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Mechanical vibration is a periodic displacement of the medium in which it takes place. In the case of conventional (electrical) vibration sensors, the response is proportional to particle velocity (for geophones) or acceleration (for accelerometers). These sensors are localized typically to a few cm, sometimes less, they respond independently of each other and they have specific angular and frequency response patterns. Quite separately, hydrophones or microphones measure fluctuations of the ambient pressure; these sensors generally respond to isostatic pressure independently of the direction from which the wave impinges on their surface.

On the other hand, the relationship between incoming acoustic or seismic waves and the Fiber Optic Distributed Acoustic Sensing response has been studied mainly in the context of geophysical measurements.  Seismic signals are low-frequency (usually <100 Hz) displacement waves traveling through a geological structure. In active seismic acquisition from the surface, vibration stimuli are transmitted into the earth, and arrays of sensors are used to capture reflections from rock strata and, in some cases, direct arrivals or refracted waves. From these seismic signals, a picture of the sub-surface can be formed, which is then interpreted in terms of the likely existence of hydrocarbon-bearing formations or other structures of interest, deep underground.

The Earth’s near-surface provides the foundation that supports our modern infrastructure. Changes to the near-surface can lead to hazardous conditions. For example, ground subsidence caused by permafrost thaw can damage buildings; subsurface dissolution processes can lead to devastating sinkholes. Because many such changes can manifest themselves as time-lapse variations in velocity and/or attenuation of seismic waves, seismic monitoring has the potential to provide early warning of near-surface hazards.

To provide warnings before failures occur, an effective near-surface seismic monitoring system needs to utilize measurements that have sufficient resolution and extent, both spatially and temporally. With the advent of fiber-optic distributed acoustic sensing (DAS) techniques, low-cost, low-maintenance dense arrays are feasible at kilometer scales and beyond. As a result, DAS offers new opportunities for seismic monitoring of the near-surface.

repurposes telecommunication optical fibers as multichannel seismic arrays. In contrast to conventional arrays that consist of spatially-discrete electronic sensors, a DAS system utilizes a single optoelectronic interrogator unit that can sample tens of kilometers of the optical fiber at sub-meter channel spacing. Easy DAS installations can avail time-lapse geophysical sensing in formerly inaccessible sites: urban, icy, and offshore areas.

And with the Fiber Optic Distributed Acoustic Sensor System (FOTAS), earthquakes in Turkey have been recorded. While earthquakes in the Marmara region of Turkey (where FOTAS is installed to monitor seismic waves) are easily detected with FOTAS, earthquakes in distant regions are also detected according to their intensity. The Elazig earthquake, which took place on January 24, 2020, was also measured from a distance of 900 km.