Seafloor fibre optical cable repositioning using target motion analysis on distributed acoustic sensing of underwater acoustic noise

SUMMARY Distributed acoustic sensing (DAS) is a recent technology that turns optical fibres into multisensor arrays. In the marine environment, it offers new possibilities for measuring seismic and environmental signals. While DAS can be applied to existing fibre optic cables used for communications, a major limitation of such efforts is that the position of the cable is not always known with sufficient accuracy. In particular, for submarine telecommunication cables, the positioning accuracy decreases with increasing depth. This problem affects the accuracy of earthquake locations and source parameters based on DAS signals. This limitation calls for methods to retrieve the cable’s position and orientation. Here, we propose a method for relocating a linear section of cable—or multiple connected segments—using incidental acoustic sources, particularly boats moving in the vicinity of the cable. The method is based on target motion analysis (TMA) for sources in uniform rectilinear motion. We consider Bearing-Only TMA (BO-TMA) and the Bearing and Frequency TMA (BF-TMA), which respectively use changes in backazimuth (called bearing in navigation) and changes in both backazimuth and Doppler frequency shift as the source moves. We adapt these methods to the 3-D case to account for the difference in depth between the fibre and the sources. Both cases lead to a nonlinear inverse problem, which we solve by the Levenberg–Marquardt method. On synthetic data, we test both TMA techniques on single and multiple source trajectories and evaluate their accuracy as a function of source trajectory and velocity. We then test the BO-TMA on real DAS recordings of acoustic signals produced by passing ships near a 42 km-long fibre optic cable off the coast of Toulon, southeastern France. In this study case, the position and characteristics of the acoustic source are known. While the Doppler frequency shift at low frequency (30 Hz) is difficult to measure with sufficient accuracy ($< 0.1^{\circ }$), we demonstrate that effective cable location can be achieved by BO-TMA using multiple ship passages with a variety of trajectories. Once the linear sections of the cable have been relocated, the stage is set to reconstruct the entire cable configuration. More generally, the 3-D TMA on linear antennas developed here can be used to locate either the sources or the antenna situated at different depths.