The injection of CO\(_2\) causes a fluid substitution within the pore space. For fluid substitution with no change in matrix properties, a change in \(P\)-wave velocity is expected due to the change in bulk modulus (compressibility), with a minimal change in \(S\)-wave velocity expected due to the lack of change in shear modulus (which is a property not affected by pore fluid). \(P\)-wave velocities (Fig. \ref{fig:results_lab} - top left) show an initial decrease in the pore pressure range 2-7 MPa for all runs and for the two samples. This is consistent with the CO\(_2\) velocity behavior (Fig. \ref{fig:velocityco2}) where the lower velocities are registered around the critical point when the CO\(_2\) is a supercritical fluid (7.5 MPa and 31.8 \(^{\circ}\)C). Once the phase transition is crossed, velocities increase with pore pressure for both runs. This trend is far more pronounced for the CH sample than for the CA sample. The almost lack of \(S\)-wave velocity changes confirms that the observed \(P\)-wave change is caused by fluid substitution of gaseous CO\(_2\) to liquid/supercritical CO\(_2\).