1. n. [Formation Evaluation]

With reference to log analysis, the resistivity of the undisturbed formation. It is derived from a resistivity log that has been corrected as far as possible for all environmental effects, such as borehole, invasion and surrounding bed effects. Hence, it is taken as the true resistivity of the undisturbed formation in situ and is called R_t.

See: in situ,  water-filled resistivity

2. n. [Formation Evaluation]

With reference to core analysis, the resistivity of a sample only partially filled with water. Called R_t, it is used in contrast to the resistivity of a sample completely filled with water, R_o. The water may be replaced by any nonconductive fluid, usually air or dead oil.

See: dead oil