ã2017CaliforniaInstituteofTechnology.Governmentsponsorshipacknowledged.

NISAR:TheNASA-ISROSARMission

IdentifyingGroundMovementAssociatedwithSinkholeFormationNaturallyformingsinkholesarecommoninareaswherethe rockbelowthegroundcanbeeasilydissolved, i.e.,limestone, carbonate rocks, or within salt domes. Acombinationofgeology(rocktype)andhydrology(waterflow) contribute to thedevelopmentof sinkholes,withmanydifferentpossiblemechanisms leading to surface

collapse into the subsurface caves. The caves can besmallorverylarge,andcanbeshallowordeepbelowthesurface. The collapse can occur abruptly, takingmereseconds, or it can take many decades, slowly formingdepressions.

SinkholesandCavernCollapse

SinkholesarenotjustaFloridaphenomenonbutcanoccurinanyofthe50U.S.states.FortypercentoftheU.S.ispronetonaturallyoccurringsinkholesbecauseoftheunderlyinggeology.Evenmoreareasareincludedwhenweconsiderthedangerfromcollapseofman-madecavernsorburiedpipes.

Sinkhole Collapse Can Be Fast or Slow, Natural or Man-Made Sinkholesforminmanyways,allofwhichleadtocollapseoftheoverburdensoilaboveanundergroundvoid.Naturallyoccurringsinkholesforminareaswheretherockcandissolveinwater,whichcouldbeduetoerosionoranundergroundstream.Thenightmarescenarioiscatastrophiccollapse,whereavastholeabruptlyandwithoutwarningformsaboveanunsuspectedcaverndeepbelowthesurface,swallowingeverythingabove.However,sinkholesalsocanformmoreslowlyasgentledepressionsthatindicatewheretherockisdissolvingbelowground.Inaddition,accidentalcollapseofminedcavernsorburiedpipesareoftencalledsinkholes,becausetheyresultinthesametypesofpitsandcaneitheroccurslowlyorquickly,justasthenaturalsinkholes.

TheNISARMission–Reliable,ConsistentObservations

TheNASA–ISROSyntheticApertureRadar(NISAR)mission,acollaborationbetweentheNationalAeronauticsandSpaceAdministration(NASA)andthe IndianSpaceResearchOrganization(ISRO),willprovideall-weather,day/nightimagingofnearlytheentirelandandicemassesoftheEarthrepeated4-6timespermonth.NISAR’sorbitingradarswillimageatresolutionsof5-10meterstoidentifyandtracksubtlemovementoftheEarth’slandanditsseaice,andevenprovideinformationaboutwhatishappeningbelowthesurface.Itsrepeatedsetofhighresolutionimagescaninform resourcemanagementandbeused todetect small-scale changesbeforetheyarevisible to theeye.Productsare expected tobeavailable1-2daysafterobservation,andwithinhoursinresponsetodisasters,providingactionable,timelydataformanyapplications..

Photos(clockwise):OnWingsofCare,USGS,USGS,NationalCave/KarstResearchInstituteS

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NISAR:TheNASA-ISROSARMission

Reports of sinkholes in Florida and Kentucky havesurfacedrecentlyinthenews.Inonecase,asinkholeswallowed a house with a man inside; in another,antique cars at the National CorvetteMuseum fellintoasinkholethatappearedovernight. But thosestates are not alone in experiencing thephenomenon.ManyareasintheU.S.andaroundtheglobe regularly experience sinkholes, and largersinkholes can be visible from space. In the UnitedStates,sinkholesarecommoninareaswithlimestoneand other easily dissolved rock. In addition to thecontinentalU.S.,AlaskaandHawaiiarealsoat risk.SinkholesinHawaiiresultfromcollapsinglavatubes,whereas in Alaska a common cause of sinkholes iserosionassociatedwiththawingofthepermafrost.WithNISAR,expertswillbeabletomeasuresurfacemovement directly through repeat imaging of anareaandprocessingthesequenceofimagesusingatechnique called synthetic aperture radarinterferometry(InSAR)thatcanmeasurechangesin

the distance between the radar antenna and theground at the scale of a fraction of the radarwavelength (24 cm for NISAR). InSAR is a remotesensingmethodthatcanbeusedtomeasuresurfacedeformation across large areas at one time. NISARhas sufficient resolution to identify localizedmovement of the sort associatedwith sinkholes atscales as small as 50 m on the surface. Radar hasadditionaladvantages,themostsignificantbeingtheabilitytoseethroughclouds,smoke,andhazeandtoimage the surface day or night without relying onsolarillumination.All NISAR data products, including the InSARinterferograms, will be available with open accessthrough a web portal. In this way, the nation’sinvestment in radar data remotely acquired fromspacecanbemostwidelyusedindisasterresponseand tohelppreventnaturalandaccidentalhazardsfrombecomingdisasters.

NationalAeronauticsandSpaceAdministration Formoreinformation,visithttp://nisar.jpl.nasa.gov/applications

JetPropulsionLaboratory/CaliforniaInstituteofTechnology/Pasadena,California/www.jpl.nasa.gov

Some,butnotall,sinkholeswillexperiencerelativelysmallscalegroundmovementatthesurfacepriortotheabruptcollapseofcatastrophicformation.Oneexampleisshownattheright,wheregroundmovementpriortoformationoftheBayouCornesinkholewascapturedusingUAVSAR,NASA’sairborneprototypefortheNISARspacemission.Thecolorsoftheinterferogramshowcontoursofgroundmovement.AregularscheduleofradarimagingwithNISAR,whichwillimageallthelandintheUnitedStatesandnearlyalllandglobally,wouldenabledetectionofsomecaseswherethemovementstarteddayspriortothecollapse.Thisimagingprogramwillalsobeabletohelpidentifytheslow-collapsesinkholesthatcancausedamagetostructuresthroughslowbutinexorablesettling.Inthisway,remotemonitoringfromspacecanhelptoidentifythisrelativelycommonandhighlydestructivehazardmorequickly,efficiently,andeconomically,protectinglivesandproperty.

InSARinterferogramshowinggroundmovementthatprecededcatastrophiccollapsebyatleast30days.ThesinkholeformednearBayouCorne,Louisiana.Theareaofthegroundthatmovedwasmuchlargerthantheinitialsinkholethatformed(greycircle,shownabovecontoursoftheNapoleonvillesaltdome)[Jones&Blom,2014].

Ground Movement Preceding Sinkhole Formation