(1) A mathematical relationship between two values, where one value is divided by the other, commonly expressed as a fraction.

(2) In [[Thermal Decay Time Log|Thermal Decay Time Logging]]. A measurement recorded with the [[Dual Spaced Thermal Decay log|Dual-Spacing Thermal Decay Time]] tool which is derived from the counting rates of the two [[detector|detectors]]. The ratio measurement has characteristics similar to those of a [[neutron log]] and for specific cases can be calibrated to serve as such. Used with the measurement of the [[Thermal Decay Time]] tool, it provides an apparent [[porosity]] and an apparent [[formation water]] [[salinity]], useful for cased-hole [[formation evaluation]].

(3) With other dual-spaced [[radiation]] detector systems, it is the count-rate ratio (of near to far detector) sometimes recorded with two-detector tools such as the [[compensated neutron log]].