Abstract: In a well logging sonde having a sonde body and wall engaging pad, the sonde body comprises two conductive sections spaced from one another by an insulating section. The pad and sonde are maintained in a relative longitudinal position such that the focussing effect from the upper sonde section is relatively weak or negligible for current emitted from the lower pad face and relatively strong for current emitted from the upper pad face. The current is returned through the lower sonde section. Additionally, the current electrodes are provided with particular individual characteristics that are defined to optimize the signal-to-noise ratio of the respective measurements. The pad may be provided with voltage electrodes on a lower insulating region of its face to allow for correction of the measurements for the resistivity of the borehole mud. The pad may be provided with additional current electrodes by which the application of the pad to the borehole wall may be monitored.

Abstract: An electrical well logging apparatus of the spherical focusing type is suspended in a borehole drilled into an earth formation and is utilized to provide information regarding the thickness of the mudcake formed on the borehole wall.