1. n. [Formation Evaluation]
The electromagnetic force created by two different metals in contact with each other. If two such metals are in contact in a logging tool, and also communicate along a conductive borehole, then a potential drop is generated in the borehole. This potential drop will appear on the spontaneous potential (SP) log, where it can be confused with the electrochemical potential. Since the magnitude of the drop depends on the formation resistivity, the effect of bimetallism is often seen as a resistivity log superimposed on the normal SP. Under usual circumstances, the effect of bimetallism on the SP is small, and care is taken to avoid it.