Signal amplitude (Fig. \ref{fig:results_lab} - bottom) shows a rapid decrease in the pressure range 5-7 MPa and once the phase transition is crossed, the decrease is smoother. This trend is valid for CH sample for both \(P\)- and \(S\)-wave signal, however, for the CA sample, the amplitude decreases steadily and there is no detected transition phase.
Both samples being very tight, variations in velocity and signal amplitude are rather small. Nevertheless, the CH sample shows a larger sensitivity to CO\(_2\) phase change compared to the CA sample. This is attributed to the larger porosity and permeability values of the former sample (Table \ref{tbl:prop}). In the next section, we explore how the higher sensitivity of the CH unit affects the time-lapse seismic response for a VSP monitoring program.