Underwater Sensing is the monitoring of aquatic environments using technologies such as fiber optic sensing and hydrophones. NEC Labs America advances underwater sensing to detect seismic events, protect subsea cables, and monitor marine ecosystems. By applying AI to underwater signals, NEC creates scalable systems that deliver real-time insights into challenging environments.
A fiber optic hydrophone array comprising four passive fiber mandrels, interrogated by a distributed acoustic sensing system was deployed in a lake for field testing. Several stationary and moving underwater acoustic sources were localized, continuously tracked, and their acoustic signals were enhanced by beamforming techniques.
A 100-meter-long fiber optic cable was installed at the bottom of a water tank at the Davidson Laboratory, together with a hydrophone for reference. The water tank is approximately 2.5 meters deep and 95 meters long; the tank also employs a 6-paddle wavemaker which can generate programmable surface waves. A 155-cm-long model boat weighing 6.5 kilograms was automatically dragged on the surface of the tank via an electrical towing mechanism. The movement of the model boat along the fiber cable and over the hydrophone was recorded using a commercially available NEC Distributed Acoustic Sensing (DAS) system and simultaneously by a hydrophone. The experiments were repeated with and without the artificially generated surface waves. The data obtained from the hydrophone and the DAS system are presented and compared. The results show the compatibility between the DAS data and the hydrophone data. More importantly, ourresults show that it is possible to measure the surface waves and to detect a surface vessel approaching the sensor by only using the time domain analysis in terms of detected total energy over time.