Gravity and/or acceleration measurements using dual interferometer configurations

1. An apparatus for estimating gravitational properties, comprising:

an optical source configured to emit electromagnetic radiation;

a first interferometer including a fixed reference reflector and a first reflector coupled to a first moveable mass, the first mass configured to move a first amount in response to a change in gravitational force;

a second interferometer including the fixed reference reflector and a second reflector coupled to a second moveable mass, the second mass configured to move a second amount in response to a change in gravitational force, the second amount being smaller than the first amount;

a first detector configured to detect a first interference pattern generated by the first interferometer;

a second detector configured to detect a second interference pattern generated by the second interferometer; and

a processor configured to estimate the change in gravitational force based on a combination of the first and second interference patterns.

2. The apparatus of claim 1 , wherein the first moveable mass has a stiffness that is less than a stiffness of the second moveable mass.

3. The apparatus of claim 2 , wherein the first moveable mass includes a first spring and the second moveable mass includes a second spring.

4. The apparatus of claim 1 , wherein the first and second moveable masses are configured to move in at least substantially parallel directions.

5. The apparatus of claim 4 , wherein the first and second interferometers are Michelson interferometers.

6. The apparatus of claim 1 , wherein the first interferometer and the second interferometer have different, overlapping dynamic ranges.

7. The apparatus of claim 6 , wherein the first interferometer has a dynamic range on the order of one g to one μg, and the second interferometer has a dynamic range on the order of one micro-g to one nano-g, wherein g is the acceleration of gravity at the earth's surface.

8. The apparatus of claim 7 , wherein the processor is configured to calculate the change in gravitational force to a precision on the order of one nano-g.

9. The apparatus of claim 6 , wherein the processor is configured to estimate a value of the change in gravitational force to a coarse resolution based on the first interference pattern, and refine the value to a fine resolution based on the second interference pattern.

10. The apparatus of claim 1 , further comprising a first beam splitter configured to direct a portion of a beam emitted by the light source to the first reflector, and a second beam splitter configured to direct another portion of the beam to the second reflector.

11. A method of estimating gravitational properties, comprising:

emitting an electromagnetic radiation beam from an optical source;

transmitting the beam to a fixed reflector;

directing a portion of the beam to a first interferometer including the fixed reflector and a first reflector coupled to a first moveable mass, the first mass configured to move a first amount in response to a change in gravitational force;

directing another portion of the beam to a second interferometer including the fixed reflector and a second reflector coupled to a second moveable mass, the second mass configured to move a second amount in response to a change in gravitational force, the second amount being smaller than the first amount;

detecting a first interference pattern generated by the first interferometer;

detecting a second interference pattern generated by the second interferometer; and

estimating the change in gravitational force based on a combination of the first and second interference patterns.

12. The method of claim 11 , wherein the first moveable mass has a stiffness that is less than a stiffness of the second moveable mass.

13. The method of claim 12 , wherein the first moveable mass includes a first spring and the second moveable mass includes a second spring.

14. The method of claim 1 , wherein the first and second moveable masses are configured to move in at least substantially parallel directions.

15. The method of claim 11 , wherein the first and second interferometers are Michelson interferometers.

16. The method of claim 11 , wherein the first interferometer and the second interferometer have different, overlapping dynamic ranges.

17. The method of claim 16 , wherein the first interferometer has a dynamic range on the order of one g to one μg, and the second interferometer has a dynamic range on the order of one micro-g to one nano-g, wherein g is the acceleration of gravity at the earth's surface.

18. The method of claim 17 , wherein estimating the change in gravitational force includes calculating the change in gravitational force to a precision on the order of one nano-g.

19. The method of claim 16 , wherein estimating the change in gravitational force includes calculating a value of the change in gravitational force to a coarse resolution based on the first interference pattern, and refining the value to a fine resolution based on the second interference pattern.

20. The method of claim 11 , wherein the optical source, the first interferometer and the second interferometer are disposed in a downhole tool, and the method includes deploying the downhole tool in a borehole in an earth formation.