Apparatus and methods for measuring formation characteristics in presence of conductive and non-conductive muds

Abstract

Tools for investigating an earth formation in the presence of either oil-base or water-base mud include a circuit which induces current into the formation in a focused manner, and a sensor electrode which is isolated from the circuit, at least partially surrounded by the circuit, and located in the area in which the current is focused. The current inducing circuit may be an oppositely polarized double-dipole circuit to which voltage is applied with the two dipoles sharing a common radiating element over which a focused area is formed. Insulators separate the outer electrodes of the double-dipole circuit from the common radiating element. Alternatively, the current inducing circuit is a coil arranged in a partial toroidal configuration in an insulated area that at least partially surrounds the focused area and which separates the focused area from return electrodes.