Traditionally, seismic acoustic surveys are performed with a vessel that tows a number of seismic streamers – flexible cables equipped with hydrophones – and one or more acoustic sources. These powerful acoustic sources generate seismic waves underneath the earth's surface, that are then picked up by hydrophones as the waves bounce off subsurface formations. Seismic streamers are typically extremely cumbersome and accurate steering becomes difficult. Recent developments have shown that there is a vast potential for groups of cooperating marine robots to radically improve ocean exploration and exploitation. Expanding and improving the functionalities of current cooperative marine robotic systems has been the main objective of the EU-funded project WiMUST. The project brought together a group of research institutions, geophysical surveying companies and small- and medium-sized enterprises with expertise in autonomous systems, marine robotics, communication, navigation and cooperative control. A swarm of autonomous underwater robots Research focused on engineering an intelligent distributed underwater array of marine robots for seismic acoustic surveys. “WiMUST’s novelty is using marine robots to capture seismic data instead of conventional towed sources and streamers. The project vision was to develop a team of cooperative autonomous marine robots, acting as intelligent sensing and communicating nodes of a reconfigurable moving acoustic network able to share information and jointly conduct geophysical surveys,” notes Dr Giovanni Indiveri. Teams of autonomous robots carrying acoustic sources and equipped with acoustic sensors can replace conventional towed seismic acquisition systems by decoupling the acoustic source from the receivers. This enables them to follow the desired paths more accurately, even in the presence of external environmental disturbances. In other words, the vehicles are equipped with hydrophone streamers so the overall network system behaves as a large distributed acoustic array for acquiring acoustic data. By actively controlling the geometry of the robot formation, it is possible to change the shape of the acoustic array as needed. “This operational flexibility holds tremendous benefits as it allows collection of a large amount of seabed and sub-bottom data at better resolution. In addition, operations at sea will be facilitated by the absence of physical ties between the surface ship and the data acquisition equipment,” explains Dr Indiveri. Communication control WiMUST’s communication architecture ensures that the positions of each AUV in the formation are known with centimetre-scale accuracy, and that the internal clocks of the vehicles are precisely synchronised. This is crucial to ensure that the acquired seismic data is assigned temporal and spatial tags that are consistent across the formation, as well as to derive relative positions of the vehicles for navigation and control. Researchers worked on the design of new solutions for AUV closkc synchronisation in addition to others supporting seismic data transfer from AUVs to surface stations. Navigation, guidance and control Cooperative navigation and control of the AUVs and ASCs was crucial for WiMUST system performance. The team developed cooperative navigation and control algorithms to precisely steer the vehicles so that the ASCs and all the AUVs with the towed streamers avoid collisions and maintain the specific geometry relative to the acoustic sources. A motion planner generates waypoints based on the required vehicle formation for optimal sensor array geometry. Future prospects Compared to traditional ship based streamer-towing, a cooperative robotic system can greatly simplify marine operations, cutting both time and cost of a survey operation. “Measuring the position of hundreds of recording nodes spread out over an area of several square kilometres, at a sample interval of about 10 seconds and with accuracy higher than 10 metres remains highly challenging,” notes Dr Indiveri. Once all technical hurdles are overcome, the WiMUST consortium expects that the use of autonomous robots for recording seismic data at sea will find widespread applications. In addition to seismic surveys, WiMUST outcomes can be exploited in other underwater applications such as search and rescue operations, environmental monitoring and surveillance, demining, spill detection and plume tracking, archaeology, and fisheries.
WiMUST, autonomous underwater vehicles (AUVs), marine robots, seismic survey, hydrophone, streamer-towing, navigation and control