Electrical apparatus for well logging

Claim 1

IN APPARATUS FOR INVESTIGATING EARTH FORMATION TRAVERSED BY A BORE HOLE, THE COMBINATION OF AN ELECTRODE ARRAY MOUNTED FOR MOVEMENT THROUGH A BORE HOLE AND INCLUDING A CENTRAL ELECTRODE AND FIRST, SECOND, AND THIRD ELECTRODE MEANS EACH INCLUDING PORTIONS SPACED APART IN OPPOSITE DIRECTIONS FROM, AND SYMMETRICALLY DISPOSED ABOUT A BORE HOLE RADIUS THROUGH SAID CENTRAL ELECTRODE, INSULATED MEANS FOR ELECTRICALLY CONNECTING THE PORTIONS OF EACH SAID ELECTRODE MEANS, THE DISTANCES BETWEEN THE SPACED APART PORTIONS OF THE RESPECTIVE ELECTRODE MEANS BEING DIFFERENT, FIRST FEEDBACK MEANS SUPPLYING A FIRST CURRENT TO THE SPACED APART PORTIONS OF SAID THIRD ELECTRODE MEANS AND TO A REFERENCE POINT AT GROUND POTENTIAL AND RESPONSIVE TO A FIRST POTENTIAL DIFFERENCE BETWEEN A REFERENCE POINT AT GROUND POTENTIAL AND THE SPACED APART PORTIONS OF ONE OF SAID FIRST AND SECOND ELECTRODE MEANS FOR MAINTAINING SAID FIRST POTENTIAL DIFFERENCE SUBSTANTIALLY CONSTANT, SECOND FEEDBACK MEANS RESPONSIVE TO A SECOND POTENTIAL DIFFERENCE BETWEEN SAID FIRST AND SECOND ELECTRODE MEANS FOR SUPPLYING A SECOND CURRENT TO SAID CENTRAL ELECTRODE AND TO A REFERENCE POINT AT GROUND POTENTIAL TO REDUCE SAID SECOND POTENTIAL DIFFERENCE SUBSTANTIALLY TO ZERO, AND MEANS PROVIDING INDICATIONS OF THE CURRENT EMITTED BY SAID CENTRAL ELECTRODE AS REPRESENTATIVE OF AN ELECTRICAL PROPERTY OF THE FORMATIONS IN THE VICINITY OF SAID CENTRAL ELECTRODE.