1. n. [Formation Evaluation]
An unfocused electrode device with small spacings, mounted on a pad and pressed against the borehole wall. The typical microlog has one current-emitting electrode and two measure electrodes in line above it, one at 1 in. [2.5 cm], the other at 2 in. [5 cm]. The potential at the 2-in. electrode gives a 2-in. micronormal log. The difference in potential between the two measure electrodes gives a 1-in. x 1-in. microinverse log. The micronormal reads deeper than the microinverse. Introduced in 1948, the microlog is used to detect permeable zones across which a mudcake has formed. Since the mudcake is usually less resistive than the invaded zone, the microinverse will read less than the micronormal opposite permeable zones. If the resistivity and thickness of the mudcake are known, it is possible to estimate the resistivity of the flushed zone. The log is usually presented on a linear scale, chosen to emphasize the lower readings often seen opposite permeable zones with mudcake.