Inductive downhole sensor with center tap for common mode rejection

1. A method of evaluating an earth formation intersected by a borehole, the method comprising:

generating an excitation in the formation with an electromagnetic (EM) signal from a transmitter assembly at at least one frequency;

making EM measurements using an EM tool on a tool string in the borehole by receiving a signal at at least one receiver assembly responsive to the excitation;

wherein the at least one receiver assembly includes an induction antenna comprising a first winding and a second winding that share a common center tap, and wherein the average potential on the induction antenna is substantially the same as a ground potential.

2. The method of claim 1 wherein the common center tap is physically connected to ground.

3. The method of claim 1 wherein the at least one receiver assembly comprises a plurality of induction antennas with each antenna of the plurality separated from the other of the antennas of the plurality by a segment of the tool string and comprising a corresponding first winding and a corresponding second winding that share a corresponding common center tap.

4. The method of claim 1 wherein the first winding and the second winding overlap along a length of the tool string.

5. The method of claim 1 wherein the receiver assembly comprises a receiver electrically connected to the induction antenna by a balun transformer.

6. The method of claim 1 comprising wherein the transmitter assembly is on the tool string, the transmitter assembly comprising at least one of: i) a transmitter operatively electrically connected to another induction antenna by a balun transformer; and ii) a transmitter operatively electrically connected to the induction antenna by a balun transformer.

7. The method of claim 1 comprising using the at least one processor to perform at least one of: i) storing the measurements in a computer memory; ii) transmitting the measurements uphole; iii) processing the measurements in the borehole; or iv) displaying of the measurements to an operating engineer.

8. The method of claim 1 wherein the at least one receiver assembly maintains the average potential on the induction antenna substantially the same as a ground potential.

9. A method of evaluating an earth formation intersected by a borehole, the method comprising:

generating an excitation in the formation with an electromagnetic (EM) signal from a transmitter assembly at at least one frequency;

making EM measurements using an EM tool on a tool string by receiving a signal at at least one receiver assembly responsive to the excitation;

wherein the at least one receiver assembly includes an induction antenna comprising a first winding and a second winding that share a common center tap, and a capacitive shield encompassing the induction antenna, the capacitive shield comprising a meander shaped isolation gap, and

wherein the average potential on the induction antenna is substantially the same as a ground potential.

10. The method of claim 9 wherein the common center tap is physically connected to ground.

11. The method of claim 9 wherein the at least one receiver assembly comprises a plurality of induction antennas with each antenna of the plurality separated from the other of the antennas of the plurality by a segment of the tool string and comprising a corresponding first winding and a corresponding second winding that share a corresponding common center tap.

12. The method of claim 9 wherein the first winding and the second winding overlap along a length of the tool string.

13. The method of claim 9 wherein the receiver assembly comprises a receiver electrically connected to the induction antenna by a balun transformer.

14. The method of claim 9 comprising wherein the transmitter assembly is on the tool string, the transmitter assembly comprising at least one of: i) a transmitter operatively electrically connected to another induction antenna by a balun transformer; and ii) a transmitter operatively electrically connected to the induction antenna by a balun transformer.

15. The method of claim 9 wherein the at least one receiver assembly maintains the average potential on the induction antenna substantially the same as a ground potential.

16. An apparatus for evaluating an earth formation intersected by a borehole, the apparatus comprising:

an electromagnetic (EM) tool configured for conveyance in the borehole on a tool string, and further configured to generate an excitation in the formation with an EM signal from a transmitter assembly on the tool at at least one frequency;

at least one receiver assembly on the EM tool configured to make EM measurements by receiving a signal at the at least one receiver assembly responsive to the excitation;

wherein the at least one receiver assembly includes an induction antenna comprising a first winding and a second winding that share a common center tap, and wherein the average potential on the induction antenna is substantially the same as a ground potential.

17. The apparatus of claim 16 further comprising the tool string.

18. The apparatus of claim 17 wherein the tool string is a drill string, and the apparatus comprises a drill bit at a distal end of the drill string.

19. A method of evaluating an earth formation intersected by a borehole, the method comprising:

generating an excitation in the formation with an electromagnetic (EM) signal from a transmitter assembly at at least one frequency;

making EM measurements using an EM tool on a tool string in the borehole by receiving a signal at at least one receiver assembly responsive to the excitation;

wherein the at least one receiver assembly includes an induction antenna comprising a first winding and a second winding that share a common center tap, and wherein the at least one receiver assembly maintains the average potential on the induction antenna substantially the same as a ground potential.