1. n. [Formation Evaluation]

A measurements-while-drilling log of formation resistivity. The log normally contains at least one attenuation and one phase-shift resistivity reading. In many cases there will be multiple curves of both, the difference being the depth of investigation. For the same nominal depth of investigation, the attenuation resistivity reads deeper than the phase-shift resistivity and is less affected by invasion, but more affected by surrounding beds and apparent dip. The attenuation measurement has a poorer vertical resolution and is less affected by anisotropy. Depths of investigation and vertical resolution of both measurements vary with the average formation resistivity. Although depths of investigation are less than with wireline resistivity logs, the invasion at the time of measurement is usually small and it is possible to derive the resistivity of the undisturbed zone.

See related terms: attenuation resistivity,  depth of investigation,  dielectric resistivity,  electromagnetic propagation,  horizontal resistivity,  phase-shift resistivity,  propagation resistivity measurement,  resistivity log,  undisturbed zone,  vertical resistivity,  vertical resolution