Differential waveform tomography for time‐lapse crosswell seismic data with application to gas hydrate production monitoring

To detect the change of physical properties in small areas, a series of high‐resolution waveform inversions is applied to time‐lapse seismic data. A procedure by use of differentiation between the time‐lapse data and normalization using reference data is proposed in this study. The procedure is derived as a straight‐forward extension of waveform inversion as the scatterer imaging. Through numerical tests, the proposed approach was found to be more accurate than the conventional approach in obtaining the velocity change in small areas. The method was applied to the time‐lapse crosswell seismic data obtained during the Mallik 2002 gas production test. A small area showing a velocity decrease near the production zone were found using the proposed method, indicating the existence of dissociated methane gas in the sand layers.