A [[resistivity]] log (run in uncased hole filled with electrically [[conductivity|conductive]] [[mud]]) made with a [[tool]] that achieves focusing through the use of additional current [[electrode|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 [[bed|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.

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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]].