Plural focusing electrode systems for measuring the dip of substurface strata

Claim 1

A WELL LOGGING APPARATUS FOR DETERMINING THE RESISTIVITY OF SUBSURFACE FORMATIONS INTERCEPTED BY A BORE HOLE, AT LEAST ONE ELECTRODE SYSTEM LOCATED IN SAID BORE HOLE AND INCLUDING A CENTER ELECTRODE, AT LEAST ONE FOCUSING ELECTRODE AND A COLLECTOR ELECTRODE, A SURFACE STATION INCLUDING A SOURCE OF CONSTANT CURRENT, A CABLE INTERCONNECTING SAID SURFACE STATION AND SAID ELECTRODE SYSTEM, MEANS FOR APPLYING A CONSTANT CURRENT FROM SAID SOURCE TO SAID CABLE, A RESISTOR CONNECTED BETWEEN SAID COLLECTOR ELECTRODE AND THE OTHER ELECTRODES OF SAID ELECTRODE SYSTEM AND ELECTRICALLY IN PARALLEL WITH THE RESISTANCE OF THE FORMATIONS TO BE INVESTIGATED SAID RESISTOR HAVING AN IMPEDANCE SUBSTANTIALLY SMALLER THAN THE SAID RESISTANCE OF THE FORMATIONS TO BE INVESTIGATED, MEANS FOR APPLYING THE CONSTANT CURRENT ON SAID CABLE THROUGH SAID RESISTOR IN PARALLEL WITH SAID RESISTANCE OF THE FORMATIONS, A TRANSFORMER HAVING A LOW IMPEDANCE PRIMARY WINDING AND A SECONDARY WINDING, MEANS CONNECTING SAID PRIMARY WINDING BETWEEN SAID CENTER ELECTRODE ONLY AND SAID RESISTOR AND MEANS FOR TRANSMITTING A SIGNAL PROPORTIONAL TO THE VOLTAGE DEVELOPED ACROSS SAID SECONDARY WINDING TO SAID SURFACE STATION.