Schematic representation of the imbibition-drainage mixing unit designed for pore pressure control. Key components include pressure pumps P2 and a gas bottle filled with air. 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 is depicted as coloured lines. C1 connects to the X-ray transparent low-frequency apparatus’s inlet, and c2 to its outlet.