Basics of Resistivity Tools
Resistivity logs use electrode arrays or coil arrays for focusing. The accuracy of the measurement is enhanced by designing the array to focus into the zone of interest.
The most significant difference between laterolog and induction devices is that laterologs put the borehole fluid, the invaded zone, and the undisturbed zone in series in the radial direction of the borehole as regards current flow, whereas the induction devices put these regions in parallel. If the injected currents are constant, the measured potentials for laterolog tools increase proportionally with resistivity. The eddy current induced in the formation and the electric potential measured by induction devices will increase as the conductivity increases (i.e., as the resistivity decreases). This chapter briefly describes laterolog and induction tool response theories.