The geometry-dependent weighting factor for determining how the conductivities of each medium in the vicinity of an induction logging tool affect the apparent conductivity measurement. The only well-logging devices for which this concept is sound are the induction tools, because when Rxo > Rt, only with these tools is the measuring geometry independent of variations in Rxo/Rt.
When conductivities are not high, skin effect may be neglected, and the response of induction logs can be described in terms of conductivities and “geometrical factors” of the volumes surrounding the tool. The geometrical factor, (G), of a volume having a specific geometrical orientation with the sonde is simply the fraction of the total signal that would originate with that volume in an infinite homogeneous medium. For computation of geometrical factor to be practical. it is necessary to assume that the volumes conform to symmetry of revolution about the sonde.
The magnitude of the signal in conductivity units is the product of the geometrical factor and the conductivity of the material, and the total signal sensed by the tool is the sum of these products for all volumes within range (which extends to infinity, but can be circumscribed to practical limits).
Since the G's add up to unity by definition, this can be stated:
;#; CIL = C1G1 + C2G2 + C3G3 . . . + CNGN ;#;
where the C's and G's refer to the zones of differing conductivity, and N is the total number of such zones.
The chief significance of this concept is the fact that a volume of space defined only by its geometry relative to the sonde has a fixed and computable geometric factor. That permits the construction of mathematically sound correction charts to account for the effects of borehole mud, the invaded zone, and adjacent beds on the R measurement, providing symmetry of revolution exists. Compare pseudo-geometrical factor.