SFL. A [[log]] of formation [[resistivity]] measured by a [[tool]] developed for the limited investigation of the [[invaded zone]].

Focusing is used to enforce an approximately spherical shape on the equipotential surfaces within the [[formation]] in spite of the presence of the borehole. Borehole effect is virtually eliminated for hole diameters up to 10 in., yet investigation of the tool is kept shallow enough that its response is, in the majority of cases, mostly from the [[invaded zone]]. SFL is a mark of Schlumberger.

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