Migration of COud2udthrough storage reservoirs can be monitored using time lapse seismicudreflection surveys. At the Sleipner Field, injected COud2udis distributed throughout nine layers within theudreservoir. These layers are too thin to be seismically resolvable by direct measurement of the separationudbetween reflections from the top and bottom of each layer. Here we develop and apply an inverse methodudfor measuring thick ness changes of the shallowest layer. Our approach combines differences in traveltimeuddown to a specific reflection together with amplitude measurements to determine layer thicknesses fromudtime lapse surveys. A series of synthetic forward models were used to test the robustness of our inverseudapproach and to quantify uncertainties. In the absence of ambient noise, this approach can unambiguouslyudresolve layer thickness. If a realistic ambient noise distribution is included, layer thicknesses of 1–6 m areudaccurately retrieved with an uncertainty of ±0.5 m. We used this approach to generate a thickness mapudof the shallowest layer. The fidelity of this result was tested using measurements of layer thicknessuddetermined from the 2010 broadband seismic survey. The calculated volume of COud2udwithin the shallowestudlayer increases at a rate that is quadratic in time, despite an approximately constant injection rate into theudbase of the reser voir. This result is consistent with a diminished growth rate of the areal extent of underlyingudlayers. Finally, the relationship between caprock topography and layer thickness is explored and potentialudmigration pathways that charge this layer are identified.
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