Apparatus for electrically logging boreholes

Claim 1

IN AN ELECTRICAL SYSTEM FOR LOGGING BORE HOLES HAVING CONDUCTIVE FLUID THEREIN AND INCLUDING MEANS FOR CAUSING ALTERNATING CURRENT TO FLOW THROUGH THE EARTH BETWEEN A FIXED POINT ON THE EARTH''S SURFACE ADJACENT THE BORE HOLE AND A MOVABLE POINT THEREIN, AND MEANS FOR DETECTING INTERMEDIATE THEREOF A POTENTIAL ESTABLISHED BY THE FLOW OF SAID ALTERNATING CURRENT, THE COMBINATION THEREWITH OF APPARATUS FOR OBTAINING A CONTINUOUS INDICATION OF THE RESISTIVITY OF THE SUBTERRANEAN FORMATIONS TRAVERSED BY THE BORE HOLE, SAID APPARATUS INCLUDING AN IMPEDANCE THROUGH WHICH SAID ALTERNATING CURRENT IS CAUSED TO FLOW TO GENERATE A STANDARD SIGNAL, SAID IMPEDENCE INCLUDING A VARIABLE ELEMENT FOR VARYING THE AMPLITUDE OF SAID STANDARD SIGNAL, MEANS FOR COMPARING THE DECECTED POTENTIAL WITH THE STANDARD SIGNAL, MEANS RESPONSIVE TO A DIFFERENCE IN AMPLITUDE BETWEEN THE DETECTED POTENTIAL AND STANDARD SIGNAL FOR VARYING SAID VARIABLE ELEMENT SO AS TO MAINTAIN SUBSTANTIALLY CONTINUOUSLY A BALANCE BETWEEN THE DETECTED POTENTIAL AND THE STANDARD SIGNAL, AND MEANS FOR CONTINUOUSLY RECORDING VARIATIONS IN SAID VARIABLE ELEMENT.