Using Remote Sensing Data to Identify Large Bottom Objects: the Case of World War II Shipwreck of General Von Steuben

geosciences

Article Using Remote Sensing Data to Identify Large Bottom Objects: The Case of World War II Shipwreck of General von Steuben

Artur Grz ˛adziel Department of Navigation and Hydrography, Faculty of Navigation and Naval Weapons, Polish Naval Academy, 81–127 Gdynia, Poland; [email protected]

Received: 25 May 2020; Accepted: 16 June 2020; Published: 19 June 2020

Abstract: The seabed of the Baltic Sea is not yet fully searched for and investigated. In 2004 the crew of the Polish Navy hydrographic ship Arctowski discovered a new shipwreck that was not listed in the official underwater objects database nor was it marked on a chart. The identity of a new wreck is most frequently established based on artefacts found in the object by divers as a part of archaeological research, or through underwater inspection with remotely operated vehicle. The aim of this paper is to show how acoustic remote sensing data is used to identify large bottom object without having to go underwater. Bathymetric survey and sonar investigation were conducted over the study area. An appropriate methodology allowed for obtaining high-resolution imagery of the wreck. A review of literature concerning the end of World War II in the Baltic Sea was carried out. Moreover, the author presents a comparative analysis and evaluation of remote sensing data with archival photos, silhouette, and ship characteristics. The proposed approach led to the identification of a new Baltic Sea wreck as the General von Steuben, which was torpedoed in 1945 by soviet submarine. The author’s findings show that state of preservation of the shipwreck, quality data as well as historical records play a key role in establishing the wreck’s identity.

Keywords: side-scan sonar; shipwreck; discovery; survey; wreck’s identiﬁcation

1. Introduction Over the centuries, ships survived storms, wars, icebergs, and pirates. However, many of them were lost and lie at the bottom of the ocean with their stories, mysteries, and secrets. Some wrecks have been there for a long time, inaccessible to anyone. About three million shipwrecks lie on the sea bottom around the world [1]. Each wreck has its own exceptional history. Because most of the world’s oceans are unreachable to divers, we must rely on advanced remote sensing technology to examine, map, and verify shipwrecks in these areas [2]. Most of the wrecks in the Polish part of the Baltic Sea come from World War II, and from even earlier [3]. The inland nature of the Baltic Sea—conducive to the development of shipping, ports and maritime trade, and numerous naval wars and conflicts, combined with unexpected storms—meant that many vessels found their end at sea bottom. This sea has always been seen as dangerous to navigate. Its weather conditions are considered difficult due to strong winds and high storm waves. This unfavorable aura, imperfection of old shipbuilding techniques, navigation errors, and, finally, simple negligence were the reasons for many maritime accidents and disasters [4]. The number of these tragedies is multiplied by wars during which military operations were also carried out at sea. They led to numerous sinking of vessels and warships. One of them was the German liner General von Steuben (GvS). After the World War II, wreck search and investigation had top priority to ensure the safety of shipping in the navigation channels that had been cleared of mines. Before the war, wrecks close to the

Geosciences 2020, 10, 240; doi:10.3390/geosciences10060240 www.mdpi.com/journal/geosciences Geosciences 2020, 10, 240 2 of 15 coast used to be mapped with deployment of divers. Wrecks lying in deep waters, which did not pose a threat to navigation, were recorded but not monitored regularly. Many vessels were lost without their position having ever been identified in systematic surveys. The traditional wreck searching method was to tow ropes across the seabed which were fastened to two ships [5]. This technique continued to be used after the introduction of the first acoustic device because it was capable of locating small or very narrow targets on the seabed which were not detected by echosounders. Wreck search has become easier after the development of sonar equipment during the last World War, which was used to locate submarines. Today wreck search or, more precisely, the investigation of underwater objects, is a special application of hydrographic surveying, that uses the advantages of a multibeam echosounder (MBES), single-beam echosounder (SBES), side-scan sonar (SSS), or remotely operated vehicle (ROV) [6–8]. The total number of shipwrecks at the bottom of the Baltic Sea is currently unknown. Numerous detected wrecks remain unidentified. There is often a lot of unverified and false information about them. Identity of the underwater objects is established on the basis of collected photographic and drawing documentation, analysis of historical facts, results of bathymetric and sonar survey, diving inspection, and movie material recorded with a TV camera mounted on an underwater vehicle. Identifying the shipwreck is often a principal issue that a court must address when deciding salvage claims over the wreck [9]. Berg [10] and Keatts [11] claim that the wreck can be reliably identified based on the artefacts found in it. Elements such as a ship’s bell, a hull number, or the name of the vessel clearly confirm the identity of the object. This involves the use of divers. Unfortunately, in the case of older wrecks a significant degree of hull growth is to be expected, which may affect the effectiveness of determining the wreck’s identity. A human diver can only work down to about 30–40 m, and the vast majority of shipwrecks are much deeper than that. What is more, it is very expensive to inspect an entire shipwreck using divers. If no element is found, the process of determining the identity of the wreck can take years. Grabiec and Olejnik [12] believe that the most effective way to identify a wreck is the use of unmanned vehicles, such as a remotely operated vehicle. The authors claim that only photographic or TV images of the wreck provide 100% certainty in the identification process. The use of ROV may be limited by the depth of the wreck’s lying, its orientation at the bottom, and hydrometeorological conditions of the environment. What is more, carrying out the mission of an underwater vehicle is associated with the danger of entangling the vehicle in the fishing nets or getting stuck inside the wreck. ROV collision with structural components such as masts or funnels is also possible. After all, using ROV also means that the fishing net can get tangled in the vehicle’s propulsion or hooked the cable to the wreck structure. To reach shipwrecks in deep water, archaeologists have rarely partnered with ocean scientists to use an ROV. The costs of ROV operations are high due to the operational expenses of the technology and day rate for the survey vessels. ROV inspection requires that the support vessel must be fitted with a dynamic positioning system (DP). Vessels equipped with DP systems are very expensive to charter [13]. If a new wreck (object) is found a side-scan sonar can provide high quality remote sensing data. [14]. Side-scan sonar can visualize and identify the wreck at a low cost. This acoustic sensor produces high-resolution images of the bottom and objects without the need for inspection dives. SSS is used to create sonar images (sonograph) for a wide range of scientific underwater applications, including deep sea, shallow waters, or inland waters [15]. This sonar is commonly used in marine surveying and mapping, submarine geological exploration, and detection of seabed features [16–18]. SSS is probably the best tool for searching and locating shipwrecks that enables the ability to “see” hundreds of meters to each side of the towfish with incredible detail. However, it is worth remembering that feature detection and recognition from sonar imagery are mostly based on manual interpretation, and, thus, its accuracy is significantly affected by the experience of the sonar map reader [19]. Geosciences 2020, 10, x FOR PEER REVIEW 3 of 15

Geosciences 2020, 10, 240 3 of 15 This study shows how remote sensing data, especially those acquired with side-scan sonar technology, is used to identify the shipwreck. The process of identifying the object is described based on theThis shipwreck study shows of World how remoteWar II, sensing General data, von especiallySteuben, lying those in acquired the Baltic with Sea. side-scan Based sonaron a comparativetechnology, is analysis used to identifyof archival the shipwreck.documents Thewith process Remote of identifyingSensing data, the objecttaking isinto described account based the conditionon the shipwreck of the wreck, of World it Waris possible II, General to carry von Steuben,out a positive lying inidentification the Baltic Sea. of Based the object. on a comparative It is worth notinganalysis that of archivalthe wreck documents identification with Remoteprocess Sensingis performed data,taking without into the account need thefor conditiondiving or ofROV the inspectionwreck, it is of possible the shipwreck. to carryout a positive identification of the object. It is worth noting that the wreck identificationThe article process is structured is performed as follows. without The theintroduc needtion for diving to the study, or ROV purpose, inspection and of key the publications shipwreck. cited Theare provided article is structuredin Section as1. Section follows. 2 Thedetails introduction the history to of the the study, German purpose, liner General and key von publications Steuben. Sectioncited are 3 providedpresent the in circumstances Section1. Section of 2discovery details the th historye wreck of by the the German Polish Navy liner Generalhydrographic von Steuben. survey ship,Section Arctowski,3 present theand circumstances introduces the of survey discovery methodology the wreck and by the equipment Polish Navy used hydrographic in investigating survey the shipwreck.ship, Arctowski, The survey and introduces results, discussion the survey as methodologywell as method and of equipment wreck’s identification used in investigating are shown the in Sectionshipwreck. 4. Section The survey 5 concludes results, the discussion paper. as well as method of wreck’s identification are shown in Section4. Section5 concludes the paper. 2. History of the General von Steuben 2. History of the General von Steuben 2.1. Early History 2.1. Early History The General von Steuben was a German steamer 14,660 tons, built as the Munchen in 1923 in VulcanThe shipyard General von(Figure Steuben 1). wasThe aluxurious German steamer passenger 14,660 ship, tons, 168 built meters as the Munchenof length, in 19.8 1923 meters in Vulcan of breadth,shipyard (Figureand 8.51 ).meters The luxurious of draught passenger could ship,navigate 168 m16 of knots length, [20]. 19.8 She m of was breadth, often and referred 8.5 m ofto draughtas “the beautiful,could navigate white 16 Steuben” knots [20 in]. Sheadvertising was often material referred from to as that “the time. beautiful, The name white of Steuben” the ship in comes advertising from thematerial German from general that time. fighting The name for independence of the ship comes of the from United the German States—General general fighting von Steuben for independence [21]. Basic ship’sof the Unitedtechnical States—General data are shown von in Steuben Table 1. [ 21]. Basic ship’s technical data are shown in Table1.

FigureFigure 1. GermanGerman passenger passenger liner liner General General von von Steuben Steuben before before sinking [22]. [22].

GvS was the firstTable German 1. Characteristics commercial of ocean passenger line linerr after General the First von Steuben.World War that opened the trade over the Atlantic once again [23]. Munchen was launched in the end of November 1922. She Parameter Characteristics had a steam propulsion and she took 1100 passengers. In the summer of 1923, Munchen went on her Name 1923: Munchen, 1930: General von Steuben, 1938: Steuben first trip to New York. On 11 February 1930, while staying in the port, she suffered a fire that Type Passenger ship destroyed much of Tonnagethe transatlantic 14,660 BRTliner. The ship was repaired and rebuilt at the Weser shipyard in Bremen. On 20 JanuaryDimension 1931, Munche Length 168n set m, sail beam on 19.8 her m, first draft cruise 8.5 m under the changed name of General von Steuben. As a luxurySpeed passenger 16 kts ship, she cruised on the Mediterranean and the Norwegian Sea until the outbreakLaunched of the war. 1922 On 20 December 1938, the passenger liner’s name was shortened to Shipyard Vulcan in Szczecin Steuben. Propulsion Steam reciprocating, exhaust turbine, twin propellers; 11,000 hp Fate Sunk 10 February 1945 Table 1. Characteristics of passenger liner General von Steuben.

GvS wasParameter the ﬁrst German commercial ocean linerCharacteristics after the First World War that opened the trade over the AtlanticName once again 1923: [23 Munchen,]. Munchen 1930: was Gene launchedral von in Steuben, the end 1938: of November Steuben 1922. She had a steam propulsionType and shePassenger took 1100 ship passengers. In the summer of 1923, Munchen went on her ﬁrst

Geosciences 2020, 10, 240 4 of 15 trip to New York. On 11 February 1930, while staying in the port, she suffered a fire that destroyed much of the transatlantic liner. The ship was repaired and rebuilt at the Weser shipyard in Bremen. On 20 January 1931, Munchen set sail on her first cruise under the changed name of General von Steuben. As a luxury passenger ship, she cruised on the Mediterranean and the Norwegian Sea until the outbreak of the war. On 20 December 1938, the passenger liner’s name was shortened to Steuben. She began her career in the German Kriegsmarine after the outbreak of World War II. Steuben was equipped with 12 anti-aircraft guns and the interior was rebuilt. From 31 July 1940, she was transferred to Hamburg, where she served as a hulk in the new naval construction department. From 1 August 1944, Steuben was used as a transport ship in the Baltic for the army and wounded people. By the end of 1944, the ship made several flights from Piława to Swinouj´scieand´ from Riga to Gdynia. From December 23, for one month, the German transport ship was assigned as a hulk to the staff of the U-Boat commander in the East. In January 1945 she was reclassified as a troop-ship for the wounded (Verwundeten-Transport-Schiff (VTS)). Steuben made her last cruise in February 1945, sharing the fate of Wilhelm Gustloff [24].

2.2. The Sinking of the General von Steuben As the war drew to a close, Steuben, like many other German ships, was pressed into service to evacuate troops and civilians from the Eastern Front. On 9 February 1945, Steuben left Baltiysk, Russia, bound for Kiel, Germany, with 2500 wounded soldiers, 2000 refugees and a crew of 450. On 10 February, shortly before midnight, she was torpedoed twice by the Soviet submarine S-13 and sank with the loss of about 3000 lives [25]. Steuben, like other two ships Wilhelm Gustloff and Goya participated in the evacuation operation from East Prussia under the operation name “Hannibal” [26]. On 8 February 1945 Steuben arrived at the port of Piława. The next day, from the morning hours, the ship began to take on board refugees, wounded soldiers, and the army. Seriously wounded soldiers were placed on the walking deck on unfolded mattresses, less injured on the lower decks. Refugees squeezed in between them. The ship had one goal: To reach Kiel as soon as possible to unload the wounded and fugitives. At around 15:30 tugs led the ship to the center of the port basin. There she was joined by escort ships, the old “T-196” torpedo boat, with a displacement of 810 tons, and the “TF-10” torpedo hunter [21]. It was thought that at high speed of Steuben, dark night, and bad weather conditions it would be possible to sneak out secretly and unnoticed. There was also no required number of life-saving equipment (lifeboats, rafts, and life jackets) onboard the transporter. If the ship was torpedoed, launching lifeboats and rafts would not be fast or efficient due to icy davit. The sea was calm, but the sky was dark, promising snow. Initially, Steuben and the torpedo boat “T-196” went at a speed of 10 to 12 knots. In the basin between Pomerania and Słupsk Bank, the speed was increased to the maximum, because here, Wilhelm Gustloff was recently sunk. At 00:50 on Saturday, 10 February 1945, S-13 fired two torpedoes from stern [20]. Both were accurate. Soon after, further explosions followed, probably under pressure boilers. A few minutes after the torpedo explosion, the bow deck sank into the front funnel. Shortly after one o’clock the ship finally began to sink. About twenty minutes after torpedoing, the ship went down. The rescue operation did not amount to much. The escorting torpedo boat, first tried to destroy the submarine, then for many it was too late. Only 300 people were saved, mostly soldiers [21]. The sinking of the Steuben just after torpedoing Wilhelm Gustloff and almost in the same place was another severe defeat for the German command. The wreck successfully hid in the Baltic Sea for 59 years [27].

3. Materials and Methods

3.1. Discovery of the Shipwreck In May 2004, the Polish Navy hydrographic ship Arctowski carried out survey measurement in the exclusive economic zones (EEZ) of Poland (Figure2). Bathymetric measurements of fairways, side-scan Geosciences 2020, 10, x FOR PEER REVIEW 5 of 15

3. Materials and Methods

3.1. Discovery of the Shipwreck Geosciences 2020 10 In May 2004,, , 240 the Polish Navy hydrographic ship Arctowski carried out survey measurement5 of in 15 the exclusive economic zones (EEZ) of Poland (Figure 2). Bathymetric measurements of fairways, side-scansonar survey sonar as survey well as as locating well as oflocating underwater of under objectswater and objects navigational and navigational obstructions obstructions were the were main thetasks main for thetasks survey for the ship. survey At 05:30, ship. 26th At 05:30, of May 26t 2004h of sonar May operator2004 sonar noticed operator depth noticed change depth in multibeam change inechosounder multibeam echosounder system display. system At the display. same timeAt the hull-mounted same time hul side-scanl-mounted sonar side-scan recorded sonar a long recorded sharp aecho long with sharp clear echo acoustic with shadow.clear acoustic There shadow. was no wreck There in was that no site wreck according in that to thesite underwater according objectsto the underwaterdatabase. There objects was database. no wreck There on the was nautical no wreck chart, on neither. the nautical At that chart, moment, neither. the At sonar that o moment,ﬃcer did the not sonarrealize officer that the did crew not realize of Arctowski that the had crew discovered of Arctowski the shipwreck had discovered of the Steuben,the shipwreck a secret of ofthe World Steuben, War aII secret in the of Baltic World Sea. War II in the Baltic Sea.

FigureFigure 2. 2. SiteSite of of discovery discovery of of the the General General von von Steuben Steuben shipwreck. shipwreck. 3.2. Survey Planning 3.2. Survey Planning Newly found wrecks or features which may be dangerous to underwater or surface navigation Newly found wrecks or features which may be dangerous to underwater or surface navigation are to be reported to the National Hydrographic Office (NHO). The role of NHO in Poland is fully are to be reported to the National Hydrographic Office (NHO). The role of NHO in Poland is fully within the Hydrographic Office of the Polish Navy (HOPN). Shipwrecks or artificial obstructions that within the Hydrographic Office of the Polish Navy (HOPN). Shipwrecks or artificial obstructions that protrude above seafloor may pose a threat to shipping over continental shelf areas. All such targets protrude above seafloor may pose a threat to shipping over continental shelf areas. All such targets must be located, investigated, and recorded [28]. must be located, investigated, and recorded [28]. After the discovery of a new, uncharted shipwreck hydrographers of the Arctowski started After the discovery of a new, uncharted shipwreck hydrographers of the Arctowski started planning the detailed hydrographic survey. The main purpose of the works after discovery of the planning the detailed hydrographic survey. The main purpose of the works after discovery of the shipwreck was to determine the exact coordinates of the object, define her geometric dimensions, shipwreck was to determine the exact coordinates of the object, define her geometric dimensions, orientation, measure the safe depth value over the wreck, and determine the type of sea bottom. orientation, measure the safe depth value over the wreck, and determine the type of sea bottom. 3.3. Mapping the Shipwreck-Site 3.3. Mapping the Shipwreck-Site Prior to the survey, the calibration of multibeam echosounder (patch test) was performed and Prior to the survey, the calibration of multibeam echosounder (patch test) was performed and the sound velocity in the water column was measured. Then the actual distribution of sound velocity the sound velocity in the water column was measured. Then the actual distribution of sound velocity was introduced into the system. A grid of several dozen parallel survey lines was planned using the was introduced into the system. A grid of several dozen parallel survey lines was planned using the QINSy software ver. 7.5 (Quality Integrated Navigation System developed by Quality Positioning QINSy software ver. 7.5 (Quality Integrated Navigation System developed by Quality Positioning Service (Zeist, the Netherlands)). The survey lines were designed to be parallel to the longer axis of the Service (Zeist, The Netherlands)). The survey lines were designed to be parallel to the longer axis of detected wreck. The use of a systematic series of parallel straight lines is the most efficient way of the detected wreck. The use of a systematic series of parallel straight lines is the most efficient way of covering a survey area. Weather conditions for the survey were fair with a slight sea state and light wind. Speed over ground was maintained as close to 4 knots as possible. In practice, it varied between 3 and 5 knots depending on the survey direction. Geosciences 2020, 10, x FOR PEER REVIEW 6 of 15 covering a survey area. Weather conditions for the survey were fair with a slight sea state and light wind.Geosciences Speed2020 ,over10, 240 ground was maintained as close to 4 knots as possible. In practice, it varied between6 of 15 3 and 5 knots depending on the survey direction. In the first stage of research, hydrographers of the Arctowski took advantage of a multibeam echosounder.In the first MBES stage is of widely research, used hydrographers for conducting of bathymetric the Arctowski surveys took and advantage offers the of possibility a multibeam of completeechosounder. bottom MBES coverage is widely [29,30]. used Multibeam for conducting data bathymetricwere acquired surveys using anda hull-mounted offers the possibility Kongsberg of Maritimecomplete EM3002D. bottom coverage The EM3002D [29,30]. Multibeamis a dual-head data configuration were acquired system using with a hull-mounted a mounting tilt Kongsberg angle of theMaritime single EM3002D.sonar head The 40° EM3002D (Figure 3). is An a dual-head angular swath configuration coverage system used withwas 140°. a mounting The array tilt anglelengths of usedthe single in an sonarEM3002D head result 40◦ (Figure in 1.5°3 by). An 1.5° angular beams swathat broadside coverage [31]. used The wassystem 140 sonar◦. The frequency array lengths was nominallyused in an EM3002D300 kHz. There result were in 1.5 ◦160by simultaneous 1.5◦ beams at broadsidereceive beams [31]. for The each system sonar sonar head. frequency They were was eithernominally spaced 300 in kHz. an equiangular There were 160or equidistant simultaneous patte receivern within beams the for coverage each sonar specified head. Theyby the were operator. either Therespaced was in an also equiangular a high-density or equidistant mode where pattern 254 within soundings the coverage are acquired specified per by head the operator.and ping There [32]. EM3002Dwas also a was high-density delivered mode with wherethe mini 254 Valeport soundings sound are acquiredvelocity sensor per head (SVS) and mounted ping [32]. in EM3002D the rear part was ofdelivered the blister. with the mini Valeport sound velocity sensor (SVS) mounted in the rear part of the blister.

FigureFigure 3. 3. Under-keelUnder-keel blister blister with with sound sound velocity velocity sens sensor,or, single-beam, and multi-beam transducer.

The exactexact position,position, heading heading and and roll, roll, and and pitch pitch and heaveand heave compensation compensation were provided were provided by Seapath by Seapath300 multifunction 300 multifunction system. system The system. The hassystem two has built-in two GPSbuilt-in receivers GPS receivers for measuring for measuring the position, the position,heading, heading, and speed and of thespeed unit. of Thethe unit. Seapath The also Seapat includesh also theincludes latest generationthe latest generation Motion Reference Motion ReferenceUnit MRU5. Unit Having MRU5. a complete Having a depth complete distribution depth di overstribution the wreck over and the in wreck her vicinity, and in several her vicinity, survey severallines were survey designed lines were to pass designed directly to over pass the directly object. over Then the we object. did measurement Then we did measurement using two-channels using two-channelssingle-beam echosounder single-beam toechosounder detect the minimum to detect the depth minimum over the depth wreck. over the wreck. SBES projects a pulse of acoustic energy from thethe transducertransducer toto thethe seabed.seabed. The The time delay interval between pulse pulse transmission and and reception reception of of the reflected reflected echo is accurately measured and converted into a distance using an estimate estimate of of th thee harmonic harmonic mean mean sound sound speed speed along along the the propagation propagation path [[33].33]. We We used two-channels single-beam ec echosounderhosounder to to detect the minimum depth over the wreck. HydrographicHydrographic SBESsSBESs are are usually usually built built with with two two channels channels of low of andlow highand frequency.high frequency. This allows This allowsfor the separationfor the separation of the bottom of the returnbottom from return soft fr layerom soft sediments layer sediments and the underlying and the underlying hard surface hard of surfacethe seabed. of the We seabed. applied We single-beam applied single-beam echosounder echosounder ATLAS Elektronik ATLAS Elektronik DESO 25 (ATLAS DESO 25 Elektronik, (ATLAS Elektronik,Bremen, Germany) Bremen, operating Germany) at operating 33 kHz and at 33 210 kHz kHz and (Figure 210 kHz4). (Figure 4). The wreck should be properly examined by echosounders first. Having a full depth distribution, the height, and orientation of the wreck, it was possible to design survey lines for the sonar survey. Our goal was to obtain as much sonar images as possible. To do that crew of the survey ship Arctowski took advantage of side-scan sonars. High-resolution seafloor mapping with side-scan sonar are commonly used to examine the seabed in coastal waters as well as to survey the shipwrecks and other obstacles [34–36]. As for the side-scan sonar, there are in general two types: (a) towed systems and (b) hull-mounted sonars [37]. Sonar emits a wide vertical (40–50◦) and narrow horizontal beam (0.2–1◦)[38] and receives seabed echoes at fixed time intervals to produce a high-resolution bottom imagery [39]. The returning echoes are displayed as one single line, where dark and light portions represent strong and weak echoes relative to the travel time. FigureTwo sonar 4. Components systems were of usedsingle-beam in the research. echosounder. EdgeTech (a) graphic DF-1000 recorder was towed and ( behindb) keel-mounted the Arctowski and thetransducers second oneof low was and hull-mounted high frequency. side-scan sonar ACSON-100 (Figure5). DF-1000 system features

Geosciences 2020, 10, x FOR PEER REVIEW 6 of 15 covering a survey area. Weather conditions for the survey were fair with a slight sea state and light wind. Speed over ground was maintained as close to 4 knots as possible. In practice, it varied between 3 and 5 knots depending on the survey direction. In the first stage of research, hydrographers of the Arctowski took advantage of a multibeam echosounder. MBES is widely used for conducting bathymetric surveys and offers the possibility of complete bottom coverage [29,30]. Multibeam data were acquired using a hull-mounted Kongsberg Maritime EM3002D. The EM3002D is a dual-head configuration system with a mounting tilt angle of the single sonar head 40° (Figure 3). An angular swath coverage used was 140°. The array lengths used in an EM3002D result in 1.5° by 1.5° beams at broadside [31]. The system sonar frequency was nominally 300 kHz. There were 160 simultaneous receive beams for each sonar head. They were either spaced in an equiangular or equidistant pattern within the coverage specified by the operator. There was also a high-density mode where 254 soundings are acquired per head and ping [32]. EM3002D was delivered with the mini Valeport sound velocity sensor (SVS) mounted in the rear part of the blister.

Figure 3. Under-keel blister with sound velocity sensor, single-beam, and multi-beam transducer.

The exact position, heading and roll, and pitch and heave compensation were provided by Seapath 300 multifunction system. The system has two built-in GPS receivers for measuring the position, heading, and speed of the unit. The Seapath also includes the latest generation Motion Reference Unit MRU5. Having a complete depth distribution over the wreck and in her vicinity, several survey lines were designed to pass directly over the object. Then we did measurement using two-channels single-beam echosounder to detect the minimum depth over the wreck. SBES projects a pulse of acoustic energy from the transducer to the seabed. The time delay interval between pulse transmission and reception of the reflected echo is accurately measured and converted into a distance using an estimate of the harmonic mean sound speed along the propagation

Geosciencespath [33].2020 We, 10 used, 240 two-channels single-beam echosounder to detect the minimum depth over7 ofthe 15 wreck. Hydrographic SBESs are usually built with two channels of low and high frequency. This Geosciencesallows for 2020 the, 10 separation, x FOR PEER of REVIEW the bottom return from soft layer sediments and the underlying 7hard of 15 twosurface frequencies of the seabed. 100 kHz We and applied 500 kHz. single-beam Pulse length echosounder is 0.1 ms ATLAS for 100 kHzElektronik and 0.01 DESO ms for25 500(ATLAS kHz. The wreck should be properly examined by echosounders first. Having a full depth distribution, HorizontalElektronik, beamwidthBremen, Germany) is 1.2◦ (for operating 100 kHz) at and33 kHz 0.5◦ and(for 210 500 kHz kHz). (Figure 4). the height, and orientation of the wreck, it was possible to design survey lines for the sonar survey. Our goal was to obtain as much sonar images as possible. To do that crew of the survey ship Arctowski took advantage of side-scan sonars. High-resolution seafloor mapping with side-scan sonar are commonly used to examine the seabed in coastal waters as well as to survey the shipwrecks and other obstacles [34–36]. As for the side-scan sonar, there are in general two types: (a) towed systems and (b) hull-mounted sonars [37]. Sonar emits a wide vertical (40–50°) and narrow horizontal beam (0.2–1°) [38] and receives seabed echoes at fixed time intervals to produce a high-resolution bottom imagery [39]. The returning echoes are displayed as one single line, where dark and light portions represent strong and weak echoes relative to the travel time. Two sonar systems were used in the research. EdgeTech DF-1000 was towed behind the Arctowski and the second one was hull-mounted side-scan sonar ACSON-100 (Figure 5). DF-1000 Figure 4.4. ComponentsComponents of single-beam echosounder. ( (aa)) graphic graphic recorder recorder and (b) keel-mountedkeel-mounted system features two frequencies 100 kHz and 500 kHz. Pulse length is 0.1 ms for 100 kHz and 0.01 transducers of low and highhigh frequency.frequency. ms for 500 kHz. Horizontal beamwidth is 1.2° (for 100 kHz) and 0.5° (for 500 kHz).

Figure 5. Side-scanSide-scan sonar systems. ( (aa)) towed towed behind behind the the survey survey vessel vessel and and ( (b)) hull-mounted hull-mounted sonar. sonar.

The sonar operator made use of thethe highesthighest resolutionresolution andand usedused 500500 kHzkHz mode.mode. Acquisition ranges of 25, 40, and 75 m were tested at the si site.te. Towed Towed deployment of the sonar was made without an acoustic tracking beacon on the sonar. Sona Sonarr position was calculated using layback and offset offset method. Recording Recording and and processing was conducted using CODA data acquisition system, based on Linux. Although Although the the manufacturer manufacturer declared declared the the ma maximumximum tow tow speed speed about 12 knots, to obtain the highest resolutionresolution andand density density of of data data possible possible over ov theer the object object the towthe tow speed speed was reducedwas reduced to 3–4 to knots. 3–4 knots.Hull-mounted side-scan sonar ACSON-100 is a low frequency system operating at 100 kHz. Sonar tracksHull-mounted were planned side-scan parallel tosonar the longACSON-100 axis of theis a wreck low frequency and at a lateral system distance operating from at the100 object. kHz. TheSonar principle tracks were from planned larger to parallel smaller to ranges, the long i.e., axis “from of the general wreck to and detail” at a was lateral applied distance to the from survey. the Theobject. primary The principle objectives from of the larger research to smaller were toranges find the, i.e., optimal “from geometrygeneral to between detail” was sonar applied mainlobe to the of survey.acoustic The energy primary and theobjectives shipwreck of the lying research on the were bottom to find to recordthe optimal good geometry quality data. between sonar main lobe of acoustic energy and the shipwreck lying on the bottom to record good quality data. 4. Results and Discussion 4. ResultsA full and and Discussion comprehensive data about the object was obtained. All records were used to identify the wreckA full and comprehensive in-depth analysis. data Bathymetric about the object data wa ands obtained. sonar images All records enabled were the used preparation to identify of thefinal wreck report. and in-depth analysis. Bathymetric data and sonar images enabled the preparation of final report. 4.1. Bathymetric Data 4.1. BathymetricBathymetric data data were logged using QINSy acquisition software. Data processing was performed usingBathymetric Qloud 3D, whichdata were is an ologgedffline toolusing fully QINSy integrated acquisition with QINSy. software. The wreckData processing lies on her portwas performedside at a depth using of Qloud 71 m, 3D, and which the hullis an reaches offline tool up tofully a depth integrated of 50 with m. TheQINSy. ship The is mainlywreck lies intact. on herIn the port post-processing side at a depth stage of 71 variousmeters, meansand the of hull MBES reaches data up presentation to a depth wereof 50 adopted,meters. The shown ship inis mainlyFigure6 .intact. One of In them the post-processing was combination stage of sun-illumination various means of and MBES colorized data gridpresentation (Figure6 a),were multibeam adopted, shownbackscatter in Figure grid data6. One (Figure of them6d), was and combination profile view of data sun-illumination (Figure6c). However, and colorized the most grid interesting(Figure 6a), multibeamis three-dimensional backscatter visualization grid data (Figure of bathymetry 6d), and presentedprofile view as adata digital (Figure terrain 6c). modelHowever, (DTM) the ofmost the interesting is three-dimensional visualization of bathymetry presented as a digital terrain model (DTM) of the Steuben wreck (Figure 6b). It provides a complete picture of the wreck. The value of the

Geosciences 2020, 10, 240 8 of 15

Geosciences 2020, 10, x FOR PEER REVIEW 8 of 15 Steuben wreck (Figure6b). It provides a complete picture of the wreck. The value of the visualization visualizationsoftware used software to produce used the to detailed produce images the detaile cannotd images be overstated. cannot be Each overstated. of sounding Each was of attributedsounding wasto given attributed shadow to ofgiven color. shadow Owing of to color. the appliedOwing to technique the applied of data technique “colorizing” of data one “colorizing” can get an one idea can of gethow an the idea wreck of how looks the like. wreck looks like.

Figure 6. DifferentDiﬀerent types of multibeam data presentation. ( (aa)) sun-illuminated sun-illuminated grid grid data, data, ( (b)) digital terrainterrain model, model, ( (c)) profile proﬁle view data, and (d) multibeam backscatter grid data.

Based on on the the survey survey with with single-beam single-beam echosounder echosounder the the least, least, safe safe depth depth over over the wreck the wreck was 51.2 was m,51.2 the m, surrounding the surrounding depth depth was wason average on average 71–72 71–72 m, and m, andthe theheight height of the of thewreck wreck was was about about 20 20m. m.If theIf the wreck wreck rested rested on on an an even even keel, keel, the the least least depth depth wa wass often often represented represented by by the the echo echo coming coming from from the the protruding mast, funnel, or superstructure.superstructure. In case of Steuben Steuben,, lying on the port port side, side, the the height height above above thethe bottom bottom is is nothing nothing other other than than the the ship’s ship’s beam, beam, which which was was 19.8 19.8 m m (Table (Table 11).). AsAs aa resultresult ofof passingpassing directly over the wreck, we .obtainedobtained a a couple couple of of echograms. echograms. One One of of them them is is presented presented in in Figure Figure 77..

4.2. Side-Scan Sonar Records and Wreck’s Identification The first passes over the wreck site were performed with a hull-mounted side-scan sonar with a low frequency 100 kHz. Sonographs recorded with this system showed only a general shape of the wreck, with no resolution (Figure8a). The next passes were done using towed, digital side-scan sonar with a frequency 500 kHz. Figure8 demonstrates the increased resolution of a digital, high frequency system over an analog, low frequency sonar with transducers mounted in the hull of the survey ship Arctowski. It revealed differences in quality and detail between two systems used. Much more detailed pictures (Figure8b) comes from sonar towed behind the stern of the survey ship. One can easily distinguish some characteristics such as davits (Figure8d), shaft, propellers (Figure8c), and quite big hole in the hull, being the result of a torpedo hit (Figure8e). Figure9 presents another sonogram of Steuben acquired while surveying parallel to the hull and in close lateral distance from the wreck. One can readily notice that the wreck remained intact with bow and stern masts, superstructure, davits, and boom. Measurements on the SSS display gave us primary dimensions and orientation of the wreck. The wreck’s length from sonar measurements was 160 m, height of the object 20 m, and the azimuth was 231◦. By surveying parallel to the shipwreck elevated from the seafloor, sufficient acoustic shadow was Figure 7. Single-beam echosounder data (echogram) showing the least depth over the wreck, produced. Shadow cast by the wreck was a function of the angle at which the sonar beam ensonified surrounding depth, and the height of the object. the wreck. The formation of acoustic shadow in side-scan sonar imagery is a key element in data interpretation and, finally, in identification of the shipwreck. Shadows will often have more detail 4.2. Side-Scan Sonar Records and Wreck’s Identification The first passes over the wreck site were performed with a hull-mounted side-scan sonar with a low frequency 100 kHz. Sonographs recorded with this system showed only a general shape of the

Geosciences 2020, 10, x FOR PEER REVIEW 8 of 15 visualization software used to produce the detailed images cannot be overstated. Each of sounding was attributed to given shadow of color. Owing to the applied technique of data “colorizing” one can get an idea of how the wreck looks like. Geosciences 2020, 10, 240 9 of 15 than the direct reﬂection from a target. By analyzing the “shadows” in a sonograph one can get some valuable information about the wreck [40]. The shadow cast by the Steuben wreck showed the deck boom at the bow, bridge superstructure, bow and stern masts, and davits on the starboard side. In addition, the shadow length was used to calculate the approximate height of the target above the surrounding bottom. The geometry of height estimation from SSS records is shown in Figure 10. The approximate height of the shipwreck OH can be calculated from the measurements made directly on the sonar image. By virtue of similar triangles characteristics, it follows that:

SH SL oH = · , (1) R + SL where SH is the height of the sonar above the seafloor (m), SL is the length of the acoustic shadow cast by the object (m), and R is a slant range (m) to the object. To calculate the shipwreck height acoustic shadow must be entirely within the sonar imagery (Figure9). If the shadow extends beyond the imaging window, then the operator cannot measure the height of the wreck (Figure8b). An acoustic shadow that reaches out beyond the edge of the sonograph implies wreck with a height that is near or Figure 6. Different types of multibeam data presentation. (a) sun-illuminated grid data, (b) digital above the altitude of the sonar and might pose a danger on subsequent passes of the sonar survey. terrain model, (c) profile view data, and (d) multibeam backscatter grid data. This is often the case with wrecks that have undamaged masts, funnels, spars, antennas, vertical booms,Based and on other the narrowsurvey with and highsingle-beam structural echosounder elements. Therefore, the least, duringsafe depth data over acquisition, the wreck the was towfish 51.2 m,should the surrounding be towed close depth to the was seabed on average to optimize 71–72 the m, ensonifying and the height geometry. of the wreck The recommended was about 20 height m. If theof the wreck sonar rested is 8to on 20% an even of the keel, selected the least range depth scale. wa Exampless often represented of sonograms by the with echo shadows coming extending from the protrudingbeyond the mast, sonar funnel, imagery or areasuperstructure. are shown In in case the Figureof Steuben 11. The, lying shadows on the port length side, in the case height of Steuben above werethe bottom measured is nothing from both other sides than of the the ship’s wreck beam, and the which results was averaged. 19.8 m (Table This 1). procedure As a result was of to passing correct directlyfor errors over introduced the wreck, by we seafloor obtained slope. a couple of echograms. One of them is presented in Figure 7.

Figure 7. Single-beamSingle-beam echosounder echosounder data data (echogram) (echogram) showing showing the the least least depth depth over over the the wreck, wreck, surrounding depth, and the height of the object.

4.2. Side-Scan Sonar Records and Wreck’s Identification The first passes over the wreck site were performed with a hull-mounted side-scan sonar with a low frequency 100 kHz. Sonographs recorded with this system showed only a general shape of the

Geosciences 2020, 10, x FOR PEER REVIEW 9 of 15

wreck, with no resolution (Figure 8a). The next passes were done using towed, digital side-scan sonar with a frequency 500 kHz. Figures 8 demonstrates the increased resolution of a digital, high frequency

Geosciencessystem over 2020 ,an 10 ,analog, x FOR PEER low REVIEW frequency sonar with transducers mounted in the hull of the survey9 shipof 15 Arctowski. It revealed differences in quality and detail between two systems used. Much more wreck,detailed with pictures no resolution (Figure 8b)(Figure comes 8a). from The nextsonar pass towees dwere behind done the using stern towed, of the digital survey side-scan ship. One sonar can witheasily a frequencydistinguish 500 some kHz. characteristics Figures 8 demonstrates such as davits the increased (Figure 8d), resolution shaft, propellersof a digital, (Figure high frequency 8c), and systemquite big over hole an in analog, the hull, low being frequency the result sonar of witha torpedo transducers hit (Figure mounted 8e). in the hull of the survey ship Arctowski. It revealed differences in quality and detail between two systems used. Much more detailed pictures (Figure 8b) comes from sonar towed behind the stern of the survey ship. One can Geoscienceseasily2020 distinguish, 10, 240 some characteristics such as davits (Figure 8d), shaft, propellers (Figure 8c), and10 of 15 quite big hole in the hull, being the result of a torpedo hit (Figure 8e).

Figure 8. Side-scan sonar images of the shipwreck of the General von Steuben. (a) sonar imagery recorded with hull-mounted, 100 kHz side-scan sonar; (b) sonar imagery recorded with towed, 500 kHz side-scan sonar; (c) displaying shafts and propellers; (d) displaying front part of the superstructure and davits; and (e) displaying torpedo hole in the shipwreck hull.

FigureFigure 8. Side-scan 8. Side-scan sonar sonar images images of of the the shipwreckshipwreck of of the the General General von von Steuben Steuben.. (a) sonar (a) sonar imagery imagery recordedFigure 9 withpresents hull-mounted, another sonogram 100 kHz side-scan of Steuben sonar; acquired (b) sonar while imagery surveying recorded pa rallelwith towed, to the hull500 and recorded with hull-mounted, 100 kHz side-scan sonar; (b) sonar imagery recorded with towed, 500 kHz in closekHz lateral side-scan distance sonar; from (c) thedisplaying wreck. Oneshafts can and readily propellers; notice (thatd) displaying the wreck front remained part ofintact the with side-scan sonar; (c) displaying shafts and propellers; (d) displaying front part of the superstructure and bow superstructureand stern masts, and davits;superstructure, and (e) displaying davits, andtorpedo boom. hole Measurements in the shipwreck on hull. the SSS display gave us davits;primary and dimensions (e) displaying and orientation torpedo hole of inthe the wreck. shipwreck The wreck’s hull. length from sonar measurements was 160 m,Figure height 9 presents of the object another 20 m, sonogram and the azimuthof Steuben was acquired 231°. while surveying parallel to the hull and in close lateral distance from the wreck. One can readily notice that the wreck remained intact with bow and stern masts, superstructure, davits, and boom. Measurements on the SSS display gave us primary dimensions and orientation of the wreck. The wreck’s length from sonar measurements was 160 m, height of the object 20 m, and the azimuth was 231°.

Geosciences 2020, 10, x FOR PEER REVIEW 10 of 15

By surveying parallel to the shipwreck elevated from the seafloor, sufficient acoustic shadow was produced. Shadow cast by the wreck was a function of the angle at which the sonar beam ensonified the wreck. The formation of acoustic shadow in side-scan sonar imagery is a key element in data interpretation and, finally, in identification of the shipwreck. Shadows will often have more detail than the direct reflection from a target. By analyzing the "shadows" in a sonograph one can get some valuable information about the wreck [40]. The shadow cast by the Steuben wreck showed the

deck boom at the bow, bridge superstructure, bow and stern masts, and davits on the starboard side. Figure 9. Identification of the wreck’s construction features. In addition, the shadowFigure length 9. Identiﬁcation was used to calculate of the wreck’s the approximate construction height features. of the target above the surrounding bottom. The geometry of height estimation from SSS records is shown in Figure 10.

Figure 9. Identification of the wreck’s construction features.

FigureFigure 10. 10.Calculation Calculation of object’s height height based based on onsimilar similar triangles. triangles.

The approximate height of the shipwreck OH can be calculated from the measurements made directly on the sonar image. By virtue of similar triangles characteristics, it follows that:

∙ , (1)

where SH is the height of the sonar above the seafloor (m), SL is the length of the acoustic shadow cast by the object (m), and R is a slant range (m) to the object. To calculate the shipwreck height acoustic shadow must be entirely within the sonar imagery (Figure 9). If the shadow extends beyond the imaging window, then the operator cannot measure the height of the wreck (Figure 8b). An acoustic shadow that reaches out beyond the edge of the sonograph implies wreck with a height that is near or above the altitude of the sonar and might pose a danger on subsequent passes of the sonar survey. This is often the case with wrecks that have undamaged masts, funnels, spars, antennas, vertical booms, and other narrow and high structural elements. Therefore, during data acquisition, the towfish should be towed close to the seabed to optimize the ensonifying geometry. The recommended height of the sonar is 8 to 20% of the selected range scale. Examples of sonograms with shadows extending beyond the sonar imagery area are shown in the Figure 11. The shadows length in case of Steuben were measured from both sides of the wreck and the results averaged. This procedure was to correct for errors introduced by seafloor slope. Figure 12 for comparison purposes presents the silhouette of the Steuben ship and part of the sonogram with the acoustic shadow. To show a high level of similarity, the silhouette of the ship was intentionally changed to black and the shadow cast by the wreck was extracted from the entire sonogram. In the area of acoustic shadow, there are no visible funnels, which probably were destroyed during the sinking of the ship and hitting the seabed.

Geosciences 2020, 10, 240 11 of 15 Geosciences 2020, 10, x FOR PEER REVIEW 11 of 15

FigureFigure 11.11. Examples of of the the wrecks wrecks with with protruding, protruding, vertical vertical structural structural elements. elements. (a) (yachta) yacht wreck; wreck; (b) (bargeb) barge with with piles piles of wood; of wood; (c) (fishingc) ﬁshing boat; boat; and and (d) ( dbarge.) barge.

Figure 12 for comparison purposes presents the silhouette of the Steuben ship and part of the sonogram with the acoustic shadow. To show a high level of similarity, the silhouette of the ship wasGeosciences intentionally 2020, 10, x FOR changed PEER REVIEW to black and the shadow cast by the wreck was extracted from the11 entire of 15 sonogram. In the area of acoustic shadow, there are no visible funnels, which probably were destroyed during the sinking of the ship and hitting the seabed. A quite important element of the wreck identification process can be a comparative analysis of remote sensing sonar data with collected archival documentation, especially ship silhouette photos and ship construction plans. The key to positive identification of the wreck is a search for dimensions match, similarities in the shape of the vessel, and common design features (Figure 13). The length and width of the investigated wreck may coincide with the real dimensions of the objects. The greater the compatibility of compared geometric features, the greater the likelihood of positive identification of the discovered wreck. One can compare the wreck’s construction features, seek compatibility and analogy in the location, and shape and size of the construction characteristic of the hull (Figures 12–14). These features visible on sonar images are compared with photos and drawing documentation of the ship. The moreFigureFigure compliance, 11. 12. Examples Comparison matching, of the of wrecksthe andacoustic with similarities protruding,shadow thatshape vertical are with obtained structural the ship's during elements. silhouette. data (a analysis, )( ayacht) hydroacoustic wreck; the more (b) the identificationbargeshadow with from of piles the the of wreck sonogram wood; can (c) of befishing the successful. shipwreck boat; and and (d) barge.(b) black silhouette of the ship General von Steuben for comparative analysis.

A quite important element of the wreck identification process can be a comparative analysis of remote sensing sonar data with collected archival documentation, especially ship silhouette photos and ship construction plans. The key to positive identification of the wreck is a search for dimensions match, similarities in the shape of the vessel, and common design features (Figure 13). The length and width of the investigated wreck may coincide with the real dimensions of the objects. The greater the compatibility of compared geometric features, the greater the likelihood of positive identification of the discovered wreck. One can compare the wreck's construction features, seek compatibility and analogy in the location, and shape and size of the construction characteristic of the hull (Figures 12– 14). These features visible on sonar images are compared with photos and drawing documentation of the ship. The more compliance, matching, and similarities that are obtained during data analysis, the more the identification of the wreck can be successful.

Figure 12. Comparison of the acoustic shadow shape withwith thethe ship’sship's silhouette.silhouette. ( a) hydroacoustic shadow from the sonogram of the shipwreck and ( b) black silhouette of the ship General von Steuben for comparative analysis.

Geosciences 2020, 10, x 240 FOR PEER REVIEW 1212 of of 15 15 Geosciences 2020, 10, x FOR PEER REVIEW 12 of 15

Figure 13. Comparison of the recorded sonar data with the photographic documentation of the ship. Figure 13. Comparison of the recorded sonar data with the photographic documentation of the ship. (Figurea) propeller; 13. Comparison (b) shaft; ( ofc) pillarsthe recorded and stern sonar decks; data ( withd) sixty the rectangular photographic side documentation windows; (e) sixof the pairs ship. of (a) propeller; (b) shaft; (c) pillars and stern decks; (d) sixty rectangular side windows; (e) six pairs of boat(a) propeller; davits; (f )( bfront) shaft; part (c )of pillars the superstructure; and stern decks; (g) (hawse-holes;d) sixty rectangular and (h )side small windows; bow superstructure. (e) six pairs of boat davits; (f) front part of the superstructure; (g) hawse-holes; and (h) small bow superstructure. boat davits; (f) front part of the superstructure; (g) hawse-holes; and (h) small bow superstructure.

Figure 14. ComparisonComparison of of the the recorded recorded sonar data with the photographic documentation of of the the bow bow partFigure of the 14. SteubenComparison ship. of the recorded sonar data with the photographic documentation of the bow part of the Steuben ship. DF-1000 side-scan sonar sonar towed towed behind behind the the survey ship Arctowski proved to be invaluable equipmentDF-1000 and and side-scan has provided sonar a towed large numberbehind the ofof high-quality high-qualitysurvey ship shipwreck shipwreckArctowski images.images. proved Theseto be sonograms invaluable wereequipment usedused to toand identify identify has provided the the wreckage wreckage a large of theofnumber the German Germ of linerhigh-qualityan liner from from World shipwreck World War II.War Basedimages. II. Based on These the sonaron sonograms the images, sonar images,thewere length, used the width,tolength, identify and width, heightthe and wreckage of height the wreck of thethe were wrec Germ measuredk anwere liner measured withoutfrom World without having War tohaving goII. underwater.Based to go onunderwater. the A sonar great Aaidimages, great in identifying aidthe inlength, identifying thewidth, wreck the and wreck turned height turned outof the to outwrec be to acoustick be were acoustic measured shadow. shadow. Thewithout The presence presencehaving of to shadows of go shadows underwater. was was an animportantA greatimportant aid clue in clueidentifying in the in recognitionthe recognitionthe wreck of wreck’sturned of wreck’s out shape to sh andbeape acoustic general and general shadow. condition condition The of thepresence object.of the of object. For shadows this For reason, wasthis reason,attentionan important attention must clue be must paid in the tobe getrecognitionpaid the to shadow get theof wreck’s withinshadow thesh withinape image. and the The general image. primary conditionThe benefitprimary of of thebenefit SSS object. was of the SSSFor ability wasthis thetoreason, quickly ability attention to and quickly effi mustciently and be efficien generatepaid totly get detailedgenerate the shadow images detailed within of images large the areas image.of large of The the areas bottomprimary of the regardless benefitbottom ofregardless ofSSS water was ofclarity.the water ability A clarity. closer to quickly passA closer and by the efficienpass wreck bytly the of generate Steubenwreck ofdetailed was Steuben made images was for detailed madeof large for imagesareas detailed of using the images bottom 40 m using range regardless 40 scale. m rangeTheof water wreck scale. clarity. was The ensonified wreck A closer was frompass ensonified dibyff erentthe fromwreck orientations different of Steuben whichorientations was resulted made which in for acquiring resulteddetailedvarious in images acquiring sonar using various images 40 m sonarcompletedrange scale.images with The completed wreck new additional was with ensonified new information. additional from different Because information. orientations towing Because speed which wastowing resulted critical speed in for acquiring optimumwas critical various along for optimumtracksonar resolution,images along completed track a good resolution, compromisewith new a goodadditional was compromise found info betweenrmation. was 3–4found Because knots. between towing The weather3–4 speed knots. was conditions The critical weather andfor conditionsseaoptimum state asalong and well sea track as state the resolution, sizeas well of theas athe surveygood size compromiseof ship the Arctowskisurvey wasship were foundArctowski of between primary were 3–4 importance.of primary knots. The importance. Moreover, weather Moreover,theconditions selection theand of selection sea the state appropriate ofas thewell appropriate surveyas the size methodology ofsurvey the survey methodology contributed ship Arctowski contributed to the finalwere to results.of the primary final results. importance. Moreover,The identity the selection of a newly of the discovered appropriate wreck survey is oftenmethodology determined contributed on the basis to the of finalthe characteristic results. features,features,The suchidentity asas artefactsartefacts of a newly foundfound discovered by by divers divers wreck in in the th ise wreck. wreck.often Anotherdetermined Another way way on of ofthe identifying identifying basis of thethe the wreckcharacteristic wreck may may be berecordingfeatures, recording such the the filmas artefactsfilm with with camera found camera installed by installeddivers on in a onth remotelye a wreck. remotely operated Another operated vehicle way vehicle of andidentifying searchingand searching the for wreck the for name may the nameorbe hullrecording or number hull numberthe of film the of vessel.with the cameravessel. Some Some discoverersinstalled discover on areaers remotely looking are looking foroperated a for ship’s a vehicleship's bell orbell and a steeringor searching a steering wheel for wheel thatthe thatname will or uniquelyhull number answer of the the vessel. question Some what discover ship weers are are dealing looking with. for aThese ship's methods bell or a were steering presented wheel that will uniquely answer the question what ship we are dealing with. These methods were presented

Geosciences 2020, 10, 240 13 of 15 will uniquely answer the question what ship we are dealing with. These methods were presented quite well in [10–12], where it was emphasized that only characteristic items found in the wreck and extracted by divers or extensive material recorded with a video camera guarantee full identification of the vessel. In this study, the author identified a newly discovered wreck in the Baltic Sea. The identification was performed without involving a team of divers. Furthermore, the wreckage was not examined with a remotely operated vehicle. The author applied the appropriate methodology for planning and using hydrographic survey equipment. As a result, very detailed and easy to interpret bathymetric and sonar data were acquired. It is worth noting that the data obtained from the side-scan sonar was high resolution. Due to the quality and completeness of the data, one can distinguish the characteristic features of the wreck construction. However, there are several limitations concerning author’s approach. The proposed method of identification may not be effective in the case of small underwater objects or if we are dealing with wreckage debris. The use of acoustic remote sensing techniques may not provide the desired results in adverse weather conditions. Full identification of the wreck will also not be possible in case of insufficient amount of historical material collected. Acoustic remote sensing of the wreck does not pose a threat to human life, does not require expensive ROV operation and is a fast, relatively cheaper method of obtaining high resolution data. Modern hydrographic equipment such as multibeam echosounder or side-scan sonar should be regarded as necessary for complete understanding the newly discovered shipwrecks and other underwater archaeological sites. It is expected that within a few or several years, the underwater survey technology will continue to develop. Improvements will probably concern increase of data resolution, portability, compactness, and autonomous vehicle applications.

5. Conclusions Almost every year we find out about discoveries of the new wrecks around the world. Many of them remain mysterious, others are inaccessible due to depth or completely destroyed with no chance to be identified. The aim of this paper was to show how acoustic remote sensing data is used to identify large bottom object such as shipwrecks. The author demonstrated a method of wreck’s identification using bathymetric data and sonar imagery. The proposed approach assumes the application of multibeam echosounder and side-scan sonar technology widely used in hydrographic surveying. This survey equipment combined with the applied methodology enabled the collection of high-resolution remote sensing data that constituted a significant contribution to identification process. The findings presented suggest that under certain conditions it is possible to identify the shipwreck without diving or using underwater vehicles. This is particularly important in terms of human safety or survey operation costs. Although the survey results validated the effectiveness of the proposed approach it should be noted that we were dealing with a large, well-preserved hull of the wreck. Identification of such a target is much simpler than establishing the identity of the debris slightly protruding above the sea bottom. Unambiguous identification of a newly discovered shipwreck will sometimes require supplementation with other systems and techniques, e.g., underwater television, diving operation, or archaeological research. The discovery of the Steuben shipwreck was a spectacular event. The wreck is well preserved and should be left unimpaired and undisturbed. A tremendous role in this matter is played by Polish maritime authorities.

Funding: This research was supported by the Minister of National Defense of Poland as part of the program called Research Grant: “Backscattering of acoustic waves in the aquatic environmental”. Geosciences 2020, 10, 240 14 of 15

Acknowledgments: Author would like to thank the crew of the Polish Navy hydrographic ship Arctowski, for their dedication, exemplary performance of tasks and a fantastic atmosphere onboard during the hydrographic investigation of the wreck of General von Steuben. Conﬂicts of Interest: The author declares no conﬂict of interest.

References

1. Ødegård, Ø.; Sørensen, A.; Hansen, R.; Ludvigsen, M. A new method for underwater archaeological surveying using sensors and unmanned platforms. IFAC-PapersOnLine 2016, 49, 486–493. [CrossRef] 2. Hutchinson, G. Threats to underwater cultural heritage. Mar. Policy 1996, 20, 287–290. [CrossRef] 3. Starnawski, K.; Poleszak, S. The dangers of diving in the sea. In Baltic Wrecks: Guide for Divers; Poleszak, S., Ed.; Eliza Poleszak: Gdynia, Poland, 2005; pp. 41–58. ISBN 83-920563-1-0. 4. Duck, B.; Klein, R.S. Shipwrecks of the Dominican Republic and A Guide to Shipwreck Identification through Recovered Artifacts, 1st ed.; Lulu Press: Morrisville, NC, USA, 2010; 156p, ISBN 978-0-9829477-0-8. 5. Dierssen, H.M.; Theberge, A.E., Jr. Bathymetry: History of Seafloor Mapping. In Encyclopedia of Natural Resources. Vol. II—Water and Air, 1st ed.; Wang, Y., Raton, B., Eds.; Taylor & Francis Group: Milton Park, UK, 2014; p. 564. [CrossRef] 6. Bates, C.R.; Lawrence, M.; Dean, M.; Robertson, P. Geophysical Methods for Wreck-Site Monitoring: The Rapid Archaeological Site Surveying and Evaluation (RASSE) programme. Int. J. Naut. Archaeol. 2011, 40, 404–416. [CrossRef] 7. Brown, C.J.; Blondel, P. Developments in the application of multibeam sonar backscatter for seafloor habitat mapping. Appl. Acoust. 2009, 70, 1242–1247. 8. Janowski, L.; Trzcinska, K.; Tegowski, J.; Kruss, A.; Rucinska-Zjadacz, M.; Pocwiardowski, P. Nearshore Benthic Habitat Mapping Based on Multi-Frequency, Multibeam Echosounder Data Using a Combined Object-Based Approach: A Case Study from the Rowy Site in the Southern Baltic Sea. Remote Sens. 2018, 10, 1983. [CrossRef] 9. Huang, J. Maritime archaeology and identification of historic shipwrecks: A legal perspective. Mar. Policy 2014, 44, 256–264. [CrossRef] 10. Berg, D. Shipwreck Diving; Instant Downloadable E-Book; Aqua Explorers: Baldwin, NY, USA, 2015; p. 90. ISBN 0-9616167-5-X. 11. Keatts, H.; Skerry, B. Complete Wreck Diving: A Guide to Diving Wrecks, 1st ed.; Aqua Quest Publications: Locust Valley, NY, USA, 1999; p. 270. ISBN 10: 0922769389. 12. Grabiec, D.; Olejnik, A. Searching for and identifying underwater objects. In Baltic Wrecks: Guide for Divers; Poleszak, S., Ed.; Eliza Poleszak: Gdynia, Poland, 2005; pp. 81–105. ISBN 83-920563-1-0. 13. Foley, B.P.; Dellaporta, K.; Sakellariou, D.; Bingham, B.S.; Camilli, R.; Eustice, R.M.; Evagelistis, D.; Ferrini, V.L.; Katsaros, K.; Kourkoumelis, D.; et al. The 2005 Chios Ancient Shipwreck Survey: New Methods for Underwater Archaeology. Hesperia J. Am. Sch. Class. Stud. Athens 2009, 78, 269–305. [CrossRef] 14. Buscombe, D. Shallow water benthic imaging and substrate characterization using recreational-grade sidescan-sonar. Environ. Model. Softw. 2017, 89, 1–18. 15. Burguera, A.; Oliver, G. High-Resolution Underwater Mapping Using Side-Scan Sonar. PLoS ONE 2016, 11, e0146396. [CrossRef] 16. Healy, C.A.; Schultz, J.J.; Parker, K. Detecting Submerged Bodies: Controlled Research Using Side-Scan Sonar to Detect Submerged Proxy Cadavers. J. Forensic Sci. 2015, 60, 743–752. [CrossRef] 17. Bryant, R. Side Scan Sonar for Hydrography-An Evaluation by the Canadian Hydrographic Service. Int. Hydrogr. Rev. 2015, 52, 43–55. 18. Nakamura, K.; Toki, T.; Mochizuki, N. Discovery of a new hydrothermal vent based on an underwater, high-resolution geophysical survey. Deep Sea Res. Part I Oceanogr. Res. Pap. 2013, 74, 1–10. [CrossRef] 19. Johnson, H.; Helferty, M. The geological interpretation of side-scan sonar. Rev. Geophys. 1990, 28, 357–380. [CrossRef] 20. Grooss, P. The Naval War in the Baltic 1939–1945; Naval Institute Press: Barnsley, UK, 2017; 416p, ISBN 10: 152670000X. 21. Gelewski, T.M. Wilhelm Gustloff i General von Steuben Statki Smierci´ czy Zbrodnia Wojenna na Morzu? A.E.L. Publishing House: Gdańsk,Poland, 1997; ISBN 83-903896-4-9. (In Polish) Geosciences 2020, 10, 240 15 of 15

22. General von Steuben. Available online: www.danzig-online.pl/gustloff/steubene.html (accessed on 17 March 2020). 23. Tagliati, M. General von Steuben. X-Ray Mag 2005, 8, 71–77. Available online: https://xray-mag.com/pdfs/ xray08/X-Ray8_Adobe6.pdf (accessed on 25 February 2020). 24. Jamkowski, M. Duchy z Gł˛ebinBałtyku. Steuben, Gustloff, Goya; Swiat´ Ksi ˛a˙zki: Warszawa, Poland, 2010; pp. 1–303. ISBN 978-83-247-1803-0. (In Polish) 25. Ostasz, A. Ostatnie wielkie odkrycie na Bałtyku. Steuben. In Nurkowanie; OKTO Sp. z o.o.: Szczecin, Poland, 2004; Volume 8, pp. 15–21. (In Polish) 26. Prince, C.J. Death in the Baltic: The World War II Sinking of the Wilhelm Gustloff, 1st ed.; Palgrave MacMillan: New York, NY, USA, 2013; 256p, ISBN 13: 9780230341562. 27. Grz ˛adziel,A. Possibilities of Using Hydrographic Side Scan Sonar to Conduct Mine Counter-Measure Tasks. Master’s Thesis, Polish Naval Academy, Gdynia, Poland, 2007. (In Polish). 28. International Hydrographic Organization. Manual on Hydrography, 1st ed.; Publication C-13; International Hydrographic Bureau: Monaco, 2005; p. 202. 29. Held, P.; Schneider von Deimling, J. New Feature Classes for Acoustic Habitat Mapping—A Multibeam Echosounder Point Cloud Analysis for Mapping Submerged Aquatic Vegetation (SAV). Geosciences 2019, 9, 235. [CrossRef] 30. Mohammadloo, T.H.; Snellen, M.; Renoud, W.; Beaudoin, J.; Simons, D.G. Correcting Multibeam Echosounder Bathymetric Measurements for Errors Induced by Inaccurate Water Column Sound Speeds. IEEE Access 2019, 7, 122052–122068. [CrossRef] 31. Grz ˛adziel,A.; W ˛a˙z,M. Estimation of Effective Swath Width for Dual-Head Multibeam Echosounder. Annu. Navig. 2016, 23, 173–183. [CrossRef] 32. Grz ˛adziel,A.; W ˛a˙z,M. System echosondy wielowi ˛azkowejw pomiarach batymetrycznych planowanych tras ˙zeglugowych. Logistyka 2014, 6, 4250–4256. Available online: https://www.researchgate.net/profile/Artur_ Grzadziel/research (accessed on 18 March 2020). 33. Demer, D.A.; Berger, L.; Bernasconi, M.; Bethke, E.; Boswell, K.; Chu, D.; Domokos, R.; Dunford, A.; Fassler, S.; Gauthier, S.; et al. Calibration of Acoustic Instruments; ICES Cooperative Research Report No. 326; International Council for the Exploration of the Sea (ICES): Copenhagen, Denmark, 2015; 133p. [CrossRef] 34. von Rönn, G.A.; Schwarzer, K.; Reimers, H.-C.; Winter, C. Limitations of Boulder Detection in Shallow Water Habitats Using High-Resolution Sidescan Sonar Images. Geosciences 2019, 9, 390. 35. Grøn, O.; Boldreel, L.O.; Cvikel, D.; Kahanov, Y.; Galili, E.; Hermand, J.-P.; Nævestad, D.; Reitan, M. Detection and mapping of shipwrecks embedded in sea-floor sediments. J. Archaeol. Sci. Rep. 2015, 4, 242–251. [CrossRef] 36. Lubis, M.J. Using Side Scan Sonar Instrument for Seabed Identification with Pattern Discrete Equi Spaced Unshaded Line Array (Pdesula) Model. J. Oceanogr. Mar. Res. 2017, 5.[CrossRef] cvon Rönn, G.A.; Schwarzer, K.; Reimers, H.-C.; Winter, C. Limitations of Boulder Detection in Shallow Water Habitats Using High-Resolution Sidescan Sonar Images. Geosciences 2019, 9, 390 37. Tamsett, D.; Hogarth, P. Sidescan sonar beam function and seabed backscatter functions from trace amplitude and vehicle roll data. IEEE J. Ocean. Eng. 2015, 411, 155–163. [CrossRef] 38. Kolouch, D. Interferometric Side-Scan Sonar. A Topographic Sea-Floor Mapping System. Int. Hydrogr. Rev. 1984, 64, 36–49. 39. Shang, X.; Zhao, J.; Zhang, H. Obtaining High-Resolution Seabed Topography and Surface Details by Co-Registration of Side-Scan Sonar and Multibeam Echo Sounder Images. Remote Sens. 2019, 11, 1496. [CrossRef] 40. Singh, H.; Adams, J.; Mindell, D.; Foley, B. Imaging Underwater for Archaeology. J. Field Archaeol. 2000, 27, 319–328. [CrossRef]

© 2020 by the author. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).