Results from Developments in the Use of a Scanning Sonar to Support Diving Operations from a Rescue Ship

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Article Results from Developments in the Use of a Scanning Sonar to Support Diving Operations from a Rescue Ship

Artur Grz ˛adziel

Department of Navigation and Hydrography, Faculty of Navigation and Naval Weapons, Polish Naval Academy, 81-127 Gdynia, Poland; [email protected]

Received: 31 January 2020; Accepted: 17 February 2020; Published: 20 February 2020

Abstract: In recent years, widespread use of scanning sonars for acoustic imaging of the seabed surface can be observed. These types of sonars are mainly used with tripods or special booms, or are mounted onboard remotely operated or unmanned vehicles. Typical scanning sonar applications include search and recovery operations, imaging of underwater infrastructure, and scour monitoring. The use of these sonars is often limited to shallow waters. Diver teams or underwater remotely operated vehicles (ROV) are commonly used to inspect shipwrecks, port wharfs, and ship hulls. However, reduced underwater visibility, submerged debris, and extreme water depths can limit divers’ capabilities. In this paper, a novel, nonstandard technique for use of a scanning sonar is proposed. The new application for scanning sonar technology is a practical solution developed on the Polish Navy’s search and rescue ship “Lech.” To verify the effectiveness of the proposed technique, the author took part in four different studies carried out in the southeastern Baltic Sea. The tests were performed using the MS 1000 scanning sonar. The results demonstrate that the proposed technique has the potential to provide detailed sonar images of the seabed and underwater objects before the descent of divers. The divers get acquainted with the underwater situation, which undoubtedly increases the safety of the entire operation. Scanning sonars are unlikely to completely replace the work of divers but may reduce the number and duration of dives. The sonar use technique turned out to be useful when rescuing a crew of a submarine that crashed and settled on the sea bottom as part of a naval exercise. The sonar data obtained during four experimental tests performed in the Baltic Sea prove the validity, usefulness, and significance of the proposed technique, especially from the standpoint of safety of underwater work.

Keywords: scanning sonar; seabed imaging; underwater object; diving operations

1. Introduction Most of the world’s oceans are inaccessible to humans. When exploring wreckages and studying the seabed or other navigation obstacles, people must rely on advanced measurement technology. With the development of measurement techniques and the progress of science, the number of sensors and platforms intended for visualization of the seabed and wreckages is increasing. If there is little knowledge of possible objects on the bottom of a water reservoir, the basic task (purpose) of hydrographic surveys is to find and detect any obstacles lying on the bottom or floating in the water. Due to the strong attenuation of electromagnetic waves in the aquatic environment, video sensors and radar equipment are not suitable for searching for underwater objects and imaging the surface of the seabed. The only practical way to visualize large areas of the seabed is to use acoustic waves. Acoustic measurement methods, due to their advantages, play a leading role in this field. The great role of hydroacoustic bottom surveying techniques is primarily due to the beneficial properties of

Remote Sens. 2020, 12, 693; doi:10.3390/rs12040693 www.mdpi.com/journal/remotesensing Remote Sens. 2020, 12, 693 2 of 18 propagation of sound waves in the aquatic environment. These devices enable complete search of relatively large areas of the seabed in a relatively short time. Currently, several acoustic devices are available for hydrographic measurements of sea and inland waters. These include multibeam echosounders (MBES) [1,2], singlebeam echosounders (SBES) [3–5], side scan sonars (SSS) [6,7], interferometric sonars, and synthetic aperture sonars (SAS) [8–10]. These systems diﬀer in terms of their use, purpose, technical measurement capabilities, and diﬀerent test results. The quickly evolving techniques of underwater space monitoring are successfully used to search for and locate underwater objects, assess the technical condition of underwater parts of hydrotechnical structures, and control the seabed clearance. Currently, underwater inspections are based on standard equipment and measurement systems mounted on towed platforms, remotely operated vehicles (ROV), and autonomous underwater vehicles (AUV) [11]. The methods and techniques used to measure depth and search for underwater objects have evolved over the years from very simple and primitive (sounding pole, lead line) to computerized, fully digital systems of the latest generation, such as multipulse sonars, chirp sonars, and multibeam probes [12,13]. Side scan sonars use the motion of the sensor host platform to form the acoustic image of the seabed and objects. The operating frequencies range from approximately 20 kHz to 1000 kHz, depending on the range (depth) and size of the seabed objects [14]. There are higher resolution imaging sonars commercially available which operate at frequencies of 1.3 MHz or even 2.1 MHz [15,16]. On the other hand, hydroacoustic measuring systems are not ideal, especially with regard to searching for and identifying very small objects. Such activities may require the use of optical underwater inspection methods. These methods consist of dive reconnaissance or use of a submarine vehicle to identify or verify the detected targets. Consequently, the characteristics of the seabed to be analyzed should be examined in detail by divers or autonomous underwater vehicles.

1.1. Acoustic Systems for Imaging of Seabed and Underwater Objects Modern survey equipment makes it possible to find, identify, and examine different objects resting on the seabed. Choice of the right hydrographic equipment depends on the task to be completed and the environmental conditions. The basic means for searching and exploring the marine environment include mostly acoustic devices, such as the singlebeam echosounder, the multibeam echosounder, and the side scan sonar [17,18]. Although multibeam echosounder systems are irreplaceable in hydrographic measurements, singlebeam echosounder systems are still used by many entities, companies, and training centers. Singlebeam echosounders have evolved from analogue to digital recording, but most importantly they have increased their accuracy. The technique of searching for underwater objects using the SBES consists in moving along closely planned parallel survey lines while recording the depth associated with precisely defined geographical coordinates [19]. However, due to the shape, size, and orientation of the acoustic beam, the ability of a singlebeam echosounder to detect small underwater objects is very limited. In recent decades, thanks to advances in sonar technology and digital signal processing, the swath bathymetric technique has been developed with equipment that includes multibeam echosounders, multichannel echosounders, side scan sonars, and bathymetric sonar. According to Calder and Mayer [20], the most effective way to conduct precise bathymetric measurements of large seabed surfaces is to use a multibeam echosounder. They are used mainly by hydrographers, navigators, engineers, marine geologists, the military sector, marine explorers, archaeologists, fishery biologists, and geomorphologists [21]. The undoubted advantage of these devices is their ability to search a much larger area compared to singlebeam echosounders in the same unit of time [22]. The first models of multibeam echosounders appeared in the 1960s as a part of a secret naval project [23]. Over the last decades, the MBES have been successfully used to obtain bathymetric data of shallow water reservoirs and deep sea areas [24–26]. Remote Sens. 2020, 12, 693 3 of 18

Today, the MBES are widely used in marine geological and oceanographic surveys [27–29], in the offshore industry, and in underwater pipeline-laying operations [30–32]. Very often measurement data sets are so large that scientists have made efforts to develop 3D measurement point reduction methods [33,34]. MBES systems also enable recording of acoustic images of the seabed by recording the strength of the backscattering signal. However, the images are of lower resolution and poorer quality than acoustic backscattering images provided by side scan sonars [35]. In general, as demonstrated by Bates et al. [36], side scan sonars are better suited for detection of objects and MBES effectively provide full bathymetric coverage of the seabed area where a wreckage is located. Side scan sonar (SSS) systems are mainly designed for seabed searching, detection of shipwrecks, pipelines, artifacts, and mine-like objects [37–40], and are considered to be an irreplaceable tool for underwater archaeological research. SSS systems first appeared in the 1950s as a result of experiments conducted using transducers tilted from the vertical [41] (p. 85). Issues related to the methodology of use of a side scan sonar and the technical capabilities and limitations are presented in publications [6,42]. In very shallow waters, multiple transducer echo sounder (MTES) systems work perfectly. This system was developed in the early 1970s and consists of a spatial array of single transducers [43]. Currently, the MTES is designed for use in ports, rivers, canals, and other shallow bodies of water. Its primary purposes are to search for and locate underwater obstacles and bottom debris, to monitor the depth of navigable waterways, and to support dredging operations. Roll and yaw angles of the platform determine the measurement accuracy of each individual transducer. Another system used for acoustic seabed surface imaging is the synthetic aperture sonar (SAS). It is well known that as the sonar range increases, the resolution of the measurement decreases. SAS sonars use advanced sonar data processing to generate a very narrow beam. The basis for the operation of SAS sonars is the combination of successive acoustic impulses (pings) as they move along a defined survey line to increase the longitudinal resolution of the system [44]. SAS sonars are capable of capturing sonar images with the resolution of several centimeters over a range of up to several hundred meters. This makes SAS sonars a seabed imaging technique designated for applications such as small object search, wreckage imaging, pipeline inspection, and underwater archaeology. A sonar with a synthetic aperture produces images whose quality does not depend on the range or the frequency. The resolution is limited by the bandwidth at a distance and by the size of the transducer, and can be equal to just a few centimeters in both dimensions [45]. The idea behind the SAS is to ensonify a single object on the seabed with a wide beam several times as the sonar moves along a planned survey line. All acoustic signals reflected from the object are combined and postprocessed. The prerequisite for proper signal processing is accurate information about the movement (position) of the sonar [46]. Between 1920 and 2020, significant technological advances were made in seabed mapping techniques. Today, new technologies make it easier for hydrographers to perform various tasks. Depending on the requirements and applications, different measuring techniques have both advantages and disadvantages, and it is the hydrographer’s job to choose the optimal measuring technology that will do best in specific conditions. There are several factors that determine the choice of the measurement system, but the most important ones to consider are (1) the required object detection, (2) the size of the area to be covered by the measurements, and (3) the depth ranges in the area covered by the acoustic survey.

1.2. Major Application Methods of Scanning Sonar The MS 1000 sonar head can be used in several versions. In a static system, it can be mounted on a special tripod and lowered to the seabed from the deck of the anchored platform or from the port wharf. For the purpose of monitoring the condition of the underwater hydrotechnical structures and bridge pillars, the sonar can be lowered from the wharf on a special boom. The scanning sonar can also be used to determine the distance from and the direction to a detected object. In this case, the sonar is Remote Sens. 2020, 12, 693 4 of 18

usually mounted on an underwater ROV. The operator of such a vehicle has the ability to obtain an acoustic image of the seabed, which certainly is an advantage, especially when the visibility in the water environment is low [47]. This version is used, among others, to inspect underwater pipelines and cables. The sonar can be mounted on a tripod, attached to a ROV platform, or used with a pole-mount

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146 147 Figure 1. Examples of standard scanning sonar use techniques. ( (aa)) A A gimbaled gimbaled sonar sonar head head tripod; tripod; ( (b)) 148 PolePole-mounted;-mounted; (c) ROV-mounted.ROV-mounted.

149 Scanning sonars are also increasingly used for inspections of vertical hydrotechnical structures 150 and for assessment of the condition of quays, weirs, bridge pillars,pillars, and other structures [[4488,,49].]. When 151 registering underwater walls of quays, a scanning sonar is is usually usually installed installed in in a a horizontal horizontal position. position. 152 Details for for us usinging the the sonar sonar head head in in these these ways ways are are presented presented in [ in49 [].49 Of]. note Of note is the is fact the that fact sonar that sonar data 153 dataobtained obtained from a from sonar a inspection sonar inspection of port ofinfrastructure port infrastructure can be the can basis be the for basisplanning for planningrepairs of repairsquays, 154 ofdocks, quays, and docks, breakwaters. and breakwaters. One application One application of scanning ofsonar scanning use is sonarto monitor use is and to monitorinspect the and technical inspect 155 thecondition technical of river condition and lake of riverbridge and supports. lake bridge In such supports. cases, sonar In such data cases, can be sonar used to data assess can the be usedcurrent to 156 assesscondition the of current scouring condition of bridge of supports scouring and of bridge to take supports appropriate and maintenance to take appropriate or protection maintenance measures or 157 protection[50]. Scanning measures sonars [ 50can]. Scanningrecord very sonars detailed can recordacoustic very images detailed of the acoustic bottom images and of of underwater the bottom 158 andobjects of underwaterwhich, in the objects case which,of poor in underwater the case of poor visibility, underwater strong visibility, currents, strong or other currents, hazards, or other is a 159 hazards,significant is advantage a significant of advantagethis measuring of this technique measuring [51 technique]. Therefore, [51 given]. Therefore, these features given these and the features high 160 andfrequency the high of the frequency acoustic of signal, the acoustic scanning signal, sonars scanning are increasingly sonars are used increasingly to search used for and to search detect forbodies and 161 detectof drowned bodies persons. of drowned Research persons. on the Research possibility on theof using possibility scanning of using sonars scanning to search sonars for human to search bodies for 162 humanis presented, bodies among is presented, others, amongin [52]. others, in [52]. 163 Scanning sonars are used primarily for their designed purpose and in the manner described and 164 recommended by the manufacturers.manufacturers. Based Based on on the literature analysis, it is concluded that scanning 165 sonars havehave not been used in the aluminum handle that is mounted to the diving platform of a rescue 166 ship,ship, and released to the bottom of the sea fromfrom thethe shipship toto searchsearch andand supportsupport divers.divers. 1.3. Study Objectives 167 1.3. Study Objectives The sonar systems available on the market today do not need to be towed or mounted on mobile 168 The sonar systems available on the market today do not need to be towed or mounted on mobile platforms. They can operate in stationary positions and, thanks to their rotary heads, they are able 169 platforms. They can operate in stationary positions and, thanks to their rotary heads, they are able to to perform imaging of the bottom by scanning its surface within a 360-degree range. Such sonars 170 perform imaging of the bottom by scanning its surface within a 360-degree range. Such sonars are are designed for searching for objects on the bottom, monitoring the scouring of bridge supports in 171 designed for searching for objects on the bottom, monitoring the scouring of bridge supports in rivers, rivers, searching for bodies of drowned persons, and inspecting the underwater parts of hydrotechnical 172 searching for bodies of drowned persons, and inspecting the underwater parts of hydrotechnical structures. The use of such sonars consists in freely lowering the sonar head on a loaded rope, mounting 173 structures. The use of such sonars consists in freely lowering the sonar head on a loaded rope, the sonar on a tripod and lowering it to the bottom, or installing the sonar on an underwater ROV and 174 mounting the sonar on a tripod and lowering it to the bottom, or installing the sonar on an performing sonar tests. 175 underwater ROV and performing sonar tests. The aim of this study is to test the effectiveness of a new technique for use of a scanning sonar 176 The aim of this study is to test the effectiveness of a new technique for use of a scanning sonar in four different cases and to assess the suitability of the new technique to search for objects and 177 in four different cases and to assess the suitability of the new technique to search for objects and support dive operations. The paper presents the technique for use of a scanning sonar mounted on 178 support dive operations. The paper presents the technique for use of a scanning sonar mounted on a a special diving platform, which is a part of a diving bell on a rescue ship. This ship performs both 179 special diving platform, which is a part of a diving bell on a rescue ship. This ship performs both search and rescue operations, and diving operations. The proposed sonar use technique helps divers 180 search and rescue operations, and diving operations. The proposed sonar use technique helps divers 181 to perform inspections and is a reliable way of detecting and pulling out small objects, often from 182 great depths. The rest of the paper is organized as follows. Section 2 describes the study areas, data 183 acquisition, survey equipment, and methodology of conducted research. Section 3 presents the 184 results and a discussion concerning four different tests conducted from the deck of the Polish Navy 185 Search and Rescue Ship. The summary and conclusions are included in Section 4.

186 2. Materials and Methods

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to perform inspections and is a reliable way of detecting and pulling out small objects, often from great depths. The rest of the paper is organized as follows. Section2 describes the study areas, data acquisition, survey equipment, and methodology of conducted research. Section3 presents the results and a discussion concerning four diﬀerent tests conducted from the deck of the Polish Navy Search and Rescue Ship. The summary and conclusions are included in Section4.

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187 2.1. Study A Areasreas 188 The study presented in the paper was conducted in didifferentfferent periodsperiods fromfrom 20122012 toto 2019.2019. The first first 189 study was carried out in the area of the wreckage of the Graf Zeppelin aircraft carrier. The object rests 190 at the depth of 87 87 meters, meters, about about 42 42 nautical nautical miles miles north north of of the the Rozewie Rozewie lighthouse. lighthouse. The The wreckage wreckage is 191 is257 257 meters meters long long and and was was discovered discovered in in 20 200606 by by the the St. St. Barbara Barbara research research vessel vessel,, which conductedconducted 192 geophysical surveys in this area. In the second case, the survey work consisted in finding finding and pulling 193 out a small underwaterunderwater object (diving apparatus) resting at the depthdepth of 8282 meters.meters. The location ofof 194 the lost apparatusapparatus was not exactly known.known. The The search operations were carried out at a distance of 195 several dozendozen meters meters from from the the PG-1 PG unmanned-1 unmanned platform platform located located 39 nautical 39 nautical miles north miles of north the Rozewie of the 196 lighthouse.Rozewie lighthouse. Further studies Further with studies the participationwith the participation of a rescue of shipa rescue were ship carried were out carried in the out waters in the of 197 thewaters Gulf of of the Gdańsk.The Gulf of Gdańsk. ship tookThe ship part took in a tacticalpart in exercise,a tactical whichexercise, was which an opportunity was an opportunity to test both to 198 thetest newboth way the ofnew use way of a of scanning use of sonara scanning and its sonar detection and its capabilities. detection Thecapabilities. most recent The experimentsmost recent 199 withexperiments the sonar with were the conducted sonar were during conducted the operation during of the search operation for and of extraction search for of and a Hall-type extraction anchor, of a 200 whichHall-type broke anchor, off the which chain broke while off another the chain ship while was at another anchor, ship near was the townat anchor, of Łeba. near Figure the town2 shows of Łeba. the 201 locationsFigure 2 shows where the the locations search and where inspection the search work and was inspection carried out. work was carried out.

202 203 Figure 2. Locations of the search and inspection work conducted from the rescue vessel.vessel.

204 2.2. Sonar E Equipmentquipment and Data Acquisition 205 For search and reconnaissance of of the the area, area, a a mobile mobile sonar sonar system system was was used, used, which which comprises comprises a 206 aKongsberg Kongsberg Simrad Simrad Mesotech Mesotech sonar sonar head, head, version version 1071, 1071, a a150 150 m m-long-long cable, cable, a apower power module, module, and and a 207 acontrol control computer computer with with MS MS 1000 1000 software software for for data data recording recording and and process processinging (Figure 33).). TheThe mainmain 208 advantage ofof the the sonar sonar used used is highis high resolution resolution resulting resulting mainly mainly from thefrom high the frequency high frequency of the acoustic of the 209 signalacoustic equal signal to 675equal kHz. to 675 The kHz basic. The technical basic technical characteristics characteristics of the sonar of the are sonar given are in Tablegiven1 in. Table 1.

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210 211 Figure 3. ConfigurationConﬁguration of the mobile scanning sonar system.system. Table 1. Characteristics of the sonar head. 212 Table 1. Characteristics of the sonar head. Parameter Value Parameter Value Operating frequency 675 kHz Operating frequency 675 kHz Beam width 0.9 30 fan ◦ × ◦ RangeBeam width 0.9° x 30° 0.5–100 fan meters Scan angleRange 0.5–100 meters 360◦ Step size Scan angle 360° 0.225 ≥ ◦ Pulse lengthsStep size 0.225° 25–2500 µs DimensionsPulse lengths 25–2500 μs Diameter 89 mm, length 488 mm Transducer width≥ 149 mm Dimensions Diameter 89 mm, length 488 mm Weight 5.7 kg in air, 2.7 kg in water Transducer width 149 mm Weight 5.7 kg in air, 2.7 kg in water The Kongsberg Mesotech sonar head of the 1071 series is characterized by its low weight and size. 213 The deviceThe Kongsberg is only 49 Mesotech cm long and sonar weighs head only of the 2.7 1071 kg in series water. is To characterized take full advantage by its low of the weight advanced and 214 capabilitiessize. The device of the is device,only 49 the cm operation, long and weighs control, only and monitoring2.7 kg in water. of the To sonar take shouldfull advantage be carried of outthe 215 fromadvanced the MS capabilities 1000 software. of the Thedevice, search the angle operation, is 360 ◦control,and the and minimum monitoring increment of the ofsonar the transducershould be 216 anglecarried is out 0.225 from◦. Thethe MS maximum 1000 software. working The range search is 100angle m, is but 360° smaller and the ranges minimum are recommended increment of the for 217 detailedtransducer inspection angle is and 0.225°. veriﬁcation The maximu of seabed.m working The sonar range can beis used100 tom,check but smaller the clearance ranges of theare 218 seabed,recommended check the for condition detailed inspection of underwater and structures,verification and of seabed. search for The objects sonar resting can be onused the to seabed check [53the]. 219 clearanceThe sonarof the uses seabed, a single check transducer the condition that emitsof underwater an acoustic structures, pulse perpendicular and search for to theobjects transducer resting 220 face.on the After seabed receiving [53]. the echoes, the stepper motor of the head rotates the transducer by a small angle, 221 settingThe a sonar new direction uses a single of acoustic transducer wave that propagation. emits an acoustic The signal pulse transmission perpendicular and to receiptthe transducer process 222 isface. repeated. After receiving To achieve the echoes, good quality the stepper sonar motor imaging, of the the head sonar rotates head the must transducer be held by stationary. a small angle, The 223 moresetting stable a new the direction sonar position, of acoustic the clearerwave propagation. and easier to The interpret signal thetransmission recorded image. and receipt The system process has is 224 therepeated. ability To to achieve scan the good bottom quality surface sonar faster imaging, with a the larger sonar angle head increment. must be held However, stationary. increasing The more the 225 speedstable ofthe the sonar transducer position, causes the clearer a decrease and easier in the to resolution interpret of the the recorded sonar imaging. image. SonarThe system data with has the 226 smbability extension to scan the were bottom recorded surface using faster the with MS a 1000 larger software angle increment. installed onHowever, a laptop increasing computer. the The speed MS 227 1000of the software transducer also causes has the a decrease ability to in record the resolution data from of external the sonar sensors imaging. via serialSonar or data USB with ports. the Eachsmb 228 timeextension before w aere sonar recorded survey, using the speed the MS of sound1000 software wave propagation installed on in a water laptop was computer. measured The with MS a CTD1000 229 (Conductivity,software also has Temperature, the ability Depth)to record Valeport data from probe. external sensors via serial or USB ports. Each time 230 before a sonar survey, the speed of sound wave propagation in water was measured with a CTD 2.3. Study Methodology 231 (Conductivity, Temperature, Depth) Valeport probe. The study methodology in all four cases was almost identical. The platform from which the 232 measurements2.3. Study Methodology were carried out was a rescue ship equipped, among others, with a special diving bell 233 withThe a platform. study methodology First, the rescue in all ship four stopped cases was with almost four anchorsidentical. directly The platform above the from wreckage—a which the 234 submarinemeasurements resting were on carried the bottom—or out was thea rescue likely ship position equipped, of the missing among underwaterothers, withobject. a special Anchor diving ropes, bell 235 eachwith 500a platform. m long, madeFirst, itthe possible rescue to ship change stopped the position with four of the anchors ship according directly above to the recommendationsthe wreckage—a 236 ofsubmarine the diving resting works on manager the bottom or the— sonaror the operator’s likely position indications. of the Whilemissing the underwater anchors were object. being Anchor placed, 237 ropes, each 500 m long, made it possible to change the position of the ship according to the

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238238 recommendationsrecommendations ofof thethe divingdiving worksworks managermanager oror thethe sonarsonar operator’soperator’s indications.indications. WhileWhile thethe anchorsanchors the hydrographic reconnaissance team prepared the sonar system. The Kongsberg Simrad Mesotech 239239 werewere beingbeing placed,placed, thethe hydrographichydrographic reconnaissancereconnaissance teamteam preparedprepared thethe sonarsonar system.system. TheThe KongsbergKongsberg scanning sonar was installed in a special aluminum rack (grip, stand), which was then combined with 240240 SimradSimrad MesotechMesotech scanningscanning sonarsonar waswas installedinstalled inin aa specialspecial aluminumaluminum rackrack (grip,(grip, stand),stand), whichwhich waswas a diving platform grating (Figure4). Before leaving the diving platform, the proper functioning of the 241241 thenthen combinedcombined withwith aa divingdiving platformplatform gratinggrating (Figure(Figure 4).4). BeforeBefore leavingleaving thethe divingdiving platform,platform, thethe sonar head was checked. First, the stabilizing anchors of the platform were lowered to the bottom, 242242 properproper functioningfunctioning ofof thethe sonarsonar headhead waswas checked.checked. First,First, thethe stabilistabilizingzing anchorsanchors ofof thethe platformplatform werewere followed by the platform with the sonar attached (Figure5). As the platform was lowered, the operator 243243 loweredlowered toto thethe bottom,bottom, followedfollowed byby thethe platformplatform withwith thethe sonarsonar attachedattached (Figure(Figure 5).5). AsAs thethe platformplatform onboard the ship was releasing the cable connecting the sonar to the onboard computer controlling 244244 waswas lowered,lowered, thethe operatoroperator onboardonboard thethe shipship waswas releasingreleasing thethe cablecable connectingconnecting thethe sonarsonar toto thethe the operation of the sonar. When the diving platform with sonar was at the bottom, the recording of 245245 onboardonboard computercomputer controllincontrollingg thethe operationoperation ofof thethe sonar.sonar. WhenWhen thethe divingdiving platformplatform withwith sonarsonar waswas measurement data began. The ﬁrst measurements were taken over the largest ranges (R 100 m, R 246246 atat thethe bottom,bottom, thethe recordingrecording ofof measurementmeasurement datadata began.began. TheThe firstfirst measurementsmeasurements werewere takentaken= overover thethe 247247 largest=largest75 m) rangesranges to provide (R(R == 100 a100 general m,m, RR = and= 7575 completem)m) toto provideprovide picture aa generalgeneral of the underwater andand completecomplete situation. picturepicture of Theof thethe operator uunderwaternderwater then 248248 situation.selectedsituation. characteristic TheThe operatoroperator sonar thenthen echoes—potentialselectedselected characteristiccharacteristic objects sonarsonar of interest—whileechoesechoes——potentialpotential reducing objectsobjects the ofof interest sonarinterest range.——whilewhile In 249249 reducingthereducing case of thethe large sonarsonar objects, range.range. such InIn asthethe an casecase aircraft ofof largelarge carrier objects,objects, wreckage suchsuch or asas a anan submarine aircraftaircraft carriercarrier resting wreckagewreckage on the seabed, oror aa 250250 submarinesectoralsubmarine sonar restingresting operation onon thethe was seabed,seabed, also sectoral performed.sectoral sonasonarr operationoperation waswas alsoalso performed.performed.

251251 252252 FigureFigure 4.4. StagesStages ofof sonarsonarsonar system systemsystem preparation preparationpreparation for forfor work: work:work: ( a (()aa) The) TheThe diving divingdiving platform; platform;platform; (b ()(bb Fixing)) FixingFixing the thethe sonar sonarsonar in 253253 inthein thethe aluminum aluminumaluminum holder; holder;holder; (c )((cc Mounting)) MountingMounting the thethe sonar sonarsonar holder holderholder to toto the thethe grating; grating;grating; ( (d(dd,e,,ee))) Placing PlacingPlacing the thethe gratinggratinggrating inin the the 254254 platform;platform; ((ff)) TheThe sonarsonar andand thethe anchorsanchors inin thethe workingworking position.position.

255255 256256 FigureFigure 5. 5. SeabedSeabed imaging imaging technique technique using using a a scanning scanning sonar sonar lowered lowered from from aa rescuerescue ship.ship.

257257 3.3. ResultsResults andand discussiondiscussion

258258 3.1.3.1. CaseCase OOnene——WWreckagereckage ofof thethe AAircraftircraft CCarrierarrier GrafGraf ZeppelinZeppelin

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3. Results and discussion Remote Sens. 2020, 12, x FOR PEER REVIEW 8 of 18 3.1. Case One—Wreckage of the Aircraft Carrier Graf Zeppelin 259 The first measurements using the new sonar scanning technique were performed during the 260 inspectionThe first and measurements verification of usingthe wreckage the new of sonar the German scanning aircraft technique carrier were Graf performed Zeppelin. duringThe rescue the 261 shipinspection of the andPolish verification Navy "Lech" of the was wreckage designated of the to Germanperform aircraftthe task. carrier During Graf the Zeppelin. underwater The works, rescue 262 samplesship of the of bottom Polish Navysediments "Lech" from was the designated area of the to wreckage perform thewere task. taken, During selected the underwaterpoints of the works,object 263 weresamples checked of bottom with sedimentsthe help of fromdivers, the video area ofmaterial the wreckage was recorded were taken, with the selected use of points a ROV, of and the objectsonar 264 weredata of checked selected with parts the of help the ofwreckage divers, videowere materialcollected. was The recorded hydrographic with the reconnaissance use of a ROV, group and sonar was 265 responsibledata of selected for the parts initial of the check wreckage of the bottom were collected. clearance The in the hydrographic diver drop locations, reconnaissance for detection group wasand 266 measurementresponsible for of the any initial underwater check of obstacles, the bottom for clearance determination in the diver of the drop height locations, of the fordiving detection platform and 267 abovemeasurement the seabed of any(wreckage), underwater and for obstacles, acquisition for determination of sonar data concerning of the height the of object the diving for the platformpurpose 268 ofabove assessment the seabed of (wreckage),its condition and and for its acquisition identification. of sonar Pairs data of concerning divers descended the object on for or the around purpose the of 269 wreckageassessment in of the its six condition positions and marked its identification. in Figure 6. Pairs of divers descended on or around the wreckage 270 in the six positions marked in Figure6.

271 272 Figure 6. "Lech""Lech" anchorage anchorage positions positions above above the wreckage wreckage..

273 The inspection worksworks werewere attendedattended by by the the crew crew of of the the ORP ORP "Lech" "Lech" rescue rescue ship, ship, representatives representatives of 274 ofthe the Naval Naval Academy, Academy, and and specialists specialists of the of Hydrographicthe Hydrographic Support Support Squadron Squadron of the of Polish the Polish Navy. Navy. First, 275 First,two anchors two anchors were loweredwere lowered to the bottom,to the bottom, followed followed by a platform by a platform with a sonar with attached.a sonar attached. At selected At 276 selecteddepths (50, depths 60, and (50, 75 m)60, theand platform 75 m) wasthestopped platform in orderwas tostopped perform in control order measurements,to perform control check 277 measurements,the height above check the seabed,the height and above acquire the sonar seabed, data and concerning acquire sonar the object. data concerning In the final the phase object. of the In 278 thesonar final inspection, phase of the the platform sonar inspection, was lowered the to platform the height was of 1–2lowered m above to the the height seabed of or 1 the–2 flightm above deck the of 279 seabedthe wreckage. or the flight At this deck stage, of the an underwaterwreckage. At vehicle this stage, providing an underwater a constant vehicle visual imageproviding when a reachingconstant 280 visuala critical image depth when was invaluable.reaching a c Theritical use depth of a ROV was forinvaluable. monitoring The improved use of a theROV safety for ofmonitoring the quite 281 improvedexpensive the measuring safety of equipment. the quite expensive measuring equipment. 282 After the platform with the sonar was lowered to the seabed, the distance of the platform from 283 the wreckage was first first determined a andnd then the clearance of the bottom around the sonar within the 284 range of thethe hydroacoustichydroacoustic beambeam was was checked. checked. The The objects objects detected detected and and located located were were dimensioned, dimensioned, in 285 particularin particular those those that that obstructed obstructed the diver the diver performing performing the underwater the underwater inspection inspection (Figure7 ).(Figure Selected 7). 286 Ssonarelected echoes sonar were echoes checked were with checked the underwater with the underwater ROV. In position ROV. 1,In images position were 1, alsoimages recorded were also that 287 recordedshowed a that part showed of the left a sidepart ofof thethe wreckage left side of (Figure the wreckage8). The sonar (Figure image 8). showsThe sonar 12 meter-long image shows niches 12 288 meteron the-long left side niches of the on aircraftthe left carrier. side of The the lifeboatsaircraft carrier. were to The be placed lifeboats in them. were Additionally,to be placed therein them. is a 289 Additionally,visible side structure there is designeda visible side for double structure trailers designed of the for 15 double cm main trailers artillery of guns.the 15 cm main artillery 290 guns.

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291291 Figure 7. A scanning sonar image of the seabed in the vicinity of the wreckage, 87 m deep (position 1, 292292 FigureFigure 7.7. AA scanning scanning sonarsonar imageimage ofof thethe seabedseabed inin thethe vicinityvicinity ofof thethe wreckage,wreckage, 8787 mm deepdeep (position(position port side). 293293 1,1, port port side) side). .

294294 295295 FigureFigure 8. 8. A AA scanning scanningscanning sonar sonarsonar image imageimage showing showing the the port port side side of of the the wreckage wreckage (position (position 1) 1).1). .

296296 InIn positionspositions 2, 2, 3,3, andand 4,4, severalseveral sonarsonar imagesimages representingrepresenting the the stern stern part part of of thethe wreckagewreckage werewere 297297 recordedrecorded (Figures(Figures 9 99 and andand 10 10).10).). In InIn position positionposition 4, 4,4, the thethe diving divingdiving platform platformplatform was wawass located locatedlocated 19 1919 meters metersmeters from fromfrom the thethe stern sternstern 298298 ofof thethe carrier.carrier. Sonar Sonar captured captured a a partpart ofof thethe sternstern withwith thethe overhangingoverhanging ﬂightflightflight deckdeck atat thisthisthis point.point.point. OfOf 299299 notenote areare thethe bracketsbrackets supportingsupporting thethe taketake-otake--offoffﬀ deck, deckdeck, , visiblevisible inin thethe sonogramsonogram (Figure(Figure9 9).).9). Thanks ThanksThanks to toto the thethe 300300 measurementsmeasurements in in ppositionspositionsositions 22 andand 3,3, interestinginteresting sonogramssonograms showingshowing octagonaloctagonal holes,holes, wherewhere aircraftaircraft 301301 elevatorselevators were were installed,installed, werewere collectedcollected (Figure(Fig(Figureure 1010). 10).). 302302 InIn positionspositions 55 andand 6,6, sonarsonar measurementsmeasurements werewere takentaken onon thethe bowbow ofof thethe GrafGraf ZeppelinZeppelin aircraftaircraft 303303 carrier.carrier. ThereThere isis aa thirdthird octoctagonalagonal openingopening ofof thethe aircraftaircraft elevatorelevator withwith visiblevisible ropesropes andand netsnets goinggoing 304304 insideinside (Figure(Figure 11).11). NextNext toto it,it, onon thethe starboardstarboard side,side, therethere isis aa superstructuresuperstructure thatthat waswas damageddamaged inin aa 305305 largelarge part.part. TheThe deckdeck inin thisthis partpart isis damaged,damaged, withwith holesholes withwith thethe diameterdiameter ofof 22 meters,meters, probablyprobably 306306 causedcaused by by Russian Russian weapons weapons in in 1947. 1947.

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307 307 308 Figure 9. A sonogram showing the stern of Graf Zeppelin with characteristic flight ﬂight deck brackets 308 Figure 9. A sonogram showing the stern of Graf Zeppelin with characteristic flight deck brackets 309 (position 4, sonar 19 mm fromfrom thethe stern).stern). 309 (position 4, sonar 19 m from the stern).

310 310 Figure 10. A sonogram of the stern section with a part of the engine room showing clearly visible 311 Figure 10. A sonogram of the stern section with a part of the engine room showing clearly visible aircraft elevator holes (position 3, octagonal aircraft elevator). 312311 aircraftFigure 10.elevator A sonogram holes (position of the stern 3, octagonal section aircraftwith a partelevator) of the. engine room showing clearly visible 312 aircraft elevator holes (position 3, octagonal aircraft elevator). In positions 5 and 6, sonar measurements were taken on the bow of the Graf Zeppelin aircraft carrier. There is a third octagonal opening of the aircraft elevator with visible ropes and nets going inside (Figure 11). Next to it, on the starboard side, there is a superstructure that was damaged in a large part. The deck in this part is damaged, with holes with the diameter of 2 meters, probably caused by Russian weapons in 1947.

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313 314 Figure 11. AA sonogram sonogram of of the the bow bow section section of of the wreckage with an aircraft elevator opening and a 315 superstructuresuperstructure (item 6) 6)..

316 As a result of the survey work, a very rich collection of sonar images was obtained and digital 317 models of the seabed were prepared,prepared, based on bathymetric data. The The sonar sonar images images of of the seabed 318 surfacesurface andand ofof the the elements elements of of the the hull hull of theof wreckagethe wreckage were were digitally digitally processed. processed. The wreckage, The wreck restingage, 319 restingat a depth at a of depth 87 meters, of 87 meters, with a 34 with◦ tilt a to 34° starboard, tilt to starboard, was additionally was additionally filmed with filmed a TV with camera a TV mounted camera 320 mountedon a Super on Achille a Super underwater Achille underwater vehicle. Thevehicle. hull The of thehull vessel of the did vessel not did lose not its lose integrity its integrity and is welland 321 ispreserved. well preserved. No objects No ofobjects military of originmilitary were orig foundin were on found the wreckage on the andwreckage in its immediate and in its vicinity.immediate On 322 vicinity.the other On hand, the numerousother hand, fishing numerous nets were fishing observed, nets were especially observed, in the especially stern and in bow the sections, stern and in somebow 323 sections,cases including in some floats cases and including fishing buoys.floats and Numerous fishing underwaterbuoys. Numerous obstacles underwater were detected obstacles which were may 324 detectedpose a risk whi toch divers may andpose to a fishingrisk to divers gear used and byto fishermen.fishing gear used by fishermen. 3.2. Case Two—Search for a Diving Apparatus 325 3.2. Case Two—Search for a Diving Apparatus Another test for the new technique was the search for a small object (a diving apparatus), just 326 Another test for the new technique was the search for a small object (a diving apparatus), just 86 86 cm long, resting at the depth of 82 meters. The location where the object sank was only probable. 327 cm long, resting at the depth of 82 meters. The location where the object sank was only probable. Considering the size of the object and the depth, this was quite a challenge and a real test for the 328 Considering the size of the object and the depth, this was quite a challenge and a real test for the proposed technique. Preliminary scans were carried out at the range of 75 m, which allowed for an 329 proposed technique. Preliminary scans were carried out at the range of 75 m, which allowed for an indicative survey of the seabed. A search was then carried out at the range of 50 and 30 m (Figure 12). 330 indicative survey of the seabed. A search was then carried out at the range of 50 and 30 m (Figure Based on an analysis of the acoustic shadow, several sonar echoes were selected (Figure 13). Not all 331 12). Based on an analysis of the acoustic shadow, several sonar echoes were selected (Figure 13). Not registered underwater objects were identified and only those that were the most similar in size to 332 all registered underwater objects were identified and only those that were the most similar in size to the object being searched for were selected. Visual identification was performed by means of a ROV 333 the object being searched for were selected. Visual identification was performed by means of a ROV underwater vehicle, which was guided to the designated positions by determining the distance and 334 underwater vehicle, which was guided to the designated positions by determining the distance and the location of the target. The first echo was a paint container, the second was an oval metal object, and 335 the location of the target. The first echo was a paint container, the second was an oval metal object, the third was the diving apparatus that was searched for (Figure 14). 336 and the third was the diving apparatus that was searched for (Figure 14).

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337337337 338338338 FigureFigureFigureFigure 12. 12.Sonar12. SonarSonar imaging imagingimaging of of the the bottom bottom inin thethe area area ofof thethe searchsearchsearch search forforfor for the thethe the lost lostlost lost diving divingdiving diving apparatu apparatuapparatu apparatus.s.s.s. ( (a(a)a) ) (a) 339339339 MeasurementsMeasurementsMeasurementsMeasurements at the atat thethe range rangerange of of R R= =75 75 m; m; ( (bb))) Measurements MeasurementsMeasurements at atat at the thethe the range rangerange range of ofof ofR RR = = R= 50 5050= m;m; 50m; (( m;c(c)c) )Measurements Measurements (Measurementsc) Measurements 340340340 at theatatat rangethe thethe range rangerange of R ofof= RR30 == 3030 m. m.m.

341341341 342 Figure 13. A sonar image showing the echo from Platform PG1 and the indication of three underwater 342342 FigureFigureFigure 13. 13.A13.sonar A sonar image image showing showing the the echo echo fromfrom Platform Platform PG1PG1 PG1 andand and thethe the indicationindication indication of of three three of three underwater underwater underwater 343 objects to be identified. 343343 objectsobjectsobjects to be toto identiﬁed.be identified.identified.

344 344 344 Figure 14. Visual identiﬁcation of the detected underwater objects. (Images from an ROV underwater vehicle: Commander Adam Olejnik, Institute of Ship Construction and Operation, Department of Diving Technology and Underwater Activities, Polish Naval University).

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345345 FigureFigure 14.14. VisualVisual identificationidentification ofof thethe detecteddetected underwaterunderwater objectsobjects.. ((IImagesmages fromfrom aann ROVROV underwaterunderwater 346346 vehicle:vehicle: CommanderCommander AdamAdam Olejnik,Olejnik, InstituteInstitute ofof ShipShip ConstructionConstruction andand Operation,Operation, DepartmentDepartment ofof 347 Diving Technology and Underwater Activities, Polish Naval University). 347 RemoteDiving Sens. 2020 Technology, 12, 693 and Underwater Activities, Polish Naval University). 13 of 18

348348 3.3.3.3. CaseCase TThreehree——RRescueescue ofof thethe CCrewrew ofof aa SubmarineSubmarine LyingLying onon thethe SSeabedeabed 3.3. Case Three—Rescue of the Crew of a Submarine Lying on the Seabed 349349 TheThe newnew techniquetechnique forfor useuse ofof thethe MSMS 10001000 scanningscanning sonarsonar providesprovides favorablefavorable conditionsconditions forfor 350350 supportsupportThe ofof new searchsearch technique forfor aa missingmissing for use submarinesubmarine of the MS andand 1000 rescuerescue scanning ofof itsits sonarcrew.crew. providesSuchSuch aa sscenariocenario favorable waswas conditions testedtested duringduring for 351351 asupporta nationalnational of maritimemaritime search for exercise.exercise. a missing AA submarinesubmarinesubmarine and withwith rescue aa defectdefect of its restedrested crew. onon Such thethe seabed. aseabed. scenario TheThe was mainmain tested objectiveobjective during ofof a 352352 thenationalthe exerciseexercise maritime waswas exercise.toto improveimprove A submarine cooperationcooperation with betweenbetween a defect restedships,ships, on maritimemaritime the seabed. aviation,aviation, The main onon-- objectivedutyduty services,services, of the 353353 specialists,exercisespecialists, was medicalmedical to improve centers,centers, cooperation andand civilcivil agencies betweenagencies ships,inin thethe event maritimeevent ofof aa aviation, failurefailure ofof on-duty aa subsubmarinemarine services, restingresting specialists, onon thethe 354354 seabed.medicalseabed. OnceOnce centers, thethe anddefectivedefective civil agenciessubmarinesubmarine in was thewas eventfoundfound of onon a thethe failure seabed,seabed, of a aa submarine rescuerescue shipship resting waswas sentsent on thetoto provideprovide seabed. 355355 help.Oncehelp. For theFor defectivethisthis purpose,purpose, submarine thethe rescuerescue was found shipship withwith on the fourfour seabed, anchorsanchors a rescue waswas shipplacedplaced was aboveabove sent tothethe provide submarine.submarine. help. ForAnAn 356356 underwaterthisunderwater purpose, reconnaissancereconnaissance the rescue ship groupgroup with composedcomposed four anchors ofof hhydrographers wasydrographers placed above usingusing the aa portableportable submarine. scanningscanning An underwater sonarsonar tooktook 357357 measurementsreconnaissancemeasurements grouptoto determinedetermine composed thethe of position hydrographersposition ofof thethe using submarinesubmarine a portable inin scanningrelationrelation to sonarto thethe took rescuerescue measurements ship.ship. TheThe 358358 submarinetosubmarine determine waswas the locatedlocated position andand of then thethen submarine thethe distancesdistances in relation toto thethe bow,bow, to the thethe rescue manhole,manhole, ship. the Thethe finfin submarine ofof aa submarinesubmarine was located,, etc.etc.,, 359359 wereandwere then determinedetermine the distancesd.d. AfterAfter to initialinitial the bow, measurements,measurements, the manhole, itit turnedturned the ﬁn outout of a thatthat submarine, thethe submarinesubmarine etc., were waswas determined. tootoo farfar awayaway Afterfromfrom 360360 theinitialthe divingdiving measurements, platform.platform. UsingUsing it turned thethe anchoranchor out that ropes,ropes, the submarine thethe positionposition was ofof toorescuerescue far awayshipship waswas from slightlyslightly the diving changed.changed. platform. TheThe 361361 targetUsingtarget thewaswas anchor dimensioned,dimensioned, ropes, the itsits position orientationorientation of rescue waswas shipdetermined,determined, was slightly andand changed. severalseveral sonson Thearar target imagesimages was werewere dimensioned, recordedrecorded 362362 (Figuresits(Figures orientation 1515 andand was 16).16). determined, and several sonar images were recorded (Figures 15 and 16).

363363 364364 FigureFigure 15. 15. ScanningScanning sonarsonar imagesimages ofof thethe submarinesubmarine resting resting on on the the seabed seabed at at the the depth depth of of 41 41 meters meters.meters..

365365 Figure 16. Determination of the distance and identiﬁcation of the structural components of the submarine, based on a sonar image.

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366 Figure 16. Determination of the distance and identification of the structural components of the 367 submarine, based on a sonar image.

368 3.4. Case Four—Search for a Hall-Type Anchor 369 The last project where the effectiveness of the new scanning sonar technique was tested was the 370 search for, detection, and extraction to the surface of a Hall-type anchor. The anchor broke off the 371Remote chain Sens. 2020while, 12 the, 693 ship was anchored near Łeba. The position of the anchor on the seabed was only14 of 18 372 probable. The task of finding and extracting the anchor was assigned to the rescue ship ORP “Lech” 373 inRemote cooperation Sens. 2020, with12, x FOR a team PEER of REVIEW hydrographers from the Hydrographic Support squadron of the 14Polish of 18 3743.4. CaseNavy. Four—Search The rescue ship for awa Hall-Types anchored Anchor in the probable position where the anchor had broken off. The 375366 MS 1000Figure scanning 16. Determination sonar was of mounted the distance to the and diving identification platform of gratingthe structural and lowered components to the of seabed.the 376367 TheSonar lastsubmarine measurements project, based where onstarted a the sonar eatff imagetheectiveness range. of of75 themeters new and scanning then reduced sonar to technique 50 meters was(Figure tested 17). wasAt the 377search the for, range detection, of 75 meters, and sonar extraction images towere the recorded, surface based of a Hall-type on which three anchor. sonar The echoes anchor were brokeselected. off the 368 3.4. Case Four—Search for a Hall-Type Anchor 378chain These while echoes the ship were wasindicated anchored to the neardiver Łeba.who carried The positionout the visual of the identification. anchor on One the seabedof the echoes was only 379369probable. indicatedThe The lastwas task project identified of finding where asand the losteffectiveness extracting Hall-type the of anchor. anchorthe new wasscanning assigned sonar to technique the rescue was ship tested ORP was “Lech” the in 380370cooperation searchDuring for, with detection, the a measurements, team and of hydrographers extraction the hydrographer to the fromsurface theincorrectly of Hydrographic a Hall -indicatedtype anchor. Supportother The sonar anchor squadron echoes, broke including of off the the Polish 381371Navy. thechain The echoes while rescue from the ship shipthewas anchor was anchored anchored of the rescue innear the Łeba.ship probable droppedThe position position during of where thethe anchoranchoring the anchoron theoperation. seabed had broken Figurewas only o18ff. The 382372MS 1000showsprobable. scanning the Thedifferences sonartask of was findingbetween mounted and the extractingimage to the of divingthe the echo anchor platform of the was anchor assigned grating of the andto rescuethe lowered rescue ship ship to(Figure the ORP seabed. 18a) “Lech and” Sonar 383 thein cooperation image of the with echo a ofteam the ofanchor hydrographers being searched from for,the restingHydrographic on the seabed Support for squadron two weeks. of theIn Figure Polish 373measurements started at the range of 75 meters and then reduced to 50 meters (Figure 17). At the 384374 18a,Navy. one The can rescue even shipsee the wa spalms anchored of the in anchor the probable in the positionsonar image. where The the sonar anchor echo had from broken the off.anchor The range of 75 meters, sonar images were recorded, based on which three sonar echoes were selected. 385375 thatMS 1000had rested scanning for 14sonar days was on themounted seabed to is theweak, diving fuzzy, platform and practically grating withoutand lowered any acoustic to the seabed. shade. 386376These TheSonar echoes diver's measurements were account indicated shows started that to at the the the diver anchor range who ofwas 75 carried almost meters completely outand thethen visual reduced covered identification. to with 50 metersseabed (Figuresediment. One of 17). the The At echoes 387377indicated sonarthe range wasimages of identified 75 in meters, Figure as sonar18a the,b lostimagesshow Hall-type the were role recorded, of anchor. time in based image on quali whichty threeand detail. sonar echoes were selected. 378 These echoes were indicated to the diver who carried out the visual identification. One of the echoes 379 indicated was identified as the lost Hall-type anchor. 380 During the measurements, the hydrographer incorrectly indicated other sonar echoes, including 381 the echoes from the anchor of the rescue ship dropped during the anchoring operation. Figure 18 382 shows the differences between the image of the echo of the anchor of the rescue ship (Figure 18a) and 383 the image of the echo of the anchor being searched for, resting on the seabed for two weeks. In Figure 384 18a, one can even see the palms of the anchor in the sonar image. The sonar echo from the anchor 385 that had rested for 14 days on the seabed is weak, fuzzy, and practically without any acoustic shade. 386 The diver's account shows that the anchor was almost completely covered with seabed sediment. The 387 sonar images in Figure 18a,b show the role of time in image quality and detail.

388 389 FigureFigure 17. 17.Sonar Sonar images images of of thethe seabed in in the the anchor anchor break break off opositionﬀ position..

During the measurements, the hydrographer incorrectly indicated other sonar echoes, including the echoes from the anchor of the rescue ship dropped during the anchoring operation. Figure 18 shows the diﬀerences between the image of the echo of the anchor of the rescue ship (Figure 18a) and the image of the echo of the anchor being searched for, resting on the seabed for two weeks. In Figure 18a, one can even see the palms of the anchor in the sonar image. The sonar echo from the anchor that had rested for 14 days on the seabed is weak, fuzzy, and practically without any acoustic shade. The 388diver’s account shows that the anchor was almost completely covered with seabed sediment. The 389sonar images in FigureFigure 18a,b 17. show Sonar theimages role of of the time seabed in in image the anchor quality break and off position detail..

390

390 Figure 18. Sonar images recorded with a scanning sonar: (a) A part of a sonogram with an echo of the anchor of the rescue vessel; (b) A sonogram with the echo of the anchor being searched for; (c) The Hall-type anchor found and pulled out to the surface. Remote Sens. 2020, 12, 693 15 of 18

4. Conclusions This paper presents a new technique for use of a scanning sonar, which was tested on a rescue ship while performing tasks at sea. So far, the sonar head had been used by hydrographers mainly on tripods and in shallow waters. The sonar use method presented in this paper makes it possible to search for small objects at depths much greater than 80 meters. The results demonstrate that a scanning sonar mounted to a diving platform grating has the potential to provide detailed sonar images of the seabed and underwater objects before the divers descend. For a diver performing underwater inspection work, information about the presence of any objects (obstacles, nets, ropes), their size, and their position is a great help and support. The diver is familiar with the situation on the seabed, sees the wreckage and the objects, which certainly improves the safety of the entire operation. Scanning sonars are unlikely to fully replace the work of divers, but they can reduce the number and duration of dives. Elimination of uncertainties related to the presence of risks will increase the divers’ efficiency and safety. The sonar use technique presented herein also proved its applicability in rescuing the crew of a submarine that had crashed and rested on the seabed (during a naval exercise). The MS 1000 sonar recorded images showing individual parts of the structure of the submarine on the seabed. Precise indication and imaging of the emergency hatch at the bow appeared to be crucial for the entire operation aimed at rescuing the trapped submarine crew. The new technique was also highly effective in search for, detection, and extraction of small underwater objects. It should be noted that an underwater ROV can provide invaluable help and significant support in recognizing and identifying the detected targets. It is extremely difficult for a diver to find an underwater object with the size of less than one meter at visibility below 0.5 meter. Preliminary sonar measurements enable accurate acoustic imaging of the seabed and indication of echoes whose shape and size match the object being searched for. In this way, a diver or an ROV can be guided to a specific target to perform visual identification. An essential requirement for obtaining high resolution sonar images is to create stable operating conditions for the sonar. When the transducer is in a fixed position, the recorded images are highly detailed and free from interferences caused by the movement of the sonar or the vessel. The proposed sonar use technique provides optimal operating conditions in terms of stability. First of all, the ORP "Lech" rescue ship floating above the wreckage, immobilized with four anchors, appeared to be standing in a dock or a slip, or to be simply moored to the wharf. Secondly, the sonar head was mounted in a special aluminum frame, which was then attached to a grating that was a part of the diving platform. The diving platform, with steel cables and anchors lowered to the seabed, formed quite a stable system for the sonar. The sonar head was almost stationary, which translated directly into the quality of the sonograms recorded. The recorded data were legible and easy to interpret. The installation of the MS 1000 scanning sonar on the diving platform and its use to support the work of divers and the search for and extraction of underwater objects is an unconventional but effective technical solution, which was successfully tested in several operations at sea.

Funding: This work 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”. Conﬂicts of Interest: The author declares no conﬂict of interest.

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