SPWLA Glossary

A type of microresistivity log recorded from a tool that uses three button electrodes spaced 1 in. apart in a line, located on a pad that is pressed against the formation wall. The lower electrode is the A current electrode. The potential of the upper electrode with respect to a remote reference electrode gives a 2 in. micronormal, and the difference between the two upper electrodes gives a 1.5 in. microinverse (lateral type) measurement. Because mud cake usually has appreciably lower resistivity than the formation, the microinverse will read less than the micronormal when mud cake is present. This difference (called positive separation) usually indicates a permeable formation. A caliper curve is usually recorded at the same time.