Distributed acoustic sensing (DAS) transforms submarine telecommunication cables into densely sampled seismic receivers. To demonstrate DAS applications for seismic imaging, we have used 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 geologic structures from the 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 (DAS) leverages an ocean-bottom telecommunication fiber-optic cable into a densely sampled array of strain sensors. We demonstrate DAS applications to passive acoustic monitoring through an experiment on a submarine fiber-optic cable in Longyearbyen, Svalbard, Norway. We find that DAS can measure many types of signals in the frequency range from 0.01 to 20 Hz generated by dynamics in the atmosphere, ocean, and solid earth. These include ocean-bottom loading pressure fluctuation of ocean surface waves generated by storms, winds, and airflow turbulence, shear-wave resonances in low-velocity near-surface sediments, acoustic resonances in the water column, and propagating seismic waves. We find that DAS can record high-quality low-frequency seismo-acoustic waves down to 0.01 Hz, which could be used for subsurface exploration. Using the shear-wave resonances recorded by DAS, we can determine the subsurface structure of near-surface sediments with low velocity. In addition, we can trace ocean swells back to their origins of distant storms as far as 13,000 km away from the cable. Because DAS is capable of seismo-acoustic monitoring with a high spatial resolution of approximately 1 m over a cable of approximately 100 km long and with a broadband sensitivity down to 0.01 Hz on the low end, it can deliver great scientific value to the ocean observation and geophysics community.

Seismic imaging is essential for detailed characterization and understanding of the subsurface. Traditionally, marine seismic data have been acquired either using towed streamers or ocean-bottom receivers. Distributed acoustic sensing (DAS) is a novel and fast-emerging technology that makes it possible to use fiber-optic cables for acoustic measurements. The technology turns the fiber-optic cable into a densely sampled receiver array. A controlled source experiment comparing DAS and conventional towed streamer data for seismic imaging purposes is described. An existing 130 km telecommunication fiber-optic cable was interrogated to acquire DAS data to analyze the signal at long distances and for subsurface imaging. A seismic source vessel was used to create seismic data along the full cable length, while at the same time acquiring conventional towed streamer data for comparison. In addition, the telecommunication fiber-optic cable was used to acquire passive DAS data. The results show that seismic signals in the telecommunication fiber-optic cable are detectable at distances above 100 km from the source point. Furthermore, comparisons of the DAS data and towed streamer data show that the DAS technology allows for the acquisition of large offset seismic data that are comparable with conventional seismic data. Analysis of the passive DAS data shows that the infragravity waves correlate with the observed weather conditions during the acquisition.

We describe DAS interrogation for non-intrusive coexistence with live C-band WDM channels. The scheme facilitates consistent high sensing sensitivity range >100 km. Surface vessels, seabed fishing gear and earthquakes are localized from the 2Africa network. © 2024 The Author(s)

Recent advances in range and performance of distributed acoustic sensing (DAS) enable new geophysical applications by measuring fiber strain in existing telecom cables and subsea power cables that incorporate optical fibers. We will  present new field data showing the usability of DAS for environmental and geophysical applications, focusing especially on seabed surface waves and the sub-Hz domain. These examples show that highly sensitive DAS technology can be a valuable tool within seismology and oceanography.The sensitive range along the fiber for DAS was previously limited to about 50 km. We will demonstrate a newly developed system (named OptoDAS) that allows for launching several orders of more optical power into the fiber, and thereby significantly improving the range beyond 150 km.This new interrogation approach allows for high degree of flexibility optimizing the interrogation parameters to optimize the noise floor, spatial and temporal resolution according to the application. The gauge length (spatial resolution) can be set from 2 to 40 m. For interrogation of 10 km fiber, we achieve a record low noise floor of 1.4 pε/√Hz with 10 m spatial resolution. For interrogation of fibers beyond 150 km, we achieve a noise floor below 50 pε/√Hz up to 100 km. Above 100 km, the noise is limited by the level of reflected optical power, and the noise increases by ~0.3-0.4 dB/km, corresponding to the dual path optical loss in the fiber.A modern instrument control interface allows for automatic optimalization of interrogation parameters based on application parameters in a few minutes. The instrument computer provides a flexible platform for different applications. The high-capacity storage system can store recorded time-series of several weeks to support e.g., geophysical investigations where extensive post-processing is required. The computational capacity can also be used for real-time visualization and advanced signal processing, for example for event detection and direct reporting of estimated parameters.The OptoDAS system can convert a submarine cable into a 100 km+ densely sampled array.  From the recordings on a telecom cable in the North Sea, we will show examples of propagating Rayleigh and Love acoustical modes bounded to the seafloor surface. These modes can be excited by acoustic sources on or above the seafloor, such as trawls and anchors. The dense spatial sampling allows for accurate estimates of the location of these sources. The system also allows for applications in seismology and earthquake monitoring. When attached to a cable with non-straight geometry, the measurements have substantial information to determine the location of seismic events. This will be demonstrated using field data from the North Sea telecom cable.From recordings on a submarine cable between Norway and Denmark, we present the DAS response in the frequency range 0.1 mHz-10Hz across a cable span of 120 km. The response in this frequency range will be a combination of temperature changes, ocean swells and tides. We show that increasing the gauge length in post-processing allows for improving the sensitivity for detecting ultra-low frequency signals.

Distributed Acoustic Sensing (DAS) leverages an ocean-bottom telecommunication fiber-optic cable into a densely-sampled massive array of strain sensors. We demonstrate DAS applications to Passive Acoustic Monitoring (PAM) through an experiment in Longyearbyen, Svalbard, Norway. We show that DAS can measure many types of signals generated by dynamics in the atmosphere, ocean, and solid earth. These include primary and secondary microseisms, Scholte waves, water-layer acoustic resonances, and seismic waves from earthquakes. In addition, we can trace the origin of primary microseisms back to distant storms a quarter of the way around the planet. We also find that the fjord acts as an amplifier for microseisms. Because DAS is capable of hydroacoustic monitoring with high spatial resolution over great distances, it can deliver great scientific value to ocean observation. We believe that DAS can and will become a valuable component of the Global Ocean Observing System.