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Widely scalable Mobile Underwater Sonar Technology

Periodic Reporting for period 3 - WiMUST (Widely scalable Mobile Underwater Sonar Technology)

Reporting period: 2017-02-01 to 2018-01-31

The WiMUST (Widely scalable Mobile Underwater Sonar Technology) project aimed at expanding and improving the functionalities of current cooperative marine robotic systems, effectively enabling distributed acoustic array technologies for geophysical surveying with a view to exploration and geotechnical applications.

Traditionally, seismic reflection surveying is performed by vessel-towed streamers equipped with hydrophones acquiring reflected acoustic signals generated by acoustic sources (either towed or onboard the same vessel). In particular, the acoustic source is constituted by a series of submerged electrodes, at a distance of few centimetres. The generation of a voltage of a few kV between the electrodes allows creating a spark, which generates an acoustic wave that propagates toward the sea bottom, penetrating below it. Every time this acoustic source encounters a different layer, an echo wave is generated, which propagates toward the sea surface and is registered by the hydrophones inside the streamers. Post-processing of this data by seismic experts allows reconstructing the properties of the layers underneath the sea bottom, which are then used for civil and commercial applications (e.g. underwater construction, infrastructure monitoring, mapping for natural hazard assessment, environmental mapping, etc.).
The major disadvantages of the current systems are the operational cost of the big vessels necessary to tow km of streamers, the difficulty to operate and steer them with accuracy, and the coupling between the acoustic sources and the receivers.

The vision of the WiMUST project was to replace the role of the big, expensive vessel with autonomous marine vehicles. Such a team of autonomous robots is equipped with acoustic sensors replacing conventional, long streamers, physically decoupling the acoustic source from the receivers (hydrophones) and affording both of them the capability to follow desired paths accurately, in the presence of external environmental disturbances.

In particular, two catamarans, an additional surface vehicle and four underwater vehicles composed the WiMUST set-up. Each catamaran was carrying one acoustic source, while each autonomous underwater vehicle was towing a short streamer. The additional surface vehicle was exploited to aid underwater vehicles in their localization. A final open sea survey of about 100x200 meters, of the duration of 2 hours, was executed outside the Sines harbour, Portugal. The successful seismic images collected during the experiment, and the robots navigation data have shown how the employment of marine robots is indeed a potential breakthrough technology in seismic reflection surveying.
As for WP2 Scenario Analysis, the work focused on detailed specifications of a robotic system performing geotechnical surveys.

As for WP3 Distributed Sensor Array, a tool for determining optimal source-receiver geometry based on low acoustic field coherence that is conditioned on existing a priori information was developed. A dual accelerometer vector sensor has been also developed as an alternative to conventional streamer of hydrophones.

As for WP4 Cooperative Control, a deviation from the initial plan was the consortium decision to use two autonomous catamarans in place of a traditional vessel for towing the WiMUST seismic sources. This decision required further work on the single vehicle motion control of the two catamarans, namely DELFIM from IST-ID and ULISSE, whose construction was completed during the final reporting period. In the WP, the inner control loops of all the involved vehicles were developed and tested. Furthermore, the formation control and cooperative motion of the overall fleet of vehicles was one of the most important output of the WP itself.

Work package WP5 Mission Planning has dealt with the generation of collision free trajectories to help the vehicles reach the initial formation without collision or streamers entanglements. Furthermore, empowerment-based control techniques were developed.

As for WP6 Communications, several achievements were reached. Among them, the integration of atomic clocks into EvoLogics’s modems has allowed a precise time synchronization between all the robotic systems. This has also simplified the overall localization mechanisms of the underwater vehicles, since thanks to the time synchronization, a one-time travel way technique has been implemented. This is important from the scalability point of view, as the localization cycle does not depend on the number of underwater vehicles.

As for WP7 Integration and Experimentation, system integration has been a very relevant task for achieving the ultimate WiMUST goals. The WP4 decision to use two autonomous catamarans in place of a traditional vessel for towing the WiMUST seismic source has created an impact in the system integration activities, as of course more work was needed to extend the WiMUST concept also to autonomous surface vehicles carrying an acoustic sourcec. This has delayed when the final survey could be executed, as it could not be done until the integration on the ASVs of the power supply and sparkers were completed. However, despite this extra effort, the WP4 milestone MS4, i.e. the delivery of the “WiMUST prototype”, has been achieved in the last month of the project, during the final survey that was executed on the 24th of January 2018. In that occasion, the consortium has executed a final demonstration in a relevant open sea scenario, consisting of a 100 x 200 m survey with 3 ASVs and 4 AUVs, where all the relevant technologies were integrated.

As for the dissemination activities within WP8, these have proceeded very well in terms of both general public dissemination on the media (including internet social sites) and scientific publications.
Overall, the consortium generated more than 60 scientific publications. Euronews gave coverage of the final experiments. Links to social channels and general dissemination information can be found on the project web page
"The project activities have impact on both research in the area of cooperative marine robotics and related industrial applications. In the long term, the WiMUST consortium expects that the use of ""Autonomous Immerged Nodes"" for recording seismic data at sea will become the dominant technology. In order for this to become reality, however, many technical, industrial and business hurdles need to be addressed. The WiMUST project has addressed technology that will ultimately be able to overcome some of the technical obstacles, while keeping in mind the industrial and business constraints. A demonstration of this technology has been performed, executing a geotechnical survey of 2 hours in an area of 100x200 [m] outside the Sines harbour, Portugal, involving 3 surface and 4 underwater vehicles. Such a technology is expected to have a big impact on how seismic reflection surveying is performed. In particular, not having a tie between the vessel and the streamers allows much more flexibility in terms of mission execution.
The use of ""Autonomous Immerged Nodes"" for recording seismic data is expected to impact also on smaller scale, higher resolution surveys. In such scenarios there will be savings on towing leads (and the winches that go with it) and mechanical additions to the vessel (outriggers, cable guides) compared to the current situation.