An underground gas blowout occurred in 1989 during a well drilling in North Sea. As a result, gas has migrated to shallower layers ( ). Repeated 2D lines were acquired as part of a site survey. We ultimately aim at estimating the gas saturation and rock frame properties for a shallow sand layer (480 meters deep) where gas has accumulated. To serve this objective, we developed a two-steps inverse approach: first, we obtain the seismic P-wave velocities (before and after blowout) using seismic full waveform inversion (FWI). The acoustic FWI was applied in the time domain ( ). We downscale these macroscale data to microscale porous medium properties. First, we estimate frame properties of the reservoir rocks using the baseline’s inverted velocity model and, then, we estimate the gas saturation change using the monitor’s velocity model. The Biot-Gassmann’s ( ) relations are the basis for the downscaling of poroelastic parameters. For partially saturated media, these equations are generalized using frequency-dependent moduli ( ). We present the forward model equations which are the basics of the inverse downscaling method. Then, we show the data obtained by time-lapse FWI which are used in the downscaling process. Finally, the results of the downscaling for baseline and monitor models are presented.


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