Electrical resistivity well logging systems and methods

Claim 1

IN A SYSTEM FOR MAKING A RESISTIVITY LOG IN A BOREHOLE IN THE EARTH, THE COMBINATION OF AN ELECTRICAL CABLE, AN ELECTRODE ASSEMBLY SUPPORTED IN THE BOREHOLE ON THE CABLE AND INCLUDING A CURRENT ELECTRODE DEVICE AND A PICKUP ELECTRODE, A SOURCE OF CURRENT AT THE SURFACE OF THE GROUND CONNECTED TO SAID CABLE FOR CAUSING CURRENT TO FLOW FROM THE CURRENT ELECTRODE DEVICE INTO FLUID IN THE BOREHOLE AND INTO THE EARTH FORMATIONS, MEASURING EQUIPMENT AT THE SURFACE OF THE GROUND, ALSO CONNECTED TO SAID CABLE FOR TAKING READINGS OF THE ELECTRICAL POTENTIAL BETWEEN THE PICK-UP ELECTRODE AND A REFERENCE POINT, AN ARRANGEMENT FOR CHANGING THE EFFECTIVE DISTANCE BETWEEN THE CURRENT ELECTRODE DEVICE AND THE PICK-UP ELECTRODE WHILE THE READINGS ARE BEING TAKEN TO ENABLE THE MAKING OF A STEP BY STEP LOG AND A CONTROL CIRCUIT CONNECTED TO SAID CABLE AND SAID MEASURING EQUIPMENT FOR CORRELATING THE CHANGES IN SAID EFFECTIVE DISTANCE WITH SAID READINGS.