When a static magnetic field and a crossed oscillating field are applied, the deformation rate at a boundary between a fluid and a porous medium depends on the amplitudes of the applied magnetic fields (to be exact, on their product), electric conductivity, porosity, and permeability. Knowing two of the three enables determination of the third. It is emphasized that this abstract is provided to comply with the rules requiring an abstract which will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. 37 CFR 1.72(b).