1. n. [Formation Evaluation]
The resistivity measured by the ring of a measurements-while-drilling (MWD) toroid device. The ring resistivity is a focused measurement with a depth of investigation and a vertical resolution of a few inches. It is not azimuthal. The measurement is similar to a wireline laterolog except that toroids are used instead of electrodes for transmitting and monitoring. Like a laterolog, the signal is proportional to resistivity, and is thus most effective at high resistivities, high formation to mud-resistivity contrast and in the presence of conductive invasion. It is usually combined with the bit resistivity, and possibly also with the button resistivities. In the same way as a laterolog, two transmitter toroids are used to force current to flow approximately perpendicularly into the formation at the ring. Other toroids measure the current flow and to balance the currents emitted by the two transmitters.