Amplitude modulated resistivity logging system

Abstract

In the preferred and illustrated embodiment of a downhole oil well resistivity logging tool having a focused electrode, the current ideally flows from the electrode radially outwardly in a thin sheet deep into the earth's formation adjacent to the well. There is an undesirable vertical current component along the electrodes which distorts the measurement of resistivity. First and second alternate embodiment are set forth which utilize additional balanced electrodes in the electrode array. They establish a vertical current in the adjacent formation. The vertical current is varied by means of an oscillator connected to the balanced electrodes, and the voltage of the electrodes is monitored to detect zero crossing. Zero crossing is indicative of reduced vertical current flowing in the formation. At this instant, the entire current flow from the electrode array is radially into the formation whereby measurements of the current flow and voltage of the current emitting electrode enables calculation of the formation resistivity.