spherical focusing

1. n. [Formation Evaluation]
A technique for focusing an electrode device based on maintaining a spherical equipotential surface centered at the main current electrode. Unlike the laterolog, which tries to maintain equipotential lines parallel to the sonde, spherical focusing tries to maintain the spherical equipotential lines that would exist in a homogeneous formation with no borehole. This is achieved with a particular arrangement of current-emitting electrode, current-return electrodes and monitor electrodes. This arrangement creates two spherical equipotential spheres with a constant voltage drop between them. The resistivity is determined from the current flowing between the spheres and the voltage drop. The depth of investigation is determined by the radii of the two spheres. Spherical focusing is used to produce shallow-reading resistivity logs and the pad-based microspherical log.