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Fibre-optic sensing technology is transforming seafloor monitoring by enabling dense, continuous measurements across vast distances using existing telecommunication infrastructure. Distributed acoustic sensing (DAS) and optical interferometry [1] have demonstrated remarkable potential for earthquake detection, ocean dynamics monitoring, and hazard early warning. However, for these technologies to be used for these applications, the transfer function between environmental perturbations and measured optical signal changes in submarine cables needs to be known.We present the, to the best of our knowledge, first controlled-environment characterisation of submarine cable responses to active seismic and acoustic sources, comparing DAS and optical interferometry measurements with ground-truth data from 58 geophones, 20 three-component seismometers, and microphones [2]. Our results reveal three key findings:In contrast with proposed theoretical models [3], our interferometric measurements show first-order sensitivity to broadside seismic sources, enabling localisation of arrivals along straight fibre links.We identify a previously unreported fast-wave phenomenon, attributed to seismic energy coupling into the cable's metal armour and propagating at velocities exceeding 3.5 km/s, significantly altering recorded waveforms.We compared measurements between adjacent fibres within the same cable. Results show significant discrepancies between the measured waveforms, which should be considered in applications operating in a similar frequency range as our tests.These findings show the complexity of submarine cable mechanics and their impact on optical sensing performance. Understanding these processes is critical for calibrating transfer functions and improving the reliability of fibre-based geophysical observations. In addition to these findings, we also discuss the limitations of our methodology, which primarily arise from the limited range of seismic source frequencies available. Our work presents a first step towards understanding the complex transfer function of environmental perturbations to optical signals in subsea cables, advancing the vision of large-scale, cost-effective Earth observation systems.[1] Marra, G. et al. Optical interferometry–based array of seafloor environmental sensors using a transoceanic submarine cable. Science 376 (6595), 874–879 (2022)[2] Fairweather, D.M., Tamussino, M., Masoudi, A. et al. Characterisation of the optical response to seismic waves of submarine telecommunications cables with distributed and integrated fibre-optic sensing. Sci Rep 14, 31843 (2024)[3] Fichtner, A., Bogris, A., Nikas, T. et al. Theory of phase transmission fibre-optic deformation sensing. Geophysical Journal International, 231(2), 1031–1039, (2022)