Schematic representation of the |$\mathrm{CO_2}$|-water mixing unit designed for pore pressure control. Key components include pressure pumps P2 and P3. Circles with directional arrows represent pressure transducers, while numbered indicators denote needle valves. Valves adjacent to the gas bottles incorporate regulators. ‘BPV’ stands for back pressure valve. Four-way connections are symbolized by rectangles, and 316 steel tubing are depicted as in colours. C1 connects to theX-ray transparent low-frequency apparatus’s inlet, and c2 to its outlet. Confining pressure regulation via P1 is detailed in Fig. 2.