Understanding Subsurface Fracture Evolution Dynamics Using Time‐Lapse Full Waveform Inversion of Continuous Active‐Source Seismic Monitoring Data

Predicting the behavior, geometry, and flow properties of subsurface fractures remains a challenging problem. Seismic models that can characterize fractures usually suffer from low spatiotemporal resolution. Here, we develop a correlative double‐difference time‐lapse full waveform inversion of continuous active source seismic monitoring data for determining high‐spatiotemporal‐resolution time‐lapse Vp models of in‐situ fracture evolution at a shallow contamination site in Wyoming, USA. Assisted by rock physics modeling, we find that (a) rapidly increasing pore pressure initializes and grows the fracture, increasing the porosity slightly (from ∼13.7% to ∼14.6%) in the tight clay formation, thus decreasing Vp (∼50 m/s); (b) the fluid injection continues decreasing Vp, likely through the introduction of gas bubbles in the injectate; and (c) final Vp reductions reach over ∼150 m/s due to a posited ∼4.5% gas saturation. Our results demonstrate that high‐resolution Vp changes are indicative of mechanical and fluid changes within the fracture zone during hydrofracturing. Plain Language Summary Induced fractures serve as the primary flow paths for subsurface fluids in many engineered systems involving injection, extraction, or long‐term storage (e.g., groundwater, carbon storage, and enhanced geothermal exploitation). A detailed understanding of the coupled thermal‐hydro‐mechanical‐chemical processes present in fractured systems requires real‐time monitoring of the fracturing process (initiation and growth), a challenging problem. In this study, we develop an approach of time‐lapse full waveform inversion (TLFWI) of continuous active source seismic monitoring data for estimating accurate seismic velocity changes induced by fracture evolution. The localized P‐wave velocity (Vp) reductions by our TLFWI are closely consistent with various fracturing stages. These dynamic reduced Vp zones are the result of fracturing‐induced subsurface porosity in the fracture opening and then introducing gas‐bubble during the fluid injection. Key Points Seismic velocity changes caused by near‐surface hydrofracturing are obtained from the correlative double‐difference time‐lapse full waveform inversion We observed small velocity reduction (∼2%) due to fracture growth and larger velocity reduction (∼7%) potentially caused by gas bubbles Rock physics modeling supports quantitative interpretations and allows critical insight to understand hydrofracturing dynamics