1. n. [Formation Evaluation]

A logging tool based on an arrangement of simple metallic electrodes working at low frequency (less than 500 Hz). The term includes conventional electrical logs, laterologs, micrologs and other microresistivity logs. Electrode devices are used for both wireline and measurements-while-drilling logs. In all electrode devices, a current (I_O) and a voltage (V_O) are measured on the appropriate electrodes or combinations of electrodes. The apparent formation resistivity is then determined by: R_a = K V_O / I_O, where K is a system constant for the device concerned.

See: departure curve,  electrical log,  electrode resistivity,  microlog,  wireline log