Focused electrode logging system for investigating earth formations, including means for monitoring the potential between the survey and focusing electrodes

Claim 1

IN APPARATUS FOR INVESTIGATING EARTH FORMATIONS TRAVERSED BY A BOREHOLE, THE COMBINATION COMPRISING: (A) AN ELECTRODE SYSTEM ADAPTED FOR MOVEMENT THROUGH THE BOREHOLE AND INCLUDING A SURVEY ELECTRODE, AT LEAST ONE FOCUSING ELECTRODE AND AT LEAST ONE PAIR OF VOLTAGE MONITORING ELECTRODES LOCATED THEREBETWEEN; (B) FIRST MEANS FOR SUPPLYING AN ALTERNATING CURRENT SIGNAL; (C) SECOND MEANS RESPONSIVE TO THE DIFFERENCE BETWEEN THE POTENTIAL OF THE SUPPLIED SIGNAL FROM THE CURRENT SUPPLYING MEANS AND THE POTENTIAL AT A POINT IN THE VICINITY OF SAID AT LEAST ONE PAIR OF MONITOR ELECTRODES FOR SUPPLYING SUFFICIENT CURRENT TO AT LEAST A FIRST ONE OF THE ELECTRODES FOR MAINTAINING THE POTENTIAL IN THE VICINITY OF SAID AT LEAST ONE PAIR OF MONITOR ELECTRODES SUBSTANTIALLY THE SAME AS THE POTENTIAL OF THE SUPPLIED SIGNAL; (D) THIRD MEANS RESPONSIVE TO THE DIFFERENCE IN POTENTIAL BETWEEN SAID AT LEAST ONE PAIR OF MONITOR ELECTRODES FOR SUPPLYING CURRENT BETWEEN AT LEAST A SECOND ONE OF THE ELECTRODES AND A POINT HAVING A POTENTIAL APPROXIMATING THE POTENTIAL ON SAID AT LEAST A SECOND ONE OF THE ELECTRODES; AND (E) MEANS RESPONSIVE TO THE CURRENT SUPPLIED TO AT LEAST ONE OF THE ELECTRODES FOR PROVIDING AN INDICATION OF THE CONDUCTIVITY OF THE SURROUNDING EARTH FORMATIONS.