Abstract

A method is proposed for a time-lapse study of Carbon dioxide Capture and Storage (CCS) and Enhanced Oil Recovery (EOR) using the seismic Accurately Controlled and Routinely Operated Signal System (ACROSS) and seismometer array. However, it is considered that near-surface effects and their temporal changes caused by water-content changes, temperature changes, and surface-wave generation could significantly affect the time-lapse estimations. Near-surface effects are evaluated and reduced by comparing surface and borehole seismometer records. In addition, simulation studies evaluate the effects of near-surface effects and heterogeneities such as man-made cavities in green tuff layers (Vp ~ 2.5 km/s). Near-surface effects of changing seismometer depths are also evaluated. The results of the simulations are as follows.

During simulations of a surface source, seismic waves suffered attenuation and diffraction due to man-made cavities. This has a heterogeneous structure like that of man-made cavities when observed seismic waves are attenuated, and shows that man-made cavities become a secondary source. The waveform amplitudes of diffracted P and S phases observed with borehole seismometers are larger than those observed with surface seismometers. When a single vertical force is applied at the surface and the offset distances of seismometers are a few kilometers, the waveform amplitudes of diffracted phases observed with horizontal components of seismometers are larger than those observed with vertical components of seismometers. During simulations of a deep source, seismic waves are also diffracted by man-made cavities.

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