Application of a Fibre Optic Distributed Acoustic Sensor (DAS) for Shallow Seismic Investigations of a Fractured Dolostone Aquifer in Guelph, Ontario.

Understanding groundwater flow and contaminant transport in fractured bedrock aquifers requires detailed characterization of the discrete features that control flow, as well as the properties of the rock matrix. This requires multiple, high-resolution, depth discrete datasets that provide different, but complementary information. Distributed fibre optic sensing is a relatively new technology used to continuously monitor properties along the entire length of an optical fibre. Technological advances over the past few years have brought the sensitivity and spatial resolution to the point where shallow (<200m) borehole applications are practicable.

Recent studies using fibre optic distributed temperature sensors (DTS) have shown excellent application of DTS for characterizing groundwater flow in both continuously sealed and open boreholes. This presentation highlights the results of a field trial at the Bedrock Aquifer Research Station on the University of Guelph campus (Ontario, Canada) where a single fibre optic cable was interrogated by both a DTS (Ultima-DTS) and a Distributed Acoustic Sensor (iDAS). DAS is a relatively recent development that allows an optical fibre to be used as a receiver for seismic imaging. These seismic images are produced by sending an optical pulse down the fibre and analyzing the effects of seismic waves on the propagating light. Numerous vertical seismic profiles were collected and the effects of different fibre optic cable structures and coupling techniques were examined. The seismic profiles will help delineate structural features and lithological contacts away from the borehole wall, and will assist in correlating other geophysical, hydraulic, or geological logs collected in the boreholes across the site. Preliminary results show promise for shallow seismic imaging and continued field trials will allow refinement of the technique.