Distributed acoustic sensing for near-surface imaging using submarine telecommunication cable: A case study in the Trondheimsfjord, Norway

Abstract

Distributed acoustic sensing (DAS) transforms submarine telecommunication cables into densely sampled seismic receivers. To demonstrate DAS applications for seismic imaging, we use an optical cable on the seafloor in the Trondheimsfjord, Norway, to record seismic data generated by a controlled seismic source. The data are simultaneously recorded by a towed hydrophone array and the fiber optic cable. Following our data processing methods, we can produce seismic images of the seafloor and underlying geological structures from both hydrophone array and DAS data. We find that the hydrophone and DAS data have a comparable signal-to-noise ratio. Moreover, DAS images can be improved by using a seismic source that has sufficiently large energy within the frequency range matching the spatial resolution of DAS. The temporal resolution of the DAS images can be improved by minimizing the crossline offset between seismic sources and the DAS cable. The seismic images from DAS can be used to support geohazard analysis and various subsurface exploration activities.

Distributed acoustic sensing for active offshore shear wave profiling

“…However, many of the areas where windfarms are planned, for example in the Irish Sea, are not located close to existing structures and the expected relatively low frequency nature of passively acquired Scholte wave data in these areas may not be suited for characterising the geophysical and geotechnical properties of the near surface, associated with the design of renewable energy infrastructure on the seabed. Conversely, the potential application of DAS, when combined with active seismic sources, is more suitable for high-resolution near surface offshore investigations 39 , 40 .…”

Eavesdropping at the Speed of Light: Distributed Acoustic Sensing of Baleen Whales in the Arctic

“…The interrogator calculates the timedifferentiated phase change of the backscattered response from consecutive sweeps at regularly spaced intervals along the fiber, further named channels (Figure 1C). The phase change is averaged over a section of the fiber, the gauge length, and converted into longitudinal strain waveforms, analogous to acoustic pressure, for each corresponding fiber section (Hartog, 2017;Taweesintananon et al, 2021). The resulting strain data, sampled in both time and space is streamed in near-real-time from the experiment site to a remote data-processing center (Figure 1D).…”

“…DAS array response to strain for a specific fiber optic/ interrogator combination is a function of the gauge length, wave frequency, wave velocity and grazing angle between the source and the receivers. Due to the nature of the coupling between acoustic stress and FO cable strain, a notch in the impulse response of the fiber can be observed for perpendicular arrivals (Taweesintananon et al, 2021). The conversion from strain to acoustic pressure is a linear relation that depends on the Young's modulus of the seafloor, which varies along the FO cable.…”

“…The conversion from strain to acoustic pressure is a linear relation that depends on the Young's modulus of the seafloor, which varies along the FO cable. During this first experiment, calibration, e.g., using a known and georeferenced active source and co-located hydrophones as demonstrated locally in (Matsumoto et al, 2021;Taweesintananon et al, 2021) was not performed. Therefore, the results are presented in nano strain (adimensional) or in dB re.…”

“…Initially applied to geophysical data collection (Schenato, 2017;Hartog et al, 2018), DAS has recently aroused interest with waterborne sound sources, e.g, it has been used for near-surface ship detection and bearing estimation in the Mediterranean sea (Rivet et al, 2021). Quality assessment of the data was performed using controlled airguns sources compared (a) to ocean bottom seismometers recordings in Japan (Matsumoto et al, 2021) and (b) to seismic streamers in Norway (Taweesintananon et al, 2021). Both studies showed similar capabilities between DAS and known instrumentation in terms of frequency response and signalto-noise ratio (SNR).…”