Abstract
Seismic ACROSS (Accurately-Controlled Routinely-Operated Signal System) enables us to continuously monitor the temporal change of the amplitude and/or travel-time of particular seismic reflectors or diffractctors from a time-variant region (e.g., oil-gas /CO2/aquifer reservoirs and a focal region) because of very accurate-and-stable seismic signals. The seismic ACROSS controlled by GPS clock is fixed type source which can generate 10-50Hz and 40 ton-f at 50Hz. To evaluate the seismic characteristics before and after the change and optimized source-receiver array design and to image the time-variant target region, we investigated the following cases with a velocity change of (i) 10 % in a CO2 reservoir with a 300-m-wide and 100-m-thick at 200m-depth, and (ii)~30 % in a deep-slip zone with a 10-km-long and 200-m-thick at 30-km-depth, by applying a imaging method using FDM back-propagating residual waveforms. We can robustly obtain the location and shape of the targets by using the velocity structure before the change even if a single source or the change in a near surface layer. We finally concluded that ACROSS technology is very useful to monitor the temporal change of physical properties in the time-variant regions with variable scale and depth.
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