Temporal Variations in Seismic Velocity via Ambient Noise Interferometry: Applications to Wastewater Injection and Induced Seismicity in Greeley, Colorado

Abstract:

Since 2014, wastewater injection into the basal sandstone units of the Denver Basin have induced earthquakes in the previous aseismic region near Greeley, Colorado (Yeck et al., 2016; Brown et al., 2017). In this study, we computed changes in relative seismic velocity (dv/v) from 2014 to 2019 to investigate if dv/v exhibits spatial and temporal correlations with wastewater injection volumes, earthquake frequency, and/or seasonal hydrologic fluctuations. We cross-correlated ambient seismic noise to construct daily virtual seismograms between all stations and calculated dv/v with MSNoise (Lecocq et al., 2014.) To constrain the depth of velocity variations and their potential causes, we consider a range of frequency bands between 0.1 and 8 Hz, with lower frequencies generally sampling deeper material. Consistent with previous observations, we find that dv/v measurements depend strongly on the choice of reference waveform and that seasonal variation dominates our initial results. We present our approach to reducing these effects, which overprint the signal of deeper perturbations within the reservoir. Additionally, the expansion of broadband sensors later in the deployment allows us to retrieve deeper signals.