DAS physics — Rayleigh scatter + gauge length
Learning objectives
- Explain phase-OTDR of coherent Rayleigh backscatter
- Define gauge length and its resolution/SNR trade-off
- Quote typical industry gauge lengths (2–20 m depending on use case)
- Distinguish channel spacing (fixed at 1 m) from gauge length (configurable)
A Distributed Acoustic Sensing (DAS) interrogator repurposes a strand of telecom-grade single-mode fibre as thousands of seismic channels. The principle: a short laser pulse is injected into the fibre at one end. As the pulse propagates, a tiny fraction of light (≈10⁻⁹) scatters back from frozen-in density fluctuations in the glass — Rayleigh backscatter. The interrogator windows the return signal and measures the phase change of the scattered light BETWEEN successive pulses at each position along the fibre.
Gauge length
The interrogator doesn’t report strain at a point. It reports strain integrated over a GAUGE LENGTH L_g — the window used to compare two positions along the fibre. A shorter gauge gives finer spatial resolution but pulls in more scatter noise, since fewer Rayleigh scatterers are averaged. A longer gauge is smoother but smears waves with wavelengths shorter than 2 L_g. For a velocity of 3000 m/s, a 10 m gauge limits frequencies to ≈ 150 Hz at normal-incidence; shorter gauges are needed for high-frequency microseismic work.
Channel spacing vs gauge length
Modern DAS systems sample every 1 m along the fibre (channel spacing) but compute each channel’s strain from a L_g-wide window centred on that position. Channel spacing is FIXED by hardware; gauge length is a PROCESSING choice. That means the same raw interrogator stream can be re-processed with different gauge lengths for different targets.
Industry defaults
Gauge length rules of thumb: 2–5 m for VSP (fine resolution, short well, OK to integrate noise down later), 10–20 m for surface-seismic imaging (wider events, coarser acceptable), 0.5–1 m for microseismic (need the high-frequency content). Commercial interrogators: Silixa iDAS / Carina, OptaSense ODH-F, Febus A1-R, Halliburton Distributed Sensing. Channel counts: 10,000–100,000 per interrogator depending on fibre length (up to 40 km unamplified).
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.
- Aki, K., Richards, P. G. (2002). Quantitative Seismology (2nd ed.). University Science Books.