1:45 PM - 2:00 PM
[SSS11-12] Time-lapse seismic full waveform inversion for monitoring near-surface velocity changes during microbubble injection
Keywords:Full waveform inversion, Seismic monitoring, Fluid injection
We apply acoustic full waveform inversion to time-lapse cross-well monitoring surveys, and estimate the velocity changes that occur during the injection of microbubble water into shallow unconsolidated Quaternary sediments in the Kanto basin of Japan at a depth of 25 m below the surface. Microbubble water is comprised of water infused with air bubbles of a diameter less than 0.1mm, and may be useful to improve resistance to ground liquefaction during major earthquakes. Monitoring the space-time distribution of microbubble injection is therefore important to understand the full potential of the technique.
The time-lapse data set consists of 17 monitoring surveys conducted over 74 hours which exhibit excellent repeatability, allowing us to analyze small time-lapse changes in the subsurface. We observe transient behaviors in the seismic waveforms during microbubble injection manifested as traveltime shifts and changes in amplitude and frequency content. Time-lapse full waveform inversion detects changes in P-wave velocity of less than 1 percent during microbubble injection, initially as velocity increases, and then subsequently as velocity decreases. The velocity changes are mainly imaged within a thin (1 m) layer between the injection well and the receiver well, inferring that microbubble water flow is constrained by the fluvial sediment depositional environment. The resulting velocity models fit the observed waveforms very well, supporting the validity of the estimated velocity changes. In order to further improve the estimation of velocity changes, we investigate the limitations of acoustic waveform inversion, and by applying and comparing elastic waveform inversion to the time-lapse data set.