Modular resistivity sensor for downhole measurement while drilling

1. A resistivity sensor comprising:

a sensor body having a longitudinal axis, wherein the sensor body is disposable in and separable from a tool body in a drillstring;

one or more transmitting coil antennas disposed along a length of the sensor body;

one or more receiving coil antennas disposed along a length of the sensor body;

a calibrating device configured to calibrate signals generated from the receiving coil antennas, and

an electronics section contained within the sensor body for generating and receiving signals to and from the transmitting and receiving coil antennas,

wherein the longitudinal axis of the sensor body is radially offset from a longitudinal axis of the tool body, and thereby when the tool body rotates, transmitting and receiving patterns from the coil antennas disposed along a length of the sensor body rotate with the tool body.

2. The sensor of claim 1 , wherein at least one coil antenna is configured to generate magnetic moments parallel with the longitudinal axis of the sensor body.

3. The sensor of claim 1 , wherein at least one coil antenna is configured to generate magnetic moments misaligned with the longitudinal axis of the sensor body.

4. The sensor of claim 1 , wherein the calibrating device comprises a signal generator configured to generate a voltage signal at a desired frequency.

5. The sensor of claim 1 , wherein the calibrating device comprises a coil antenna configured to generate a voltage signal at a desired frequency in the receiving coil antennas.

6. The sensor of claim 1 , wherein the calibrating device is substantially equally spaced between a pair of receiving antennas.

7. The sensor of claim 1 , wherein at least part of the sensor body comprises a non-conducting material.

8. The sensor of claim 1 , further comprising circumferential grooves in an outer surface of the sensor body, each groove having an insulating layer deposited therein on which at least one of the transmitting and receiving coil antennas are wound.

9. The sensor of claim 1 , further comprising a sensor cover that fits over the sensor body, wherein at least part of the sensor cover comprises a non-conducting material.

10. The sensor of claim 9 , wherein the sensor cover further comprises one or more slots aligned substantially along a direction of magnetic moments generated by respective coil antennas underneath.

11. The sensor of claim 1 , wherein a radially outermost surface of the resistivity sensor is at a distance from the longitudinal axis of the drillstring substantially equal to or less than an outer diameter of the drillstring.

12. The sensor of claim 1 , further comprising a tool face measurement sensor configured to measure the angular position of the drill string relative to the wellbore.

13. The sensor of claim 1 , further comprising multiple sensor bodies disposed in and separable from drillstring pockets, wherein electromagnetic signals are transmitted and received at multiple coil antennas on the multiple sensor bodies.

14. A method of making resistivity measurements of a formation from a wellbore being drilled by a tool having a drill bit disposed on a distal end, the method comprising:

providing a resistivity sensor disposed within and separable from a tool body and radially offset from a longitudinal axis of the tool body, the resistivity sensor comprising at least one transmitting coil antenna, one or more receiving coil antennas, a calibrating coil antenna located longitudinally between the receiving coil antennas, and an electronics section contained within a body of the resistivity sensor;

firing the at least one transmitting coil antenna for generating a current loop in the formation, thereby inducing a first set of independent voltage signals in the receiving coil antennas related to a parameter of the formation;

firing the calibrating antenna at a desired frequency, thereby inducing a second set of independent voltage signals in the receiving coil antennas; and

calibrating the first set of independent voltage signals relative to the second set of voltage signals.

15. The method of claim 14 , further comprising calibrating the first set of independent voltage signals relative to air hung measurements.

16. The method of claim 14 , further comprising firing the coil antennas within a frequency range of between 100 kHz and 1 GHz.

17. A method of making resistivity measurements of a formation from a wellbore being drilled by a tool having a drill bit disposed on a distal end, the method comprising:

providing a resistivity sensor disposed within and separable from a tool body and radially offset from a longitudinal axis of the tool body, the resistivity sensor comprising at least one transmitting coil antenna, one or more receiving coil antennas, a calibrating signal generator, and an electronics section contained within the resistivity sensor;

firing the at least one transmitting coil antenna for generating a current loop in the formation, thereby inducing independent voltage signals in the receiving coil antennas related to a parameter of the formation;

generating a voltage signal at a desired frequency in the calibrating signal generator;

transmitting the generated voltage signal to electronics coupled to the receiving antennas; and

calibrating the independent voltage signals induced in the receiving coil antennas.

18. The method of claim 17 , further comprising calibrating the independent voltage signals relative to air hung measurements.

19. The method of claim 17 , further comprising firing the coil antennas within a frequency range of between 100 kHz and 1 GHz.