Exploring the limits of fiber-optic sensing in Cascadia: Borehole passive seismic monitoring using co-located DAS, low noise optical accelerometers, and geophones

Abstract:

Continuous monitoring is necessary for accurate characterization of the source mechanisms of smaller magnitude seismic events, and for such, an appropriate set of instruments are crucial for detecting a wide range of seismic signals. The Cascadia optical sensing project aims to demonstrate long-term monitoring for improved microseismic detection by using a combination of ultra-low noise fiber-optic accelerometers (ULN - Silicon Audio), fiber-optic distributed acoustic sensors (DAS), and 3-component geophones. In August 2017, a 310 m uncased borehole was drilled in granite near Sidney, British Columbia, and the instruments were cemented in the borehole. Near-continuous ULN data have been recorded from August 2017 to December 2019, and two months of continuous ambient data were recorded on DAS starting in late August 2017. This presentation gives an overview of the Cascadia optical sensing project experiment, and performance intercomparisons of each sensor type.The ULN optical accelerometers present a low noise floor with high sensitivity and are able to detect small events. The data recorded by DAS contains significantly higher noise levels. In particular, at tidal periods, DAS data show strong optical noise that obscures weaker solid earth tides. We present a method to estimate and remove this noise.