A substantially cylindrical sample of a porous material is surrounded by an elastic jacket and placed in a confining pressure cell. Pressure is supplied to the cell to press the jacket into contact with the surface of the sample. The pressure is varied over a plurality of pressure points and is scanned at a plurality of locations with X-rays at each such pressure point. A computed tomographic image is produced for each X-ray scan. Conformance of the jacket to the sample is determined from these computed tomographic images and a range of confining pressure is determined over which parameters of the sample may be measured without being affected by improper conformance of the jacket to the surface of the sample. The sample structure is monitored with these tomographic images for fractures produced due to the increase of pressure. The fracture starting point is determined to be interior or surface related.