Phulpin,1980;Price,1984].Usingthisprocess, we obtaina realtemperature mapat the AVHRR spatialresolution.

Knowingtherealgroundsurface temperature, 0g•o•a, in eachpixel of the imageandthe brightness temperature, 04, in AVHRR channel4 (10.3 - 11.3gm), we maycompute an atmospheric effectiveabsorptioncoefficientK,• validfor the corresponding LandsatTM spectralwindow (band6= 10.42 - 11.66 I.tm):

04

subterminal andlateraltypesstartingOctober,30 1986.The September1986 eruptionstartedon the 14thandended10 dayslater,comingfromtheNortheast Craterandproducing a small amount of lava, estimated to be 1• m3 in volume. The October 1986 event started on the 30th and ended 4 months later. The eastern and nor,h-eastern flanks of the volcano were involved between the altitudes of 2900 and

2200 m. Duringthateruptionmorethan60x10• m3 of lava were produced.

Methodology

Kat m=Ogrotmd (2) The mostimportantatmospheric effectis dueto adiabaticcooling with altitude which is indicatedto first orderby surfacegroundtemperature.This stronggradient masksthe undergroundthermal contribution.For the TM6 image,thiseffectmay be estimatedby a stat/sticalapproach which consistsof determiningthe correlationbetweenthe absolutepixel temperatureand its altitude, z•x•.. We then determinea regionaladiabaticcoolinggradient:

Our fn'st step in reducing the data was to remove atmosphericeffects.

(1) One atmospheric effectis dueto the absorption of energyat somediscretewavelengths by activecomponents suchas H20 andCO2.Sincethisabsorption depends on wavelength,we can use multispectralalgorithmsfor modelingthe atmospheric effectsand thus compute real-at-ground temperature. The TM radiometer, having

grad=

d0ground

dz

whichis verycloseto thetheoretical one(0.006'C.m4).A 225 km2 digitalterrainmodel,accurateto 10 metersin elevationhas beencompiled.Thus, for eachTM6 pixel at temperature06, the resultingcorrectedsurfacetemperam may be expressedas:

d0ground 0cør = Katm'06 + '•lz ' ZPmm This temperaturecould be consideredas the best estimate of AO(Figure1), thequantitywe haveto dealwith in our survey. Results

The efficiencyof thismethodis shownby therelative

O, ,,,'10 km' -&•e:••

enhancementof thermal anomalies from the uncalibrated

TM image(Figure3) to the final image(Figure4). The mostsignificant resultis the evidencefor five anomalous thermalfieldslabeledA1 throughA5:

IA

AI: In the summitzone,threehigh-amplitude-small-

spatial-extentanomaliescorrespondto the north-east,

central, and south-eastactive cratersof Mount Etna, very

Fig. 2. Simplifiedtopographic mapof MountEtnawith the locationof the TM imageindicatedby the box.

accurately l•ated. The lava flowsextending fromthe Northeast Crater in a north-west direction [September 1986

Bonneville andGouze: Thermal Survey ofMt Etna

727

clouds

Fig.3. Thematic Mapperband6 (TM6) imageof Mount Etnavolcano gathered onOctober23, 22:00localtime.The dataarenot calibratedand are represented by color levels fromdark blue (cold) to pink (warm). Note the noise between linesand the two saturatedpixelsin the centerof

theimagethatcorrespond to activesummitcraters.The pixelsizeis 120x120m.

Fig. 4. Final TM6 image corrected for atmospheric absorptionand adiabaticair cooling with altitude. All the temperatures havebeenreducedto sealevel usinga Digital Terrain Model. Each color level conespondto I'C from dark blue (14.50'C) to pink (27'C). Five positive anomalous zones are evidenced A1, A2, A3, A4 and A5

(see text for explanations).The jagged line representthe edgesof Valle del Bove; heavy lines are roads and fine lines are the altitude contours. TR= Torre del Filosofo;

eruption; ScanBulletin, 1986a]as well as thosefrom the Southeast Crater are identified.The conesare particularly

NEC=Northeast Crater; SEC= SoutheastCrater, BN=Bocca

Nova Crater;PR=PiccoloRifugio;RS=RifugioSapienza.

evident as hot areas around all the craters.

Themeantemperatureanomalyreachesabout3'C with respect to the surroundingarea. The total anomalousheat flow on the summit zone between altitude levels 3000 and

3300m (1 km2) canbe estimated at 200 MW usingan empiricalmethod[Sekioka and Yuhara, 1974]. This heat budget maybe a goodindicatorof the MountEtna activity in its all.

A2: This anomalyis relatedto a lava field extending through thehigherpartof ValledelBove.Previous workby Piefietal. [1990],usingTM bands5 and7 data,hasclearly shownthe thermalsignature of a 1984 lava flow in this zone.

A3: This anomalouszone on the northernedge of Valle

clelBovehas a shortmaximumspatialextension(100200m)to the east,closeto the 2000 m altitudelevel, This zoneexperienced a large fissureactivity [ScanBulletin, 1986b]on October30, 1986, one week after the data acquisition. Thesefissurezonesareknownasbeingareasof eruptive eventscomparable to the Hawaiianrift zones [Kieffer, 1975]. A4:Thisanomaly in thesouthern edgeof ValledelBove

isnotassociated withanyrecent volcanic activity, butit is elatedto knownfissurezoneslike anomaliesA2 and A3.

AS:Thislargeanomaly around RifugioSapienza maybe

thereis a real thermaleffect, or whetherwe are observing slopeeffects,or differentialthermalinertialeffects.If the zones are indeed not thermal anomalies, then we must

explain severalfeatures:Why are the zonesso closely linked with volcano-structural features?Why are there no anomaliesobservable on the westernand northernslopesof the volcano?What has causedthe disappearanceof the anomalouszone in the north detected during 1981 [Bonnevilleet al., 1985] which was strongly linked to

eruptiveactivityin themonthsbeforetheobservations took place?For thesereasons,we assumethat the anomalies really do represent hot-spots, eventhougha minor part of themcouldalwaysbe attributed to surfaceeffects. Theseanomalies,exceptthe summitanomaliesA1, are

onlydetectable afterthealtitudecorrection described above andpresenta minimumamplitudeof 2'C with respectto the environment,which is significantowing to the applied

processing. They correspond, to a fin'stapproximation, to heatflow anomalies of about130Wm'2. The anomalyA3 with its short extension maximum could be considered as a thermal forerunner of the October 30, 1986 event, even

thoughonlya repetitivesurveywouldhavepermittedthe

of thissituation.Note that the lack of vegetation linked torecent lavafields(eruptions of 1983),bigcinder assessment

cones likeMountSilvestri,andnumerous fracturezones.

Forthese lasttwoanomalous zones, wemayaskwhether

within these zones (altitude above 2000 m) allows an

interpretation witha greatdegreeof confidence.

728

BonnevilleandGouze:ThermalSurveyof Mt Ema Conclusion

Francis, P. W., Infra-red techniques forvolcano monitoring andprediction-a review,J.Geol. $oc. Lond., 136, 355-

We presentevidencethat some thermal anomaliesare linked to volcanic activity. However, only a continuous surveyover a long time (3 to 4 years)would prove the efficacy of the method for forecastingvolcaniceventsas well as give bettercluesfor the thermalorigin of the large

and apparentlypermanentanomalies.Our application suggests that significant progress in the methodology will be achievedwhen voleanologists have at their disposal frequentnight-timepassesover volcanoeswith a wide rangeof spectralwindows.This couldbe a realityin the near future due to the US Earth ObservationSatellite(EOS)

program[Mouginis-Market al., 1991], and perhapsat a later date, when the EuropeanPolar Platform becomes operational.

359, 1979.

Francis,P. W., andD. A. Rothery,Using LandsatThematic

Mapper to detect and monitor active volcanoes:an

example fromLascarvolcano,northern Chile,Geology, 15, 614-617, 1987.

Kieffer,G., Surl'existence d'unerift-zone•t l'Ema,Sicfie, C•R. Acad. $ci. Paris, D, 280, 263-266, 1975.

Mouginis-Mark,P., S. Rowland, P. W. Francis,T.

Friedman,I. Gradie,S. Self, L. Wilson,J. Crisp,L. Glaze, K. Jones, A. Kahle, D. Pieri, H. Zebker, A. Kreuger,L. Waiter, C. Wood,W. Rose,J. AdamsandR. Wolff, Analysis of active volcanoesfrom the Earth Observing System, Remote $ens. Environ. 36, 1-!2 1991.

Pieri, D.C.,

Acknowledgements. This work has been supported by the C.N.R.S. and the C.N.E.S. (A.T.P. T•l•dOectionSpatiale 1985). The authorswish to thank Y. Kerr, G. Macedonio and J. VandeMelbroukfor their help in obtainingthe data; P. Filmer, P. Francis,D. Pieri andone anonymous reviewer for their careful comments.

L. S. Glaze and M. J. Abrams, Thermal

radianceobservations of an activelava flow duringthe June 1984 eruptionof Mount Ema, Geology,18, 10181022, 1990.

Price,J. C., Land surfacetemperaturemeasurements from the split window channelsof the NOAA7 Advanced Very High ResolutionRadiometer,J. Geophys. Res.,89, 7231-7237, 1984.

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Alain BOnneville and Philippe Gouze, C.N.R.S. - Centre G•ologiqueet Gertphysique, Universit•Montpellierii, PlaceE.Bataillon,34095MontpellierC•dex05, France. (ReceivedDecember 9, 1991;

acceptedFebruary12, 1992.)