A method and apparatus for determining the fluid permeability of an earth formation is described. An instrument comprising an acoustic source and acoustic and electrokinetic sensors is located in a borehole at a site of interest. The acoustic source is fired and the resulting variations in pressure and electric field generated are measured and recorded. The Biot equation, and an equation for the streaming potential coupled to fluid displacement terms, is solved using finite element analysis to give an expected pressure waveform and electrokinetic waveform based on estimated earth formation properties. By comparison of the measured waveforms and those calculated from the equations, the actual permeability of the earth formation can be deduced.