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Submarine ARA San Juan located

How the ARA San Juan was ultimately found?







Source: Clarin (2017)





Updated 21 November 2018

It is with a sense of a little relief but also deep sadness to learn today that the ARA San Juan submarine missing for a year was finally located


On 23 Nov, the Argentine Navy released the tragic news of the loss of the submarine, where it confirmed a "singular, abnormal, short, violent & non-nuclear event" consistent with an explosion, took place on 15th November.


The ‘Seabed Constructor’ contracted by the Argentine government located the submarine after an extensive and exhaustive detailed search of two areas off the Patagonian coast. 







Source: Armada Argentina (2018)




"Argentina’s Defence Ministry and Navy report that after examining point of interest (POI) no. 24, located by the the ‘Seabed Constructor’ with the help of a remotely operated vehicle (ROV), at a depth of 800 metres [2,625 feet], the location of the ARA San Juan submarine was positively identified," 


Twitter translation




PART 1 — The first series of searches


Last year, numerous ships with various search capabilities crisscrossed a huge swath of the seabed, as determined by the last reported position of the submarine and also acoustics findings reported by the CTBTO.  See image for the intensity of the search by 30th December 2017:







Source: Mike Chillit (2017)



As I remarked last year, these are just the AIS tracks of ships involved. There were other ships who weren’t on AIS, such as some of the other participating search support Argentine Navy ships- (ARA La Argentina, ARA Robinson, ARA Sarandí, ARA Puerto Argentino).  








A couple of months later the ‘Yantar’ also covered again a lot of the search area. 







Source: unknown (2018)



Initial 2017-2018 search


Here a roll of honour for the ships involved in the prolonged initial search phase from November 2017 to end of December 2017. Several of these ships also continued their search into the first quarter of 2018. (In alphabetical order):



  • RV Dr. Victor Angelescu – Argentine fisheries oceanographic research vessel

  • RV Atlantis- US - WHOI research oceanographic ship

  • ARA Austral (Argentine Navy) 

  • RV Cabo de Hornos (Chilean Navy)

  • ARA Islas Malvinas (Argentine Navy)

  • ARA Puerto Argentino (Argentine Navy)

  • ARA Puerto Deseado (Argentine Navy)

  • MV Skandi Patagonia (Offshore Support Vessel contracted by US Navy)

  • RFS ‘Yantar’ - Russia - Naval oceanographic ship


Back-up support (Argentina Navy)



  • ARA Sarandi

  • ARA La Argentina

  • ARA Robinson

  • ARA Rosales

  • ARA Drummond


Other ships



  • NSS "Felinto Perry" (Brazilian Navy)

  • Rademaker (Brazilian Navy)

  • Icebreaker Almirante Maximiano (Brazilian Navy)

  • “Sophie Siem” (Offshore Support Vessel contracted by US Navy)

  • HMS Clyde (Royal Navy — UK)







List of main search participants with type of equipment ( unconfirmed data)



The series of initial searches were ultimately inconclusive.  Subsequently, the Argentinian authorities took the decision to resume the search for the submarine in late October following the signing of a "no cure no fee basis" contract with the company Ocean Infinity. 



PART 2: Searching the seabed - techniques


How was the search done?


I outlined the principles of seafloor survey and searching in a blog post last year, but I will summarize it here.  The steps in locating a sunken ship:


1. bathymetric profiling with multibeam echo-sounder of the seabed;


by detailed profiling multi-beam sonar array  (combined  with side scan either with an AUV or on a ‘sledge’ towed astern of mothership).


The images obtained are digitally processed & analysed for subsequent verification;


2. Verification is done by sending down an ROV for visual inspection of the object.


Multi-beam sonar-  (MBES) a ship equipped with this type of sonar will travel along certain tracks, (called mowing the lawn), to survey a band of the seabed.  The ship sends out multiple ‘sonar’ pings that go down to the seabed at all different angles  (fan-shaped pattern). Each individual ping is returned back up as backscatter and they are received back onboard.  The returned echoes are thus recorded as a function of time.  The approximate result is shown in the image below:







Source:EduGIS.nl (2017) http://www.edugis.nl/lesmodules/Hydrografie/Diepte/di2.html



Essentially it is a continuous band of individual backscattered returns (swath) of depth soundings, (based on the time of first return). 


Side-scan sonar — (SSS).  There are several names for this type of sonar.  SSS differs to MBES in that can not accurately measure depth.  As the ship gradually moves forward, an image (a sort of 'photographic scan') of the morphology of the sea floor is obtained and objects that stand out  (in this case side-scan imagery of  2 sunken Chinese fishing boats).  



The side-scan sonar images of the ARA San Juan show the sea-floor details very well.

An interesting explanation is provided by the company, EdgeTech on how its side scan sonar technology was used in the deep water search to find the ARA San Juan.

Similar to an MBES,  sound waves are usually transmitted and received from a special device (called a "fish") which is towed close to the seafloor, like the one shown here:







Source: Huibert-Jan Lekkerkerk (2016) Hydro International



Side-Scan Sonar is an (almost) continuous signal, whereas MBES is not so quite continuous because it is consists of one backscatter value per beam.







Source:



The above image shows how ridges and projections on the seafloor are 'seen' by SSS, where these are reflected more by the sonar energy, resulting in a brighter colour on a 'scan'. Equally they also leave a shadow zone. This is due to the fact that no sound energy is abled to reach this part and nothing is reflected back. 


Details of the features including the height can be analysed given the height of the towed unit (towfish), above the seafloor and also the length of the shadow.


However, due to the distance between a ship and the sea floor, the resolution of the sonar ‘beams’ returned are hampered (limited) for various reasons and there is also 'noise' that affects the sonar data.  So, to get a finer resolution, especially in areas of complex sea floor morphology, (as it is the case of the ARA San Juan location), surveys 'scans' closer to the sea-floor are needed.  







Source: NOAA



The seabed profile of the ARA San Juan search area is complex: from a fairly flat continental shelf (top of image) that drops down, with numerous ravines, (gullies / canyons).







Profile of sea-floor in search area. Source: Armada Argentina (November 2017)



Hence, the use of autonomous underwater vehicle (AUV or UUV) that are able to ‘fly’ closer to the complex seafloor morphology. For even more finer resolution scan and collecting images in a specific location, a remotely operated vehicle (ROV) can be used.


The AUV also sends out the same kind of sonar pings, but the data obtained is of much higher resolution.  This data is received on the mothership and it is also sent over to shore-side experts for post-processing.  Thus, specific aspects of the sea floor can then be looked at in greater detail.  This process is required to obtain the desired mapping product, (in this case to identify a "Points of Interest" for subsequent ROV close-up inspections).  The AUVs used in the search for the ARA San Juan had both side-scan and multi-beam bathymetric sonar. 



This is was what the ‘Seabed Constructor’ doing for several months.


This combination of technology is not only used to search for wrecks, but it is also used in the marine biology, offshore industry and geophysical fields.







Source: Marine Technology News (accessed 2017)



It is of interest to note that only about 15% of the world’s seas and oceans have been surveyed with multi-beam sonar systems. 


Tags: #LaBúsquedaContinúa, #arasanjuan, #armadaargentina, #ministeriodedefensa, ara san juan, argentina, search, uav
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