Apparatus and method for measuring the properties of rocks such as permeability, porosity and fluid properties downhole around a borehole (3) in which a seismic source (6) which radiates a seismic signal substantially radially within the borehole is lowered down the borehole (3) and the electrical signal generated by the seismic signal within the surrounding rock is detected by detectors (4, 5) mounted above and below the seismic source (6) so that signals are received from substantially all radial directions. Preferably a continuous wave signal is generated and the signal detected whilst the apparatus (1) is raised or lowered. The apparatus (1) gives a much better and more accurate measurement than previous methods.