A weighting factor used for estimating uninvaded [[formation resistivity factor|formation resistivity]] from the response (R<sub>a</sub>) recorded on a [[laterolog]] (or [[guard log]]). [[True resistivity]] can be estimated from the relationship

;#;
R<sub>a</sub> = R<sub>xo</sub>J + R<sub>t</sub>(1 - J)

;#;


\\
where R<sub>xo</sub> = [[flushed zone]] resistivity, R<sub>t</sub> = uncontaminated zone resistivity, and J = pseudo-geometrical factor, a function of invasion depth. It must be emphasized that a pseudogeometrical factor relating to an electrode-type resistivity device is applicable in only one set of conditions, and therefore charts of this type are not valid as general-purpose invaded-zone correction charts. The most useful feature of such a chart is the graphic comparison it allows of the relative contrihution of invaded zones to the responses of the various tools. Compare [[geometrical factor]].