ORCID Profile
0000-0002-9188-9030
Current Organisations
University of Tokyo
,
Massachusetts Institute of Technology
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Publisher: American Geophysical Union (AGU)
Date: 19-12-2022
DOI: 10.1029/2022GL098734
Abstract: Monitoring time‐lapse changes in subsurface physical properties of the near‐surface critical zone is increasingly important with respect to climate change, environmental conservation/remediation, geohazard mitigation, and geotechnical engineering activities. Innovative controlled‐source cross‐well seismic monitoring surveys combined with full waveform inversion analysis enable us to map small and highly localized changes by repeatedly scanning the subsurface between borehole sensors at depth. In the Kanto Basin, Japan, we successfully monitor the dynamic transient fluid‐flow effects of the subsurface injection of microbubble water, which is of interest for soil contamination remediation and preventing earthquake liquefaction. The fluid migration is detected by observing P‐wave velocity changes (∼1%) within a very thin (∼1 m) sediment layer at a depth of ∼25 m. The injected microbubble water of the differential physical properties (temperature) is observed to follow geological and hydrologic preferential fluid‐flow paths rather than diffusing equally in all directions away from the injection well.
Publisher: Society of Exploration Geophysicists
Date: 14-05-2020
Abstract: Estimating [Formula: see text] using downgoing waves in zero-offset vertical seismic profiles (VSPs) can be challenging when scattered waves from near-borehole heterogeneities interfere with direct arrivals. In any [Formula: see text] estimation method that assumes a downgoing plane wave, constructive and destructive wave-mode interference can cause errors in the estimate. For ex le, in the spectral-ratio method, such interference modulates the litude spectra introducing significant variations and even nonphysical negative [Formula: see text] ( lification) estimates. We have investigated this phenomenon using synthetic and field data sets from offshore Australia and developed a two-step waveform-based method to characterize scattering anomalies and improve [Formula: see text] estimates. Waveform information is key to deal with closely spaced band-limited seismic events. First, we solve an inverse problem to locate and characterize scatterers by minimizing the traveltime and waveform misfits. Then, using the estimated parameters, we model the scatterers’ contribution to the VSP data and remove it from the observed waveforms. The resulting spectra resemble those that would have been acquired in the absence of the scatterers and are much more suitable for the spectral-ratio method. By assuming a 1D medium and a simple scatterer shape (i.e., circular), we parameterize a scattering heterogeneity using five parameters (depth, distance, size, velocity, and density) and seek a solution using a grid search to handle the nonuniqueness of the VSP inversion. Instead, adaptive subtraction is required to fine-tune the modeled interference to better fit the observation. We successfully use this method to characterize and mitigate the strongest wave interference in the field data. The final [Formula: see text] estimates contain milder variations and much less nonphysical negative [Formula: see text]. Our results demonstrate that the proposed method, readily extendible to multiple scatterer cases, can locate discrete scatterers, remove the effects of their interference, and thus significantly improve the [Formula: see text] estimates from VSP data.
Publisher: American Geophysical Union (AGU)
Date: 16-06-2022
DOI: 10.1029/2022GL098062
Abstract: Despite public concerns, only a few CO 2 injections into saline aquifers have reported microseismicity. We analyze passive seismic monitoring of a small (15,000 tonnes and 0.15 MPa pressure) injection of supercritical CO 2 ‐rich mixture for Stage 2C of the CO2CRC Otway Project (Victoria, Australia), which induced 19 detectable events with maximum moment magnitude M W ‐0.5. The locations and dynamic parameters of the triggered events indicate a reactivation of a small fault patch where CO 2 flowed through the fault. Time‐lapse seismic images of the plume and reservoir simulations show that the reactivation occurred when the CO 2 plume reached this fault. This might be indicative of a fault weakening by the plume that enabled subsequent reactivation by pressure variations. Our observations suggest that a leakage from a commercial‐scale storage may trigger felt seismicity in the overburden without strong overpressure, thus, the de‐risking workflows should involve a detailed study of small faults.
Location: United States of America
No related grants have been discovered for Rie Nakata.