Who would believe that the scarcity of fish poses a threat to some three million wrecks dotting the bottom of the world’s seas? However, it is industrial trawl fishing, often practiced at more than 1,000 meters deep and now partially limited to 800 meters in Europe, which constitutes one of the main threats to the submerged heritage of the abyss. In a few seconds, a multi-millennial site can be turned upside down, or even destroyed, while it has so far been remarkably preserved, especially by the darkness and the low temperature, far from violent tidal currents or surface weather phenomena…
The remarkable state of conservation of the deep wrecks gives them a major archaeological interest, but also, unfortunately, arouses the interest of some private companies of “treasure hunt”, who recover the cargoes to resell them, without concern for archaeological study and often in violation of the 2001 UNESCO Convention on the Protection of the Underwater Cultural Heritage.
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Preserve immersed heritage
Faced with these threats, states must urgently locate and assess their immersed cultural heritage in order to preserve its invaluable scientific and cultural value. In recent years, the underwater archeology services of many countries have thus begun to conquer the abyss.
France, the second-largest maritime area in the world, has a special position in this race. In 1966, Andre Malraux, then Minister of Culture, created the DRASSM, the Department of underwater and underwater archaeological research. For over 50 years, French underwater archaeologists have forged their expertise and know-how, which are today recognized worldwide. Until recently, most of the archaeological campaigns were limited to human diving, with some occasional deeper incursions on the margins of oceanographic ( Ifremer ) or industrial operations ( Comex, in particular).
Since the 2010s, taking into account the urgency associated with the protection of the deep heritage, the underwater archaeologist Michel L’Hour, Director of the DRAM, has launched an ambitious program of development of innovative means dedicated to archeology deep sea. The first phase consisted of designing a ship certainly suited to human archaeological excavation, between 5 and 50 meters deep, but also the deployment of robots to assess wrecks up to a depth of over 1000 meters. This 36-meter vessel was launched in 2012 and legitimately bears the name of André Malraux.
Detection of wrecks
For the detection of wrecks, since electromagnetic waves hardly penetrate the sea, most technologies are based on the use of acoustic waves. This is the case, for example, with lateral sonar ( side-scan sonar ), an acoustic antenna towed a few meters away from the seabed. It makes it possible to map the latter and detect anomalies in the relief. When you want to map beyond 300 or 400 meters deep, the side sonar is installed on an autonomous robot called AUV (Autonomous Underwater Vehicle). The lateral sonar is often associated with a magnetometer which makes it possible to detect magnetic anomalies, possibly caused by the metallic masses of a wreck.
These so-called “survey” operations are carried out either in the context of preventive archaeological operations (for example prior to the laying of an underwater cable) or in the context of the search for specific wrecks. The Cordelière and Regent wrecks, which have been lying open to the Brest Gully since 1512, are currently the subject of a vast research campaign. For this type of wreck, clearly identified, the prospecting areas are restricted by a careful study of the archives (accounts of witnesses or survivors, meteorological archives, study of sea currents, morphology, old nautical charts, period newspapers…), but also by analyzing oceanographic data, such as currents, tides or prevailing winds.
The expertise phase
The “survey” is followed by the inspection, which makes it possible to visually determine whether the magnetic or acoustic anomalies correspond to wrecks. Beyond the limits of human diving, this inspection is most often accomplished by a remotely operated robot, an ROV ( remotely operated vehicle ). Thanks to its umbilical, the cable that connects it to the surface, the ROV transfers its videos live to the pilot who is on board the ship stationed above the wreck.
If the site is interesting, hundreds, even thousands of photos are taken to build, a posteriori, a 3D model of the wreck. This technique, photogrammetry, is now perfectly mastered. It is used both with a reflex camera ( Ifremer, ipso facto ) and by coupling several digital cameras ( Comex ). The models are then scientifically exploited, or delivered to the general public for visits in virtual reality, as the DRAM did for the wreck of the Moon or for that of the battleship Danton, which has rested since 1917 by 1025 meters deep.
In 2019, Onera (National Office for Aerospace Studies and Research), LIRMM (Laboratory of Robotics and Microelectronics in Montpellier) and DRASSM designed a new single-camera system, the size of a bottle of mineral water, capable of producing a 3D model in real-time, while precisely calculating the position of the robot. This technology facilitates the management and accelerates the interpretation of the sites visited.
Take without breaking
During the appraisal of a wreck, it is sometimes necessary to take samples. The existing ROVs were developed mainly for the petroleum industry and are unsuitable for archaeological work. The slowness and the weak dexterity of their hydraulic manipulator arms force the pilots to place the robots on the bottom, therefore on the wreck itself. Hydraulic clamps on the arms are also very often incompatible with the fragility and shape of the most delicate archaeological objects (glass, wood, leather, ropes, fabrics, etc.). For these reasons, since 2012, the DRAM has started developing specialized robotic tools, with the support of the LIRMM.
A laboratory site 91 meters deep
Most of the tests in this program are carried out near Toulon, on the mythical wreck of the Moon, 91 meters deep. This ship of Louis XIV, whose sinking dates from 1664, brings together most of the constraints encountered on deep wrecks and allows them to experiment with new robotic tools, after their validation in the laboratory.
Thus, in 2014, archaeological samples were taken using a robotic hand, designed by the company Techno Concept (Loupian, Hérault), then a claw.
Thanks to a computer-assisted piloting algorithm developed at the LIRMM, the carrying robot achieves horizontal and vertical precision of the order of 1 to 2 cm, which makes it possible to get rid of the manipulator arms and work “on the fly” Without touching the bottom. It is in fact directly the robot which pivots and moves to gently bring the claw or the hand onto the object to be sampled. The absence of arms considerably reduces the size of the robots, which allows them to more easily access the cramped or complex areas of wrecks.
Building on these first results, French archaeologists have expanded their collaborations. Thus, in 2016, the submarine humanoid Ocean One, entirely developed by the team of Professor Oussama Khatib of Stanford University, carried out its first dive on the wreck of the Moon, accompanied by the DRAM and the LIRMM. The robot has two unique arms, quickly and precisely actuated by electric motors and equipped with force sensors. The efforts are restored thanks to haptic interfaces, kinds of motorized joysticks allowing to control the robot in translation and in rotation, similar to those used for the control of robot surgeons.
Still in the field of manipulation, this time as part of the ANR SeaHand project, the prime Institute finalized, in early 2020, a robotic hand specifically designed for underwater archeology. Measuring the forces perceived by each finger, the SeaHand hand opens the way for “touch” searches in turbid environments. It was unthinkable only a few years ago.
All these advances meet a large part of the needs of deep archeology, but there are still many challenges to be met, such as the delicate release of large volumes of sediment during a methodical excavation or the automatic analysis of large amounts of data. generated during survey operations.