Microseismic monitoring with fibre-optic cables

Anna Williams1, Anna L. Stork*1, Alan F. Baird*1, J.-Michael Kendall1, James P. Verdon1and Andy Clarke2

1School of Earth Sciences, University of Bristol, UK (anna.stork@bristol.ac.uk/alan.baird@bristol.ac.uk), 2Silixa Ltd

Fibre-optic distributed temperature sensing (DTS) has been used for borehole monitoring since the 1980s. In the last decade fibre-optic technology has developed to enable the measurement of acoustic sound and vibration by monitoring the dynamic strain induced in a fibre-optic cable. This exciting new technology is now being deployed for the purpose of microseismic monitoring of industrial activities such as hydraulic fracturing and CO2geological storage (CCS). The technology has many advantages, such as high-fold data, since the fibre provides a continuous line of sensors, often with thousands of receivers. However, some data characteristics require a different approach to the traditional geophone recordings. For example, DAS arrays record axial strain in the fibre so only one-component (1C) seismic data is available and, consequently, the azimuthal information provided by a standard three-component (3C) geophone is lost. Here we model locating microseismic events recorded by DAS arrays. We test array geometry, station spacing, number of wells and the effects of the availability of P and/or S- wave arrival times. Deviated cables (e.g., non-linear) always perform better than unidirectional cables. Sparse DAS receiver arrays in multiple linear boreholes perform much better than single borehole arrays. This is true even in comparison with traditional 3C geophone sensors, which have the advantage of the availability of azimuthal information derived from particle motion analysis. In general, we find cable geometry is more important than station spacing.