A weighting factor used for estimating uninvaded formation resistivity from the response (R_a) recorded on a laterolog (or guard log). True resistivity can be estimated from the relationship

;#; R_a = R_xoJ + R_t(1 - J)

;#;

where R_xo = flushed zone resistivity, R_t = 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.