Fibre optic cables as Distributed Acoustic Sensors

: Applications in seismology

Abstract

The application of fibre optic Distributed Acoustic Sensors (DAS) is a rapidly developing field in seismology. DAS cables are cheap and non-intrusive and are capable of rapid deployment on the surface, or in permanent or semi-permanent deployment into the subsurface. As seismic arrays, DAS cables offer wide coverage at high spatial resolution allowing the simultaneous monitoring of entire wellbores. Consequently there has been growing interest in quantifying the performance of DAS arrays relative to conventional geophone arrays. This thesis aims to assess the viability of DAS as a tool for seismic monitoring and microseismic location. In contrast to conventional workflows using geophones, certain characteristics of DAS - including gauge length effects, increased spatial density and one-component measurements - cause additional considerations in how we process DAS data. In addition, continuous DAS monitoring produces huge data volumes in comparison with conventional geophone arrays. This increased data volume requires the development of automated processing methods in order to process seismic events in near real-time.
This thesis investigates these differences in data characteristics in a near-surface refraction survey and in downhole arrays during microseismic monitoring. In our first study we are able to directly compare geophone and DAS seismic data through the acquisition of a near-surface seismic refraction dataset in North London in March, 2019. This allows exploration of the differences in first break arrivals in a controlled small-scale setting. In the subsequent chapters we investigate downhole monitoring of a microseismic dataset via data provided by Silixa Ltd., with the aim to identify different processing requirements and develop a best automated workflow that would allow the picking and locating of DAS data in close to real-time.

Date of Award	22 Mar 2022
Original language	English
Awarding Institution	University of Bristol
Sponsors	Silixa Ltd.
Supervisor	James P Verdon (Supervisor) & James M Wookey (Supervisor)