We detect maritime vessels on a 41.5 km long fiber optic telecom cable using distributed acoustic sensing. We measure noise emitted by the same tanker cruising one day at a distance of 5.7 km from the shore, above 85m of water, and the following day at a distance of 20 km from the shore, above 2000 m of water. The acoustic emissions of the tanker result in measurable strain rate on the optical fiber cable laid on the seafloor and probably, for some sections, covered by sediments. The spectral signature of the engines of the ships, the doppler effect, and the apparent velocity of the acoustic waves on the fiber allow to separate the ship noise from the environmental and the DAS interrogator noises. The broad band sensitivity of the distributed acoustic sensing allows to identify tanker acoustic noise at frequencies ranging from 16 to 100 Hz on the noise content measured up to 1kHz. At 85m water depth, the signal to noise ratio is high, and the trajectory of the boat is recovered from beam forming analysis of the distributed strain rate measured every 6.4m. At this shallow depth it is possible to track the position of the tanker up to 2 km away from the cable. The main features of the intensity of the acoustic noise in space and time is well model using a ray-based model of the acoustic wave propagation in the sea and converted into longitudinal strain rate, as it is seen by the cable. On the open sea, at 2000m water depth, the acoustic signal of the ship is more attenuated, however narrow bands signals below 50Hz are still detected by the DAS. In both route of the cruising tanker, we assess the possibly to use the Doppler shift of the emitted frequency to estimate the tanker velocity. These results confirm the high potential of DAS technology using telecom cable for a remote and quantitative monitoring of the maritime traffic and for vessel tracking on shallow water but also at great depth.

The use of larger numbers of sensors is becoming more common at the large, continental scale for deep-structure imaging in seismology, and at a smaller scale with exploration geophysics objectives. Seismic arrays require array processing from which new types of observables contribute to a better understanding of the wave propagation complexity. From among these array processing techniques, this study focuses on a way to select and identify different phases between two source-receiver arrays based on the double beamforming (DBF) method. At the exploration geophysics scale, the goal is to identify and separate low-amplitude body waves from high-amplitude dispersive surface waves. A synthetic data set from a finite-difference time-domain simulation is first used to validate the array processing method. From directional information obtained with DBF, and due to the double-plane wave projection, it is demonstrated that surface and body waves can be extracted with a higher efficacy compared to classical beamforming even at short offset. A seismic prospecting data set in a laterally heterogeneous medium is then investigated. This data set is a high-resolution survey which provides a perfect control on source and receiver arrays geometry. The separation between the direct surface and body waves is observed after DBF and ray bending is discussed from the additional azimuthal information.

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