Apparatus and method of determining casing thickness and permeability

1. An apparatus for evaluating a ferromagnetic tubular within a borehole, the apparatus comprising:

(a) a tool conveyed in the borehole, the tool having at least one magnet configured to produce a magnetic flux in the tubular;

(b) at least one multicomponent flux sensor responsive to magnetic flux and configured to provide an output indicative of an absolute thickness of the tubular; and

(c) a processor configured to use the output of the at least one multicomponent flux sensor to determine the absolute thickness of the tubular by defining a mapping function between a first component and a second component of the output from the multicomponent flux sensor.

2. The apparatus of claim 1 wherein the at least one magnet and the at least one multicomponent flux sensor are positioned on an inspection member extendable from a body of the tool.

3. The apparatus of claim 1 wherein the at least one magnet comprises a plurality of pairs of magnets disposed on at least one inspection module having a plurality of inspection members extendable from a body of the tool.

4. The apparatus of claim 3 wherein the tool is configured to be substantially self centralizing.

5. The apparatus of claim 3 wherein the at least one inspection module comprises two spaced apart inspection modules.

6. The apparatus of claim 5 wherein the plurality of inspection members on one of the inspection modules are in a staggered configuration relative to the plurality of inspection members on another one of the inspection modules.

7. The apparatus of claim 1 wherein the at least one multicomponent flux sensor comprises a Hall effect sensor.

8. The apparatus of claim 1 wherein the processor is further configured to determine a change in magnetic permeability of the tubular.

9. The apparatus of claim 1 further comprising a wireline which is configured to convey the tool into the borehole.

10. A method of evaluating a ferromagnetic tubular within a borehole, the method comprising:

(a) producing a magnetic flux in the tubular using at least one magnet on a tool conveyed in the borehole;

(b) obtaining a signal indicative of an absolute thickness of the tubular using a multicomponent flux sensor; and

(c) using the signal to estimate the absolute thickness of the tubular by defining a mapping function between a first component and a second component of the signal.

11. The method of claim 10 wherein producing the magnetic flux further comprises positioning at least one pair of magnets on an inspection member extendable from a body of the tool.

12. The method of claim 10 wherein producing the magnetic flux further comprises positioning a plurality of pairs of magnets on a first inspection module having a plurality of inspection members extendable from a body of the tool.

13. The method of claim 12 further comprising positioning a plurality of magnets on a plurality of inspection members on a second inspection module spaced apart from the first inspection module and wherein the plurality of inspection members on the first inspection module are in a staggered configuration relative to the plurality of inspection modules on the second inspection module.

14. The method of claim 10 wherein obtaining the signal further comprises using a Hall effect sensor.

15. The method of claim 10 further comprising estimating a change in magnetic permeability of the tubular.

16. The method of claim 10 further comprising conveying the tool into the borehole on a wireline.

17. The method of claim 10 wherein estimating the thickness of the tubular further comprises using a function that maps a feature of one component of the signal from the multi-component flux sensor onto a feature of a second component of the signal from the multi-component flux sensor.

18. A machine readable medium for use with an apparatus which evaluates a ferromagnetic tubular within a borehole, the apparatus including:

(a) a tool configured to be conveyed within the tubular;

(b) at least one magnet on the tool which is configured to produce a magnetic flux in the tubular; and

(c) a multicomponent flux sensor responsive to magnetic flux;

the medium comprising instructions that enable a processor to estimate from an output of the multicomponent flux sensor:

(d) an absolute thickness of the tubular by defining a mapping function between a first and second component of the output from the multicomponent flux sensor; and

(e) a magnetic permeability of the tubular.

19. The medium of claim 18 wherein the medium is selected from the group consisting of (i) a ROM, (ii) an EPROM, (iii) an EEPROM, (iv)a Flash Memory, and (v) an Optical disk.