a method of determining the permeability of a particular stratum in an earth formation traversed by a borehole includes injecting a liquid into the borehole at a first pressure thereby causing liquid flow into the stratum . a first flow rate of the liquid is determined at the first pressure . the pressure of the liquid being injected into the borehole is then changed to a second pressure level and a second flow rate of the liquid flowing into the stratum is determined at the second pressure . an indication of the permeability of the stratum is then derived in accordance with the two pressures , the two flow rates and known characteristics of the stratum .