Sensitivity distributions of different borehole electrode configurations considering a model with a cylindrical coaxial boundary

Abstract

The sensitivity distributions of different electrode configurations are computed for both a homogeneous resistivity distribution and a model consisting of two vertical zones of homogeneous resistivity. The inner zone around the borehole axis represents a borehole filled with mud and the outer zone is the undisturbed formation. The sensitivity of the homogeneous model is independent of resistivity. Whereas the sensitivity in the case of the cylindrical coaxial boundary depends on the contrast between formation resistivity and mud resistivity. With increasing contrast, the sensitivity distribution changes dramatically for all investigated electrode configurations. The sensitivity patterns are used to illustrate the ability of different electrode configurations to delineate thin layers. The superiority of focused tools in comparison to normal logs can clearly be shown if the effect of variable bucking currents is included.