geosciences

Article SAR Imaging of Archaeological Sites on Intertidal Flats in the German

Martin Gade 1,*, Jörn Kohlus 2 and Cornelia Kost 3 1 Universität Hamburg, Institut für Meereskunde, Bundesstraße 53, 20146 Hamburg, 2 Landesbetrieb für Küstenschutz, Nationalpark und Meeresschutz Schleswig-Holstein (LKN), Nationalparkverwaltung, 25832 Tönning, Germany; [email protected] 3 Wördemanns Weg 23a, 22527 Hamburg, Germany; [email protected] * Correspondence: [email protected]; Tel.: +49-40-42838-5450

Received: 4 August 2017; Accepted: 2 October 2017; Published: 13 October 2017

Abstract: We show that high-resolution space-borne synthetic aperture radar (SAR) imagery with pixel sizes smaller than 1 m2 can be used to complement archaeological surveys on intertidal flats. After major storm surges in the 14th and 17th centuries (“Grote Mandrenke”), vast areas on the German coast were lost to the sea. Areas of settlements and historical farmland were buried under sediments for centuries, but when the surface layer is driven away under the action of wind, currents, and waves, they appear again on the Wadden Sea surface. However, frequent flooding and erosion of the intertidal flats make any archaeological monitoring a difficult task, so that remote sensing techniques appear to be an efficient and cost-effective instrument for any archaeological surveillance of that area. Space-borne SAR images clearly show remains of farmhouse foundations and of former systems of ditches, dating back to the times before the “Grote Mandrenke”. In particular, the very high-resolution acquisition (“staring spotlight”) mode of the TerraSAR/TanDEM-X satellites allows detecting various kinds of remains of historical land use at high precision. Moreover, SARs working at lower microwave frequencies (e.g., that on Radarsat-2) may complement archaeological surveys of historical cultural traces, some of which have been unknown so far.

Keywords: storm surge; cultural traces; German Wadden Sea; intertidal flats; synthetic aperture radar; TerraSAR-X; St. Marcellus Flood; Burchardi Flood

1. Introduction In the Medieval Period, the German North Sea coastline looked very different from how it looks today: the North did not yet exist, but were part of the so-called “” (outer lands) that belonged to, or were connected with, the mainland [1]. Vast areas along the coast were dominated by swamps, marshes, and swamp forests, which often made any settlements difficult or impossible. From the 11th and 12th centuries on, however, many settlements on the German North Sea coast appeared. In those settlements houses were often built on dwelling mounds and were protected by small “summer dikes”, which could provide protection against high water only during summer, when storms are rare. Later on, coastal protection was improved through a system of “winter dikes” that were strong enough to prevent the marsh lands from frequent flooding [2]. Systems of drainage ditches were built to remove the water from the farmlands, thereby allowing for any kind of agriculture. Salt extraction from salty peat, which was found and mined all along the North Sea coast, became an important economical factor, but the peat mining also caused a decrease in surface level in the protected (diked) areas [2,3]. Together with the draining the extensive land use caused the land surface to be below the mean high tide level at many places. In mid-January 1362, severe westerly storms hit the Northern German coast, and on January 16 an immense storm tide flooded the coast, causing the small dikes to break at many places and killing a

Geosciences 2017, 7, 105; doi:10.3390/geosciences7040105 www.mdpi.com/journal/geosciences Geosciences 2017, 7, 105 2 of 14

Geosciences 2017, 7, 105 2 of 14 In mid-January 1362, severe westerly storms hit the Northern German coast, and on January 16 an immense storm tide flooded the coast, causing the small dikes to break at many places and killing greata great number number of peopleof people and cattle.and cattle. This majorThis ma stormjor surgestorm issurge known is inknown history in as history the Saint as Marcellus’the Saint FloodMarcellus’or “Grote Flood Mandrenke or “Grote ”Mandrenke (“great drowning” (“great ofdrowning men”) and of men”) caused and that caused vast areas that werevast areas lost to were the sea.lost Thoseto the areassea. Those haven’t areas been haven’t diked sincebeen thendiked (compare since th theen (compare left and middle the left panels and middle of Figure panels1). Thereafter, of Figure it1). took Thereafter, a long time it took until a long new time dikes until were new built dikes to protect were built the remaining to protect marshthe remaining land. The marsh new farmlandland. The wasnew characterized farmland was by characterized wider plots of by land, wider the plots dikes of enclosed land, the larger dikes polders enclosed than larger in the polders centuries than before, in the andcenturies farmhouses before, on and terps farmhouses were connected on terps by were narrow connected lanes. by narrow lanes.

Figure 1. Changes in the German North Sea coastline during the past 700 years (note the upper left Figure 1. Changes in the German North Sea coastline during the past 700 years (note the upper left part of each panel), after [1]. The left panel shows vast coastal wetlands that were lost after the first part of each panel), after [1]. The left panel shows vast coastal wetlands that were lost after the first “Grote Mandrenke” in 1362. In the middle panel, the horseshoe-shaped island of (white arrow) “Grote Mandrenke” in 1362. In the middle panel, the horseshoe-shaped island of Strand (white arrow) can be seen, whose major, central parts got lost after the second “Grote Mandrenke” in 1634. can be seen, whose major, central parts got lost after the second “Grote Mandrenke” in 1634. Another major storm surge occurred on 11 October 1634, again, causing death of people and cattle,Another after the major dikes storm had surgebroken occurred at several on places. 11 October This second 1634, again, “Grote causing Mandrenke death” (also of people known and as cattle,Burchardi after Flood the) dikes hit the had area broken of North at several places.in an economically This second “weakGrote period, Mandrenke after” (alsothe plague known had as Burchardicaused many Flood deaths) hit the only area about of North 30 years Frisia before. in an Th economicallye island of Strand, weak period,a horseshoe-shaped after the plague island had in causedthe center many ofdeaths the North only aboutFrisian 30 coast years (see before. the Thewhite island arrow of Strand,in the amiddle horseshoe-shaped panel of Figure island 1), in was the centerflooded of and the Northeventually Frisian broke coast up (see into the pieces white (compa arrowre in the the middle middle and panel right of panels Figure 1of), wasFigure flooded 1) and andfarmland, eventually farms, broke and up whole into piecesvillages (compare were destroyed. the middle Consequently, and right panels the ofBurchardi Figure1 )Flood and farmland,is still the farms,most-known and whole storm villages surge in were history destroyed. in the area Consequently, of the North the FrisianBurchardi Wadden Flood Sea.is still the most-known stormOver surge the in historyfollowing in thecenturies, area of thegreat North parts Frisian of this Wadden former Sea. agricultural area have been buried underOver muddy the followingand sandy centuries, sediments, great which parts nowadays of this formerform the agricultural German Wadden area have Sea. been This buriedarea is underbeing flooded, muddy andand sandyfalls dry, sediments, once during which each nowadays tidal cycle, form thereby the Germanmaking archaeological Wadden Sea. Thisexcavations area is beingvery difficult. flooded, However, and falls dry, under once the during permanent each tidalaction cycle, of the thereby tidal forces making the archaeological muddy and sandy excavations marine verysediments difficult. are However, partly driven under theaway, permanent and traces action of offormer the tidal peat forces digging, the muddy drainage and sandysystems, marine and sedimentssettlements are appear partly drivenagain on away, the andsurface traces of of the former intert peatidal digging,flats [3]. drainage Since thos systems,e areas and are settlementsdifficult to appearreach, againand thus on the to surfaceobserve of from the intertidal ground, flatsairborne [3]. Since sensors those have areas proven are difficult to be toadvantageous reach, and thus for to a observesystematic from observation ground, airborne of sensorsthose historic have proven places to be[4,5]. advantageous Airborne for surveillance, a systematic observationhowever, ofis thosecost-intensive historic places and the [4, 5use]. Airborne of unmanned surveillance, aerial vehicl however,es (UAV) is cost-intensive is prohibited and in thethe useNational of unmanned Park (to aerialwhich vehicles the study (UAV) area is belongs). prohibited Therefore, in the National high-r Parkesolution (to which space-borne the study sensors area belongs). are an alternative Therefore, high-resolutionsource of data that space-borne can be used sensors by archaeologists are an alternative for sourcetheir frequent of data thatsurveillance can be used of that by archaeologists area [6,7]. for theirSpaceborne frequent synthetic surveillance aperture of that radar area [(SAR)6,7]. imagery has been used for archaeological studies at variousSpaceborne places worldwide synthetic since aperture the 1980s radar [8,9]. (SAR) SAR imagery sensors has image been the used Earth’s for archaeological surface at high studies spatial atresolution various placesand independent worldwide sinceof daylight the 1980s and [8,9 ].weather SAR sensors conditions, image ther theeby Earth’s making surface dedicated at high spatialacquisitions resolution easier. and Particularly independent their of daylightall-weathe andr capabilities weather conditions, can make thereby them advantageous making dedicated over acquisitionsoptical sensors easier. in Particularlyareas where their those all-weather do not perfor capabilitiesm effectively can make [10]. them In advantageous this respect, overSAR-based optical sensorsarchaeological in areas studies where thosehave dobeen not focusing perform mainly effectively on [mountainous,10]. In this respect, arid, SAR-basedand forested archaeological regions [11– studies14], partly have using been SAR focusing data mainlyof very on high mountainous, resolution arid,[15,16]. and In forested order to regions complement [11–14 ],those partly studies, using SARhere, data we introduce of very high a new resolution domain [ 15for,16 SAR-driven]. In order toarchaeology. complement those studies, here, we introduce a new domain for SAR-driven archaeology.

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2.2. Materials andand MethodsMethods WeWe alreadyalready usedused SARSAR imagesimages toto derivederive surfacesurface characteristicscharacteristics ofof exposedexposed intertidalintertidal flatsflats andand toto detectdetect bivalvebivalve bedsbeds [[17].17]. During our analysesanalyses of SARSAR imageryimagery ofof intertidalintertidal flatsflats northnorth ofof PellwormPellworm wewe discovereddiscovered brightbright andand darkdark linearlinear featuresfeatures thatthat couldcould notnot bebe ofof naturalnatural origin,origin, butbut ratherrather bebe manifestationsmanifestations of anthropogenic structures dating back to the times before the “Grote Mandrenke”. TheseThese findingsfindings calledcalled forfor furtherfurther studiesstudies thatthat werewere basedbased onon high-resolutionhigh-resolution SAR SAR imagery. imagery.

2.1.2.1. Region ofof InterestInterest AA mapmap ofof thethe studystudy areaarea isis shownshown inin FigureFigure2 2,, with with the the location location of of the the SAR SAR image image of of Figure Figure3 3 inserted.inserted. This area lies in thethe centercenter ofof thethe NorthNorth FrisianFrisian WaddenWadden Sea,Sea, i.e.,i.e., inin anan areaarea thatthat waswas mostmost affectedaffected byby thethe majormajor stormstorm surgessurges inin thethe 14th14th andand 17th17th centuriescenturies (Figure(Figure1 1).). Remains Remains ofof the the former former islandisland ofof StrandStrand areare PellwormPellworm andand Nordstrand,Nordstrand, thethe latterlatter meanwhilemeanwhile beingbeing attachedattached toto thethe mainlandmainland afterafter long-standinglong-standing landland reclamation.reclamation. In In the area of interest traces of formerformer landland useuse havehave beenbeen observedobserved forfor somesome decadesdecades [[3,18].3,18].

FigureFigure 2.2.Area Area of of interest interest on on the the German German North North Sea coast,Sea coast, north north off off Pellworm island, i.e.,island, at the i.e., location at the oflocation the former of the island former of Strand.island of The Strand. red rectangle The red inrectangle the close-up in the on close-up the upper on leftthe marksupper theleft locationmarks the of thelocation SAR imageof the SAR shown imag in Figuree shown3. in Figure 3.

A TerraSAR-X image (11.6 km × 5.2 km) of the area of interest, acquired on 12 December 2012 A TerraSAR-X image (11.6 km × 5.2 km) of the area of interest, acquired on 12 December 2012 (at (at 05:33 UTC, 18 minutes after low tide), is shown in Figure 3. The islands of Pellworm and Hooge 05:33 UTC, 18 minutes after low tide), is shown in Figure3. The islands of Pellworm and Hooge can be can be seen in the lower and left parts of the image, respectively. Two red squares mark (1.0 km × 1.0 seen in the lower and left parts of the image, respectively. Two red squares mark (1.0 km × 1.0 km) km) areas, in which remains of former land use were found. areas, in which remains of former land use were found. Because of the low wind speed during that image acquisition (4 m/s), tidal channels and creeks Because of the low wind speed during that image acquisition (4 m/s), tidal channels and creeks show up dark: the radar backscattering from the water surface and moist soil mainly depends on its show up dark: the radar backscattering from the water surface and moist soil mainly depends on its surface roughness; therefore, a flat water surface at low wind speeds causes low radar backscatter surface roughness; therefore, a flat water surface at low wind speeds causes low radar backscatter and, and, thus, dark image areas. Bright features in the right image half mark the rough edges of tidal thus, dark image areas. Bright features in the right image half mark the rough edges of tidal creeks creeks and dry, sandy sediments [19]; however, they are not of interest herein. and dry, sandy sediments [19]; however, they are not of interest herein.

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Figure 3. TerraSAR-XTerraSAR-X imageimage ofof thethe areaarea ofof interest,interest, nortnorth of Pellworm and east of Hooge, acquired on Figure 3. TerraSAR-X image of the area of interest, north of Pellworm and east of Hooge, acquired on 12 December 2012. The red squares denote two areass thatthat werewere observedobserved inin greatergreater detail.detail. ©© DLRDLR 12 December 2012. The red squares denote two areas that were observed in greater detail. © DLR 2012. 2012.

TheThe locations locations of of all all (known) (known) culturalcultural tracestraces are depicted depicted in in Figure Figure 4,4, where where the the light-green light-green backgroundbackground color color marks marks the islandsthe islands of Pellworm of Pellworm (bottom) (bottom) and Hooge and (left)Hooge and (left) the yellowand the background yellow colorbackground the former color island the former of Strand island (Figure of Strand1). Several (Figure of 1). those Several traces of those are meanwhile traces are meanwhile lost, particularly lost, aroundparticularly the small around island the (so-called small island Hallig) (so-called of Südfall, Hallig) because of Südfall, of the because strong of erosion the strong by the erosion tidal by currents, the wind,tidaltidal and currents,currents, waves. wind,wind, The redandandrectangle, waves.waves. TheThe again, redred rectangl marksrectangl thee, again, location marks of the the SAR location image of shown the SAR in Figureimage 3. Fourshown dark in blue Figure arrows 3. Four mark dark those blue locations, arrowswhere mark those thethose dikes locations,locations, broke duringwherewhere thethe dikes seconddikes brokebroke “Grote duringduring Mandrenke thethe ” insecond 1634. “Grote Mandrenke” in 1634.

Figure 4. MapMap showingshowing inin yellowyellow thethe formerformer IslandIsland ofof Strand,Strand, andand inin greengreen today’stoday’s islandsislands PellwormPellworm Figure 4. Map showing in yellow the former Island of Strand, and in green today’s islands Pellworm and Hooge. The mainland and Nordstrand are located on the right. Locations of residuals of former and Hooge. The mainland and Nordstrand are located on the right. Locations of residuals of former landuse,landuse, alongalong withwith roads,roads, dikes,dikes, andand namesnames ofof formformer settlements are also inserted. The blue arrows landuse, along with roads, dikes, and names of former settlements are also inserted. The blue arrows denote those main locations, at which the dikess brokebroke inin 1634,1634, andand thethe redred rectanglerectangle denotesdenotes thethe denote those main locations, at which the dikes broke in 1634, and the red rectangle denotes the locationslocations ofof thethe SARSAR imageimage shownshown inin FigureFigure 3.3. Map:Map: C.C. Kost.Kost. locations of the SAR image shown in Figure3. Map: C. Kost.

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Many lines mark the (known) locations of former dikes and lanes, as derived from historical maps of that area. Within our study area, i.e., within the red rectangle, groups of parallel lines denote known locations of former drainage systems. The systems consisted of a mesh of ditches, whose remains have become visible after they appeared again on the surface of the intertidal flats.

2.2. Synthetic Aperture Radar Data A total of 19 SAR images acquired between 2012 and 2017 by the German TerraSAR/TanDEM-X and the Canadian Radarsat-2 satellites in high-resolution modes were available for our analyses of SAR signatures of historical land use. The pixel sizes of all images are on the order of 1 m2, or even below, thereby allowing for the detection of fine structures such as remains of narrow ditches, settlements, etc. All images were calibrated and geocoded, and the speckle noise was reduced applying a Lee filter [20]. Of particular use was the new “staring spotlight” mode of the TerraSAR/TanDEM-X missions, with pixel sizes reaching down to 0.05 m2 (Table1) and spatial resolutions down to 0.5 m. This made it possible to detect fine structures even after speckle filtering. SAR data from the European ENVISAT ASAR [21] and the Japanese ALOS PALSAR [22] missions have been successfully used for archaeological prospection and monitoring. However, since the pixel sizes of those images are on the order of 10 to 100 m2, they did not allow studying the fine structures that are of interest herein. Table1 gives an overview of all SAR images used, including the acquisition times and modes, pixel sizes, and the times of the closest low tide.

Table 1. SAR data used in the present study.

Sensor Date/Time Low Tide Mode 1 Pixel Size TerraSAR-X 16 June 2012, 17:10 UTC 16:47 UTC Spotlight 1.00 m × 1.00 m TerraSAR-X 30 July 2012, 17:10 UTC 16:04 UTC Hires SL 1.00 m × 1.00 m TerraSAR-X 15 August 2012, 17:19 UTC 17:29 UTC Hires SL 1.00 m × 1.00 m Radarsat-2 28 October 2012, 05:44 UTC 05:39 UTC Ultra Fine 1.00 m × 1.00 m TerraSAR-X 28 October 2012, 05:50 UTC 05:39 UTC Spotlight 1.00 m × 1.00 m TerraSAR-X 25 November 2012, 05:42 UTC 04:13 UTC Hires SL 0.75 m × 0.75 m TerraSAR-X 11 December 2012, 05:50 UTC 04:09 UTC Hires SL 0.75 m × 0.75 m TerraSAR-X 12 December 2012, 05:33 UTC 05:15 UTC Hires SL 0.50 m × 0.50 m TanDEM-X 25 December 2012, 17:19 UTC 16:48 UTC Hires SL 0.50 m × 0.50 m TanDEM-X 25 April 2013, 17:19 UTC 18:42 UTC Hires SL 1.00 m × 1.00 m TanDEM-X 6 May 2013, 17:19 UTC 16:22 UTC Hires SL 1.00 m × 1.00 m TerraSAR-X 7 May 2013, 17:02 UTC 17:15 UTC Hires SL 1.00 m × 1.00 m TerraSAR-X 25 May 2013, 05:50 UTC 06:38 UTC Hires SL 1.00 m × 1.00 m TanDEM -X 4 October 2013, 05:50 UTC 06:14 UTC Hires SL 1.00 m × 1.00 m TanDEM -X 19 November 2014, 17:02 UTC 16:35 UTC Staring SL 0.26 m × 0.26 m TanDEM -X 21 November 2014, 05:42 UTC 05:42 UTC Staring SL 0.22 m × 0.22 m TanDEM -X 20 January 2015, 05:51 UTC 06:27 UTC Staring SL 0.28 m × 0.28 m TerraSAR-X 18 July 2016, 05:42 UTC 05:32 UTC Staring SL 0.22 m × 0.22 m TerraSAR-X 22 June 2017, 17:02 UTC 17:13 UTC Staring SL 0.26 m × 0.26 m 1 Hires SL = High Resolution Spotlight; Staring SL = Staring Spotlight.

Intertidal flats are highly morphodynamic, and when the muddy and sandy marine sediments of the flats’ upper layer are moved away, banks of peat, old clay, and remains of farmland and settlements appear again on the dry-fallen surface. Moreover, the deposition of fine sediments along those morphologically harder structures can increase the local contrast, thereby making them (better) visible in aerial and satellite imagery. Analyzing the high-resolution SAR imagery we found at several places fine linear structures, which are clearly anthropogenic. Geosciences 2017, 7, 105 6 of 14

Geosciences 2017, 7, 105 6 of 14 3. Examples of Cultural Traces Found on SAR Imagery 3. Examples of Cultural Traces Found on SAR Imagery The two SAR images in Figure5 were acquired on the same day, 28 October 2012, only 7 mins The two SAR images in Figure 5 were acquired on the same day, 28 October 2012, only 7 mins apart, by Radarsat-2 (upper panel) and TerraSAR-X (lower panel), both with pixel sizes of 1 m × 1 m. apart, by Radarsat-2 (upper panel) and TerraSAR-X (lower panel), both with pixel sizes of 1 m × 1 m. × TheyThey show show the the same same 1.7 1.7 km km ×1.3 1.3 km km area,area, northnorth of Pellworm, containing containing the the lower lower left left red red square square in in FigureFigure3. The 3. The diagonal diagonal parallel parallel lines lines in in the the images’ images’ centers centers areare manifestationsmanifestations of of remains remains of of historical historical ditchesditches (compare (compare with with Figure Figure4 as 4 as well) well) that that were were buried buried for for centuries centuries underunder the sandy sediment. sediment.

FigureFigure 5. Radarsat-25. Radarsat-2 (upper) (upper) and and TerraSAR-X TerraSAR-X (lower) (lower)SAR SAR imagesimages of the same (1.7 (1.7 km km ×× 1.31.3 km) km) spot, spot, northnorth off off Pellworm, Pellworm, acquired acquired on on 28 28 October October 2012.2012. ParallelParallel diagonal lines lines in in the the images’ images’ centers centers are are remains of historical ditches. Radarsat-2 data and products © MacDonald, Dettwiler and Associates remains of historical ditches. Radarsat-2 data and products © MacDonald, Dettwiler and Associates Ltd. 2012—All Rights Reserved; TerraSAR-X data © DLR 2012. Ltd. 2012—All Rights Reserved; TerraSAR-X data © DLR 2012. These SAR image examples demonstrate that cultural traces on exposed intertidal flats can be detectedThese SARby space-borne image examples SAR, if demonstratethe spatial resolution that cultural is high traces enough. on We exposed also note intertidal that the flats two canSAR be detectedsystems by operate space-borne at different SAR, microwave if the spatial frequencies resolution (Radarsat-2: is high enough. 5.41 GHz; We also TerraSAR-X: note that the9.65 two GHz), SAR systemsbut show operate similar at differentimage contrasts microwave caused frequencies by the cultural (Radarsat-2: traces. The 5.41 radar GHz; backscattering TerraSAR-X: depends 9.65 GHz), on butthe show surface similar roughness, image contrasts whose scales caused are by of the the cultural same order traces. than The the radar radar backscattering wavelength depends(some oncentimeters). the surface roughness,Therefore, we whose can conclude scales arethatof the the roughness same order caused than by the cultural radar wavelength traces must be (some of centimeters).scales of several Therefore, centimeters, we can too conclude (see below). that the roughness caused by the cultural traces must be of scales of several centimeters, too (see below). 3.1. Remains of Settlements in “Bupsee” 3.1. Remains of Settlements in “Bupsee” A series of 1 km × 1 km details of TerraSAR/TanDEM-X images is included in Figure 6. The SAR imagesA series show of 1the km same× 1 spot, km details north- ofeast TerraSAR/TanDEM-X of Pellworm, corresponding images to is the included upper inright Figure red6 square. The SAR in imagesFigure show 3. In the the samelower spot, part north-eastof each panel of the Pellworm, tidal channel corresponding “Rummelloch” to the can upper be delineated, right red squareas well in Figureas a3 smaller. In the tidal lower channel part of entering each panel from the the tidal north. channel The intertidal “Rummelloch” flat in the can panels’ be delineated, centers had as wellfallen as a smallerdry to different tidal channel extent, entering depending from on thethe north.water leve Thel at intertidal the time flatof image in the acquisition. panels’ centers A rectangular had fallen dryfeature, to different which extent, appears depending in some of on the the panels water (but level not at in the all timeof them), of image is marked acquisition. by the red A rectangular arrow in feature,the lower which left appears panel. This in some feature of thewas panels observed (but in not high-resolution in all of them), TerraSAR-X is marked imagery by the first, red arrow and it in was studied in greater detail using data acquired in the ‘staring spotlight’ mode (lower right panel of the lower left panel. This feature was observed in high-resolution TerraSAR-X imagery first, and it Figure 6). was studied in greater detail using data acquired in the ‘staring spotlight’ mode (lower right panel of Figure6).

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FigureFigure 6. Examples6. Examples of of (1 (1 km km× ×1 1 km) km) TerraSAR-XTerraSAR-X imageryimagery of the the area area of of interest interest marked marked by by the the right right Figuresquare 6. Examplesin Figure 3. of The (1 km red ×arrow 1 km) in TerraSAR-X the lower left imagery panel denotes of the areacultural of interest traces, whosemarked imprints by the canright square in Figure3. The red arrow in the lower left panel denotes cultural traces, whose imprints can be squarebe seen in Figure on some 3. Theof the red SAR arrow images, in the and lower best left on panelthe ‘staring denotes spotlight’ cultural scene traces, on whosethe lower imprints right. ©can seen on some of the SAR images, and best on the ‘staring spotlight’ scene on the lower right. © DLR be seenDLR 2012,on some 2013, of 2014. the SAR images, and best on the ‘staring spotlight’ scene on the lower right. © 2012, 2013, 2014. DLR 2012, 2013, 2014. Figure 7 shows an aerial photograph of the same intertidal flat, taken at low tide on 29 July 2009 (i.e.,Figure some7 7 showsyearsshows before anan aerialaerial the SAR photographphotograph images were ofof thethe acquir samesameed). intertidalintertidal Here, it flat,flat,is obvious takentaken atthatat lowlow the tidetide above-mentioned onon 2929 JulyJuly 20092009 (i.e.,linear some structures years before originate the SAR from images foundations were acquired).acquirof formered). constructions. Here, it is obvious In addition, that the several above-mentioned dark spots linearare structuresstructuresvisible, which originateoriginate originate from from from foundations foundations former wells, of of former former pits, constructions. cisterns, constructions. etc. Those In In addition, addition,spots can several severalalso darkbe darkfound spots spots as are arevisible,bright visible, which spots which originateon SARoriginate imagery from from former (Figure former wells, 8) wells,if pits, the cisterns,spatialpits, cisterns, resolution etc. Those etc. is Those spotshigh enough.spots can also can Note be also found that be foundin as SAR bright as brightspotsimages onspots SAR of onlower imagery SAR resolutions imagery (Figure (Figurethey8) if thecould 8) spatial ifeasily the resolutionspatial be confused resolution is highwith enough.isthe high speckle enough. Note noise that Notetypical in SARthat for imagesin SAR SAR imagesof lowerimagery. of resolutions lower Three resolutions arrows they are could theyincluded easily could in be easilythe confused aerial be confusedphotograph with the with speckle for thebetter noisespeckle comparison typical noise for typicalwith SAR the imagery.for SAR SAR images shown in Figure 8. Also visible in the photograph is the sandy sediment, under which those imagery.Three arrows Three are arrows included are in included the aerial in photograph the aerial forphotograph better comparison for better with comparison the SAR imageswith the shown SAR structures were buried for long, and which was driven apart by currents, wind, and waves. imagesin Figure shown8. Also in visible Figure in 8. the Also photograph visible in the is the ph sandyotograph sediment, is the sandy under sediment, which those under structures which werethose structuresburied for were long, buried and which for long, was drivenand which apart was by currents,driven apart wind, by andcurrents, waves. wind, and waves.

Figure 7. Aerial photograph of exposed intertidal flats north of Pellworm, taken in July 2009. Residuals of former settlements, close to a tidal creek (upper left), can be clearly seen. The arrows were included for easier comparison with the SAR images shown in Figure 8. Photograph: B. Figure 7. Aerial photograph of exposed intertidal flats north of Pellworm, taken in July 2009. FigureHälterlein, 7. Aerial LKN. photograph of exposed intertidal flats north of Pellworm, taken in July 2009. Residuals of former settlements, close to a tidal creek (upper left), can be clearly seen. The arrows Residuals of former settlements, close to a tidal creek (upper left), can be clearly seen. The arrows were were included for easier comparison with the SAR images shown in Figure 8. Photograph: B. included for easier comparison with the SAR images shown in Figure8. Photograph: B. Hälterlein, LKN. Hälterlein, LKN.

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Figure 8. Subsections (1000 m × 1000 m) of four TerraSAR-X/TanDEM-X staring spotlight scenes Figure 8. Subsections (1000 m × 1000 m) of four TerraSAR-X/TanDEM-X staring spotlight scenes acquired on (upper left) 19 November 2014, (upper right) 20 January 2015, (lower left) 18 July 2016, acquired on (upper left) 19 November 2014, (upper right) 20 January 2015, (lower left) 18 July 2016, and (lower right) 22 June 2017. The linear structures are cultural traces. The letter (A) is introduced and (lower right) 22 June 2017. The linear structures are cultural traces. The letter (A) is introduced for for convenience (see text), the arrows are inserted for intercomparison with Figure 7. © DLR 2014, convenience (see text), the arrows are inserted for intercomparison with Figure7. © DLR 2014, 2015. 2015.

FigureFigure8 contains 8 contains 1000 1000 m m× ×1000 1000 m m details details ofof fourfour TerraSAR-X/TanDEM-XTerraSAR-X/TanDEM-X images images acquired acquired in in staringstaring spotlight spotlight mode mode on on 19 19 November November 2014, 2014, at at 17:0217:02 UTCUTC (upper left; left; 27 27 minutes minutes after after low low tide), tide), on on 20 January20 January 2015, 2015, at at 05:51 05:51 UTC UTC (upper (upper right; right; 36 36minutes minutes beforebefore low tide), tide), on on 18 18 July July 2016, 2016, at at 05:42 05:42 UTC UTC (lower(lower left; left; 10 10 minutes minutes after after low low tide), tide), and and onon 2222 JuneJune 2017,2017, at 17:02 UTC UTC (lower (lower right; right; 11 11 minutes minutes beforebefore low low tide). tide). The The location location of of these these 1 1 km km22 detailsdetails is the same as as in in Fi Figuregure 66 (upper(upper right right red red square square in Figurein Figure3). 3). TheThe very very fine fine pixel pixel sizes sizes of of 28 28 cm cm× × 2828 cmcm and below allow allow imaging imaging of of remains remains of of historical historical land land useuse (e.g., (e.g., houses, houses, ditches, ditches, lanes), lanes), which which usually usually areare tootoo narrow to be be delineated delineated on on SAR SAR imagery imagery of of conventional resolution, with pixel sizes on the order of 10 m × 10 m. Clearly visible in all panels of conventional resolution, with pixel sizes on the order of 10 m × 10 m. Clearly visible in all panels of Figure 8 are linear, partly rectangular structures in the lower left image centers, close to the tidal Figure8 are linear, partly rectangular structures in the lower left image centers, close to the tidal creek creek (note the red arrows and compare with Figure 7). A close comparison of the aerial photograph (note the red arrows and compare with Figure7). A close comparison of the aerial photograph (Figure7) (Figure 7) with the high-resolution SAR images (Figure 8) also reveals that, during the approx. five with the high-resolution SAR images (Figure8) also reveals that, during the approx. Five years years between the acquisitions, parts of the exposed remains were already lost, due to the permanent betweenerosion, the sedimentation, acquisitions, partsand morphological of the exposed changes. remains However, were already the lost,comparison due to the of the permanent four panels erosion, of sedimentation,Figure 8 also and reveals morphological that those changes. rectangular However, structures the comparisonare expanding of the eastward, four panels i.e., ofthat Figure an 8 alsoincreasing reveals that amount those of rectangular those trac structureses is appearing are expanding at the surface. eastward, This i.e., example that an demonstrates increasing amount that of thoseerosion traces may iserase, appearing but may at thealso surface. produce, This the exampletraces on demonstratesthe surface that that are erosion visible on may SAR erase, imagery. but may also produce,Groups the of tracesparallel on vertical the surface lines can that be are seen visible at different on SAR locations imagery. in all image centers. The mean distanceGroups between of parallel those vertical parallel lines line cans is beabout seen 15 at m, different thus indicating locations that in allthey image are remains centers. of The former mean distanceditches between and drainage those channels parallel [18]. lines The is about letter 15(A) m, mark thuss the indicating same location that theyin all are panels; remains note ofthat former the ditchesspatial and extent drainage of the channels parallel vertical [18]. The lines letter surroun (A) marksding this the letter same has location been increasing, in all panels; due note to the that theerosion spatial of extent the upper of the sediment parallel layers. vertical A linescloser surrounding comparison with this letterthe map has of been all known increasing, cultural due traces to the erosion(see below) of the upperalso reveals sediment that these layers. are A st closerructures comparison that had not with been the known mapof so all far. known cultural traces (see below)The alsoabove reveals comparisons that these illustrate are structures the strong that morphodynamics had not been known on intertidal so far. flats, but also the needThe for above a frequent comparisons monitoring illustrate of those the archaeological strong morphodynamics sites. on intertidal flats, but also the need for a frequent monitoring of those archaeological sites.

Geosciences 2017, 7, 105 9 of 14

Geosciences 2017, 7, 105 9 of 14 3.2. Remains of Drainage Systems and Lanes in “Waldhusen” 3.2. Remains of Drainage Systems and Lanes in “Waldhusen” A series of 12 TerraSAR/TanDEM-X images (1 km × 1 km details, corresponding to the lower left A series of 12 TerraSAR/TanDEM-X images (1 km × 1 km details, corresponding to the lower red square in Figure3) is shown in Figure9. The red arrows mark the same diagonal parallel lines that left red square in Figure 3) is shown in Figure 9. The red arrows mark the same diagonal parallel werelines shown that inwere Figure shown5 and in thatFigure are 5 due and to that former are du ditchese to former and lanes. ditches However, and lanes. different However, environmental different conditions,environmental namely conditions, different water namely levels different and wind wa speedster levels during and the wind image speeds acquisitions, during maythe stronglyimage reduceacquisitions, the radar may contrast strongly of reduce those structures,the radar contrast thereby of making those structures, it difficult thereby to extract making them it fromdifficult SAR images.to extract Also them note from that theSAR brightness images. Also of thenote exposed that the intertidalbrightness flats of the varies, expos dependinged intertidal not flats only varies, on the typedepending of sediment not [only19], buton the also type on the of environmentalsediment [19], but conditions also on the [23 ].environmental These variations, conditions however, [23]. are notThese of interest variations, herein. however, are not of interest herein.

Figure 9. Examples of TerraSAR-X imagery of the area of interest marked by the left square in Figure Figure 9. Examples of TerraSAR-X imagery of the area of interest marked by the left square in Figure3. 3. The red arrows mark several diagonal bright lines that can be identified on all SAR images, and The red arrows mark several diagonal bright lines that can be identified on all SAR images, and that that are due to traces of old drainage systems. They are best visible on the “staring spotlight” scene are due to traces of old drainage systems. They are best visible on the “staring spotlight” scene on the on the lower right. © DLR 2012, 2013, 2014. lower right. © DLR 2012, 2013, 2014. Figure 10 shows two photographs taken during low tide on 15 June 2017 in the study area. Dark parallelFigure structures 10 shows (marked two photographs by the red arrows) taken duringon the sand low and tide mud on 15 flats June can 2017 be delineated, in the study which area. Darkcorrespond parallel structuresto those seen (marked on the bySAR the images. red arrows) Note the on differences the sand and in sediment mud flats composition can be delineated, of the whichlinear correspond structures toand those the surrounding seen on the SARsandy images. flats. The Note ditch the remains differences are marked in sediment by denser composition (harder) of thesediment linear structures causing andhigher the surface surrounding roughness, sandy which flats. Thein turn ditch results remains in arehigher marked radar by backscattering. denser (harder) sedimentAlso note causing that those higher lines surface show roughness,higher concentrat whichion in of turn bivalves results and in vegetation higher radar (sea backscattering. grass). Also Alsovisible note is that remnant those lineswater show in between higher those concentration structures of and bivalves in small and tidal vegetation pools scattered (sea grass). over Alsothe study visible is remnantarea. This water water in effectively between those flattens structures the surface and seen in small by the tidal SAR pools (at scatteredlow wind over speeds), the study thereby area. leading to a stronger contrast between the parallel bright lines and the dark areas in between (Figure This water effectively flattens the surface seen by the SAR (at low wind speeds), thereby leading to a 11). stronger contrast between the parallel bright lines and the dark areas in between (Figure 11).

Geosciences 2017, 7, 105 10 of 14

GeosciencesGeosciences 2017 2017, 7,, 1057, 105 10 of10 14 of 14

Figure 10. Two photographs of traces of historical ditches in the area of interest, taken at low tide Figure 10.duringTwo a field photographs excursion in of June traces 2017. of The historical red arro ditchesws in both in thepanels area mark of interest,manifestations taken of atthe low tide Figure 10. Two photographs of traces of historical ditches in the area of interest, taken at low tide during a(former) field excursion ditches’ rims in Junethat can 2017. be seen The on red SAR arrows imagery. in both Photographs: panels mark M. Gade. manifestations of the (former) during a field excursion in June 2017. The red arrows in both panels mark manifestations of the ditches’ rims that can be seen on SAR imagery. Photographs: M. Gade. (former) ditches’ rims that can be seen on SAR imagery. Photographs: M. Gade.

Figure 11. Subsection (1000 m × 1000 m) of a TanDEM-X staring-spotlight scene acquired on 21 November 2014, north of Pellworm Island. Letters (A) and (B) are described in the text. The photographs shown in Figure 10 were taken in the area marked by the red arrow. © DLR 2014. Figure 11. Subsection (1000 m × 1000 m) of a TanDEM-X staring-spotlight scene acquired on 21 Figure 11. Subsection (1000 m × 1000 m) of a TanDEM-X staring-spotlight scene acquired on NovemberAn example 2014, of north very highof Pellworm resolution Island. SAR imageryLetters (A) of theand same (B) aresite described(corresponding in the to text. the lowerThe 21 November 2014, north of Pellworm Island. Letters (A) and (B) are described in the text. The photographs rightphotographs panel of Figureshown in9) Figureis shown 10 were in Figure taken in11. the Inserted area marked is a red by thearrow red thatarrow. marks © DLR the 2014. location, at shownwhich in the Figure photographs 10 were taken of Figure in the 10 areawere marked taken. Th byis the small red (1000 arrow. m ©× 1000 DLR m) 2014. section of a TanDEM-X staringAn example spotlight of scene very was high acquired resolution on 21 SAR November imagery 2014, of the at 05:42 same UTC site (low(corresponding tide) and shows to the many lower Anrightbright example panel and of dark ofFigure very parallel 9) high is lines shown resolution all overin Figure the SAR image 11. imagery Insertedcenter. The of isthe adistance red same arrow of site those that (corresponding lines marks is between the location, to10 them at lower which the photographs of Figure 10 were taken. This small (1000 m × 1000 m) section of a TanDEM-X right panel of Figure9) is shown in Figure 11. Inserted is a red arrow that marks the location, at which staring spotlight scene was acquired on 21 November 2014, at 05:42 UTC (low tide) and shows many the photographs of Figure 10 were taken. This small (1000 m × 1000 m) section of a TanDEM-X staring bright and dark parallel lines all over the image center. The distance of those lines is between 10 m spotlight scene was acquired on 21 November 2014, at 05:42 UTC (low tide) and shows many bright and dark parallel lines all over the image center. The distance of those lines is between 10 m and 20 m, again, indicating that they are remains of a former mesh of draws and ditches built for the drainage of Geosciences 2017, 7, 105 11 of 14 Geosciences 2017, 7, 105 11 of 14 theand farmland 20 m, again, [18]. However,indicating wethat also they note are remains that, once of thea former exposed mesh space of draws in between and ditches is partly built filled for the with drainage of the farmland [18]. However, we also note that, once the exposed space in between is sandy sediments, some of the lines may also appear dark (seen in the image center of Figure 11, below partlyGeosciences filled 2017 with, 7, 105sandy sediments, some of the lines may also appear dark (seen in the image11 ofcenter 14 letter (A)). of Figure 11, below letter (A)). Aand part 20 m, of again, the same indicating area, that north they of are the remains red arrow of a former in Figure mesh 11 of, draws is shown and ditches on the built photograph for the in A part of the same area, north of the red arrow in Figure 11, is shown on the photograph in Figuredrainage 12 that of was the taken farmland from [18]. a vantage However, point we on also Pellworm’s note that, northernonce the exposed dike at lowspace tide in onbetween 15 May is 2009. Figure 12 that was taken from a vantage point on Pellworm’s northern dike at low tide on 15 May Duringpartly that filled time, with easterly sandy sediments, winds were some driving of the thelines water may also out appear of the studydark (seen area in and the image caused center very low 2009.of Figure During 11, that below time, letter easterly (A)). winds were driving the water out of the study area and caused very water levels. Under such favorable environmental conditions the parallel features could be easily low waterA part levels. of the Under same area,such northfavorable of the environmental red arrow in Figure conditions 11, is shownthe parallel on the features photograph could in be observedeasilyFigure observed from 12 that land. fromwas We taken land. also from note,We alsoa however, vantage note, pointhowever, that theon Pellworm’s prevailingthat the prevailing northern westerly dikewesterly winds at low in winds that tide area, on in 15that along May area, with thealong greater2009. with During distance the greaterthat of time, most distance easterly of the of winds remains most wereof fromthe drivin rema thegins coast,the from water usually the out coast, of make the usually study such area land-basedmake and such caused land-based observations very veryobservationslow difficult. water verylevels. difficult. Under such favorable environmental conditions the parallel features could be easily observed from land. We also note, however, that the prevailing westerly winds in that area, along with the greater distance of most of the remains from the coast, usually make such land-based observations very difficult.

Figure 12. Photograph of the study area north of the red arrow in Figure 11. The picture was taken at Figure 12. Photograph of the study area north of the red arrow in Figure 11. The picture was taken at low tide on 15 May 2009, when easterly winds caused a very low water level. Photograph: M. Gade. low tide on 15 May 2009, when easterly winds caused a very low water level. Photograph: M. Gade. Figure 12. Photograph of the study area north of the red arrow in Figure 11. The picture was taken at Figurelow tide 13 onshows 15 May a reconstruction2009, when easterly of awinds historical caused la a ne,very with low waterditches level. on Photograph: either sides, M. whichGade. can be foundFigure on the13 shows intertidal a reconstruction flats north of Pellworm of a historical and which lane, withmay cause ditches structures on either like sides, those which observed can be foundin the on SARFigure the imagery intertidal 13 shows (below flats a reconstruction northletter (B) of Pellwormin ofFigure a historical 11). and Residuals whichlane, with may of ditches fossil cause farmlandon structures either sides,structures, like which those mostly can observed be of inditches, thefound SAR buton imagery thealso intertidal of (below lanes flatsor letter dikes, north (B) cannotof in Pellworm Figure be observed 11 and). Residuals which through may of causetheir fossil structuresrelief farmland of less like structures,than those 10 observed cm mostlyheight. of ditches,Instead,in the but itSAR is also the imagery of sediments lanes (below or on dikes, letter the lost cannot(B) inpastures Figure be observed that11). Residualsare different through of fossil from their farmland those relief in of thestructures, less linear than structures mostly 10 cm of height. of Instead,ditches.ditches, it isTypical but the also sediments Wadden of lanes onsedimentsor thedikes, lost cannot on pastures the be flat observed that sand are banks through different consist their from reliefof thosemarine of less in fine thethan linearsand, 10 cm which structures height. had of ditches.beenInstead, the Typical basic it is Waddenthe compound sediments sediments onof the lostonold thepastures marsh flat sand thatland banksare and different which consist from is of still marinethose a inmajor finethe linear sand,part structuresof which the coastal had of been theenvironment basicditches. compound Typical on the Wadden of German the old sediments North marsh Sea landon coast. the and flat In which sandcontra banks isst, still the consist asurface major of ofmarine part the offossil fine the ditches coastalsand, which is environment different: had been the basic compound of the old marsh land and which is still a major part of the coastal onin the their German center North pillow-like Sea coast. sediments In contrast, can be thefound, surface while of the the ditch fossil edges ditches are isoften different: stabilized in their by fossil center environment on the German North Sea coast. In contrast, the surface of the fossil ditches is different: pillow-likeroots and sediments other plant can material be found, connected while with the ditch the se edgesdiment are (Figure often stabilized10). This causes by fossil narrow roots ridges and other of onlyin their10 cm center to 30 pillow-likecm width, sedimentswhich can can still be be found, found while today the and ditch which edges show are often up on stabilized SAR imagery, by fossil if its plantroots material and other connected plant material withthe connected sediment with (Figure the sediment 10). This(Figure causes 10). This narrow causes ridges narrow of ridges only of 10 cm spatial resolution is high enough. to 30only cm 10 width, cm to which30 cm width, can still which be can found still todaybe found and today which and showwhich upshow on up SAR on SAR imagery, imagery, if its if its spatial resolutionspatial is resolution high enough. is high enough.

Figure 13. Reconstruction of a historical lane with ditches on either side. Photograph and sketch: J. Kohlus.

Geosciences 2017, 7, 105 12 of 14

Figure 13. Reconstruction of a historical lane with ditches on either side. Photograph and sketch: J. GeosciencesKohlus.2017 , 7, 105 12 of 14

4. Comparison with Existing Data 4. Comparison with Existing Data A detail of the map in Figure 4 is shown in Figure 14. Two red squares mark the areas investigatedA detail ofherein the map in greater in Figure detail,4 is shown “Waldhusen” in Figure (lower14. Two left) red and squares “Bupsee” mark the (upper areas right), investigated where hereinremains in greaterof former detail, ditches “Waldhusen” and lanes (lower and of left) former and “Bupsee” settlements, (upper respectively, right), where have remains been offound former on ditchesSAR imagery. and lanes and of former settlements, respectively, have been found on SAR imagery.

Figure 14. Detail of the map shown in Figure 4. Two red squares denote the locations of the SAR Figure 14. Detail of the map shown in Figure4. Two red squares denote the locations of the SAR images shown in Figure 6 and in Figure 9, respectively. Map: C. Kost. images shown in Figure6 and in Figure9, respectively. Map: C. Kost. In the area “Waldhusen” (lower left square) parallel linear features on the map correspond well withIn the the SAR area observations; “Waldhusen” however, (lower left new square) features parallel were linear found features in the onarea the “Bupsee” map correspond (upper right well withsquare). the Here, SAR observations;more vertical however,lines (i.e., newnorth-southerly features were oriented found structures) in the area were “Bupsee” found (upper on theright SAR square).images (Figure Here, more 8) than vertical known lines before (i.e., [18]. north-southerly This is a clear oriented indication structures) that SAR were imagery found can on be the used SAR to imagesupdate (Figureund to 8improve) than known maps beforeof remains [ 18]. of This historical is a clear land indication use. that SAR imagery can be used to updateWe und also to note, improve however, maps that of remains the SAR of did historical not capt landure use. all features that are included in the map: the extentWe also of note,the linear however, features that theon the SAR SAR did imagery not capture shown all features in Figure that 9 areis smaller included than in thein the map: lower the extentleft square of the on linear the map. features Again, on thethis SAR proves imagery the strong shown morphodynamic in Figure9 is smaller changes than in inthat the area, lower which left squaremay lead on to the an map. appearance, Again, this but provesalso to thea disappearance, strong morphodynamic of exposed changescultural intraces. that area, which may lead to an appearance, but also to a disappearance, of exposed cultural traces. 5. Conclusions 5. Conclusions We have demonstrated that high-resolution SAR imagery can be used to complement We have demonstrated that high-resolution SAR imagery can be used to complement archaeological surveys on intertidal flats on the German North Sea coast. The new high-resolution archaeological surveys on intertidal flats on the German North Sea coast. The new high-resolution TerraSAR-X acquisition mode (“staring spotlight”) allows detecting various kinds of remains of TerraSAR-X acquisition mode (“staring spotlight”) allows detecting various kinds of remains of historical land use, some of which have been unknown so far. Signatures of remains of both former historical land use, some of which have been unknown so far. Signatures of remains of both former settlements and former systems of ditches and of peat cutting, dating back to periods before major settlements and former systems of ditches and of peat cutting, dating back to periods before major storm surges in the 14th and 17th centuries, can be found on high-resolution TerraSAR/TanDEM-X storm surges in the 14th and 17th centuries, can be found on high-resolution TerraSAR/TanDEM-X images. In this respect, best results were obtained (i.e., strongest and clearest signatures were found) images. In this respect, best results were obtained (i.e., strongest and clearest signatures were found) when SAR images acquired in the new “staring spotlight” mode were used, with pixel sizes on the when SAR images acquired in the new “staring spotlight” mode were used, with pixel sizes on the order of 0.1 m2. order of 0.1 m2. In many cases the observed signatures of former ditches are due to different sediment types, In many cases the observed signatures of former ditches are due to different sediment types, which in turn are due to the actual ditch morphology. The presence of fossil roots and other organic which in turn are due to the actual ditch morphology. The presence of fossil roots and other organic material results in denser and harder sediments, which may be directly sensed by space-borne SAR, Geosciences 2017, 7, 105 13 of 14 or which may cause additional sedimentation (i.e., a deposition of sandy sediments) that can be seen on SAR imagery. We also note that different sediments may cause different biological productivity, and are therefore often marked by benthic organisms causing different surface roughness patterns. Also, bivalves tend to settle on those sediments, thereby causing higher surface roughness sensed by the SAR. SAR sensors are advantageous over optical sensors, because their use is independent of daylight and weather conditions. This is a particular advantage for the studies presented herein, since data acquisitions of the study area always have to be made at low water levels. Moreover, acquisitions made by spaceborne SAR sensors allow a frequent surveillance of the study areas, which is required in a strongly morphodynamic environment. Further research will see thorough analyses of more SAR imagery of the area of interest, but also of other locations further north (close to the island of Langeneß), where more cultural traces were reported before. Automated detection of those remains that manifest in linear structures on SAR imagery seems possible through an application of Fourier and Principal Component Analyses as well as Hough Transformations, but only if the image resolution is high enough.

Acknowledgments: Parts of this research received funding from the German Ministry of Economy (BMWi) under contract 50 EE 0817, RS2 and TSX data were provided by the Canadian Space Agency (CSA) under contract 5077 and by the German Aerospace Center (DLR) under contract COA0118, respectively. RADARSAT is an official mark of the Canadian Space Agency. Author Contributions: Martin Gade designed and performed all SAR image analyses and wrote most parts of the text. Jörn Kohlus initiated this research and provided a-priori knowledge about the site and cultural traces. Cornelia Kost provided knowledge about all known cultural traces in the study area. All authors participated in the data interpretation and in the final text editing. Conflicts of Interest: The authors declare no conflict of interest. The funding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.

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