Development of 3-C Fibre Optic Sensors, a Comparative Analysis with 3-C Geophones and DAS FO Cable

A combined DAS-1C on hybrid wireline with two 3-C optical seismic sensors at the bottom was used to acquire a zero offset VSP survey in the Avalon test site in Cornwall, Rosemanowes Quarry using a seismic vibrator and airgun source. The test site comprises multiple deep deviated wells with 9 5/8” casing, wells drilled in the 80s as part of the UK Hot Dry Rock Geothermal Energy Research program.

The reference downhole antenna is our 3-component geophones array with 4 levels run on wireline from TD to surface and used as benchmark for the analysis.

The Avalon system is an all-optical solution without downhole electronic components. The sensors are discrete 3-component fibre-optic point receiver accelerometers designed for slim hole tools to cope with any sized wellbore and borehole deviations.

The acquired vibrator and airgun datasets enabled many quantitative comparisons between the three sensor types. These comparisons allowed the relative merits of cable-deployed DAS systems and 3-C vector sensors to be evaluated using a defined data analysis workflow. In this paper we describe our findings in terms of SNR, frequency responses, vector fidelity, sensitivity, and comment on the current state of DAS technology and 3-C optical point sensors versus 3-C geophones