Horizontal-to-Vertical Spectral Ratio (HVSR) Seasonal Variations at a Shallow CO2 Injection Site, the CaMI Field Research Station in Alberta, Canada

The Field Research Station (FRS) is a CO2 storage site in southeastern Alberta, Canada, which was developed by the Containment and Monitoring Institute (CaMI) in collaboration with the University of Calgary. At the FRS, CO2 is being injected at a depth of ~300 m to improve monitoring techniques for the purpose of de-risking CO2 storage. Injection of CO2 changes the pore pressure and state of stress in the subsurface, which can lead to faulting, induced seismicity, and leaks. To establish a secure CO2 injection operation, continuous monitoring of structural variations is crucial. Ambient seismic noise provides a way to monitor CO2 injection. Horizontal to Vertical Spectral Ratio (HVSR) curves are obtained using the ambient seismic noise to estimate the fundamental frequency of the FRS and the effects of environmental changes such as temperature and precipitation. An array of 3-component 10 Hz geophones is deployed within a 1 by 1 km area surrounding the injection well. The frequency of HVSR peaks and their amplitudes are sensitive to impedance contrasts between and within soil and bedrock layers and can be used to estimate the shear wave velocity and thickness of the resonating layers. HVSR curves from the FRS during 2020 exhibit two distinct peaks at frequencies of 2-3 and 7-8 Hz and a third peak occasionally is present at higher frequencies (~20 Hz). We suggest that the peak at 7-8 Hz is related to an interface within the shallow soil and till at a depth of ~5m. The peak at 2-3 Hz is related to the top of the Dinosaur Park formation at the depth of ~25m. The sporadic high frequency peak would correspond to a shallow layer 2-3 m thick with low seismic velocity. Noting a correlation between the appearance of this high frequency peak and episodes of precipitation suggests that the presence of ground water is responsible for reducing velocities in this uppermost layer. We find lower HVSR amplitudes during periods when surface temperatures drop below freezing between November and March, which means that at these times the impedance contrast between layers is lower. Variations in HVSR spectra that follow the same pattern as changing temperature show that these observations are sensitive to environmental conditions. Accurately monitoring of CO2 sequestration operations requires accounting for these environmental influences on structure.