Efficient signal detection and visualisation for fibre-optic seismology: exploring multiple environments and applications

Ground motion signals acquired through Distributed Acoustic Sensing (DAS) provide unprecedented spatial resolution over kilometric distances, particularly in environments traditionally difficult to reach, such as the ocean bottom. Although this is a substantial upside on its own, DAS experiments come with data storage costs that translate into processing costs that have to be addressed. Whether we choose to adapt former tools or develop new tools, we often leverage computational infrastructures that may not be readily available or easily accessible to every researcher, and thus there is still a need for tools that can reliably run on the average workstation.

This presentation introduces a multiscale, kurtosis-based picking algorithm designed for detection on arbitrary-length traces. Using this newly developed picker, we propose pick scatter maps as a novel method for visualizing DAS data. These maps combine individual picks from traces to reveal patterns and facilitate the interpretation of signals recorded by DAS. It is infeasible to reliably plot a full-day 2D strain map due to resolution and memory issues, but it is possible to plot a full-day scatter map where overlapping points (picks) appear to form lines that correspond to individual signals, with their corresponding apparent velocities. Examples include extremely vertical lines (large apparent velocities) spanning the whole cable, which are expected for earthquakes, or localised lines with a visible slope (lower apparent velocities), which will usually correspond to vehicles. Scatter maps from some environments may feature signals of interest to other fields of research, such as marine life on ocean bottom cables.

Scatter maps provide a way to highlight specific segments within month-long strain recordings. In the particular case of earthquakes, curve fitting in pick clusters produced by P- and S-wave arrivals lets us obtain phase picks by keeping those close enough to the fitting curve, discarding the rest to reduce delayed or consecutive triggers. These phase picks can be used for location purposes, sometimes combined with traditional stations to ensure proper azimuthal coverage. For other types of signals, each specific application will determine how its corresponding picks can be used. Speed tracking or near-cable activity monitoring are examples of such applications.