Dual focused resistivity logging method and apparatus with dynamic range

Abstract

Method and apparatus for reducing the dynamic range of measure signals encountered in simultaneously obtaining two focused logs of formation resistivity with different depths of investigation and means for providing the required electrical connections of the electrode system. The apparatus includes an electrode system having a central electrode, a near and far pair of current electrodes, respectively shortcircuited, and aligned symmetrically on both sides of the central electrode, and a near and far pair of measure electrodes, respectively shortcircuited, and aligned symmetrically between the central electrode and the near pair of current electrodes. A signal detected on the near measure electrode pair at a preselected frequency and a first and second signal are combined to produce first and second combined voltages which are used in the production of a first and second frequency, f.sub.1 and f.sub.2, respectively. Electrode drive signals, each at a different, preselected frequency are generated, each having an amplitude functionally related to the deep resistivity measure signal with the frequencies selected to permit simultaneous measurement in a borehole of the electrical resistivities of the subsurface earth formations over relatively small and relatively large lateral distances from the wall of the borehole. The varying amplitude of the electrode drive signals acts to reduce the dynamic ranges in the measured currents and voltages impressed across the formation permitting an increase in the measurement accuracy over a smaller measurement range.