Time lapse seismic data has been acquired for the monitoring of an underground blow-out. Using time-lapse FWI techniques, we obtain P-wave velocity images for baseline (before the blow-out) and monitor models (after). The difference between both velocities is about 100 m/s in a 480 m deep sand layer. Using poroelastic Biot-Gassmann based relations and a semi-global optimisation method, we estimate the frame properties of the sand layer for the baseline model. We obtain a range of values for the porosity, the bulk and shear dry moduli. Then, we estimate the gas saturation in the sand layer due to the blow-out using patchy saturation and Brie models. The gas saturation is low, between 0.5 and 3.5 % depending on the rock physics model. Nevertheless, the baseline poroelastic model (values of porosity and moduli) has a very low influence on the gas saturation estimation.


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