When it comes to measuring seismic activities, areas distant from major fault lines tend to be under sampled, hampering the ability to study localized and small natural events. This under sampling is particularly problematic for studying the effects of oil and gas production, wastewater injection, and carbon dioxide sequestration on ground movement, activities which tend to be located far from seismically active areas.
Dark fiber are the unused fiber-optic cables that were laid in the 1990s by telecommunication companies. Thousands of kilometers of cabling remains unused. Researchers wanted to test if unlit fiber cable could be used to collect measurements of seismic activity to fill in spatial gaps in the data. Researchers used a new technique called distributed acoustic sensing, or DAS to measure the effect of seismic waves on these subsurface cables by measuring the phase and amplitude of vibrations. The test this methodology, researchers spent seven months measuring passive seismic data along a 27 km section of dark fiber stretching from West Sacramento, CA to Woodland, CA.
Researchers were able to measure low and high-frequency seismic activity, even earthquakes happening over 7,700 km away. Researchers concluded that, “DAS-based seismic measurements acquired using dark fiber can provide a wealth of information relevant to near-surface seismic property estimation, hydrologic state, and natural seismicity.”
Ajo-Franklin, J. B., Dou, S., Lindsey, N. J., Monga, I., Tracy, C., Robertson, M., … & Li, X. (2019). Distributed Acoustic Sensing Using Dark Fiber for Near-Surface Characterization and Broadband Seismic Event Detection. Scientific reports, 9(1), 1328. Doi: https://doi.org/10.1038/s41598-018-36675-8