Long Duration Time-lapse Experiment in Al Wasse, Saudi Arabia Using an Ultra-stable Seismic Source

We carried out a long-duration seismic time-lapse experiment consisting of two periods in 2012-2013 and 2015 in a water pumping field in the Kingdom of Saudi Arabia using an ultra-stable ACROSS seismic source and a geophone array. The comparison of travel times and amplitudes of P waves show quite small but distinct temporal variation with time. The maximum change is approximately 1.5 ms during two months both in the first and the second periods. Because the total travel time is 0.2 s, the change of 1.5 ms corresponds to 0.75%. The resolution of travel-time change is ~0.1 ms and it is good resolution to detect the change of subsurface caused by CO2 or high-temperature H2O injection to the heavy oil reservoir. We also discuss the NRMS repeatability of the observation system using the first arrivals at the stations within 700 m from the source and found that the NRMS repeatability was better than 5% during 2 month periods. The source itself might have roughly 2% NRMS repeatability. The NRMS repeatability is based on the waveform change. The present result supports that the ACROSS source has excellent repeatability well enough to discuss the time lapse of the subsurface.