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\section{Ultrasonic measurements}  Laboratory measurements on core samples under CO$_2$ flooding can be used to calibrate seismic data. Most experiments reported in the literature so far imply injection of CO$_2$ in a porous media pre-saturated with another fluid and the acoustic variations observed are usually a combination of pore pressure and fluid substitution effect \citep{Wang1989,Xue2005,Lei2009,Purcell2010,Shi2011,Ivanova2012}. However, it is difficult to apply the results of those studies to theseismic  interpretation of seismic data  since observed velocity changes due to CO$_2$ solely cannot be quantitatively determined as the CO$_2$ partial saturation of the injected fluid is generally unknown. \citet{Yam2011b} and \citet{Njiekak2013} investigate the seismic effects associated with the different phase states of CO$_2$ by measuring the ultrasonic response of Berea sandstone and samples from Weyburn-Midale geologic project, fully saturated with CO$_2$ at different temperatures and pressures. In both works, particular attention has been given to the separation of the seismic effect due to changes in the pore fluid, from those induced by the pore pressure build-up. \citet{Lebedev2013} tested the effects of supercritical CO$_2$ injection into sandstones from the Otway Basin on acoustic responses. For this study, laboratory measurements were carried out at the Experimental Geophysics Group laboratory (Institute for Geophysical Research, Department of Physics, University of Alberta) on two representative core samples of the Potsdam Group, following the approach proposed by \citet{Yam2011b} and \citet{Njiekak2013}.