Bollettino di Geofisica Teorica ed Applicata Vol. 57, n. 1, pp. 31-42; March 2016

DOI 10.4430/bgta0172

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Hyperspectral images to monitor oil spills in the River Po

C. PietraPertosa1, a. sPisni2, V. PanCioli3, a. PaVan4, P. sterzai4, P. Paganini4, M. VelliCo4, a. Monni3 and F. Coren4

1 CNR-IMAA, Istituto di Metodologie per l’Analisi Ambientale, Tito-Scalo (PZ), Italy2 Servizio Idro-Meteo-Clima, ARPA Emilia-Romagna, Bologna, Italy3 Agenzia Regionale di Protezione Civile, Regione Emilia Romagna, Bologna, Italy4 Istituto Nazionale di Oceanografia e di Geofisica Sperimentale (OGS), Trieste, Italy

(Received: June 24, 2015; accepted: January 26, 2016)

ABSTRACT Knowledge of an oil spill’s extent and its quantification are fundamental to limitdamageandassess impacts.Remote sensingpermits theobservationof large areasinashorttime,tolocateandquantifythephenomenon.WepresentthecasestudyoftheRiverLambro,whereanoilspilloccurredonFebruary23,2010andthenflowedintotheRiverPo.TheAgenziadiProtezioneCiviledellaRegioneEmilia-Romagnaquicklycommissionedtwoaerialsurveysoverthepollutedarea,performedbyIstitutoNazionalediOceanografiaediGeofisicaSperimentale (OGS)withahyperspectralsensor,AISAEagle1K,inordertoobtainqualitativeandquantitativeassessmentofthespilledsubstancesandtosupportrapiddecision-makingwithreal-timemonitoring.ThemethodusedaSpectralAngleMapper(SAM)classificationtolocatethepollution.Resultsshowedasuccessfulapplicabilityintheproductionofthepollutionmapusedforthecontainmentphase.

Key words:oilspills,remotesensing,hyperspectralimagery,SpectralAngleMapper(SAM),oilthickness,RapidResponseSystems.

© 2016 – OGS

1. Introduction

TheRiverLambro’soilspillhappenedduringthenightbetweenFebruary22andFebruary23,2010;itwasdischargedfromLombardaPetrolioilstoragefacilityinVillasanta(Monza,Italy).ItreachedtheRiverPoonFebruary24,2010.Theestimatedquantityofoilspilledwas2,600tons:1,800tonsofheatingoilanddieselfuel,lighterthanwater,and800tonsoffueloil,heavierthanwater.About300tonswererecoveredfromcontainmenttanksintheLombardaPetroliyard.AnotherpartoftheoilspillreachedthesewagetreatmentplantofBrianzAcque,whichretainedaquantityofapproximately1,250tonsofmaterial.Aquantityof1,050tonsreachedtheRiverLambro.Thankstobarriers,another200tonswerecollectedbeforetheLambromettheRiverPo.AlongtheRiverPo,containmentactionsallowedtherecoveryof450tonsofmaterialatthedamofIsolaSerafini(Po).Unspecifiedquantitiesoftheremaining400tonsevaporatedorweredepositedon the sidesof the river (DipartimentodellaProtezioneCivile, 2010).Thedamagewasveryseriousfortheriverecosystem,requiringurgentremediationandrecoverymeasures.Duringtheemergencyphase,AgenziadiProtezioneCiviledellaRegioneEmilia-Romagnasetupanactionplan.TworemotesensingaerialsurveyswereperformedbyIstitutoNazionaledi

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OceanografiaediGeofisicaSperimentale(OGS),usingitsownAISAEagle1KAirborneImagingSpectrometerforApplications,tosupportthemeasuresusedtodealwiththeemergency(Dondiet al.,2010),mappingtheoilalongtheriverflow.TheflightstookplaceonFebruary27andMarch2,2010.ThecoverageoftheimagesacquiredduringthefirstsurveyfollowstheRiverPoflowfromitsconfluencewiththeRiverLambro(PiacenzaProvince)tothetownofCorbola(RovigoProvince);thesecondsurveygoesfromCorbolatothePodelta.Duetotheemergencysituation,itwasnotpossibletoplantheflightsunderidealweatherconditions.Inthispaperwediscussthe resultsobtained, analyzing theFebruary27data set,because itwasacquiredunderbetterconditions.

2. Background

Oil spill detectiononwater canbe quite a difficult task, due toweather conditions,watermovements, theextentof thepollutedarea,andoilspectral response.Oilscanbebeneath thewatersurfaceoremulsifiedwithit.

Mostoilsquicklyspreadhorizontallyontopofthewater.Thisthinsurfaceiscalledaslick.Astimepasses,theslickbecomesthinner,formingalayercalledasheen,whichreflectsrainbowcolours.Floatingonwater,anoilslickspreadsrapidly:theheavyparttendstoformsediment,theverylightportiontendstoevaporateinafewdays.Dependingontheturbulenceofthewater,theoil’slightportionformsdropletsthatcanhaveadiameterintherangeof5μmtoafewhundredμm(Korotenkoet al.,2002).Duringmovementanddispersionoftheoilslick,dropletscancoalesce,combiningthisphenomenonwithevaporationanddegradation;thethicknessoftheslickcaninthiswaybereducedtoafewtensofμm.Naturaldispersionofoilisaverycomplexprocess:theoilslickbreaksinashowerof“oil”dropletsthatdiffusedownwardsandthatsubsequentlycanfloatbacktothesurfacetocoalesceorremaindispersedinwater(Korotenkoet al.,2002).Thethicknessoftheoilspilldependsonthechemicalandphysicalpropertiesofthepollutant.

Surfacetension,specificgravity,andviscositydeterminehowtheoildiffusesintowaterorintotheatmosphere.Lowsurfacetensionwillspreadoilmoreeasily,highspecificgravitywillformtarballsonthebottomofthewaterbody.Oilwithhighviscositywilltendtostayinoneplace(EPA,1999).

Inwaterenvironments,oil’snaturaldegradationcanoccurinmanyways,throughphysical,chemical,andbiologicalprocesses.Themostcommonarethefollowing:

• weathering:chemicalandphysicalreactionsbreakdownoilchainsthatbecomeheavierthanwater.Thesedropletscanbedispersedbywavesorformthinfilms;

• evaporation: this occurs when the more volatile substances of oil become vapours andleavethewatersurface.Thisistypicalofrefinedproductssuchaskeroseneandgasoline,becausetheycontainahighpercentageofverytoxicandflammablecomponentsthatusuallyevaporateinafewhours.Theheavierpartoftheoilmaymovedowntothewatercolumnandthenbeweathered;

• oxidation:thisoccursattheedgeoftheslicks,whereoilinteractswithwaterandoxygentoproducewater-solublecompounds;

• biodegradation:whenbacteriafeedonhydrocarbons.Thisprocessisstrictlydependentonwateroxygenationandontheecosystem.Thisbiodegradationprocessisrapidandremoves

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most of the hydrocarbons (Gearing et al., 1980). However, about 20-30% of branchedalkanes,cycloalkanes,andaromaticsmayremainformorethanoneyear.Thesolublepart(mostlyaromatics)canbeabsorbedbyundergroundwater(Eganhouseet al.,1996);

• emulsification: thisprocess formsamixtureofoildropletsandwater. It canbewater inoiloroilinwater.Inthefirstcase,theoilslickisthickandviscousandcanremainintheenvironmentforalongtime.Inthesecondcase,oildisappearsfromthesurfacebutisstillpresentatthebottomofit,givingtheillusionthattheissueisresolved.

Aquaticenvironmentshavecomplexinteractionsbetweenplantandanimalspecies.Damagetotheenvironmenthasanimpactonthefoodchainuptohumans.Inopenwater,fishesmaymoveawayfromthepollutedareaandnotbehurt,butincoastalareas,inlakes,orinshallowwater,oilmayhaveanimpactonvegetation,benthicbiota,andotheranimals,suchasreptiles,fishes,amphibians,andmammals.Alongariver,theoiltendstodepositnearriversidesandsandbanks.Itcanbetrappedbyvegetationsuchascanethicketorshrubs(EPA,1999).

Usuallyoilspillsaremorecommoninsaltwaterbodiesorcoastalareas(marineenvironments)wherehugetankersarepresent.Inthiskindofenvironment,oilcanbeseenusingactiveremotesensingasSyntheticApertureRadar(SAR)data,abletodetectthedifferencesinwavemotion.The fluvial/lake bodies (freshwater environments) have a different dynamic compared to themarineone; theanalysisphase forfluvial/lakebodies ismoredifficultbecauseof the smallerwaterextension,thewavemotionabsence,thecontinuouswaterflows,andthedifferentdensityofwater(seawaterisdenserthantheriverwaterbecauseofsaltconcentration).

Oilspillscanalsotakeplaceonland.Thisiscommoninoilfields,refineries,storagefacilities,oralongpipelines.Onthesoilsurface,thespillappearsasablackpatch,butitcanalsopenetratethesoilprofile.

Pollutant identificationandmonitoringcanbeperformedbyusingdifferentremotesensinginstruments:ultraviolet,visibleandinfraredsensors,microwaveradiometers,laserfluorosensors,andlaser-acousticthicknesssensors(Jhaet al.,2008).Actually,thelaserfluorosensoristheonlyinstrumentthatcanpositivelydetectdifferenttypesofoilonmostbackgrounds,butithasthedisadvantageofalargesize,andhighweightandcost.Forthesereasons,itisusuallylesslikelytobepurchased(FingasandBrown,2000).Meanwhile,handlaser-acousticinstrumentsarebeingdevelopedbecausethisistheonlytechnologyabletomeasureabsoluteoilthickness(FingasandBrown,2000).

Airborne oil tracking remains the most common method (Fingas and Brown, 2000) forstudying oil spill evolution.TheBonnAgreement established in northernEuropean countriestriestodefineacrisisprotocolincaseofanoilspill.Itsetstheguidelinesforaerialsurveillance(BonnAgreement,2007).

Inthevisiblespectrum,oilhasahigherresponsethanwater,butalsohasnodistinctivefeatures(Brownet al.,1996).Sheenappearsasabrightsurface,reflectinglightoverawidespectralregion.

False alarms can be detected due to sun glint,wind sheen, and biogenicmaterial such asweeds.Evenwiththoselimitations,visiblecamerasarewidelyusedtomonitorandtrackoilspillsbecausetheyarecheaperthanotherinstruments.

Thehyperspectralpassivetechniquefurnishesdetailedspectral/spatialsignaturesfordifferentmaterials,thankstothecombinationofhighspatialandspectralresolutions(Salemet al.,2005).Thiscanbeveryuseful forGeographical InformationSystems (GISs)where techniciansmaymonitortheevolutionofthedisasterphenomena.

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Remotesensingdatacanbeprocessedindifferentways,suchasmultispectralclassificationsupported by in situ data to calibrate oil thickness (Svejkovsky et al., 2012), or computationof ratiosandresults related to laboratory testsonoil thickness (Lennonet al.,2003).Remotesensingacquisitionsshould,infact,besupportedbygroundspectroscopicmeasurementsoftheslickfromonboardaboat(Lennonet al.,2006).Thisapproachcouldbepossibleinexperimentalsites,butwhenarapidresponseisneededitcouldbedifficulttoperform,especiallywhenthespillisnotwideandisconfinedalongariver,inwhichcasemostoftheeffortsarededicatedtooilremoval.

Thegoaloftheseproceduresistoidentifytheoilspillandtoprovideinformationaboutthelocationandextentofthepollutionphenomenon,inordertoplansubsequentoperations.Sincetheoperationaltargetwastodefineapictureofthepollutionspreadacrosstheriverflow,itwascrucial to adopt amethodology thatdidnot require a time-consumingprocedure.The remotesensingacquisitionshadtoprovideinformationintimetoensuretheeffectivenessofcontainmentmeasures.

Under these conditions, the choice was to adopt a methodology based on multispectralclassification applied to radiance data, following the approach of Filizzola et al. (2002); thismethodologymaps surfacematerials using hyperspectral radiance on airborne sensor imagesandconsidersalimitedsetoffieldobservations.Noatmospheric,topographic,orilluminationcorrectionswereperformedonthedata.

3. Data acquisition

Data were acquired by a hyperspectral sensor mounted on a Beechcraft Sundowner C23aircraft.ThehyperspectralsystemAirborneImagingSpectrometer(AISA)Eagle1K(developedbySpectralImagingLtd,Specim,Finland)iscomposedofapush-broomsensorabletocollectupto244bandsintheVisibleNearInfrared(VNIR)spectrum,adata-acquisitionunitinaPC,aGPSreceiver,andaninertialnavigationsystem.

TheacquisitionwasperformedonFebruary27,2010,theaverageflightaltitudewas1,500mabovegroundlevel,swathwidthwas1kmandthepixelresolutionwas1m;16datastripswerecollected,startingat11:00andfinishingat15:08GMT.Twenty-fourhyperspectralbandswereacquiredforeverystrip, inthecontinuousVNIRspectralrange,from400to970nm,withanaveragebandwidthof24.Theacquiredhyperspectraldatasetwascorrectedtospectralradiancevalues(mW/cm2·sr·nm)andgeo-referencedusingtheHyper-SpectralProcessor(HSP)softwarepackage,developedbyOGS.Theamountofacquiredandprocessedrawdatawas15Gb.

Adigitalcamera(CanonEOS1DsMarkIII),adaptedforaerialacquisitions,wasthenusedtoobtainorthophotos.ItwasconnectedtotheGPSnavigationsystem,inordertogettheshuttertimeandtheattitudeoftheaircraft,foreachsingleframeacquired. 928 photographical frames were.928 photographical frames were928photographicalframeswereacquiredsimultaneouslywiththehyperspectraldata,withagroundresolutionof0.20mperpixel.Thesedataweremainlyusedtoassistvisualinterpretationofphenomena.

Duringtheflight,theheadofthespillreachedPontelagoscuro(FE),whileahugeslickwasstillpresent,justbeyondPiacenzaandnearSanNazzaro(PC).ThePohadariverflowof1370m3/satPiacenzaand1870m3/satPontelagoscuro.Theriverflowvelocitywasapproximately5-6km/h.

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Fig.1showstheflightpathoftheacquisitiononFebruary27.Thephenomenonwasalsomonitoredthroughaseriesofin situmeasurementsalongthecourse

of theRiver Po and theRiver Lambro.On February 27, some chemicalmeasurementsweredone,thesamplesgatheredatthewatersurface,at-1,and-2m.Table1showsthepresenceofhydrocarbonsinthewater;sampleswerecollectedinthreelocationsfromwesttoeast,atPolesineParmense(PR),TorricelladiSissa(PR)andBoretto(RE).Nomeasurementsofoilslickthicknessweredone.Unfortunately,thesampleswerecollectedmainlyfordrinkingwateranalysis,sotheycouldbeusedonlyasareferenceforthepresenceofthepollutants.Anidealsamplingmethodusefulforremotesensinganalysisshouldhavemeasuredthesurfacequantityofoilandestimatedthethickness.Atthesametime,sincetheriverflowedatthespeedof5km/h,thefieldsurveyshouldhavetakenplaceatthetimeofthepassageoftheplane.Theseconditionswerenotviabledue to the ongoing emergencymeasures that were focused primarily on oil containment, oilremoval,andcitizensafety.

Fig.1-February27,2010flightpath:blackcorrespondstotheriver,magentaandgreentotheflightpath.

Table1-RiverPowatersamplescollectedduringtheFebruary27fieldstudy.

TIME LOCATION DEPTH DISSOLVED HC (mg/L) TOTAL HC (mg/L)

7.00 Boretto - Pontile Giudecca 0 m 7.80 11.5

7.00 Boretto - Pontile Giudecca - 1 m 0.06 0.1

7.00 Boretto - Pontile Giudecca - 2 m 0.07 0.11

9.10 Polesine P.se - pontile attracco 0 m 0.29 0.63

9.10 Polesine P.se - pontile attracco - 2 m 0.10 0.35

9.50 Torricella di Sissa - pontile attracco 0 m 0.11 0.49

9.50 Torricella di Sissa - pontile attracco - 2 m 0.03 0.03

10.30 Torricella di Sissa - pontile attracco 0 m 0.15 0.52

13.30 Boretto - Pontile Giudecca 0 m 2.7 4.8

13.30 Boretto - Pontile Giudecca - 1 m 0.08 0.15

4. Classification: methodology and results

Tomaptheoilontheriversurfaceusinghyperspectraldata,it isnecessarytocomparethespectralsignaturebetweenacquireddataandlaboratorydataorbetweenacquireddataandground-basedmeasurements.Thisprocedurerequiresdatathatiscorrectedforatmosphericinterference,illumination,andtopographiceffects.Typically,thetraditionalradiometricmethodsarebasedon

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standardatmosphericmodelsorontheground-basedmeasurementsofatmosphericparameters,butfrequentlydifficultiesariseincomparingthespectralreflectancesignaturemeasuredinthelaboratorytotheonemeasuredbythesensor,becauseofthedifferentatmosphericandancillaryconditions.

In this study, the imagesarecorrected for radianceat sensor;noatmosphericcorrection isperformedaccordingtotheapproachproposedbyFilizzolaet al. (2002).Thisapproachisbasedon the following requirement: to have samples of thematerial of interest uniquely identifiedindependentofdifferentilluminationconditionsintheacquireddataset.

Thefirst step is tocollect theoil spectral signatureson thedifferentlyacquired images, inordertobuildaspectrallibrary.Thiswasahardtask,becausethesurveywasperformedinanemergencysituation,sowehadonlyone imagewherefielddatawerecollected less than twohoursbeforetheflighttime(locationTorricelladiSissa,Parma)andthesedatawerenotcollectedatthesurface.Inordertocollectahighernumberofspectralsignaturesindifferentconditions,weexaminedalltheacquiredimagesandidentifiedoilonthewaterusingtheknownpropertiesofoilandtheappearanceandcolourofoilassuggestedbytheBonnAgreement(2009)..

Svejkovsky and Muskat (2006) report the difficulties of thickness measurements in situ.In their study, theymeasure thicknessfirst in a special pool subdivided in plotswith varioussamplethicknesses.Inthesamestudytheyalsoreportthecomparisonbetweenappearanceandthickness.

Oil is affected by the fluorescence phenomenon: it absorbs electromagnetic radiation atwavelengths<400nmandemitsintherangebetween400and650nm(Lennonet al.,2003).Asaneteffect,anopticalcontrastbetweenoilandthesurroundingwaterappears(Fig.2).Theenhancements of different colour bandspointedout the presenceof oil on the river.Detailedanalysisofimagespointedoutmacroscopicphenomenaintwoofthem,oneacquiredneartheRiverLambro,andanotheroneacquirednearthebridgeofSanNazzaro,whereskimmersandfloatingbarrierswerepresent.Thesimultaneousrecognitionofthesephenomenainthehyperspectraldataandorthophotoshelpeddelineateasetofregionsofinterest.

Fig.2reportsanorthophotoontheleftsideandanAISAimageinfalsecolour(red:801nm,green: 670nm,blue: 552nm)on the right; theorthophoto and thehyperspectral imagewere

Fig.2-SanNazzaronearPiacenza(12:34GMT).Ontheleft:theorthophotoinrealcoloursshowsunpollutedwaterwithagreencircle,whilethemagentacircleindicatesoil.Ontheright:correspondingAISAradianceimageinfalsecolours.

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acquiredsimultaneouslyontheSanNazzarobridge,wherethespectralsignatureswereacquired.Inbothimages,thegreencircleindicatestheriverwater,whilethemagentacircleindicatestheoilonthewater:astrongreflectanceofwateratblueandgreenwavelengthsisevident.

In therangeof thevisiblespectrum,from400to700nm,oilslickappearancedependsondifferentfactors:weather,oilconcentration,altitude,andmeteorologicalconditions.Acontrastenhancementof falsecolourswasapplied to the imageandunderlined thedifferencesamongthedetectedareas.Basedonthevisualcolourimageinterpretationandknownoiltextures,weidentifiedfivesituationsorclassesforthesuperficialstateoftheriverinthepreviousimages:

• water:ithastobedistinguishedbetweenclearwaterandturbidwater;• film:thewaterassumesthecolourbrownishiridescentorbrownishsheen;• concentrate:thecolourisbrownishconcentrate;• aggregate:brownishaggregateordarkaggregate;• deposit:heterogeneoussuspension(hydrocarbonsandbrushwood).According to theBonnAgreement (2009),oil appearanceon theacquired imageschanges

fromdark-browntosilver-greysheenwithrespecttoslickthickness.InTable2,wereportthefivecategoriesoftheBonnAgreementOilAppearanceCode(BAOAC);thesecategoriesdescribetherelationshipbetweentheappearanceofoilontheseasurfaceandthethicknessoftheoillayer(BonnAgreement,2009).

Table2-BonnAgreementoilappearancecode.

Code Description appearance Layer thickness interval (µm) Litres per km2

1 Sheen (silver/green) 0.040 to 0.30 40 - 300

2 rainbow 0.3 to 5.0 300 - 5000

3 metallic 5 to 50 5000 – 50.000

4 Discontinuous true oil colour 50 to 200 50.000 – 200.000

5 Continuous true oil colour More than 200 More than 200.000

Theregionsofinterest(Rois)usedweredefinedintheimageswherepollutionwasvisuallyevident,especiallyintheareaofIsolaSerafiniandthebridgeofSanNazzaro,nearPiacenza.Theareawascoveredbyflightstripsat12:22and12:34GMTapproximately,duringtheacquisition.

TheRoisstatisticsarerepresentedbyanaverageofmorethanonepolygon.SomeRoiswereselected as a reference, like vegetation, to prevent misclassification.We identified 5 classesreportedinTable3.

Table3-Classesidentification.

Classes Polygons Pixel

Water 15 1,673

Film 15 1,125

Concentrate 31 3,270

Aggregate 11 236

Vegetation 3 32,040

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TheRoiswereprocessedtogeneratespectralsignaturesappliedto thewholedataset.Thespectral radiancesignaturesacquiredbyAISA(except forvegetation)aredisplayed inFig.3.Theyareplottedintherangefrom412to696nm.Thehighvalueassumedbythefilmclassintherangefrom500to670nmindicatesthattheoilsheenhasareflectionhigherthanotherclasses;thefilmreflectiontendstocoincidewithclearwaterforwavelengthscloseto696nm.Topreventthemisclassificationofclasses1and2(Table2),theywereaggregated,sinceitwasdifficulttodistinguishbetweenthem.

Fig.3-Oilspillclassesvs. water (in the range412-696nm).

ThegeocodedimageswereclassifiedonthebasisoftheforegoingidentifiedclassesusingtheSpectralAngleMapper (SAM) technique(Kruseet al.,1993), included in theENVIsoftwarepackage(EXELISVisualInformationSolutions).TheSAMclassificationalgorithmwasapplied,because it is not influenced by solar illumination and accentuates the spectral characteristics,supportingFilizzolaet al. (2002)approachandpreventingissuesregardingthetimeofacquisitionduringtheflight.Thesimilaritybetweenpixelspectralradiancesignatureandtheidentifiedtargetswasfoundselectingpurepixelsinanyclassduringtheclassificationphase.Amaskwasappliedto every image in order to eliminate the groundpixels corresponding to ambiguities, such asriversides,isles,piersandothermanmadestructures.Adepositclasswasnotconsideredintheanalysisbecauseofthepresenceofalotofspuriouspixels(sand,shadows,foliage,trunks,etc.).

Theinitialresultsprovethatthemethodisefficientfordistinguishingclearwaterfrompollutedwater.Itisinterestingtonotethattheharbourareaandasecondbranchnottouchedbythestreamareclassifiedaspurewater(seeFig.4).Theseresultsarecomparableto in situmeasurementspresentedinTable1;thesamplestakeninthesameareaatdifferenttimesconfirmthepresenceofhydrocarbons.IntheTorricellediSissaarea,theclassificationshowsdiffuseoilasconcentrateandfilmflowingalongtheriver’smainstream.

The whole classification is represented in Fig. 5, where pollution and its extension arehighlighted.ThecorrespondentvaluesaresummarizedinTable4.

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Fig.4-TorricelladiSissa:ontheleftthetruecolourAISAimage,ontherightitsclassification.

Fig.5-Classificationmap.

Table4-Waterandpollutionextensionestimation.

Classes km2 extension % extension

Water 31 55%

Film 12 22%

Concentrate 12 22%

Aggregate 0.6 1%

Thepercentageofpollutantextensionintheimagepermittedtheidentificationofoildistributionand features at that time.As shown in Fig. 6,we can divide themap into threemacro areasaccordingtothepollutantdistribution:Sector1,Sector2,andSector3.Sector1ischaracterizedbythegreatestpollutionextensionpercentage.Thepercentagehasbeencalculatedaccordingtotheoverallsurveyedriverarea.

5. Quantitative analysis estimation

Inthisstudy,wemadeanattempttoestimatetheamountoffloatingoilonwaterinthegeocodedclassificationmap.AccordingtoSvejkovskyet al.(2012),simultaneousfieldmeasurementsaredifficulttoobtainduringarealoilspill,andadifficultchallengeremainsalsoduetothevariabilityofthewaterbackgroundandillumination.Atthesametime,thewayinwhichoilsinteractwithwaterdependsontemperature,waves,salinity,andotherenvironmentalparameters.

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Fig.6-Classificationmap:a)Sector1,b)Sector2,c)Sector3.

Inourclassification,theappearanceofoilvariesfrombrownishsheentodark.ConsideringtheBonnAgreementoilappearancecodeasareference(BonnAgreement,2009),Table5relatesclasses tocodes,appearancedescription,and theaverage layer thicknessusedfor theoilspillalongtheriver.InaccordancewithTable2,weconsideredanaveragethicknessvalueforeachclassofpollutants.

Anaveragedensityequalto0.85t/m3wasfixedforthethreedifferentclasses:film,concentrate,and aggregate (http://www.safewater.org/PDFS/resourcesknowthefacts/Oil_Spills.pdf). We

Table5-BAOAClayerthicknessappliedtoPocase.

Classes Code (Bonn Agreement) Description appearance Average layer thickness interval (µm)

water

film 1 and 2 Sheen (silver/green) and rainbow 2.5

concentrate 3 metallic 25

aggregate 4 Discontinuous true oil colour 100

deposit hydrocarbons and brushwood

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calculatedthevolumesofpollutantsforeveryclass,multiplyingtheaveragelayerthicknessofTable5bytheextensionofTable4(seeTable6).Finally,thankstotheaveragedensityinformation,weestimatedthetotalpollutantmasspresentonthewatersurface;itisabout338t.ThisresultiscoherentwiththequantityofpollutantestimatedbytheDipartimentodellaProtezioneCivile(2010).

6. Conclusions

TheoilspillalongtheRiverPoin2010wasmonitoredusinganAISAhyperspectralsensorandaveryhigh-resolutionRGBcamera.TheAISAimageswereclassifiedaccordingtoaspectrallibrarydefinedontheacquireddatasets.Thefirstoilspilldistributionmapwasdeliveredthreedaysaftertheevent.ApplyingtheBonnAgreementoilappearancecodetotheidentifiedclasses,itwaspossibletoidentifydifferentoilthicknessesandtoapproximatelyestimatethequantityofoilspillintheriveratthemomentoftheaerialsurvey.Thelocationofoilanditsthicknesscanbehelpfultogroundoperations(containmentmonitoringsystems,positioningofnewbarriers,etc.)inrealtimeduringthefirstemergencyphase.

Somedifficultiesaroseintheanalysisphaseduetothefluvialenvironment,inparticularthepresenceofturbidwater,centralcurrents,andriverbends.Turbiditycanaffectthecolourofthewaterandthereforethespectralsignaturedetection.

This experience revealed how important it is to set up a local remote sensing protocol topreciselydefinetheplanningandallof therelatednecessaryprocessingstepsforasuccessfulaerial survey.From this experience and consideringwhatwas suggestedbyBajić (2012),wecanstatethatitisnecessarytodefineaprotocolforhyperspectraldataacquisitioninemergencysituationsfocusedonthefollowingkeyissues:

- adetailedairborneflightplanrelatedtothetargetrequiringinvestigation;- anaccurateselectionofthespectralrangebandstoacquireonlythemostsignificantdata;- an estimate of the pre- andpost-processingphase time requirement,which is significantwhenthesurveyedareaislargeandhyperspectraldataaretobeprocessed;

- the possibility of performing a simultaneous acquisition of geo-referred data in situ, theminimuminformationpossiblybeingthepresenceorabsenceofoilatthesurface.

The organization and synergy of these activities constitutes an important added value toenvironmentalprotection.

Acknowledgements. Many thanks toA. Spisni (Dept. ofExperimentalMedicine,University of Parma,Italy)forhissupportinthedefinitionofoilschemistryanditsbehaviourinwater.Preliminaryoutcomeswerepresentedin:PietrapertosaC.,SpisniA.,PancioliV.,SterzaiP.,PavanA.,PaganiniP.,MonniA.andCorenF.(2010),UtilizzodiimmaginiiperspettraliperilmonitoraggiodisversamentodiidrocarburinelfiumePo,Atti14ªConf.NazionaleASITA,Brescia,Italy,pp.1457-1462.

Table6-Volumeofeachclass.

Classes Volume (m3)

Film 30

Concentrate 309

Aggregate 58

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Corresponding author: Paolo Paganini Istituto Nazionale di Oceanografia e di Geofisica Sperimentale Borgo Grotta Gigante 42c, 34010 Sgonico (TS), Italy Phone: +39 040 2140343; fax: +39 040 327307 e-mail: ppaganini@inogs.it