ABSTRACT

We have analyzed the time-dependent properties of the ambient seismic wavefield between 0.1 and 8 Hz to detect, resolve, monitor, and image the deformation induced by the water injection associated with the stimulation of the 2006 Deep Heat Mining Project in the city of Basel, Switzerland. The application of passive methods allowed the detection of an aseismic transient of approximately 35 days’ duration that began with the onset of the reservoir stimulation. Peak deformation was reached some 15 days after the bleed-off and after the induced seismicity ceased. We resolved a significant increase in seismic velocities and a simultaneous decorrelation of the noise correlation coda waveforms. The wavefield properties implied that the material response was monitored mainly in the sedimentary layer (<2.5km) above the stimulated volume that was approximately 4.5 km deep. We inverted the velocity-change and decorrelation data to estimate the spatial distribution of the medium changes. The resulting images showed that the strong velocity variations and medium perturbations were generally colocated with the lateral distribution of the induced seismicity. Positive velocity changes and damage around the injection site indicated subsidence, settling, and compaction of the material overlying the stimulated volume. Our results demonstrate that noise-based analysis tools can provide important observables that are complementary to results obtained with standard microseismicity tools. Passive monitoring and imaging have the potential to mature into routinely applied observation techniques that support reservoir management in a variety of geotechnical contexts, such as for mining, fluid injection, hydraulic fracturing, nuclear waste management, and CO2 storage.

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