Formation evaluation using a bit-based active radiation source and a gamma ray detector

1. A drill bit, comprising:

a bit body;

a source in the bit body configured to induce radiation into a formation surrounding the drill bit during drilling of a wellbore;

a movable member that in a first position exposes the formation to the radiation from the source and in a second position prevents the formation from being exposed to the radiation from the source;

a control unit configured to move the movable member to selectively expose the formation to the radiation from the source; and

at least one sensor in the bit body configured to detect radiation responsive to the induced radiation in the formation during drilling of the wellbore.

2. The drill bit of claim 1 , wherein the source is selected from a group consisting of: a gamma ray source; and a neutron source.

3. The drill bit of claim 1 , wherein the at least one radiation sensor includes a sensor from a group consisting of a: neutron sensor; and gamma ray sensor.

4. The drill bit of claim 1 , wherein the control unit includes a power unit configured to move the movable member between the first position and the second position.

5. The drill bit of claim 4 , wherein the power unit is selected from a group consisting of: an electrically-operated unit; and a hydraulically-operated unit.

6. The drill bit of claim 5 further comprising a valve configured to control supply of a fluid to the movable member to move the movable member between the first and second positions.

7. The drill bit of claim 1 , wherein the control circuit is further configured to process signals provided by the at least one sensor.

8. The drill bit of claim 1 , wherein the at least one sensor is integrated into a cutter of the drill bit.

9. The drill bit of claim 1 , wherein the at least one sensor is positioned in one of a shank, cone, blade or cutter of the drill bit.

10. The drill bit of claim 1 further comprising a communication link configured to provide data communication between a remote device and the at least one sensor.

11. A method of providing a drill bit, comprising:

placing a source in the drill bit for inducing radiation into a formation surrounding the drill bit during drilling of a wellbore;

placing a movable member in the drill bit that is configured to move between a first position that exposes the formation to the radiation from the source and a second position that prevents the formation from being exposed to the radiation from the source;

placing a control unit in the drill bit configured to move the movable member to selectively expose the formation to the radiation from the source;

placing in the bit body a sensor configured to detect radiation responsive to the induced radiation in the formation and provide signals representative of the detected radiation; and

processing the signals to estimate a property of the formation.

12. The method of claim 11 further comprising providing a control unit in the drill bit configured to selectively expose radiation from the source to the formation.

13. A method of drilling a wellbore in an earth formation, comprising:

drilling the wellbore with a drill bit in the formation that includes a radiation source;

moving a movable member to selectively expose the formation to the radiation from the source, wherein, in a first position, the movable member exposes the formation to the radiation from the radiation source and, in a second position, the movable member prevents the formation from being exposed to the radiation from the radiation source;

inducing in the formation radiation from the radiation source in the drill bit;

detecting radiation from the formation responsive to the induced radiation by a sensor in the drill bit and providing signals representative of the detected radiation; and

processing the signals to estimate a property of the formation.

14. The method of claim 13 further comprising:

stopping inducing radiation into the formation from the source for a selected period of time;

detecting naturally-occurring gamma rays in the formation and providing signals representative of such detected naturally-occurring gamma rays; and

estimating a property of the formation using the signals representative of the detected-naturally-occurring gamma rays.

15. The method of claim 14 further comprising processing signals representative of naturally-occurring gamma rays and signals representative of radiation detected from the formation in response to induced radiation to estimate a property of interest of the formation.

16. The method of claim 15 , wherein the property of the formation is selected from a group consisting of: a bed boundary condition; a rock property of the formation; formation density; formation porosity; and formation sigma.

17. The method of claim 13 further comprising adjusting at least one drilling parameter based on information obtained from processing of the signals.

18. The method of claim 17 , wherein the at least one drilling parameter is selected from a group consisting of: (i) weight-on-bit and (ii) rotational speed of the drill bit; and (iii) supply of a fluid.