Comparison of Fiber-Optic Cable Deployment Strategies inside a Geothermal Borehole using Active DAS VSP

Distributed Acoustic Sensing (DAS) has become a key technology for downhole seismic monitoring over the past decade, owing to its dense spatial sampling, operational simplicity, and cost efficiency. By converting fibre‑optic cables into continuous seismic sensors, DAS enables full‑depth borehole measurements that significantly exceed the spatial coverage of conventional geophone arrays. However, the quality of DAS measurements is highly dependent on fibre–formation coupling, which in turn is controlled by the method used to deploy the fibre within the borehole. 

This study investigates the impact of different fibre‑optic deployment strategies on the quality and detectability of active DAS signals in a geothermal context. Experiments were conducted at a newly developed geothermal test site in Pau, France, equipped with six shallow (130 m) boreholes supplying a heating and cooling system. Two deployment methods were tested during probe installation: fibre cemented alongside the geothermal probe and fibre installed inside the geothermal probe tube. Multiple fibre‑optic cables (Single‑Mode and Multi‑Mode) were deployed, with this study focusing on three wells instrumented with different cable types and geometries (cemented looped, cemented single‑pass, and retrievable looped inside the probe). 

An active seismic Vertical Seismic Profiling (VSP) survey was conducted in September 2025 using a weight‑drop source, with 43 repeated shots stacked to enhance signal‑to‑noise ratio. DAS data were acquired with a 5 m gauge length, 1 kHz sampling rate, and 2.4 m spatial sampling, yielding over 500 measurement channels along a continuous 1.2 km fibre. Despite ongoing geothermal circulation during the experiment, coherent seismic energy was detected along all boreholes down to 130 m depth. 

Preliminary analysis of the stacked DAS data reveals coherent seismic arrivals in all instrumented boreholes, with detectable signals down to 130 m depth. Two main wave types are observed across all configurations: a fast first‑arrival wave (~2,270 m/s) and a slower guided mode (~550 m/s). Qualitative comparisons between boreholes suggest that signal amplitude and continuity vary with fibre deployment configuration. Cemented fibres generally display clearer first arrivals, while fibres installed inside the HDPE probe show localized attenuation and reduced amplitudes, potentially linked to bending and coupling conditions. Further analysis will include wavefield separation and time-lapse analysis to help quantify the effects of fibre coupling and geothermal operations