Rie Nakata (Kamei)
What to Expect
Advances in acquisition systems and computational resources are bringing us to a new era of seismic imaging, inversion and monitoring. We are now able to utilize seismic records more comprehensively, and hence able to extract more detailed subsurface information. Seismic full waveform inversion (FWI) is one of such advanced methods that estimates a spatial distribution of subsurface elastic properties (velocity, attenuation, and anisotropy) of the Earth at an unprecedented level of resolution by fitting seismic waveforms based on the numerical solution of the wave equation. Seismic monitoring especially benefits from FWI. Time-varying changes in subsurface elastic parameters are often small both in terms of magnitude and spatial extent, leading to minimal traveltime changes, and making waveform information critical to detect such changes. Crosshole monitoring datasets are favourable for FWI monitoring because of dominating transmitted body-wave arrivals, but face challenges because of source radiation patterns and strong tube-wave arrivals. I show a successful FWI monitoring of microbubble water injection into shallow unconsolidated sediments. Time-lapse FWI detects transient velocity changes of less than one percent within a thin (1m) layer, inferring the temperature and fluid-flow influences. With another crosshole monitoring dataset for carbon geosequestration, I demonstrate that a source-radiation pattern can be crucial for velocity estimation, and that tube-waves can be used to monitor borehole conditions. Lastly, with zero-offset VSP datasets, I show that even when FWI is difficult to apply, using waveform information is critical and enables to characterize near-borehole anomalies and obtain attenuation values.
Rie Nakata (Kamei) is an assistant professor at Earthquake Research Institute of University of Tokyo. She obtained a PhD at University of Western Ontario on the topic of seismic full waveform inversion. After a half-year post-doctoral position at University of Western Ontario, she was a research fellow at University of Western Australia in 2013-2018, University of Oklahoma in 2018, and MIT in 2019, before joining University of Tokyo in May 2019. She is interested in imaging, inversion and monitoring of Earth’s seismic properties in various scales ranging from rock samples to crustal scales. She works on scientific investigation of seismogenic and volcanic zones, seismic monitoring of Carbon Capture and Storage programs, oil/gas exploration, and near-surface engineering problems.