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.
