Abstract: A method for determining the permeability profile of an uncased borehole penetrating a subterranean formation by a natural gamma ray well logging technique in which particles containing gamma ray emitting nuclides are added to a thixotropic drilling mud used to drill the wellbore such that a radioactive filter cake of the particles forms along the face of the wellbore having substantially increased thickness and intensity of radiation emissions at depths corresponding to zones of high permeability in the subterranean formation. Preferably the additive is a mud weighting particle, such as barite, bearing adsorbed or mechanically mixed, naturally-occurring radioactive material, such as thorium-bearing bastnasite or thorium-bearing cerium oxide.