Damage Proxy Map of the Beirut Explosion on 4Th of August 2020 As Observed from the Copernicus Sensors

sensors

Communication Damage Proxy Map of the Beirut Explosion on 4th of August 2020 as Observed from the Copernicus Sensors

Athos Agapiou 1,2

1 Department of Civil Engineering and Geomatics, Faculty of Engineering and Technology, Cyprus University of Technology, Saripolou 2-8, Limassol 3036, Cyprus; [email protected] 2 Eratosthenes Centre of Excellence, Saripolou 2-8, Limassol 3036, Cyprus

Received: 15 September 2020; Accepted: 7 November 2020; Published: 9 November 2020

Abstract: On the 4th of August 2020, a massive explosion occurred in the harbor area of Beirut, Lebanon, killing more than 100 people and damaging numerous buildings in its proximity. The current article aims to showcase how open access and freely distributed satellite data, such as those of the Copernicus radar and optical sensors, can deliver a damage proxy map of this devastating event. Sentinel-1 radar images acquired just prior (the 24th of July 2020) and after the event (5th of August 2020) were processed and analyzed, indicating areas with significant changes of the VV (vertical transmit, vertical receive) and VH (vertical transmit, horizontal receive) backscattering signal. In addition, an Interferometric Synthetic Aperture Radar (InSAR) analysis was performed for both descending (31st of July 2020 and 6th of August 2020) and ascending (29th of July 2020 and 10th of August 2020) orbits of Sentinel-1 images, indicating relative small ground displacements in the area near the harbor. Moreover, low coherence for these images is mapped around the blast zone. The current study uses the Hybrid Pluggable Processing Pipeline (HyP3) cloud-based system provided by the Alaska Satellite Facility (ASF) for the processing of the radar datasets. In addition, medium-resolution Sentinel-2 optical data were used to support thorough visual inspection and Principal Component Analysis (PCA) the damage in the area. While the overall findings are well aligned with other official reports found on the World Wide Web, which were mainly delivered by international space agencies, those reports were generated after the processing of either optical or radar datasets. In contrast, the current communication showcases how both optical and radar satellite data can be parallel used to map other devastating events. The use of open access and freely distributed Sentinel mission data was found very promising for delivering damage proxies maps after devastating events worldwide.

Keywords: Copernicus; Sentinel-1; Sentinel-2; InSAR; change detection; damage proxy map; Beirut; Lebanon; explosion

1. Introduction In emerging situations such as industrial and technological accidences, earth observation sensors can provide near-real-time information to local stakeholders and international organizations. This has been already demonstrated in the past in several examples such as the case study of the blast at Cyprus Naval Base (11th of July 2011, at Mari area, Cyprus), at the Fukushima Daiichi nuclear disaster (11th of March 2011, Okuma,¯ Japan), etc. High-resolution satellite sensors are mainly used for rapid mapping and recording of the damage over large extents. However, the use of freely distributed sensors such as those of the Copernicus Programme did not thoroughly investigate in the past. This is in contrast with their wide use in monitoring other hazards all over the world, which has been demonstrated in several articles [1–4]. This communication article aims to investigate whether the use of these sensors, namely the Sentinel-1 and Sentinel-2, can provide reliable information, in case of emerging situations.

Sensors 2020, 20, 6382; doi:10.3390/s20216382 www.mdpi.com/journal/sensors Sensors 2020, 20, x 2 of 22 Sensors 2020, 20, 6382 2 of 21 investigate whether the use of these sensors, namely the Sentinel‐1 and Sentinel‐2, can provide reliable information, in case of emerging situations. For this reason, a recent catastrophic event was Forexamined this reason, in Lebanon, a recent aiming catastrophic to produce event Damage was examined Proxy Maps in Lebanon, (DPM) over aiming the area. to produce Damage ProxyThe Maps event (DPM) took overplace the in the area. evening of the 4th of August 2020 in the area near the main harbor of Beirut,The the event capital took of place Lebanon. in the According evening of theto the 4th media, of August a fire 2020 near in thethe areaport near ignited the maina large harbor nearby of Beirut,store of the ammonium capital of Lebanon. nitrate, which According is a highly to the media,explosive a ﬁre chemical near the often port ignitedused in a fertilizer. large nearby More store than of ammonium100 people died, nitrate, while which more is a than highly 5000 explosive were wounded. chemical oftenIt was used estimated in fertilizer. that at More least than 300,000 100 people died,were left while homeless more than from 5000 this were event, wounded. while the It destruction was estimated was thatestimated at least to 300,000 cost between people 10 were and left 15 homelessbillion USA from dollars this event,[5]. The while Beirut the explosion destruction generated was estimated seismic to waves cost between equivalent 10 andto a 15magnitude billion USA 3.3 dollarsearthquake, [5]. The according Beirut explosionto the United generated States seismicGeological waves Survey equivalent (USGS) to Earthquake a magnitude Hazard 3.3 earthquake, Program according[6]. to the United States Geological Survey (USGS) Earthquake Hazard Program [6]. From the early beginning, several agencies have tried to support ground rescue investigations. Examples of this eeffortﬀort are byby thethe CenterCenter forfor SatelliteSatellite BasedBased CrisisCrisis InformationInformation (ZKI) of the German Aerospace Centre (DLR) [[7]7] thatthat usedused high-resolutionhigh‐resolution WorldView-2WorldView‐2 multispectralmultispectral imagesimages acquired just a day afterafter thethe event.event. The Advanced Rapid Imaging and Analysis (ARIA) team at NASA’s Jet Propulsion LaboratoryLaboratory and and California California Institute Institute of Technology of Technology in Pasadena, in California,Pasadena, inCalifornia, collaboration in withcollaboration the Earth with Observatory the Earth of Observatory Singapore (EOS), of Singapore have also (EOS), processed have Sentinel-1 also processed radar images Sentinel indicating‐1 radar areasimages that indicating are likely areas damaged that are caused likely by damaged the explosion caused in by Lebanon the explosion [8]. in Lebanon [8]. Figure1 1 shows shows high-resolution high‐resolution red-green-bluered‐green‐blue (RGB)(RGB) WorldView-2WorldView‐2 images images over over the the area area of of the the harbor (Figure 11,, top)top) andand thethe broaderbroader areaarea of of the the explosion explosion (Figure(Figure1 ,1, bottom) bottom) before before (Figure (Figure1a,c) 1a,c) and after (Figure1 1b,d)theb,d) the event.event. These images were releasedreleased somesome hourshours afterafter thethe explosionexplosion byby the the European Space Imaging [[9],9], thusthus allowingallowing visualizationvisualization ofof thethe destruction.destruction.

(a) (b)

Figure 1. Top: (a) WorldView-2WorldView‐2 high-resolutionhigh‐resolution optical image over the Beirut harbor area before thethe explosion and (b) afterafter thethe event.event. Bottom: ( (cc)) WorldView WorldView-2‐2 high-resolutionhigh‐resolution optical image over the broader area of the Beirut harbor before the explosionexplosion and (d)) afterafter thethe eventevent (copyrights(copyrights European Space Imaging [[9]).9]).

Sensors 2020, 20, x 3 of 22 Sensors 2020, 20, 6382 3 of 21 While the above‐mentioned results are very important for local stakeholders as they provide damageWhile proxy the above-mentionedmaps in a short resultstime, the are detection very important of damaged for local areas stakeholders is usually as based they provideon the processingdamage proxy on mapseither in optical a short or time, radar the detectionproducts. of In damaged this paper, areas we is usuallyaim to basedinvestigate on the the processing Beirut explosionon either opticalthat took or radar place products. on 4th Inof thisAugust paper, 2020 we aimby tousing investigate in parallel the Beirutboth explosionoptical and that radar took Copernicusplace on 4th Sentinel of August data. 2020 The by detection using in of parallel damage both areas optical in different and radar wavelengths Copernicus of the Sentinel spectrum data. canThe further detection strengthen of damage the areasfinal damage in different proxy wavelengths maps. of the spectrum can further strengthen the finalThe damage paper proxy is organized maps. as follow: initially, the methodology and the data used are presented, whileThe the paper results is from organized the change as follow: detection initially, and the InSAR methodology analysis andfrom the the data Sentinel used‐1 are datasets presented, are shown.while the These results results from the are change also detection compared and with InSAR optical analysis Sentinel from the‐2 Sentinel-1images as datasets well areas shown.other published—inThese results are the also media—high compared‐resolution with optical results. Sentinel-2 Finally, images a general as well discussion as other for published—in the potential the of spacemedia—high-resolution‐based applications for results. monitoring Finally, areas a generalunder threat discussion is presented. for the potential of space-based applications for monitoring areas under threat is presented. 2. Study Area 2. Study Area The study area was focused on the harbor area of Beirut in Lebanon, where the explosion occurred,The study and its area surrounding was focused area on the(Figure harbor 2). areaWhile of much Beirut damage in Lebanon, was where observed the explosion in the harbor occurred, area (seeand also its surrounding Figure 1a,b), area other (Figure damages2). While have much been damage also reported was observed and mapped in the from harbor high area‐resolution (see also satelliteFigure1 a,b),images other in damagesan area 2000 have m been away also from reported the blast and zone, mapped by the from DLR high-resolution team (see [7]). satellite In this images study, wein an extended area 2000 the m area away of frominterest the to blast cover zone, a circular by the area DLR of team a radius (see [of7]). 3000 In this m around study, wethe extendedharbor area. the Fourarea ofmain interest zones to were cover defined a circular as follows: area of a Zone radius A ofis an 3000 area m aroundup to 500 the m harbor from the area. blast Four site, main Zone zones B is anwere area defined from as500 follows: m to 1000 Zone m Afrom is an the area site up of to the 500 explosion, m from the and blast Zone site, C Zone covers B is areas an area up from to 2000 500 m m awayto 1000 from m from the harbor. the site ofFinally, the explosion, Zone D andis defined Zone C as covers the area areas from up to2000 2000 to m 3000 away m fromaway the from harbor. the harborFinally, site. Zone The D is northern defined as part the areaof the from study 2000 area to 3000 is the m away Mediterranean from the harbor Sea; site.therefore, The northern no images part analysisof the study was areacarried is the out Mediterranean at this part. Sea; therefore, no images analysis was carried out at this part.

FigureFigure 2. StudyStudy area area indicating indicating the the harbor harbor area area (blast (blast site) site) with with a a yellow yellow star star and and the the various zones zones createdcreated for for further further consideration consideration covering distances from 0 to 3000 m away from the blast site.

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3. Materials and Methods

3.1. Methodology For the needs of the study, Copernicus radar freely distributed datasets were used. Once the available images were selected from the Alaska Satellite Facility (ASF) services [10], these were further processed by the Hybrid Pluggable Processing Pipeline (HyP3) cloud platform [11] (see more in Section 3.3). The radar processing at the HyP3 cloud platform included two products as follows. A change detection map based on a pair of Radiometrically Terrain-Correct (RTC) Sentinel-1 data and an Interferometric Synthetic Aperture Radar (InSAR) analysis map, using both ascending and descending Sentinel-1 images, was used to map changes in the area. For the ﬁrst product, the log diﬀerence for both VV (vertical transmit, vertical receive) and VH (vertical transmit, horizontal receive) backscattering gamma 0 amplitude polarizations of the RTC Sentinel-1 images was estimated based on the following equation for each image pixel:

Change detection = Log10 (D2/D1), (1) where D1 refers to the Sentinel-1 with the earlier acquisition date (before the explosion) and D2 refers to the Sentinel-1 image taken after the explosion. Due to the small temporal window of the images, any changes can be linked to the blast event. A significant change of the gamma-0 amplitude can indicate areas of damage. Positive values indicate an increase in radar backscatter from the first date to the second, while negative values indicate a decrease. It should be mentioned that Sentinel-1 images are first filtered using the Enhanced Lee speckle filter, while the images are corrected based on the input Digital Elevation Model (DEM) of the area of interest. Then, the images are co-registered and radiometrically corrected (removal of radiometric distortions). The log difference of the gamma-0 amplitude for the VV and VH polarizations input images is estimated following Equation (1). In addition, the coherence values were also mapped over the area. The degree of coherence is defined as the normalized complex correlation coefficient of the complex backscatter intensities S1 and S2 [12]. Coherence values can be estimated using Equation (2), where the * denotes the complex conjugate. γ = | S S * /√( S S * S S * )| (2) h 2 × 1 i h 1 × 1 ih 2 × 2 i Coherence values may range from 0 to 1; the larger the number, the higher the coherence. A low coherence value may indicate areas that have changed between the two overpasses of the Sentinel-1 radar sensors. The InSAR analysis of an ascending and a descending pair of Sentinel-1 images was executed—as mentioned before in the HyP3 platform—using the Gamma software. In short, the InSAR Gamma algorithm for the Sentinel-1 images comprises of eleven (11) steps as follows. (1) determine the overlapping area using the two Sentinel-1 images. (2) Download the European Union Digital Elevation Model (EU-DEM) v1.1, a hybrid Shuttle Radar Topography Mission digital elevation model (SRTM), and ASTER Global Digital Elevation Map (ASTER GDEM) data fused by a weighted averaging approach over the area of interest [13]. (3) Create a lookup table between the DEM and Sentinel-1 imagery. A lookup table for SLC co-registration, considering terrain heights, is created. The data are filtered with adaptive data filtering (ADF). (4) Create a differential interferogram using the DEM height along with the co-registration with the DEM. (5–6) Removal of the flat earth phase (5) and the topographic phase (6). (7) Refinement of the slave image (prior to the event) with the master image (after the event). A check for convergence is performed using the azimuth offset as a limit (less than 0.02 pixels). (8) Resampling of the slave to match the master image. (9) Create the final interferogram. (10) Unwrap the phase using the Minimum Cost Flow (MCF) algorithm. The MCF algorithm permits a global automatic optimization robust phase unwrapping, taking into consideration disconnected areas of high coherence. (11) Geocode the results. The end products, as well as the sub-products Sensors 2020, 20, 6382 5 of 21 developed through this processing chain of the InSAR analysis, are also available for downloading by the end-user (see more in [11]). Then, the final products from the change detection and the InSAR analysis were imported into a Geographical Information System (GIS), namely the ArcGIS v.10 software, whereas zonal statistics per each zone, namely Zone A to Zone D, were estimated. The overall results generated a damage detection proxy map that was also compared with available high-resolution RGB WorldView-2 images as well as other reports from international agencies, namely the German Aerospace Center (DLR) and the ARIA teams. In addition, Sentinel-2 optical images, taken before and after the explosion, were downloaded from the Sentinel Data Hub [14] in order to compare the results from the Sentinel-1 analysis.

3.2. Datasets As mentioned earlier, two pairs of Sentinel-1 images were used for the change detection and the InSAR analysis. Table1 indicates the characteristics of the two Sentinel-1B Interferometric Wide (IW) images used for the change detection analysis (see Equation (1)). An image with an acquisition date of the 24th of July 2020 and an image taken on the 5th of August 2020 were used. The images were taken from the same sensor (S1B) as well as with the same pass direction (ascending orbit) to minimize any noise.

Table 1. Sentinel-1 data used for the change detection analysis.

Characteristics Characteristics Name Prior to the Event (Reference-D1) After the Event (Secondary–D2) Date The 24th of July 2020 The 5th of August 2020 Mode Interferometric Wide swath (IW) Interferometric Wide swath (IW) Satellite Sentinel-1B Sentinel-1B Absolute Orbit number 22,615 22,790 Pass direction ASCENDING ASCENDING Polarization VV + VH VV + VH Product type ground range (GRD) ground range (GRD) Path 14 14 Frame 107 107

In addition, two pairs of Sentinel-1B images in descending and ascending orbits were processed for the InSAR analysis. The images were taken on the 31st of July 2020 and the 6th of August 2020 with a descending orbit (see Table2), while another pair of Sentinel-1B images in ascending orbit, with an overpass at the 29th of July 2020 and the 10th of August 2020, was used (see Table3).

Table 2. Sentinel-1 data used for the Interferometric Synthetic Aperture Radar (InSAR) analysis (descending orbit).

Characteristics Characteristics Name Prior to the Event (Reference—D1) After the Event (Secondary—D2) Date The 31st of July 2020 The 6th of August 2020 Mode Interferometric Wide swath (IW) Interferometric Wide swath (IW) Satellite Sentinel-1B Sentinel-1B Absolute Orbit number 33,693 22,797 Pass direction DESCENDING DESCENDING Polarization VV + VH VV + VH Product type Single Look Complex (SLC) Single Look Complex (SLC) Path 21 21 Frame 480 480 Sensors 2020, 20, 6382 6 of 21

Table 3. Sentinel-1 data used for the InSAR analysis (ascending orbit).

Characteristics Characteristics Name Prior to the Event (Reference—D1) After the Event (Secondary—D2) Date The 29th of July 2020 The 10th of August 2020 Mode Interferometric Wide swath (IW) Interferometric Wide swath (IW) Satellite Sentinel-1B Sentinel-1B Absolute Orbit number 22,688 22,863 Pass direction ASCENDING ASCENDING Polarization VV + VH VV + VH Product type Single Look Complex (SLC) Single Look Complex (SLC) Path 87 87 Frame 107 107

Finally, Sentinel-2 images of Level 2A (Bottom of Atmosphere (BOA) reﬂectance images) with a spatial resolution of 10/20 m acquired before (24th of July 2020) and after (8th of August 2020) the explosion, with limited cloud coverage over the area of interest were downloaded from the Sentinel Data Hub. The RGB optical composite of the Sentinel-2 image taken after the explosion and the NIR-R-G pseudo color composite of the same image were used for comparison purposes through interpretation with the Sentinel-1 image analysis. In addition, Principal Component Analysis (PCA) was applied to the integrated Sentinel-2 dataset (images of 24th of July 2020 and 8th of August 2020 together) to detect any signiﬁcant spectral changes near the harbor area. The PCA is a well-known statistical analysis process that takes into account the spectral variations within the image [15]. While this type of analysis is implemented in single image processing, it can also be tested for a multi-temporal dataset, whereas the temporal variance will be taken into consideration. Therefore, the PCA can be used as a fast change detection method, in cases where the radiometric noise and the time span between the images is minimum [16,17]. In our example, we used Sentinel 2 optical images of Level 2A processing, meaning that the images are geometric, radiometric, and atmospherically corrected. Therefore, the PCA can explain the changes due to the explosion over the area.

3.3. Big-Data Cloud Platform The HyP3 big data cloud platform provided by the ASF was used to process the radar Sentinel-1 datasets. The platform is designed on Amazon cloud services, and upon submitting the request form, the end users may select several products and processing algorithms. Further details for the potentials of this platform can be found in [11]. The ﬁnal products are distributed through Amazon’s simple storage service (S3) and are available to the end users for downloading. The HyP3 platform currently operates in a beta version, and its access is limited to restricted users. It runs a series of diﬀerent radar processing chains such as Interferometric SAR (InSAR), Radiometric Terrain Correction (RTC), and change detection. The processing of the radar images is based on either the Sentinel Toolbox [18] or Gamma software [19].

4. Results This section provides an overview of the results generated for this analysis of the radar images. These results are also compared with optical Sentinel-2 data as well as other high-resolution images such as those of WorldView-2 and the public products from the DLR and ARIA teams.

4.1. Sentinel-1 Analysis Below, the results from the Sentinel-1 radar images is provided, following the change detection image analysis in Section 4.1.1 and the InSAR analysis in Section 4.1.2. SensorsSensors 20202020,, 2020,, 6382 x 77 ofof 2122

4.1.1. Sentinel‐1 Change Detection Analysis 4.1.1. Sentinel-1 Change Detection Analysis Figure 3 presents the results of the change detection analysis. The two Sentinel‐1B images with acquisitionFigure3 dates presents of 24th the of results July 2020 of the and change 5th of detection August 2020 analysis. (Table The 1) have two Sentinel-1Bbeen processed images based with on acquisitionEquation (1). dates Two of threshold 24th of July values 2020 were and selected 5th of August to classify 2020 the (Table change1) have detection been processed results for based both onVV Equationand VH polarizations (1). Two threshold of the values Sentinel were‐1 images. selected The to classifydefault thevalue change of −0.25 detection and +0.25 results from for the both HyP3 VV andplatform VH polarizations was kept, while of the another Sentinel-1 lower images. threshold The of default −0.15 valueand +0.15 of 0.25 was and also+ applied0.25 from to the visualize HyP3 − platformany minor was changes kept, while in the another magnitude lower thresholdof the backscattered of 0.15 and signals+0.15 was of the also two applied images. to visualize Areas that any − minorundergo changes a negative in the magnitudechange between of the backscatteredthe two images, signals and of therefore the two images. recorded Areas a decrease that undergo in the a negativebackscatter change returns, between are displayed the two images, in red, and while therefore those displaying recorded a a decrease positive in change the backscatter that indicates returns, an areincrease displayed in backscatter in red, while returns those are displaying displayed a positive in blue changecolor. In that Figure indicates 3, the an four increase zones in under backscatter study returns(Zone A are to displayedZone D) are in bluealso presented. color. In Figure Figure3, the 3 is four a focus zones of underthe results study around (Zone Athe to harbor Zone D) area are (blast also presented.site) indicating Figure the3 issame a focus results. of the The results extent around of Figure the 4 harbor is indicated area (blast in Figure site) indicating3, with a white the same dash results.line around The extentthe blast of Figuresite. As4 isthe indicated individual in Figureproducts3, with from a whitethe VV dash and line the aroundVH change, the blast detection site. Aspolarizations the individual tend products to give different from the accuracy VV and levels; the VH these change, can detectionbe found polarizationsin Appendix A. tend However, to give dialthoughfferent accuracy the VH polarization levels; these signals can be found(Figures in AppendixA1 and A3)A. tend However, to have although much lower the VH coherence polarization than signalsthe VV (Figurespolarization A1 andsignals A3 )(Figures tend to A2 have and much A4), lowerboth of coherence them were than able the to indicate VV polarization areas of changes signals (Figuresnear the A2blast and zone. A4), both of them were able to indicate areas of changes near the blast zone. IntegratingIntegrating both both these these polarization polarization results, results, the the majority majority of the of di thefferences differences of the of backscattered the backscattered signal (VVsignal and (VV VH and polarizations) VH polarizations) are around are around the blast the zone blast mainly zone mainly in Zone in A Zone and ZoneA and B Zone(Figures B (Figures3 and4). 3 Thisand 4). is alsoThis evident is also evident in the focus in the area focus in area Figure in 4Figure. However, 4. However, damages damages have also have been also recorded been recorded on a broaderon a broader area mainlyarea mainly to the to east the and east south and south of the of blast the site,blast extended site, extended in Zones in Zones C and C D, and and D, in and some in areassome evenareas beyondeven beyond Zone Zone D (> 3000D (>3000 m). m). It is It also is also important important to to highlight highlight that that the the majority majority of of thesethese changeschanges areare consideredconsidered quite quite significant significant ( >(>−0.25 or >>0.25,0.25, indicated indicated with red color in Figures 3 3 and and4 ),4), − whichwhich areare thethe resultresult fromfrom thethe suddensudden catastrophecatastrophe aroundaround the the harbor harbor zone. zone. DetailsDetails forfor thethe individualindividual resultsresults ofof thethe VVVV andand thethe VHVH polarizationspolarizations cancan bebe found found in in Appendix AppendixA A (see (see Figures Figures A1 A1–A4).–A4).

FigureFigure 3.3. Change detection results using the log didifferencefference ofof thethe VVVV andand thethe VHVH backscatteringbackscattering amplitude.amplitude. Significant Significant changes changes of the pair of images are presented with dark redred andand blueblue colorscolors (> 0.25 or >0.25 differences), while other minor changes in the range of 0.25 to 0.15 and 0.15 to 0.25 (>−0.25 or >0.25 differences), while other minor changes in the range of− −0.25 to −−0.15 and 0.15 to 0.25 areare alsoalso presentedpresented inin lightlight redred andand blueblue colors,colors, respectively.respectively. TheThe fourfour zoneszones underunder studystudy (Zone(Zone AA toto ZoneZone D) are also given. The The location location of of the the blast blast size size is isshown shown with with the the yellow yellow star star at the at the center center of the of thefigure. figure. The The white white dashed dashed rectangle rectangle around around the the blast blast site site is is indicating indicating the the zoom zoom area presented inin FigureFigure4 4..

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Figure 4. ChangeChange detection detection results—as results—as in Figure Figure 33,, seesee before—forbefore—for thethe areaarea aroundaround thethe blastblast sitesite nearnear the harbor of Beirut (indicated with a white dashed line in Figure3 3).).

Table4 4 summarizessummarizes the damaged areas areas for for each each zone zone (Zone (Zone A A to to Zone Zone D) D) as asestimated estimated from from the GISthe GISenvironment environment using using the log the difference log difference of the of VH the polarizations VH polarizations (see also (see alsoFigures Figures A1 and A1 A3).and A3The). 2 areaThe area of destruction of destruction for each for each zone zone is estimated is estimated in hectares in hectares units units (10,000 (10,000 square m ). meters). As shown, As the shown, majority the majorityof the damaged of the damaged area is found area inis found Zone A in with Zone an A area with of an 17.9 area hectares, of 17.9 hectares, which equals which 37.2% equals of 37.2% the total of thedamaged total damaged area, while area, another while 24.5% another of the 24.5% total of damage the total area damage is reported area in is Zone reported B that in extends Zone B up that to 1000extends m awayup to from1000 m the away blast from site. the Zonal blast statistics site. Zonal analysis statistics indicate analysis a similar indicate pattern a similar for Zone pattern C and for Zone DC and with Zone an approximately D with an approximately average damage average area ofdamage 9 hectares area (18.5%); of 9 hectareshowever, (18.5%); the majority however,from the these areas are in the medium log difference threshold units ( 0.25 to 0.15 and 0.15 to 0.25). Zone A majority from these areas are in the medium log difference threshold− units− (−0.25 to −0.15 and 0.15 to also reports the highest percentage in terms of areas of having a log difference more than > 0.25 and 0.25). Zone A also reports the highest percentage in terms of areas of having a log difference− more >than0.25 > (17.9−0.25 hectares and >0.25 or (17.9 62.0% hectares and 11.8 or hectares 62.0% and or 76.9%,11.8 hectares respectively). or 76.9%, respectively).

Table 4.4. Zonal statisticsstatistics usingusing the the VH VH polarization polarization for for each each log log di differenceﬀerence threshold threshold and and zone zone (Zone (Zone A 2 Ato to Zone Zone D). D). Areas Areas are are measured measured in in hectares hectares (10,000 (10,000 m square). meters).

Log DifferenceLog DiﬀerenceZone A Zone AZone Zone B B ZoneZone C C Zone DZone Total D Total >−0.25 > 0.254.4 4.40.5 0.5 0.60.6 1.61.6 7.1 7.1 − 0.25 to 0.15 4.4 7.2 6.2 5.6 23.4 −0.25 to −0.15− − 4.4 7.2 6.2 5.6 23.4 0.15 to 0.25 5.5 3.8 1.8 1.9 13.0 0.15 to 0.25 5.5 3.8 1.8 1.9 13.0 >0.25 3.6 0.4 0.3 0.5 4.7 >0.25 Total3.6 17.90.4 11.8 8.90.3 9.50.5 48.2 4.7 Total 17.9 11.8 8.9 9.5 48.2

The next table (Table5) shows the zonal statistics for the log di ﬀerence of the VV polarizations The next table (Table 5) shows the zonal statistics for the log difference of the VV polarizations (see also Figures A2 and A4). The VV polarizations reported a larger area of damage (99.0 hectares) (see also Figures A2 and A4). The VV polarizations reported a larger area of damage (99.0 hectares) against the 48.2 hectares of the VH polarization analysis (Table4). Zone A reports the highest percentage against the 48.2 hectares of the VH polarization analysis (Table 4). Zone A reports the highest of the top threshold value (> 0.25) with an area of 14.4 hectares out of the total 25.8 hectares of Zone A. percentage of the top threshold− value (>−0.25) with an area of 14.4 hectares out of the total 25.8 It is worth noting also that statistics from all zones (Zone A to Zone D) tend to give similar total areas hectares of Zone A. It is worth noting also that statistics from all zones (Zone A to Zone D) tend to of damage ranging from 20.9 hectares (Zone D) to 26.9 hectares (Zone C). give similar total areas of damage ranging from 20.9 hectares (Zone D) to 26.9 hectares (Zone C).

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Table 5. Zonal statistics using the VV polarization for each log diﬀerence threshold and zone (Zone A to Zone D). Areas are measured in hectares (10,000 m2).

Log Diﬀerence Zone A Zone B Zone C Zone D Total > 0.25 14.4 6.0 3.8 2.5 26.7 − 0.25 to 0.15 6.0 12.6 16.1 10.7 45.5 − − 0.15 to 0.25 3.1 4.7 5.3 5.9 19.0 >0.25 2.3 2.1 1.7 1.7 7.8 Total 25.8 25.4 26.9 20.9 99.0

Synthesizing the results from Tables4 and5, we can see the overall changes both for the VV and the VH polarizations. It should be mentioned that the new outcome that corresponds to the total damaged areas (shown in Table6) is not the sum of Tables4 and5, but the statistics after the spatial union of these tables. This affects the overlapping areas, whereas the higher-ranking threshold was kept. Based on the findings of Table6, we can estimate the total damaged areas within 3000 m away from the blast site to be around 117 hectares. In detail, an area of 29.0 hectares was mapped in Zone A (0 500 m from the blast site), while an area of approximately 32.0 hectares was mapped for Zones B − and C (500–1000 m and 1000 2000 m from the blast site, respectively). Finally, an area of 24.2 hectares − was estimated in Zone D (2000 3000 m). − Table 6. Zonal statistics using the synthesis of the VH and VV polarizations for each log difference threshold and zone (Zone A to Zone D). Areas are measured in hectares (10,000 m2).

Log Diﬀerence Zone A Zone B Zone C Zone D Total > 0.25 11.0 5.2 4.1 2.9 23.3 − 0.25 to 0.15 8.3 17.0 19.6 12.6 57.4 − − 0.15 to 0.25 5.9 7.8 6.4 6.9 27.0 >0.25 3.7 2.1 1.6 1.9 9.2 Total 29.0 32.0 31.7 24.2 116.9

The majority of the damaged areas are found between a medium threshold value (from 0.25 to − 0.15 and 0.15 to 0.25 log diﬀerence of the VV and VH polarization) with a total area of 84.4 hectares − (57.4 and 27.0 hectares, respectively) that equals 72.1% of the total damaged area. The remaining 27.9% of the damaged area (32.5 hectares) is found at the higher threshold values (> 0.25 and >0.25). − 4.1.2. Sentinel-1 InSAR Image Analysis Following the change detection analysis, the InSAR analysis for mapping the micromovements of the area because of the generated seismic waves from the explosion was performed. As mentioned earlier for the needs of the InSAR analysis, a pair of Sentinel-1B Single Look Complex (SLC) images was used, with acquisition dates of 31st July 2020 and the 6th of August 2020. Figure5 shows the wrapped interferogram around the harbor site for the descending orbit (see Table2). The wrapped interferogram is proportional to the di ﬀerence in path lengths for the SAR Sentinel-1 image pair [12]. Interferometric fringes, visible in Figure5, represent a full 2 π cycle of phase change. The deformation fringes, visible around the area of the harbor in Figure5, can be linked to the ground movement of that area due to the explosion. It should be mentioned that this observation is only visible in the area around the harbor site. The relative ground movement between the blast site and any other point of the area can be calculated by counting the fringes and multiplying them by half of the wavelength. The closer together the fringes, the higher the deformation on the ground. Sensors 2020, 20, x 10 of 22

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Figure 5. Wrapped interferogram showing the deformation fringes as a result of the Beirut explosion as derived from the Sentinel‐1 SAR images. The color shows a full 2π cycle of phase change.

Based on this wrapped interferogram, which is mainly useful for visualization purposes, the unwrapped interferogram was calculated for the descending orbit Sentinel‐1 images (Figure 6a), which corresponds to the change in the distance along the line of sight of the sensor. An unwrapped interferogramFigure 5. WrappedWrapped converts interferogram the wrapped showing 2‐π thescale deformation into a continuous fringes as a scaleresult (ofof the multiples Beirut explosion of pi). The Minimumas derived Cost from Flow the (MCF) Sentinel Sentinel-1 and‐1 SAR triangulation images. The methodscolor shows (see a full more 2 π π cyclein [20]) of phase are applied change. through the HyP3 platform for the phase unwrapping process. BasedFigure on 6b this shows wrapped the results interferogram, from the ascendingwhich is mainly orbit Sentinel useful for‐1 images. visualization Values purposes, greater than the unwrappedzero (positive) interferogram indicate relative was was movementcalculated calculated awayfor for the the from descending the sensor orbit orbit (subsidence), Sentinel Sentinel-1‐ 1while images images negative (Figure (Figure values 66a),a), whichindicate corresponds movement to toward the change the sensor in the (uplift).distance Figure along 6cthe shows line of the sight same of the area sensor. with theAn resultsunwrapped from interferogramthe change detection convertsconverts analysis. the the wrapped wrapped The ground 2-π 2scale‐π relativescale into ainto continuous displacements a continuous scale around (of scale multiples the(of areamultiples of pi). of the The ofharbor Minimum pi). The(see MinimumCostthe yellow Flow (MCF)Cost rectangle Flow and triangulationin(MCF) Figure and 6a,b) triangulation methods were estimated (see methods more between in [(see20]) more are −15 applied mmin [20]) relative through are applied displacement the HyP3 through platform along the HyP3forwith the the platform phase line of unwrapping forsight the of phase the process. sensor unwrapping for the descendingprocess. orbit and −5 mm for the ascending orbit. Figure 6b shows the results from the ascending orbit Sentinel‐1 images. Values greater than zero (positive) indicate relative movement away from the sensor (subsidence), while negative values indicate movement toward the sensor (uplift). Figure 6c shows the same area with the results from the change detection analysis. The ground relative displacements around the area of the harbor (see the yellow rectangle in Figure 6a,b) were estimated between −15 mm relative displacement along with the line of sight of the sensor for the descending orbit and −5 mm for the ascending orbit.

Figure 6.6. ((aa)) Unwrapped Unwrapped interferogram interferogram as as derived derived from from the the Sentinel-1 Sentinel SAR‐1 SAR images images in descending in descending orbit (seeorbit Table (see2 Table). ( b) Unwrapped2). (b) Unwrapped interferogram interferogram as derived as fromderived the from Sentinel-1 the Sentinel SAR images‐1 SAR in ascendingimages in orbitascending (see Table orbit3 (see), and Table ( c) VV3), and and ( VHc) VV log and di ﬀVHerence log difference polarizations polarizations of the same of area the same (as in area Figure (as3 in). HarborFigure 3). area Harbor is indicated area is in indicated a yellow in square. a yellow square.

Figure6b shows the results from the ascending orbit Sentinel-1 images. Values greater than zero (positive) indicate relative movement away from the sensor (subsidence), while negative values Figure 6. (a) Unwrapped interferogram as derived from the Sentinel‐1 SAR images in descending indicate movement toward the sensor (uplift). Figure6c shows the same area with the results from the orbit (see Table 2). (b) Unwrapped interferogram as derived from the Sentinel‐1 SAR images in changeascending detection orbit analysis. (see Table The 3), groundand (c) VV relative and VH displacements log difference aroundpolarizations the area of the of same the harbor area (as (see in the Figure 3). Harbor area is indicated in a yellow square.

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The line of sight (LOS) displacement map was estimated for both the ascending and the

Sensorsdescending2020, 20 pairs, 6382 of SLC Sentinel‐1 (Figure 7a,b respectively). For estimating the LOS, we convert11 of the 21 unwrapped differential phase (Figure 6) into measurements of ground movement along the look vector (line of sight). Positive values indicate movement toward the sensor (such as uplift), while yellownegative rectangle values indicate in Figure movement6a,b) were away estimated from between the sensor15 (such mm as relative subsidence). displacement along with the − line ofIn sight the area of the of interest, sensor for and the based descending on the descending orbit and 5orbit mm Sentinel for the ascending‐1 images (see orbit. Table 2), we can − observeThe only line ofpositive sight (LOS) movements displacement (toward map the was sensor) estimated that range for both between the ascending 5 and 9 and cm; the however, descending the pairsmore ofsignificant SLC Sentinel-1 movement (Figure is7 a,bfound respectively). in the area For over estimating the harbor the LOS,area. weSimilar convert findings the unwrapped were also retrieveddiﬀerential from phase the (Figure analysis6) intoof the measurements ascending orbit of ground Sentinel movement‐1 images alongof Table the 3. look These vector results (line are of shownsight). Positivein Figure values 7b. Once indicate again, movement the positive toward values the sensor indicate (such uplift, as uplift), while while the negative negative values indicate subsidence. movement away Specific from pixels the sensorwithin (such the harbor as subsidence). area indicate the higher relative displacements.

Figure 7. (a(a) )Line Line of of sight sight (LOS) (LOS) displacements displacements as as derived derived from from the Sentinel Sentinel-1‐1 SAR images (descending orbit).orbit). Harbor Harbor area, area, as indicated as indicated in a yellow in square.a yellow (b )square. Line of sight (b) (LOS)Line displacementsof sight (LOS) as displacementsderived from the as Sentinel-1 derived SARfrom images the Sentinel (ascending‐1 SAR orbit). images Harbor (ascending area, as indicated orbit). in Harbor a yellow area, square. as indicated in a yellow square. In the area of interest, and based on the descending orbit Sentinel-1 images (see Table2), we can observeOf course, only positive these movementsexamples should (toward be the taken sensor) with that great range caution, between as a 5 low and 9coherence cm; however, is reported the more at thissignificant site due movement to the VV isand found VH changes in the area observed over the from harbor the area.explosion, Similar as findingswell as the were sensitivity also retrieved of the Sentinelfrom the‐1 analysis sensors of(in the relation ascending to their orbit wavelength Sentinel-1 imagesthat corresponds of Table3. to These ≈5.54 results cm). areHowever, shown the in findingsFigure7b. here Once are again, quite the noteworthy, positive values as indicateeven with uplift, low while coherence, the negative the InSAR values analysis indicate was subsidence. able to detectSpecific the pixels blast within site and the locate harbor significant area indicate changes, the higher such as relative those reported displacements. from the change detection analysis.Of course, these examples should be taken with great caution, as a low coherence is reported at this siteThe due coherence to the VV map and for VH both changes descending observed and from ascending the explosion, orbits as wellare shown as the sensitivityin Figure of8a,b, the Sentinel-1 sensors (in relation to their wavelength that corresponds to 5.54 cm). However, the findings respectively. Areas highlighted with purple color indicate high coherence≈ values, while the blue colorhere are indicates quite noteworthy, regions with as evenlow coherence. with low coherence, The latest the (regions InSAR analysiswith low was coherence), able to detect as shown the blast in bothsite and Figure locate 8a,b, significant are around changes, the blast such zone as those at the reported Beirut fromharbor, the indicating change detection a significant analysis. degree of changeThe between coherence the map two for overpasses both descending of the and Sentinel ascending‐1 (both orbits in are the shown ascending in Figure as8 a,b,well respectively. as for the Areasdescending highlighted orbits). with purple color indicate high coherence values, while the blue color indicates regions with low coherence. The latest (regions with low coherence), as shown in both Figure8a,b, are around the blast zone at the Beirut harbor, indicating a significant degree of change between the two overpasses of the Sentinel-1 (both in the ascending as well as for the descending orbits).

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Figure 8. 8. (a) Coherence(a) Coherence map generated map generated from the from descending the descending orbit images orbit (see Table images 2). (see(b) Coherence Table2). map(b) Coherence generated map fromgenerated the ascending from theorbit ascending images (see orbit Table images 3). (see(c) High Table‐resolution3). ( c) High-resolution WorldView‐2 image.WorldView-2 image.

4.2. Sentinel‐2 Image Analysis The results from the Sentinel‐1 images have also been compared with the Sentinel‐2 optical image taken after the 4th of August 2020. The RGB composite of the image is shown in Figure 9a. Destroyed buildings can be observed in the area near the exposition site; however, these are more

Sensors 2020, 20, 6382 13 of 21

4.2. Sentinel-2 Image Analysis Sensors 2020, 20, x 13 of 22 The results from the Sentinel-1 images have also been compared with the Sentinel-2 optical image taken after the 4th of August 2020. The RGB composite of the image is shown in Figure9a. evident in the NIR‐R‐G pseudo color composite shown in Figure 9b. Vegetated areas are shown with Destroyed buildings can be observed in the area near the exposition site; however, these are more red color in this figure. The correlation between the visual inspection of the Sentinel‐2 and the evident in the NIR-R-G pseudo color composite shown in Figure9b. Vegetated areas are shown Sentinel‐1 change detection results can be seen in Figure 9c. with red color in this ﬁgure. The correlation between the visual inspection of the Sentinel-2 and the Visual interpretation of the optical image can depict some of the most apparent damages over Sentinel-1 change detection results can be seen in Figure9c. the area (see black arrows in Figure 9).

Figure 9. (a()a )Sentinel Sentinel-2‐2 optical optical image image taken taken over over the thearea area of interest of interest on the on 8th the of 8th August of August 2020 (RGB 2020 (RGBcomposite). composite). (b) The ( bNIR) The‐R‐G NIR-R-G pseudo pseudocolor composite color composite of the same of the image same and image (c) the and change (c) the detection change detectionresults from results the from Sentinel the Sentinel-1‐1 image imageanalysis. analysis. Black Black arrows arrows indicate indicate destroyed destroyed areas areas from from image interpretation ofof the the Sentinel-2 Sentinel‐2 image, image, while while yellow yellow arrows arrows indicate indicate the non-detectablethe non‐detectable destroyed destroyed areas fromareas thisfrom analysis. this analysis. The location The location of the of blast the sizeblast is size shown is shown with the with yellow the yellow star. star.

VisualTo further interpretation process the of the optical optical Sentinel image‐ can2 images, depict somePCA of was the mostimplemented apparent in damages the integrated over the areadatasets (see of black the arrowsSentinel in‐2 Figure images9). taken on 24th of July 2020 and 8th of August 2020. The results are shown in Figure 10. Figure 10a shows the results from the first principal component (PC1) while Figure 10b shows an RGB pseudo color composite of PC1–PC3. Figure 10c shows the high‐resolution optical WorldView‐2 image over the area. The red color in Figure 10 shows higher PC1 values, thus indicating changes in the integrated temporal Sentinel‐2 dataset (i.e., 24th of July 2020 and 8th of August 2020 together). As evident, higher PC1 values are located around the blast area as well as in

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To further process the optical Sentinel-2 images, PCA was implemented in the integrated datasets of the Sentinel-2 images taken on 24th of July 2020 and 8th of August 2020. The results are shown in Figure 10. Figure 10a shows the results from the first principal component (PC1) while Figure 10b shows an RGB pseudo color composite of PC1–PC3. Figure 10c shows the high-resolution optical WorldView-2 image over the area. The red color in Figure 10 shows higher PC1 values, thus indicating changesSensors 2020 in, 20 the, x integrated temporal Sentinel-2 dataset (i.e., 24th of July 2020 and 8th of August14 2020 of 22 together). As evident, higher PC1 values are located around the blast area as well as in the western partthe ofwestern the harbor. part Similarof the findingsharbor. Similar can be alsofindings reported can from be also the pseudoreported color from composite the pseudo where color the firstcomposite three principal where the components, first three principal namely PC1 components, to PC3, have namely been PC1 used. to PC3, have been used.

FigureFigure 10.10. ((aa)) TheThe ﬁrstfirst principalprincipal componentcomponent (PC1)(PC1) resultsresults overover thethe areaarea aroundaround thethe harborharbor afterafter thethe PrincipalPrincipal ComponentComponent AnalysisAnalysis (PCA) (PCA) of of the the integrated integrated Sentinel-2 Sentinel‐2 optical optical images images ofof 24th24th ofof JulyJuly 20202020 andand 8th8th ofof August. ( (bb)) The The PC1–PC3 PC1–PC3 pseudo pseudo color color composite composite of of the the same same integrated integrated dataset dataset and and (c) (thec) the high high-resolution‐resolution WorldView WorldView-2‐2 image. image.

5. Comparison with Other Results The explosion at Beirut harbor has left significant and extensive damage in the surrounding buildings but also in other areas far away from the blast site. In the previous section, we have presented how this destruction could be detected by the Copernicus Sentinel‐1 sensor via two difference analyses: (a) a change detection approach and (b) an InSAR analysis. The overall results were also confirmed through other studies that have been reported only some hours after the event, such as the case of the ARIA and the DLR teams. In addition, the availability of a high‐resolution

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5. Comparison with Other Results The explosion at Beirut harbor has left signiﬁcant and extensive damage in the surrounding buildings but also in other areas far away from the blast site. In the previous section, we have presented how this destruction could be detected by the Copernicus Sentinel-1 sensor via two diﬀerence analyses:

Sensors(a) a change 2020, 20, detectionx approach and (b) an InSAR analysis. The overall results were also confirmed15 of 22 through other studies that have been reported only some hours after the event, such as the case of the WorldViewARIA and the‐2 image DLR teams. taken In over addition, the area the of availability interest has of aconfirmed high-resolution the results WorldView-2 generated image from taken the analysisover the of area the of Sentinel interest‐1 has sensors. confirmed the results generated from the analysis of the Sentinel-1 sensors. ToTo evaluate the overall performance of of this this study, study, we have compared the results presented aboveabove with with other other published published material material (maps) (maps) and and available available high high-resolution‐resolution WorldView WorldView-2‐2 images. images. Initially,Initially, we havehave comparedcompared ourour changechange detection results (Damage Proxy Map) with the one of the ARIA team team [8], [8], as as these these have have been been generated generated with with the the Sentinel Sentinel-1‐1 sensor sensor (as (as here). here). The The comparison comparison is shownis shown in inFigure Figure 11. 11 Figure. Figure 11a 11 showsa shows the the change change detection detection results results with with the the threshold threshold used used in in our our study,study, while Figure Figure 11b11b shows shows the the damaged damaged area area as as reported reported from from the the ARIA ARIA team. team. Although Although there there is nois no threshold threshold unit unit for for this this classification, classification, it itwas was evident evident that that a a similar similar pattern pattern is is reported reported from from both studies,studies, while while adjusting the the thresholds values would generate similar results. Once Once again, again, the the area with the most damages is the one around the blast site.

FigureFigure 11. ((aa)) Change Change detection detection results results as as generated generated from from the the VV VV and and VH VH log log differences diﬀerences of of the SentinelSentinel-1‐1 images, images, while while ( (bb)) indicates indicates the the Damage Damage Proxy Proxy Map Map generated generated from from the the Advanced Rapid ImagingImaging and and Analysis (ARIA) (ARIA) team team [8]. [8]. The The location location of of the the blast blast size size is is shown shown with the the yellow star.

WithinWithin thethe area area around around the the harbor harbor of Beirut, of Beirut, digitization digitization of the of buildings the buildings that have that been have damaged been damagedhas been reportedhas been by reported the MapAction by the MapAction platform [ 21platform]. Similar [21]. findings Similar with findings this report with this are alsoreport found are alsoin the found maps in generated the maps bygenerated the DLR by team, the DLR which team, are notwhich shown are not here. shown Figure here. 12 shows Figure the 12 focusshows area, the focuswhereas area, the whereas VV and the the VV VH and log the diff VHerence log polarizationsdifference polarizations (as found from(as found this study) from this are study) presented, are presented,and the digitized and the results digitized from results the MapAction from the platform. MapAction Comparing platform. these Comparing two results, these we two can results, see a high we cancorrespondence see a high correspondence between the two between products; the however, two products; these were however, generated these from were diff erentgenerated resolutions. from different resolutions.

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Figure 12.12.Change Change detection detection results results as generatedas generated from from the VV the and VV VH and log VH diﬀerences log differences of the Sentinel-1 of the imagesSentinel around‐1 images the around blast site, the while blast orange site, while polygons orange indicate polygons the indicate buildings the that buildings have been that damaged have been as reporteddamaged by as the reported MapAction by the platform MapAction [21] platform (digitized [21] by (digitized the author). by the author).

A directdirect comparisoncomparison of the high-resolutionhigh‐resolution WorldView-2WorldView‐2 imageimage thatthat waswas takentaken justjust afterafter thethe explosionexplosion over thethe harborharbor ofof BeirutBeirut waswas carriedcarried out.out. This image is shown in Figure 1313a,a, whereaswhereas thethe destroyeddestroyed buildingsbuildings can bebe observedobserved inin thethe image.image. The blast site, indicated with a yellow star inin Figure 1313a,a, has now been vanished due to the highly explosive chemicals that were kept in the site. Both thethe changechange detectiondetection analysis (Figure 1313b),b), as well as thethe InSARInSAR analysisanalysis (Figure(Figure 1313c),c), showsshows aa good correlationcorrelation with the visualvisual inspectioninspection of the high-resolutionhigh‐resolution WorldView-2WorldView‐2 imageimage despitedespite thethe didifferenceﬀerence inin theirtheir spatialspatial resolutionresolution (0.30(0.30 m of the WorldView-2WorldView‐2 imageimage againstagainst thethe 10m10m resolutionresolution of thethe Sentinel-1Sentinel‐1 results).results).

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FigureFigure 13. 13.(a ()a High-resolution) High‐resolution WorldView-2 WorldView‐2 imageimage takentaken somesome hours after the exposition over over the the harborharbor of of Beirut. Beirut. TheThe blast site site is is indicated indicated with with a yellow a yellow star. star. (b) Change (b) Change detection detection as generated as generated from fromthe theVV VVand and VH VH log log differences diﬀerences of ofthe the Sentinel Sentinel-1‐1 image image and and (c ()c )the the InSAR InSAR analysis analysis (LOS (LOS of of the the descendingdescending orbit orbit images) images) over over the the area. area. The The location location of of thethe blastblast sizesize is shown with the yellow yellow star. star.

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6. Conclusions A damage proxy map based on the processing of Sentinel-1 images was carried out in this study over the harbor area of Beirut, Lebanon after the high explosion occurred in the area on the 4th of August 2020. The analysis of this study was achieved through a change detection analysis of Sentinel-1 images, while the seismic waves results were also mapped using an InSAR Sentinel-1 image analysis. The analysis was able to be carried out due to the systematic observations of the Copernicus Sentinel-1 sensors operated by the European Space Agency (ESA), while the processing chain was carried out by the big-data cloud HyP3 platform operated by ASF. This combination allows end users to process in a short time (less than 1-h computational time) a series of radar processing chains in almost global coverage. Of course, this is feasible due to the free and fully open policy (FFO) of the Sentinel datasets. The findings of this study are well aligned with other products delivered by specialized space centers such as the DLR and the ARIA teams, while a comparison was also performed using a high-resolution WorldView-2 image. While the medium-resolution Sentinel-1 images and their products do not allow us to detect individual destruction in a building level, the 10-m resolution is enough for estimating with high accuracy the damaged area. The use of Sentinel-2 images could support the detection analysis through visual interpretation of significant changes in the landscape as well as through a PCA temporal analysis. This was more obvious using the NIR-R-G pseudo color composite; however, due to the spatial resolution of the image (20 m), the detection of smaller areas was quite challenging. The use of both sensors, namely the Sentinel-1 and Sentinel-2, can be further utilized in the future to support emergency situations, with an almost global coverage. In this case, other events not so well known can be monitored by the scientific local community to support specific emergency needs. Of course, these maps can be only indicative of the destruction, as ground verifications are needed. In addition, the analysis of the results from this series of analyses should be seen with great caution, as the images might suffer from other factors that influence the final outcomes. For instance, the relative micromovements can be due to other sources, such as the atmospheric component and even DEM accuracy, while the change detection of the VV and the VH polarizations need to be linked also with their coherence. Overall, the use of Sentinel data was found very promising for supporting ground investigations after an event (such as the one presented here, or even a natural hazard). Future steps can include considering the automation of the whole procedure, minimizing the time between the data processing and the event itself.

Funding: This communication is submitted under the NAVIGATOR project. The project is being co-funded by the Republic of Cyprus and the Structural Funds of the European Union in Cyprus under the Research and Innovation Foundation grant agreement EXCELLENCE/0918/0052 (Copernicus Earth Observation Big Data for Cultural Heritage). Acknowledgments: The author would like to acknowledge the ASF DAAC 2020 using GAMMA software. The articles contain modified Copernicus Sentinel data 2020, processed by ESA. Acknowledgements are also given to the European Space Imaging for the WorldView-2 images. Conflicts of Interest: The author declares no conflict of interest. Sensors 2020, 20, 6382 19 of 21

Sensors 2020, 20, x 19 of 22 AppendixSensors A 2020, 20, x 19 of 22 Appendix A Appendix A

Figure A1.FigureChange A1. Change detection detection results results using using the the loglog difference difference of of the the VH VH backscattering backscattering amplitude. amplitude. Figure A1. Change detection results using the log difference of the VH backscattering amplitude. SignificantSignificant changes changes of the of pair the pair of images of images are are presented presented with with dark red red and and blue blue colors colors (>−0.25 (> or0.25 >0.25 or >0.25 Significant changes of the pair of images are presented with dark red and blue colors (>−0.25 −or >0.25 differences),differences), while other while minor other minor changes changes in the in range the range of 0.25 of − to0.25 0.15to −0.15 and and 0.15 0.15 to 0.25 to 0.25 are alsoare also presented differences),presented in whilelight red other and minor blue colors, changes respectively. in the range The− offour −0.25 zones− to −under0.15 andstudy 0.15 (Zone to 0.25A to areZone also D) in light red and blue colors, respectively. The four zones under study (Zone A to Zone D) are also given. presentedare also given. in light The red location and blue of colors, the blast respectively. size is shown The four with zones the underyellow study star. (Zone The whiteA to Zone dashed D) The locationarerectangle also of given. the around blast The the size locationblast is site shown of is indicatingthe withblast the sizethe yellowzoomedis shown‐ star.in witharea The presentedthe white yellow in dashed star.Figure The A3. rectangle white dashed around the blast siterectangle is indicating around the the zoomed-inblast site is indicating area presented the zoomed in Figure‐in area presentedA3. in Figure A3.

Figure A2. Change detection results using the log difference of the VV backscattering amplitude. Figure A2.FigureSignificantChange A2. changesChange detection ofdetection the pair results resultsof images using using are the thepresented log difference di ffwitherence dark of ofredthe the andVV VV bluebackscattering backscattering colors (>−0.25 amplitude. or >0.25 amplitude. SignificantSignificantdifferences), changes changes while of the ofother pair the pair ofminor images of imageschanges are are presentedin presented the range withwith of − dark dark0.25 redto red − and0.15 and blue and blue colors 0.15 colors to(> −0.250.25 (> areor0.25 >0.25 also or >0.25 differences), while other minor changes in the range of −0.25 to −0.15 and 0.15 to 0.25 are− also differences), while other minor changes in the range of 0.25 to 0.15 and 0.15 to 0.25 are also presented − − in light red and blue colors, respectively. The four zones under study (Zone A to Zone D) are also given. The location of the blast size is shown with the yellow star. The white dashed rectangle around the blast site is indicating the zoomed-in area presented in Figure A4. Sensors 2020, 20, x 20 of 22 Sensors 2020, 20, x 20 of 22 presented in light red and blue colors, respectively. The four zones under study (Zone A to Zone D) arepresented also given. in light The red location and blue of colors, the blast respectively. size is shown The four with zones the underyellow study star. (ZoneThe white A to Zonedashed D) are also given. The location of the blast size is shown with the yellow star. The white dashed Sensorsrectangle2020, 20, 6382around the blast site is indicating the zoomed‐in area presented in Figure A4. 20 of 21 rectangle around the blast site is indicating the zoomed‐in area presented in Figure A4.

Figure A3. Change detection results as shown in Figure A1 (see before) for the area around the blast Figure A3. Change detection results as shown in Figure A1 (see before) for the area around the blast Figuresite near A3. the Change harbor detection of Beirut results(indicated as shown with a in white Figure dashed A1 (see line before) in Figure for the A1). area The around location the of blast the site near the harbor of Beirut (indicated with a white dashed line in Figure A1). The location of the blastsite near size theis shown harbor with of Beirut the yellow (indicated star. with a white dashed line in Figure A1). The location of the blast size is shown with the yellow star.

Figure A4. Change detection results—as results—as in in Figure Figure A2,A2, see see before—for the the area area around the the blast site Figurenear the A4. harbor Change of Beirut detection (indicated results—as with ina white Figure dashed A2, see line before—for in Figure theA2).A2 ).area The around location location the of blastthe the blast site sizenearsize is isthe shown harbor with of Beirutthe yellow (indicated star. with a white dashed line in Figure A2). The location of the blast size is shown with the yellow star.

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