DUAL FOCUSED LOG HAVING ALTERNATELY ENERGIZED FOCUSING APPARATUS THEREIN

摘要

Methods and apparatus for simultaneously obtaining two focused logs of formation resistivity with different depths of investigation. A pair of nearby focusing current return electrodes is added to the subsurface apparatus of a conventional focused logging system and a second focusing current supply is added to the surface instrumentation. The preferred embodiment uses commutator devices to effect, alternately and repetitively, two different interconnections of the subsurface electrodes with the surface equipment whereby two logs are recorded on a single traverse of the borehole by the subsurface instrument. The two logs are combined to obtain a compensated focused current log representing improved measurements of the true resistivities of the formations. All subsurface elements of one focused logging system, except its focusing current return electrode, are time-shared with the other focused logging system.