1 Explorando la Tierra desde el espacio usando la interferometría de radarJill Pearse Departamento de Geociencias Universidad de Los Andes Bogotá, Colombia

2 Primer terremoto visto del espacio1992 Landers, California Satelite ERS-1 Patrón de interferencía causado por la deformación de la superficie durante el terremoto Massenet et al. 1993

3 ¿Como se puede usar el InSAR para entender los procesos dinámicos de la Tierra?

4 Webinar 1 Introducción – deformación de la cortezaRadar de apertura sintética (SAR): teoría Propiedades de imágenes de SAR Interferometría (InSAR) - teoría

5 Deformación de la superficieLa tectónica de placas, convección del manto, terremotos, actividad volcánica… procesos físicos que ocurren bajo la superficie causan efectos superficiales

6 movimientos de las placas tectónicas y terremotos Todos los procesos que ocurren bajo la superficie terrestre pueden causar deformación de la superficie que puede ser medida Ejemplos: movimientos de las placas tectónicas y terremotos Magma subiendo en la corteza Actividades humanas (extracción de agua o recursos naturales…) Con observaciones de satélites, podemos medir y monitorear la deformación para inferir lo que está pasando debajo Snapshot of a piece of the eruption cycle.

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8 Radar de apertura sintética Synthetic Aperture Radar (SAR)El satélite gira alrededor de la tierra en órbita polar Puede ver la misma escena subiendo o bajando – asi tiene vistas de la escena por dos lados Pasa encima del mismo lugar cada mes Note that sometimes when the satellite is crossing the equator it is on its way up (ascending) and sometimes it is on its way down (descending) – so you can look at the same spot from 2 different angles

9 Radar de apertura sintética Synthetic Aperture Radar (SAR)El radar es un sistema activo. -emite luz en bandas de microondas, y recoge las reflecciones (“backscattering”) de la superficie rugosa. - no requiere iluminación del sol - Las longitudes de onda: 3-20 cm - Estas ondas atraviesan la atmósfera El satelite mira hacia un lado, y crea imágines de la amplitud y la fase de las ondas reflejadas.

10 Unlike Landsat and other optical systems Radar is ACTIVE

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12 ¿Por qué microndas?

13 ¿Qué es la “apertura sintética”?

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15 Real-Aperture Radar (RAR): la resolución se mejora con la anchura del rayo (entre más estrecho el rayo, mejor la resolución). Implica que la resolución es peor con mas distancia, y se mejora con antenas mas largas Angular beam width Proportional to Wavelength/antenna length

16 Exercise: Calculate the REAL antenna size that would be required in order for the for the Canadian satellite RADARSAT (orbit height of 792 km) to obtain an azimuth resolution of 10 m at a ground range of 40 km. (Wavelength used by RADARSAT = 5.6 cm) La longitud de la antena tiene límitaciones prácticas: en un avion, puede ser 1-2 m; en un satélite, puede ser de 10 a 15 metres de longitud. ¿La solución?

17 Radar de Apertura Sintética (SAR)

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19 En este caso, la resolución es mejor si se usa una antena corta!

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22 ¿Qué propiedades influye en la interacción de la luz con la superficie?1) La rugosidad 2) Geometría de avistamiento con Respecto a la geometría de la superficie 3) Humedad y propiedades eléctricas de la superficie (generalmente, hay más reflección en las superficies húmedas)

23 Wavelength and Surface Roughness© DLR © DLR rough smooth

24 SAR Image Examples Sensor: ERS-1 Mojave Desert CA, USAazimuth range Sensor: ERS-1 Mojave Desert CA, USA Size » 40 km x 40 km ERS-1 © ESA

25 Generally, for moist targets, surface scattering is the dominant processFor drier targets, the EM waves can penetrate into the subsurface. For a given target, longer wavelengths tend to penetrate furthest If the energy penetrates the surface, then volume scattering becomes the dominant process (multiple bounces off the different particles within the volume) L-band SAR found ancient river channels several meters below the surface of the Sahara desert

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27 SAR Image characteristicsSpeckle: “salt and pepper” texture caused by random constructive and destructive interference from the many scattering returns occurring within each ground resolution cell A type of noise that can be reduced by filtering or multi-looking (but at the expense of resolution!)

28 Relief displacement and layover both create RADAR SHADOW

29 ¿Qué es la interferometría de radar?(InSAR)

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35 El satellite graba la amplitud y la fase de la luz reflejada en cada punto de la superficie

36 InSAR = InterferometríaEl satélite pasa por cada lugar muchas veces (una vez por mes) Si la superficie se desplaza entre los dos momentos, la fase de la onda reflejada cambia. Puede medir desplazamientos pequeños (milimetros)

37 Repeat-pass InSAR for deformationLa Fase depende de la distancia entre el satélite y la superficie ERS-1 ~6 cm Recall that one wavelength of path length is equivalent to 2π of phase. Given that radar satellites orbit at several hundred km of elevation, it is not feasible to count the number of phase cycles for a single image . . . ~780 km From: Gareth Funning, UC Riverside

38 Repeat-pass InSAR Pass 1 Pass 2 From: Gareth Funning, UC Riverside. . . So what we do is compare (difference) the phase from two different passes of the same satellite over the same point on the ground. Assuming that we can correct for the position of the satellite (and we can), then any difference in phase between the two passes must be related to a change in distance between the satellite and the ground (i.e. a movement of the ground). From: Gareth Funning, UC Riverside

39 Repeat-pass InSAR Pass 1: pre-movement Pass 2: post-movement phase=1phase shift due to ground motion So if the ground moves, we can detect a phase shift when we subtract the phase of the second image from the phase of the first! It is important to note that we can only measure displacement of the ground toward or away from the satellite, in the line-of-sight direction between the two. Everything we measure is a “line-of-sight” displacement. From: Gareth Funning, UC Riverside

40 Repeat-pass InSAR phase shift varies with distance from the faultIt is important to note that the only way we know that there has been movement of the ground is if different areas on the ground move different amounts—InSAR makes measurements of relative deformation. If the whole area moved by the same amount we would not see any relative phase shift. The cartoon here shows a slipping normal fault, and we would expect that the ground would move farther away from the satellite for an area directly next to the fault than an area farther away. The pattern of relative displacement that we see recorded in the InSAR data is typically diagnostic of the deformation source (e.g. for earthquakes, normal, reverse and strike-slip earthquakes all have different and distinct patterns). From: Gareth Funning, UC Riverside

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43 Requirements to form a differential interferogramImages have to be acquired by the same satellite using the same acquisition mode and properties (beam, polarization, off-nadir angle, etc). Images have to be acquired with the satellite in the same nominal orbit. The baseline separation between the master scene and any of the slave scenes must be no more than the ‘critical baseline’ (a parameter that varies with the SAR sensor in use); the baseline being the distance between the satellite paths. You need a DEM in order to subtract the contribution of topography to the interferogram

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45 First, remove effect of curvature of the earth: “Interferogram Flattening”… thenInterferometric phase (Δφ) has the following contributions: topographic distortions arising from slightly different viewing angles of the two satellite passes (t) atmospheric effects (α) arising from the wavelength distortion that occurs when signals enter and leave a moisture-bearing layer Any range (distance between the sensor and the target) displacement of the radar target (∆R) noise These factors, expressed more precisely, are given in the equation below: Atmospheric effects Topographic contribution Range displacement

46 What do the fringes mean?Aug 1995-Jul 2005 ERS data at Uturuncu volcano May 1996-Aug 2006 Apr 1996-Mar 2007 (Pearse and Fialko, 2009)

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49 Current and Future Civil Spaceborne SARssatellite owner band resolution look angle swath lifetime ERS-1 ESA C 25 m 23° 100 km ERS-2 ESA C 25 m 23° 100 km Radarsat-1 Canada C 10 m m 20°- 59° km ENVISAT ESA C 25 m - 1 km 15°- 40° km ALOS Japan L 10 m -100 m 35°- 41° km Cosmo Italy X ca. 1 m - 16 m … … 2007- TerraSAR-X Germany X 1 m - 16 m 15°- 60° km 2007/2010- & TanDEM-X Radarsat-2 Canada C 3 m m 15°- 59° km 2007- ALOS-2 Japan L 3 m – 100 m 8°-70° 25 – 350 km 2014?- Sentinel-1 ESA C 5 m – 50 m 20°-46° km 2014?-

50 Sentinel SENTINEL-1A and -1B European Space AgencyLaunch Q2 2014, 2016 Sentinel This is the current state-of-the-art: Sentinel-1, a dedicated satellite mission launched by the European Space Agency (ESA) for studying natural hazards. Due to advances in radar instrument design and improvements in the efficiency of solar panels, much more data can be acquired over a wider area on each pass of the satellite, meaning that the repeat interval between image acquisitions can be reduced to just 12 days (compared with 35 days for earlier missions). To further reduce the delay between acquisitions, there will be two satellites in orbit simultaneously, flying six days apart. With two satellites plus ascending and descending coverage, and tight orbital control, we should be able to produce interferograms and earthquake models within a few days of an earthquake, with very little opportunity for decorrelation. Image source: European Space Agency, public domain. 12 (and then 6) day repeat in same orbit mean post event wait => 3 days ascending + descending => mean wait < 3 days

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54 References and further readingSAR-EDU – SAR Remote Sensing Educational Initiative https://saredu.dlr.de/ Natural Resources Canada