(n.) Formation Evaluation. An electrode device with small spacings from which the current flow, and hence the measurement, is focused a short distance into the formation. The proximity log measures the resistivity of the flushed zone with minimum influence from themudcake or the undisturbed zone. The central current-emitting electrode (A0) is surrounded by a guard electrode that emits sufficient current to focus the current from A0 a certain distance into the formation. The electrodes are mounted on a pad that is pressed against the borehole wall. In a typical tool design, 90% of the signal comes from within 5 to 10 in. [13 to 25 cm] of the pad. This is deeper than the microlaterolog, which ensures that the mudcake has less effect but means that the proximity log is more often affected by the undisturbed zone. See: Electrode Resistivity, Flushed Zone, Microresistivity, Undisturbed Zone