The process of recording the formations traversed by a drill hole, based on the measurements of two basic parameters observable in uncased holes; namely, the spontaneous potential and the resistivity of the formations to the flow of electric currents. The detailed study in situ of the formations penetrated by a drill hole, based on 1058 measurements made systematically by lowering an apparatus in the hole responding to the following physical factors or parameters: (1) the resistivities of the rocks; (2) their porosity; (3) their electrical anisotropy; (4) their temperature; and (5) the resistivity of the drilling muds.