A resistivity log (run in uncased hole filled with electrically conductive mud) made with a tool that achieves focusing through the use of additional current electrodes above and below a central measure-current electrode. Bucking currents from the additional electrodes serve to confine the measure current to essentially a narrow disc of current flowing outwardly perpendicular to the sonde. Should the survey current tend to flow vertically in the mud column (because of highly resistive beds), currents from the symmetrically positioned bucking electrodes are caused to increase or decrease in order to maintain the horizontal attitude of the survey-current flow.

The currents from the bucking electrodes are automatically adjusted for proper focusing of the measure-current beam by use of a monitor voltage signal, either from monitorelectrode pairs on either side of the measure-current electrode, or from the voltage difference between bucking and measure electrodes. Because of the comparatively small thickness of the focused sheet of current (which is usually a few inches to a few feet thick. the laterolog gives a very detailed curve and puts clearly in evidence the sharp contrasts between successive beds, however conductive the mud may be.

The laterolog tool differs from the guarded electrode logging tool, or guard tool, in the use of smaller-sized electrodes and the use of a monitored bucking-current system to achieve focusing. Compare guard tool.