1. n. [Formation Evaluation]
The resistivity measured at the drill bit by a measurements-while-drilling (MWD) tool. The bit resistivity measurement responds to resistivity changes as the bit penetrates the formation, or when the time after bit is zero. It is thus an early indication of formation change. The measurement is similar to a wireline electrode device except that toroids are used instead of electrodes. A transmitter toroid induces a low-frequency current in the drillstring, which flows out of the bit and returns farther up the string. The magnitude of the current depends on the resistivity near the bit, and is measured by another toroid. The vertical resolution and depth of investigation depend on the distances between the toroids and the bit, which, in turn, depend on the type of bottomhole assembly (BHA) used. The depth of investigation is sufficient that the effect of the borehole is normally small. The measurement is unfocused and usually not borehole-corrected. Since both bit and drillstring are in physical contact with the formation, it is possible to make the measurement in oil-based muds.