ã2017CaliforniaInstituteofTechnology.Governmentsponsorshipacknowledged.

NISAR:TheNASA-ISROSARMission

ASinkingFeelingLand subsidence frequently occurs at a very slow rate,much tooslowly tobe identifiedvisuallyandoftenonlydetectablebyadvancedsensors.Itoftengoesunnoticeduntil the consequences become manifestly evident –buckling bridges, cracked foundations, the too-frequent

occurrence of 100-year (or more) floods. Often at thatpoint theprocesses causing the ground to sink arewellunderway,anddamagecontinues toaccruebefore theycanbestopped.This insidioushazard isbothdifficult toidentifyandcostlyifnotdiscovered.

SubsidenceandaSinkingLandscape

Subsidenceoftengoesunnoticeduntilthedamageisdone.Becauselandsinkstooslowlyorovertoobroadanareatobevisibletotheeye,theeffectsofsubsidencearerarelyrecognizedwhentheybegin.Thosesamechangesinlandelevationcanbedetectedfromspacebornesyntheticapertureradar.

Subsidence: How Low Will It Go? Subsidence,orthegradualsinkingofthelandsurface,affectsland,buildingsandroadsthroughouttheU.S.EventheWashingtonMonumenthasbeenaffected.Theiconicstructure,builtonthemarshyshoresofthePotomacRiver,hassubsidedby~2”sinceitsconstruction.

Subsidenceoccursnaturallyinareaswithhighorganicmattercontentinthesoilthroughacombinationofcompactionandoxidation.Forthisreason,subsidenceisoftenassociatedwithswampyandcoastalareas.However,therangeofunderlyingcausesismuchmorediverse,andincludesgeologicalfactors,e.g.,faultsorsinkholes,plusahostofcausesrelatedtoman’suseoflandandresources.Regardlessofthecause,theconsequencescanbesevere:crackedfoundations,warpedroadsandbridges,andmorefrequentfloodingofcitiesandtowns,allofwhichtakeaneconomicandsometimespersonaltollonpeople.

TheNISARMission–Reliable,ConsistentObservations

TheNASA–ISROSyntheticApertureRadar (NISAR)mission,acollaborationbetweentheNationalAeronauticsandSpaceAdministration(NASA)andtheIndian Space Research Organization (ISRO), will provide all-weather,day/night imaging of nearly the entire land and ice masses of the Earthrepeated 4-6 times per month. NISAR’s orbiting radars will image atresolutions of 5-10meters to identify and track subtlemovement of theEarth’s land and its sea ice, and even provide information about what ishappeningbelowthesurface.Itsrepeatedsetofhighresolutionimagescaninform resourcemanagement and be used to detect small-scale changesbeforetheyarevisibletotheeye.Productsareexpectedtobeavailable1-2daysafterobservation,andwithinhoursinresponsetodisasters,providingactionable,timelydataformanyapplications..

Photos(clockwise):CarolM.HighsmithArchive/LibraryofCongress,Calif.DWR,USGS,NASA/JPL-Caltech

NISAR:TheNASA-ISROSARMission

ã2017CaliforniaInstituteofTechnology.Governmentsponsorshipacknowledged.

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Subsidenceisaproblemthatexistsatdifferentscales innumerouslocationsaroundtheworld.ThesinkingofSanFrancisco’sMillenniumTowerandacrossthecityofNewOrleansarejusttwoexamples.Decreasedlandelevation,the cumulative effect of long-term subsidence, hasresulted inmorefrequentanddeeperflooding.Thiscancause tidal waters or rainfall to generate nuisancefloodingandevendangerouslyhighwater in some low-lyingcoastalcities.Knowledgeoftheextentandrateofthe problem can help in making better choices forinfrastructure investment and restoration activities thatprotectpeopleandproperty.However,atthemostbasiclevel, improving resiliency and dealing with subsidencerequiresunderstanding theunderlyingprocessesdrivingthechange,whichwillinformbothimmediateactionandbettermodelingoffuturesubsidence,floodrisk,andwaystohaltorreversethesinking.The sinking can be caused both by natural events andhumanactions,soelevationchangeatanygivenlocationdependsonauniquesetoflocalandregionalconditions.Many measurements, spread across a broad scale andextending for a long time, are needed to tease out therelative contributions when multiple processes areinvolved. Human activities are often the primary

contributor,includingoilandgasextraction,groundwaterpumping,andevendrainageprojectsintendedtoreducestandingwater.Alloftheseactivitiescanbeperformedinwaysthatdonotcausedamage,givenproperinformationabout the relationship between the activity and thegroundmovement.Modernradarremotesensingmethodscanrevolutionizethewaythatlandismonitored,significantlyincreasingthenumberofareaswhere subsidence ratesaremeasured.Instead of costly in-ground instruments, satellite-basedinstrumentscandetectchangesinelevationofthegroundand structures across large areas via a technique calledsynthetic aperture radar interferometry (InSAR). InSARrelies upon repeat imaging of an area from the samevantagepointinspacetomeasurechangesinthedistancebetweentheradarantennaandtheground.Tomeasureslowsubsidence,alargenumberofrepeatedimages collected at regular intervals for several years isneeded. Space-basedEarthobserving radar instrumentsare able to measure subsidence hazards across a largeenoughregiontoencompassallat-riskcitiesintheUnitedStates. NASA’s NISAR radar will be able to collect theneeded images, and its products will be open access,availablethroughtheAlaskaSatelliteFacility.

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

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

MapshowingsubsidenceinasuburbofNewOrleans,Louisiana,producedusingdatafromtheNASAUAVSARinstrument[C.Jonesetal.,2016].UAVSARisanairborneprototypefortheupcomingNISARmission,whichwillprovideregularlyrepeatedimagesofcitiessuchasNewOrleans.

Localized Land Subsidence Seen by RadarNewOrleansisoneofthemostendangeredcitiesintheworld.Alargeportionof themetropolitan area lies belowmean sea level,whileitslocationontheMississippiRiverdeltaalsoputsitatriskoffloodingbothfromgulfwaterspushedinlandduringhurricanesandtropicalstorms,andfromhighwaterintheMississippiRiverduringspringtimefloods. Knowledgeofsubsidence‘hotspots’andtheirprobable cause is particularly critical because, if undetected,unexpectedfloodingcanendangerresidentsandproperty.Highresolutionradarinstrumentscanbeusedtoidentifytheareasthataresubsidingmostrapidly,andcanalsoidentifysubsidingordeformingstructures,suchasinaresidentialneighborhoodshownat theright. In thisexample, radar interferometry (InSAR)showsthathousesinthemostrecentlydevelopedarea(yellowtoorange)aresubsidingmorerapidlythanhousesinadjacentneighborhoodsbuilt in earlier years. This type of change could be normalconsolidationofmaterialaddedduringconstruction,whichwouldstop after a few years, or could indicate deeper, longer-lastingprocessesatwork.Sustained,multi-yearobservationswillprovidetheinformationneededtodeterminethecause.