Guided wave testing

1. Apparatus for guided wave testing a test object comprising:

a linear array of receiver electromagnetic acoustic transducers, each of said receiver electromagnetic acoustic transducers having a receiver coil and serving to convert vibrations in said test object into received electrical signals in said receiver coil; and

at least one linear array of transmitter electromagnetic acoustic transducers disposed substantially parallel to said linear array of receiver electromagnetic acoustic transducers and configured to launch guided waves in said test object in a direction substantially perpendicular to said at least one linear array of transmitter electromagnetic acoustic transducers, each of said transmitter electromagnetic acoustic transducers having a transmitter coil and serving to convert driving electrical signals in said transmitter coil into vibrations in said test object,

wherein

transmitter coils of electromagnetic acoustic transducers within said at least one linear array of transmitter electromagnetic acoustic transducers have a common winding direction, receiver coils of adjacent receiver electromagnetic acoustic transducers within said linear array of receiver electromagnetic acoustic transducers have alternating winding directions, and receiver coils of at least two adjacent electromagnetic acoustic transducers within said linear array of receiver electromagnetic acoustic transducers are connected in series.

2. Apparatus as claimed in claim 1 , wherein said test object is a pipe, said linear array of receiver electromagnetic transducers and said at least one linear array of transmitter electromagnetic acoustic transducers are configured to be circumferentially arranged around said pipe.

3. Apparatus as claimed in claim 1 , wherein all receivers coils within said linear array of receiver electromagnetic acoustic transducers are connected in series.

4. Apparatus as claimed in claim 1 , wherein a sum of electromotive force induced in said receiver coils of said linear array of receiver electromagnetic acoustic transducers connected in series by electromagnetic coupling with said transmitter coils of said at least one linear array of transmitter electromagnetic acoustic transducers is substantially zero.

5. Apparatus as claimed in claim 1 , wherein at least one of:

said transmitter coils have a same number of coil turns and a substantially common shape and size; and

said receiver coils have a same number of coil turns and a substantially common shape and size.

6. Apparatus as claimed in claim 1 , wherein

said receiver coil has a racetrack shape having two substantially straight sections aligned substantially perpendicular to said linear array of receiver electromagnetic acoustic transducers; and

a magnet pole disposed over each of said substantially straight portions and arranged to provide a magnetic field passing through a corresponding one of said two substantially straight portions substantially orthogonally to an outer surface of said test object when said linear array of receiver electromagnetic acoustic transducers is coupled to said test object.

7. Apparatus as claimed in claim 6 , wherein

said magnet poles disposed over said two substantially straight portions of said receiver coil have opposite magnetic field orientations.

8. Apparatus as claimed in claim 7 , wherein

adjacent magnet poles of adjacent receiver coils have a common magnetic field orientation.

9. Apparatus as claimed in claim 6 , wherein

said transmitter coil has a racetrack shape having two substantially straight sections aligned substantially perpendicular to said linear array of receiver electromagnetic acoustic; and

a magnet pole disposed over each of said substantially straight portions and arranged to provide a magnetic field passing through a corresponding one of said two substantially straight portions substantially orthogonally to an outer surface of said test object when said linear array of receiver electromagnetic acoustic transducers is couple to said test object.

10. Apparatus as claimed in claim 9 , wherein

said magnet poles disposed over said two substantially straight portions of said transmitter coil have opposite magnetic field orientations.

11. Apparatus as claimed in claim 10 , wherein

adjacent magnet poles of adjacent transmitter coils have opposite magnetic field orientation.

12. Apparatus as claimed in claim 6 , comprising at least one of:

a drive circuit to generate said driving electrical signals as differential signals;

a receiver circuit to receive said received electrical signals as differential signals; and

electromagnetic shielding to protect said receiver coils from electro-magnetic fields external from said pipe and said apparatus.

13. Apparatus as claimed in claim 6 , wherein said at least one linear array of transmitter electromagnetic acoustic transducers comprises at least two linear arrays of transmitter electromagnetic acoustic transducers separated by a longitudinal separation distance in a direction substantially perpendicular to said linear array of receiver electromagnetic acoustic transducers.

14. Apparatus as claimed in claim 13 , wherein said driving electrical signals and said longitudinal separation distance act together to control a transmission direction of vibrations in said test object generated by said at least one linear array of transmitter electromagnetic acoustic transducers.

15. Apparatus as claimed in claim 2 , wherein all receivers coils within said linear array of receiver electromagnetic acoustic transducers are connected in series, and said vibrations correspond to a fundamental torsional wave mode vibration T(0,1) within said pipe.

16. Apparatus as claimed in claim 6 , wherein propagating waves excited by said vibrations are non-dispersive.

17. Apparatus as claimed in claim 6 , comprising a flexible sheet housing said linear array of receiver electromagnetic acoustic transducers and including a plurality of ribs oriented to permit flexing of said sheet parallel to a direction of surface curvature of said test object when said linear array of receiver electromagnetic acoustic transducers is coupled to said test object and to resist flexing of said sheet perpendicular to said direction of surface curvature.

18. Apparatus as claimed in claim 6 , wherein said apparatus comprises components having an operating temperature including temperatures greater than 100° C.

19. A method of guided wave testing a test object comprising:

coupling a linear array of receiver electromagnetic acoustic transducers to said test object, each of said receiver electromagnetic acoustic transducers having a receiver coil;

coupling at least one linear array of transmitter electromagnetic acoustic transducers to said test object, each of said transmitter electromagnetic acoustic transducers having a transmitter coil;

converting driving electrical signals in said transmitter coil into transmitted vibrations in said test object; and

converting received vibrations in said test object into received electrical signals in said receiver coil,

wherein

transmitter coils of electromagnetic acoustic transducers within said at least one linear array of transmitter electromagnetic acoustic transducers have a common winding direction, receiver coils of adjacent receiver electromagnetic acoustic transducers within said linear array of receiver electromagnetic acoustic transducers have alternating winding directions, and receiver coils of at least two adjacent electromagnetic acoustic transducers within said linear array of receiver electromagnetic acoustic transducers are connected in series.

20. Apparatus for guided wave testing a test object comprising:

a linear array of receiver electromagnetic acoustic transducers, each of said receiver electromagnetic acoustic transducers having a receiver coil and serving to convert vibrations in said test object into received electrical signals in said receiver coil; and

at least one linear array of transmitter electromagnetic acoustic transducers disposed substantially parallel to said linear array of receiver electromagnetic acoustic transducers and configured to launch guided waves in said test object in a direction substantially perpendicular to said at least one linear array of transmitter electromagnetic acoustic transducers, each of said transmitter electromagnetic acoustic transducers having a transmitter coil and serving to convert driving electrical signals in said transmitter coil into vibrations in said test object,

wherein transmitter coils of adjacent transmitter electromagnetic acoustic transducers within said at least one linear array of transmitter electromagnetic acoustic transducers have alternating winding directions, receiver coils of adjacent electromagnetic acoustic transducers within said linear array of receiver electromagnetic acoustic transducers have a common winding direction and receiver coils of at least two adjacent electromagnetic acoustic transducers within said linear array of receiver electromagnetic acoustic transducers are connected in series.