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\subsection{Experimental apparatus and procedure}  The experimental setup included several components such as the pressure vessel to encase and apply an hydrostatic confining pressure to core sample, a heat tape wrapped around the pressure vessel, tanks as pore fluids source, independent Quizix\textsuperscript{\texttrademark} dual cylinder pumping system for regulating confining and pore pressure, a pulse generator and a digital oscilloscope for recording waveform. Some of this elements are shown on Fig. \ref{fig 2}. \ref{fig2}.  The sample consists of a cylinder which ends are polished and made as parallel as possible. The sample is then dried under vacuum at modest temperatures ($\sim$70 $^{\circ}$C). It is prepared for the measurement suite by attaching the set of ultrasonic transducers to its ends.The sample-transducers assemblage (Fig. \ref{fig 2}) \ref{fig2})  is then hermetically sealed to prevent any contamination by the hydraulic fluid once in the pressure vessel. The assemblage is then placed inside the pressure vessel (Fig. \ref{fig 2}) \ref{fig2})  in a cylindrical cavity filled with hydraulic oil. The hydraulic oil serves as the pressurizing medium for providing hydrostatic confining pressure on the sample. The CO$_2$ is introduced into the sample via stainless steel tubing that connects the pore space of the sample with the pore fluid reservoir located outside of the vessel. The transmitted seismic signal was generated by triggering the transmitting transducer with a square wave using a pulse generator. For detailed description of the apparatus see \citet{Njiekak2013} and \citet{Yam2011}.