Electrical well logging apparatus and method in which the measuring current density is controlled a substantial distance from the borehole

Claim 1

IN A METHOD FOR INVESTIGATING THE ELECTRICAL RESISTIVITIES OF EARTH FORMATIONS TRAVERSED BY A BOREHOLE, THE STEPS COMPRISING PASSING CURRENT BETWEEN A FIRST LOCATION IN THE BOREHOLE AND A POINT REMOTE FROM SAID FIRST LOCATION SO AS TO ESTABLISH AN ELECTRICAL FIELD IN THE EARTH FORMATION LATERALLY OF THE FIRST LOCATION, PASSING CURRENT BETWEEN A SECOND LOCATION IN THE BOREHOLE AND A THIRD LOCATION ALSO IN THE BOREHOLE BUT INTERMEDIATE SAID FIRST AND SECOND LOCATIONS AND ALSO SIMULTANEOUSLY PASSING CURRENT BETWEEN SAID SECOND LOCATION AND A POINT REMOTE THEREFROM, CONTROLLING THE AMOUNT OF CURRENT FLOWING BETWEEN SAID SECOND AND THIRD LOCATIONS SO AS TO MAINTAIN SAID FIRST AND THIRD LOCATIONS AT SUBSTANTIALLY EQUAL POTENTIAL AS SAID LOCATIONS ARE MOVED AS A GROUP PAST NON-HOMOGENOUS EARTHEN FORMATIONS, AND OBTAINING AN INDICATION OF VARIATIONS IN AN ELECTRICAL PROPERTY EXISTING BETWEEN SAID FIRST LOCATION AND A POINT REMOTE THEREFROM.