DAS: fiber-optic distributed sensing

DAS (Distributed Acoustic Sensing) turns a single-mode optical fibre into a continuous array of "virtual geophones". An interrogator unit (a laser plus photodetector plus phase-demodulator) sends coherent light pulses down a fibre many kilometres long. Every point along the fibre backscatters a tiny fraction of the light from intrinsic refractive-index inhomogeneities (the Rayleigh backscatter). The backscatter from a small fibre region returns to the interrogator with a phase that depends on the integrated optical-path length — which depends on the local strain on the fibre.

How the strain measurement works

Consider two consecutive laser pulses separated by a few microseconds. Both probe the same fibre segment. Between them, the fibre may have been strained by a seismic wave. The phase difference between the two backscatter returns from that segment is proportional to the strain change — i.e. the strain-rate. By processing the return light as a function of its time-of-flight (which maps one-to-one to position along the fibre), the interrogator produces, for every shot, a strain-rate trace at every point along the fibre.

Gauge length

The strain is computed by differentiating the phase over a gauge length L_g (typically 1–30 m; 10 m is common). Shorter L_g gives finer spatial resolution but more noise; longer L_g averages noise but blurs wavelets. The rule of thumb: pick L_g ~ λ/4 for the dominant signal frequency — any longer and you start to cancel your wavelet within the gauge.

What DAS is, and isn’t

• Is: continuous strain sensing every ~1 m over 10–50 km of fibre. No active electronics in the field; the fibre is passive. Survives well in wells, subsea cables, and hostile environments where electronics die.
• Is not: a 3C sensor. It measures axial strain only — the component of particle motion along the fibre axis. Broadside-arriving waves are heavily attenuated; dip-sensitive.
• Noise: per-channel DAS SNR is roughly 10× worse than a premium 3C geophone. But you get 1,000× more channels at similar total cost.

Where DAS has changed seismic acquisition

Downhole VSP is where DAS hit first — a fibre cemented behind casing records every depth in every shot, replacing expensive tool runs. Permanent-reservoir-monitoring (PRM) fibres on the sea bed (Valhall) give repeatable low-cost surveys. Onshore, DAS laid along public right-of-ways (telecom dark fibre) has enabled “ ambient-noise” surveys at almost zero acquisition cost. The technology is the biggest change to seismic sensing since MEMS — still maturing, but the trajectory is clear.

References

• Mougenot, D. (2013). MEMS-based 3C accelerometers for land seismic acquisition. The Leading Edge, 32(4), 388–396.
• Sheriff, R. E., Geldart, L. P. (1995). Exploration Seismology (2nd ed.). Cambridge University Press.
• Berg, E., Svenning, B., Martin, J. (2010). OBN technology — recent developments. EAGE Workshop on Permanent Reservoir Monitoring.