Electrical well logging apparatus including a drill collar having spaced electrodes mounted thereon for making resistivity measurements while drilling

Claim 1

APPARATUS FOR USE IN MAKING ELECTRICAL MEASUREMENTS IN WELLS COMPRISING A DRILL BIT COLLAR, A DRILL BIT SECURED TO SAID COLLAR, THREE ELECTRODES MOUNTED ON THE EXTERIOR SURFACE OF SAID COLLAR SO AS TO BE SPACED VERTICALLY FROM ONE ANOTHER WHEN SAID COLLAR IS LOWERED INTO A WELL, SAID ELECTRODES BEING INSULATED FROM ONE ANOTHER AND FROM SAID BIT, AN ALTERNATING CURRENT SOURFE, A RESISTANCE ELEMENT CONNECTED BETWEEN ONE TERMINAL OF SAID ALTERNATING CURRENT SOURCE AND SAID BIT, SAID RESISTANCE ELEMENT HAVING A RESISTANCE SUFFICIENTLY LARGE THAT A SUBSTANTIALLY CONSTANT CURRENT IS EMITTED INTO SURROUNDING FORMATIONS FROM THE DRILL BIT WHEN SAID APPARATUS IS LOWERED INTO A WELL, A FIRST TRANSFORMER HAVING ONE TERMINAL OF THE PRIMARY COIL THEREOF CONNECTED TO THE SECOND TERMINAL OF SAID ALTERNATING CURRENT SOURCE, MEANS CONNECTING THE SECOND TERMINAL OF THE PRIMARY COIL OF SAID FIRST TRANSFORMER TO THE OUTER TWO OF SAID THREE ELECTRODES, A SECOND TRANSFORMER HAVING THE PRIMARY COIL THEREOF CONNECTED BETWEEN THE SECOND TERMINAL OF SAID ALTERNATING CURRENT SOURCE AND THE CENTER ONE OF SAID THREE ELECTRODES, MEANS TO MEASURE THE POTENTIAL DIFFERENCE BETWEEN THE SECOND TERMINAL OF SAID VOLTAGE SOURCE AND SAID DRILL BIT, AND VOLTAGE DIVIDING MEANS CONNECTED TO THE SECONDARY COILS OF SAID FIRST AND SECOND TRANSFORMERS TO DIVIDE THE POTENTIAL ACROSS THE SECONDARY COIL OF SAID SECOND TRANSFORMER BY THE POTENTIAL ACROSS THE SECONDARY COIL OF SAID FIRST TRANSFORMER.