Monitoring a shallow subsurface gas flow by time-lapse refraction analysis

Abstract

Studying time-lapse time shifts for refracted events is a complementary method to conventional 4D analysis. Especially for detecting changes in relatively shallow subsurface layers, we find that this method has potential. By studying data associated with an old underground blowout, we find consistent time shifts of up to 4 ms when comparing a base survey that was acquired before the underground blowout with a monitor survey acquired 20 months after the blowout. These time shifts are interpreted as caused by shallow gas migration. The 4D refraction time shifts are estimated by a standard windowed crosscorrelation technique. Observed time shifts are compared with a simple synthetic modeling to find the possible property and location of the anomaly. The synthetic result was qualitatively matched with real data. We believe that more quantitative matches can obtained when the method is combined with full waveform inversions. Time-lapse refraction analysis has some clear limitations, for instance, the presence of refracted events, which requires a positive velocity contrast for the interface to be studied. Another limitation is that for deeper events, the signal from shallower refractions and noise associated with the water layer will distort the 4D time shifts. However, we found that these results demonstrate the potential of refraction time shifts as a complementary 4D analysis technique. We believe that the method has a significant potential for quantitative 4D interpretation.