Time-lapse Full Waveform Inversion for Monitoring Near-surface Microbubble Injection

Seismic monitoring provides valuable information regarding time-varying changes in subsurface physical properties during hydrocarbon production, CO2 geosequestration, groundwater injection/subtraction etc. However, the resulting changes in subsurface properties are often small both in terms of magnitude and spatial extent, and exploiting the full seismic waveform information can be critical, since detected amplitude or traveltime changes may be minimal. We apply time-lapse acoustic full waveform inversion (FWI) to repeating cross-well surveys, and monitor microbubble injection into shallow unconsolidated sediments. Repeated monitoring surveys provide exceptionally high data repeatability, and allow us to reveal transient behaviours in waveforms and then P-wave velocities during the injection. High-resolution time-lapse FWI detects changes in P-wave velocity of less than 1 percent, initially as velocity increases and subsequently as velocity decreases. The velocity changes are mainly imaged within a thin layer between the injection and receiver wells, inferring the fluid-flow influence of the fluvial sediment depositional environment. The resulting velocity models fit the observed waveforms very well, supporting the validity of the estimated velocity changes.