In order to estimate the physical property changes in the subsurface, the time-lapse measurement is quite useful for various cases such as CCS (Carbon Capture and Sequestration) zone, shale gas exploration, oil production, EOR (Enhanced Oil Recovery) and rock mines. We carried out the time-lapse experiments in some weeks from August in 2013 to February 2014. In this study, we used an improved conventional electromagnetic vibrator in the green tuff mining area with 3 km in EW × 5.5 km NS in Japan.
We obtained Vp and Vs structure down to 200 m depth using VSP. The comparison of seismic records by walkaway and fixed source at the source B indicates large surface wave arrivals at surface, but borehole seismometers do not indicate distinct surface wave arrivals.
We calculated transfer functions from the fixed source at A or B to all 126 seismic stations buried at 30 m and two borehole seismometers at 70 m and 200 m. The temporal changes in the original transfer functions were not distinct. If we look the 12 hours waveforms, it is difficult to find the change of those with time. If we calculate residuals waveforms from the first day, we can clearly identify the change of waveforms with time. Very large temporal change is identified at F11 station and this waveform change travel with 300m/s possibly due to surface wave origin or sound wave in air.
Other temporal changes are identified. The transfer functions every hour during December show large waveform changes even in 200m deep borehole seismometer. The average velocity for this phase is 250m/s and the temporal change is distinct during the midnight of observation suggesting possibly caused by frozen of near surface layer.
