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Thermal survey of Mount Etna Volcano fromspace

ARTICLE in GEOPHYSICAL RESEARCH LETTERS APRIL 1992

Impact Factor: 4.2 DOI: 10.1029/92GL00580

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Pacific Northwest National Laboratory

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GEOPHYSICALESEARCHETTERS,OL.19,NO.7, PAGES25-728,PRIL , 992

THERMAL URVEY FMOUNT TNA OLCANOROM PACE

A. Bonneville and P. Gouze

C.N.R.S.Centre6ologiquetGophysique,niversit6ontpellierI, France

Abstract.Surveys of ground thermal anomaliesand the

monitoringf theirevolution re of great mportancen the

study f volcanoes. hermalmonitoringechniquesould

beusedn conjunction ith classicalmonitoringools i.e.

seismologicalnd deformationnetworks), o give better

predictionsf the onsetof a volcanic vent. n order o

detect and emphasize small anomalies in the surface

temperaturef the ground,we have developed new

method asedon the oint use of two satellite adiometers:

the NOAA-AdvancedVery High ResolutionRadiometer

(AVHRR) or the qualityof its thermalbandcalibration nd

theLandsat hematicMapper TM) for its high spatial

resolution. his method is applied to the Mount Ema

volcano, icily, and revealsseveral hermalanomalies.One

anomalys already known, and is associatedwith the

permanentlyctivecraters f the summit one.The second

is a largerarea where an eruptionoccurredone week after

thedata acquisition 10/23/86). The general rend of the

thermalanomalies eads to recognition of a large,

semi-circularntrusive onecorrespondingo the borders f

the we l-known Valle del Bore.

Introduction

Temperatures one of the most logical physical

parameterso monitor on an active volcano becauseof the

relationshipetween he volumeof hot intrusivemagma

and he groundsurface emperaturee.g. Francis,1979].

However,he large thermalgradientshat prevailat the

soil-air oundary revent large hermalsignaturerom

appearingt theground urface, xceptn thecase f strong

groundwateronvection etween he magmatic ntrusion

and urface,r, of course, uring n eruption.Wherestrong

groundwateronvectionoccurs, hermal anomaliescan be

detectedt hesurfacendareoften xpressedy umarolic

emissionsFigure1). The imeevolution f these nomalies

couldea good ndication f magninmovementoward he

ground urface.

Thermalmapping an be achieved y remote ensing

from naircraft r froma satellite. he wavelengthf the

maximumnergy ecorded t the radiometer n-boardhe

remoteensing latformdepends n the temperaturef the

emittingurface. his mplieshatwe have o usedifferent

spectral indows or studyingphenomena ccurring t

differentemperatures.or example,high temperature

phenomenaike lava lakes or lava flows can be studied n

short avelenghtnfraredwindows 1.2-2.5 gin) [e.g.

Francisnd othery,987; otheryta .,1988; ieri tal.,

1990] hereasow temperaturehenomenauch ssmall

Copyright992y heAmericaneophysicalnion.

Paperumber2GL00580

00t)4-8534/92/92GL-0050503.00

groundhermal nomaliesustbe studiedn t th-eriiaai

infraredwindow (8-14 grn) [Bonneville f al., 1985;

Bonneville ndKerr, 987].

There have been few studies f low-terpeulre

phenomenaecauseheonly hermalR sensorsv'ailableo

thecivil sector avea low spatialesolutione.g.1- 0 km

forweatheratellites).incehe aunchingf Landsat, we

havehad at our disposal igh resolutionhermal nfrared

data image izeof 185x185 m with a pixel si of

120x120 ) whichs better uitedor studyingolcanoes

However, he small amplitude f the observedhermal

anomalies requires highly accurate radiometric

temperatures,hichmust e determinedromnight:time

data n ordero minimizehe direct olai' eating.o

achieve his g9al, well-calibratednsmmientafionnd

atmosphericorrectionsiaust e made.Theseattereffects

could e of the same rder f magnitude.s he expec.t

anomai.'esue o the arge ltitticleariationseneral

presentedy volcoes. n view of thesediffibulties, e

have ev61opedmethodhat imultarieo/as ,mployswo

satelliteadiometers:he NOAA-Advancedery High

ResolutionadiometerAVHRR), hichaks di.antag

of highprecisionhermal ensors,nd heLdsat Thematic

MapperTM) .with highspatialesolutiofi..Weresenthe

resultsrom heprocessingf data athe.redverMount

Etna (Figure 2) on October,23 1986, one month after a

summit ruption ndoneweekbeforea flankeruption.

Activity of Mount Etna

MountEtnavolcano asbeencharacterizedn recent

historicalimesby persistent,ctivity r0m the summit

craters, mosfiy consistingof mild strombolianand

hydromagmaficutgassing,nd by flank eruptions,

frequenfiy occurring on its upper-to-middleslopes.

FollowingArmientiet al. [1989], the eruptiveactivity

observed etween1971 and 1987 can be described sing

the classificationf Ritmmnn1965]. Amongdie flank

eruptions,hreeypes f eruptive vents anbe ound:

(1) Subterminal effusions, Which consist O[ shallow

(1) Subterminal effusions, which consist of shallow

magmaticnjectionsriginatingrom heuppermostartof

themaineedingonduit.he ava s hotami luidand

flowsuiefiyromhallowissuresthat.x.teld0mslope.

(2) Lateraleruptions, hich originate roi radial dike

injectionshatpropagatepwardo thesurfacendproduce

both ava effusionand outgassing ctivity. Transition rom

subterminalo lateralstages ftenoccurs urin the same

eruption.

(3) Eccentric ruptions,Whichare of deep origin and not

related o themain eeding onduit.

For heperiod f time n whithwe are nterested,ount

Ema had experienced lateral type empfi0n dinSrig

Septemberof 1986, and a transitionalevent between

725

https://www.researchgate.net/publication/248747185_Infrared_techniques_for_volcano_monitoring_and_prediction--a_review?el=1_x_8&enrichId=rgreq-8af148f3-6781-4508-aac0-f20b1571461b&enrichSource=Y292ZXJQYWdlOzI0ODgxMTMzNztBUzo5ODU0MDgyMjk5MDg1NEAxNDAwNTA1MzcxMTgxhttps://www.researchgate.net/publication/248747185_Infrared_techniques_for_volcano_monitoring_and_prediction--a_review?el=1_x_8&enrichId=rgreq-8af148f3-6781-4508-aac0-f20b1571461b&enrichSource=Y292ZXJQYWdlOzI0ODgxMTMzNztBUzo5ODU0MDgyMjk5MDg1NEAxNDAwNTA1MzcxMTgxhttps://www.researchgate.net/publication/248747185_Infrared_techniques_for_volcano_monitoring_and_prediction--a_review?el=1_x_8&enrichId=rgreq-8af148f3-6781-4508-aac0-f20b1571461b&enrichSource=Y292ZXJQYWdlOzI0ODgxMTMzNztBUzo5ODU0MDgyMjk5MDg1NEAxNDAwNTA1MzcxMTgxhttps://www.researchgate.net/publication/248747185_Infrared_techniques_for_volcano_monitoring_and_prediction--a_review?el=1_x_8&enrichId=rgreq-8af148f3-6781-4508-aac0-f20b1571461b&enrichSource=Y292ZXJQYWdlOzI0ODgxMTMzNztBUzo5ODU0MDgyMjk5MDg1NEAxNDAwNTA1MzcxMTgx

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726 Bonneville ndGouze: hermalSurvey f Mt Etna

,,,

.:icratr furnerolic

I"v.low',', (. field

VOLCANO// 1,/';/ intrulion'0 J

Fig. 1. Sketchof thermal xchangesn a volcano. is the

temperature, 0 is the difference etween ir temperature

and surfaceground emperature. o a fn'st approximation

and n normalconditions 0--0;butwith a largeheatsupply

(magmatic xtrusion r naturalconvectionn a porousor

fracturedmedium), A0 becomes tronglypositive.This

temperature nomalycan be detected y a satellite nfrared

radiometer.

subterminalnd ateral ypesstarting ctober, 0 1986.The

September 986 eruption tarted n the 14thandended10

days ater,coming rom heNortheast raterandproducing

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. During hateruptionmore han60x10 m3 of lava

were produced.

Methodology

onlyone hermal and TM6), prevents s romusing uch

processing.However, this is possible with NOAA

meteorologicalatellites hichhave radiometer,VHRR,

with two thermalbands 4 and 5). AVHRR data, taken90

minutesbefore he TM image, at 20:30 local time, showno

clouds nd a smallvapourplume rom the volcano. irst,

these data are corrected or geometrical distortionsand

re-sampled, sing he nearest eighbor echnique,n order

to coincidewith the LandsatTM image pixels.We thenuse

a linearcombination f band4 (10.3 - 11.3 I. m) andband

(11.5 -12.5 I. m) accordingo the Split WindowAlgorithm

to correct or atmospheric bsorptionsDeschampsnd

Phulpin, 980;Price,1984].Using hisprocess, e obtain

real emperaturemapat the AVHRR spatial esolution.

Knowinghe ealground urfaceemperature,goa,n

eachpixel of the mageand the brightnessemperature,4,

in AVHRR channel (10.3 - 11.3 gm), we may compute n

atmosphericffectiveabsorption oefficientK, valid for

the correspondingandsatTM spectralwindow (band6=

10.42 - 11.66 I. m):

04

Kat Ogrotmd

(2) The most mportant tmosphericffect s due o

adiabaticcooling with altitude which is indicated o first

order by surfaceground emperature. his stronggradient

masks he underground hermal contribution.For the TM6

image, his effectmay be estimated y a stat/stical pproach

which consistsof determining he correlationbetween he

absolutepixel temperature nd its altitude, zx.. We then

determine regionaladiabaticcoolinggradient:

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

atmospheric ffects.

(1) One atmosphericffect s due o the absorptionf

energy t somediscretewavelengthsy active omponents

suchas H20 and CO2.Since his absorptionependsn

wavelength,we can use multispectral lgorithmsor

modeling he atmosphericffectsand thus compute

real-at-groundemperature.he TM radiometer,aving

d0ground

grad dz

which s veryclose o the heoreticalne 0.006 C.m4).A

225 km2 digital errainmodel,accurateo 10 metersn

elevationhas beencompiled.Thus, for eachTM6 pixel at

temperature 6, the resultingcorrectedsurface emperam

may be expressed s:

O,,10m'&e:

A

Fig. 2. Simplified opographic apof MountEtnawith the

locationof the TM image ndicatedby the box.

d0ground

0crKatm' 6 + 'lz ' ZPmm

This temperature ould be considered s the best

estimate f AO Figure1), thequantitywe have o dealwith

in our survey.

Results

The efficiency f thismethods shown y therelative

enhancementof thermal anomalies from the uncalibrated

TM image Figure ) to the final image Figure ). The

mostsignificantesult s the evidenceor five anomalous

thermal ields abeledA1 throughA5:

AI: In the summit one, hreehigh-amplitude-sm

spatial-extent nomaliescorrespond o the north-east,

central, and south-east ctive cratersof Mount Etna, very

accuratelyated. The lava flowsextendingrom he

Northeastratern a north-westirectionSeptember986

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BonnevillendGouze:hermalurveyfMt Etna

727

clouds

Fig. . Thematic apper and (TM6) image f Mount

Etna olcano atherednOctober 3, 22:00 ocal ime.The

data re not calibrated nd are represented y color evels

fromdark blue (cold) to pink (warm). Note the noise

betweenines and the two saturated ixels n the centerof

the mage hat correspondo activesummit raters. he

pixel ize s 120x120m.

eruption;canBulletin, 1986a]as well as those rom the

Southeastrater are identified. The conesare particularly

evident as hot areas around all the craters.

The mean emperature nomaly eaches bout3'C with

respecto the surrounding rea. The total anomalous eat

flow on the summit zone between altitude levels 3000 and

3300m (1 km ) canbe estimatedt 200 MW using n

empiricalmethod[Sekioka and Yuhara, 1974].This heat

budget aybe a good ndicator f the MountEtna activity

in its all.

A2: This anomaly s related o a lava field extending

throughhehigher artof ValledelBove.Previous orkby

Piefi t al. [1990],using M bands and7 data,hasclearly

shown he thermalsignature f a 1984 ava flow in this

zone.

A3: This anomalous one on the northern dge of Valle

clelBovehas a shortmaximumspatialextension100-

200m) o the east,close o the 2000 m altitude evel, This

zone xperienced large fissureactivity [Scan Bulletin,

1986b]on October30, 1986, one week after the data

acquisition.hese issure ones reknownasbeingareas f

eruptive ventscomparableo the Hawaiianrift zones

[Kieffer,975].

A4:Thisanomalyn thesoutherndge f ValledelBove

isnot ssociatedithany ecentolcanicctivity,ut t is

elated o known issurezones ike anomaliesA2 and A3.

AS:This arge nomalyround ifugio apienza aybe

linkedorecentava ieldseruptionsf 1983), igcinder

conesike MountSilvestri, ndnumerousracture ones.

For heseast woanomalousones, emayaskwhether

Fig. 4. Final TM6 image corrected for atmospheric

absorption nd adiabaticair cooling with altitude. All the

temperaturesavebeen educed o sea evel usinga Digital

Terrain Model. Each color level conespond o 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). he jagged line represent he

edgesof Valle del Bove; heavy lines are roads and fine

lines are the altitude contours. TR= Torre del Filosofo;

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

Nova Crater;PR=Piccolo ifugio;RS=RifugioSapienza.

there s a real thermaleffect, or whetherwe are observing

slopeeffects,or differential hermal nertial effects. f the

zones are indeed not thermal anomalies, then we must

explain several eatures:Why are the zonesso closely

linked with volcano-structuraleatures?Why are there no

anomalies bservable n the westernand northern lopesof

the volcano?What has caused he disappearance f the

anomalous zone in the north detected during 1981

[Bonnevilleet al., 1985] which was strongly inked to

eruptive ctivityn themonths efore heobservationsook

place?For these easons, e assume hat the anomalies

really do represent ot-spots,ven hougha minor part of

themcouldalwaysbe attributedo surface ffects.

Theseanomalies, xcept he summit anomaliesA1, are

onlydetectablefter healtitude orrectionescribedbove

andpresent minimum mplitude f 2'C with respecto

the environment,which s significantowing to the applied

processing.hey correspond,o a fin'st pproximation,o

heat low anomaliesf about 30Wm 2. The anomaly 3

with its short extension maximum could be considered as a

thermal forerunner of the October 30, 1986 event, even

though nly a repetitive urveywouldhavepermittedhe

assessmentf thissituation. ote that the ack of vegetation

within these zones (altitude above 2000 m) allows an

interpretationitha great egree f confidence.

https://www.researchgate.net/publication/248786643_Heat_Flux_Estimation_in_Geothermal_Areas_Based_on_the_Heat_Balance_of_the_Ground_Surface?el=1_x_8&enrichId=rgreq-8af148f3-6781-4508-aac0-f20b1571461b&enrichSource=Y292ZXJQYWdlOzI0ODgxMTMzNztBUzo5ODU0MDgyMjk5MDg1NEAxNDAwNTA1MzcxMTgxhttps://www.researchgate.net/publication/248786643_Heat_Flux_Estimation_in_Geothermal_Areas_Based_on_the_Heat_Balance_of_the_Ground_Surface?el=1_x_8&enrichId=rgreq-8af148f3-6781-4508-aac0-f20b1571461b&enrichSource=Y292ZXJQYWdlOzI0ODgxMTMzNztBUzo5ODU0MDgyMjk5MDg1NEAxNDAwNTA1MzcxMTgx

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728 Bonneville ndGouze: hermal urvey f Mt Ema

Conclusion

We presentevidence hat some thermal anomalies re

linked to volcanic activity. However, only a continuous

surveyover a long time (3 to 4 years)would prove the

efficacy of the method for forecasting olcaniceventsas

well as give betterclues or the thermalorigin of the large

and apparentlypermanentanomalies.Our application

suggestshat significant rogressn the methodologyill

be achievedwhen voleanologists ave at their disposal

frequentnight-timepasses ver volcanoes ith a wide

rangeof spectralwindows.This couldbe a reality n the

near future due to the US Earth ObservationSatellite EOS)

program Mouginis-Market al., 1991], and perhaps t a

later date, when the EuropeanPolar Platform becomes

operational.

Acknowledgements.his work has been supported y the

C.N.R.S. and the C.N.E.S. (A.T.P. TldOection Spatiale

1985). The authorswish to thank Y. Kerr, G. Macedonio

and J. VandeMelbrouk or their help n obtaining he data;

P. Filmer, P. Francis,D. Pieri andone anonymouseviewer

for their careful comments.

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Alain Onnevillend hilippeouze,.N.R.S.Centre

Gologique t Gertphysique,niversitMontpellier i,

PlaceE.Bataillon, 4095MontpellierCdex 05, France.

(Received December 9, 1991;

accepted ebruary12, 1992.)