DAS in wells vs on the surface

Part 9 — DAS and emerging technology

Learning objectives

Explain the cos²θ sensitivity pattern of axial-strain sensing
Distinguish the strengths of well-mounted vs surface-mounted DAS
Quote typical well-DAS channel counts (3000+ for a 3 km well)
Identify applications that specifically need each configuration

DAS is a directional sensor: only axial strain (along the fibre direction) registers. Wave motion transverse to the fibre produces no signal. Sensitivity varies as cos²θ where θ is the angle between the wave-propagation direction and the fibre axis. This single physical fact dictates where you should put the fibre.

Well-mounted DAS

A fibre cemented behind casing in a vertical well has a VERTICAL axis. Near-vertical P-wave reflections from surface shots hit the fibre nearly parallel → cos²(≪θ≫) ≈ 1. A 3 km well with 1 m channel spacing gives 3000 simultaneous channels from a single fibre — unmatched by any geophone string. Primary uses: zero-offset / walkaway / 3D VSP (§7.2–7.3), hydraulic-frac microseismic monitoring, Permanent Reservoir Monitoring on producing wells, casing-integrity monitoring.

Surface-mounted DAS

A fibre trenched or laid along roads and pipelines has a HORIZONTAL axis. Reflections from depth arrive nearly vertically at the surface → cos²(≈ 90°) ≈ 0, almost no response. But horizontally-travelling waves (ground roll, refractions, surface-wave tomography, urban ambient noise) hit the fibre along its axis → strong signal. Surface DAS shines for: near-surface / static-correction work, passive-source tomography, urban infrastructure monitoring. Dark-fibre DAS on existing telecom infrastructure (no new cable in the ground) is currently the fastest-growing segment.

Practical installation

Wells: fibre is cemented behind casing at drilling time (permanent, decades of use) or deployed on a wireline (days per run). Surface: direct-bury trenching at 50 cm depth for survey lines, or existing dark-fibre rental from telecom providers (Google Cloud, CenturyLink, regional ISPs) for passive monitoring.

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.