Abstract:
The electromagnetic acoustic transducer (EMAT) transducer disclosed herein is useful for the non-destructive analysis of objects. The transducer comprises a core having a winding and a coil disposed between the core and the object to be analyzed. One transducer can be used as a transmitter and another transducer as a receiver. Then selectively switching static magnetic field in either transmitter or receiver and processing data with and without static magnetic field allows for eliminating artifacts due to parasitic coupling between the transmitter/receiver pair. The switching of the static magnetic field can be implemented either by using electromagnet or a pair of permanent magnets where magnetization of one permanent magnet is reversed to provide cancellation of the static magnetic field.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The disclosure herein relates generally to the field of electromagnetic transducers. More specifically, the present disclosure relates to an electromagnetic acoustic transducer used in non-destructive testing. Yet more specifically, described herein is a method and apparatus for eliminating interference between separate electromagnetic acoustic transducers. 
         [0003]    2. Description of Related Art 
         [0004]    Monitoring the behavior of acoustic waves in a solid is useful in detecting potential flaws in the solid. One example of use includes propagating an acoustic wave into a member being testing, receiving the resulting wave, and analyzing the wave. Determining the resulting wave&#39;s attenuation can yield useful information concerning flaws in the member. The flaws may include cracks, pitting, corrosion, or other discontinuities in the solid. The members being tested include structural members, vessels, piping and other tubulars. Other applications include measuring solid dimensions and identifying the material through which the wave propagates. 
         [0005]    One device useful for inducing acoustic waves in solids for non-destructive testing is an electromagnetic acoustic transducer (EMAT).  FIG. 1  illustrates in a side cut-away view an example of a prior art EMAT  10 . The EMAT  10  comprises a permanent magnet  14  that extends substantially parallel to an electrically conductive object  12 . Members disposed on the terminal ends of the magnet  14  form a magnetic yoke  16  extending downward toward the object  12 . A coil  18  comprising a series of wires  20  is disposed in the space between the permanent magnet  14  and the object  12 . 
         [0006]    EMAT function comprises flowing electrical current through the coil  18  thereby inducing eddy currents in the object  12  proximate to the electrically conducting wire  20 . Interaction between a magnetic field and induced eddy currents in turn creates Lorentz forces that acoustically excite the object. The magnetic field is produced by the magnet  14 . Acoustic excitation typically results in acoustic waves that propagate in the object  12 . Similarly, placing an EMAT proximate to an object excited by acoustic waves can induce an alternating magnetic flux that in turn results in an electromotive force applied to the receiver coil wires. Thus by measuring this electromotive force an EMAT may also act as an acoustic receiver. Recording and analyzing these waves is useful in detecting flaws in the solid. 
         [0007]    One drawback of currently used EMATs is if an EMAT transmitter and an EMAT receiver are sufficiently proximate on another, parasitic coupling, or cross-talk, occurs between the respective windings of the transmitter and receiver. The resulting cross talk can have deleterious effects on data received by the receiver. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    Disclosed herein is an electromagnetic acoustic transducer useful for analyzing an object comprising, a magnetic core, a core winding circumscribing the core, wherein the winding is configured for flowing current therethrough, and a coil configured for conducting an alternating current. The transducer may optionally further comprise a permanent magnet, wherein selectively flowing current through the winding can change polarity of magnetization of the magnetic core thereby selectively canceling the resulting magnetic field of the core and magnet. The core preferably comprises a permanent magnet or a soft magnetic material. 
         [0009]    A method of analyzing a solid is disclosed comprising disposing first and second electromagnetic acoustic transducer proximate to the solid, wherein the first electromagnetic acoustic is a transmitter and the second electromagnetic acoustic transducer is a receiver; selectively switching off a static magnetic field in one of the electromagnetic acoustic transducers; generating an acoustic signal with the first electromagnetic acoustic transducer; and receiving acoustic data with and without the presence of said static magnetic field in order to eliminate the data artifacts due to parasitic coupling between the transmitting electromagnetic acoustic transducer and the receiving electromagnetic acoustic transducer. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
         [0010]      FIG. 1 . is a cut-away side view of a prior art electromagnetic acoustic transducer. 
           [0011]      FIGS. 2   a  and  2   b  are cut-away side views of operational modes of an embodiment of an electromagnetic acoustic transducer. 
           [0012]      FIG. 3  is a cut-away side view of an alternative embodiment of an electromagnetic acoustic transducer. 
           [0013]      FIG. 4  is an embodiment of a downhole tool having an electromagnetic acoustic transducer. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0014]    The present disclosure concerns an electromagnetic transducer with the capability of eliminating cross-talk that sometimes occurs between a transmitting transducer and a receiving transducer. Cross-talk is also referred to herein as a parasitic signal due to direct coupling between the transmit and receive coils of a respective transmitter and receiver. One manner of eliminating artifacts due to the cross-talk involves neutralizing or removing the magnetic field induced by a transducer in the object being analyzed. The signal recording without the magnetic field (either on transmitter or receiver side) represents the cross-talk related signal only. Subtracting this signal from the signal recording in full magnetic field mode gives acoustic propagation signal without artifacts due transmitter-receiver electromagnetic cross-talk. For the purposes of discussion herein, the term “artifact”: refers to an unwanted signal or a portion of a signal that is unwanted. One example of an artifact is noise or coherent noise. As discussed below, the magnetic field can be removed either by removing current in the electromagnet, or by creating a “canceling” permanent magnetic field. A canceling magnetic field refers to one created proximate to interact with a first magnetic field, where the canceling magnetic field has a magnitude and polarity that substantially negates the first magnetic field. 
         [0015]    With reference to  FIGS. 2   a  and  2   b  one embodiment of an electromagnetic transducer in accordance with the present disclosure is provided in a side cutaway view. In  FIG. 2   a  a transducer  22  is shown comprising a core  26  bound by a winding  28 . In this embodiment the core  26  comprises a magnetic material with substantial magnetic hysteresis. In the embodiment shown, the winding  28  comprises an elongated length of wire  30  coaxially wrapped along a portion of the core  26 . A magnetic yoke  24 , comprising a soft magnetic material, is coupled with the terminal ends of the core  26 . The magnetic yoke  24  extends downward towards the surface of the object  40  that is being examined by the transducer  22 . The yoke  24  conducts magnetic flux from the core end poles to the object being tested. 
         [0016]    Disposed substantially parallel to and below the core  26  is a permanent magnet  32 , in the embodiment shown the magnet  32  operates as a magnetic field source that generates a magnetic field. The permanent magnet  32  is also bound on its terminal ends by the magnetic yoke  24 . 
         [0017]    As shown via the double-headed arrow, the windings  28  are in electrical communication with a pulsed current source  29 . The current source  29  selectively provides electrical pulsed power to the windings  28 . The pulsed power is sufficient to magnetize/re-magnetize the magnetic core  26 . Due to magnetic hysteresis the magnetic core  26  remains magnetized after the pulsed current produced by the current source  29  ends. The magnetization directions of the core  26  and the permanent magnet  32  are shown by arrows  27  and  33  respectively in  FIG. 2   a . In the acoustic wave generation mode of the transducer  22  presented in  FIG. 2   a , the residual magnetization of the core  26  and magnetization of the permanent magnet  32  are in the same direction. The resulting magnetic field produced by the magnetized core  26  and the permanent magnet  32  is illustrated by the series of flux lines  38  extending through the object. 
         [0018]    A coupling winding  34  is shown in the embodiment of  FIG. 2   a  in the space provided between the permanent magnet  32  the object upper surface  49 . In this embodiment the coupling winding  34  comprises a coupling winding wire  36 , wherein the wire  36  is elongated and electrically conducting. This wire  36  is shown formed in a standard series of loops, in one embodiment the wire  36  may comprise a meander wire. A current source  35  of RF current is shown in electrical communication with the coupling winding  34  via the double-headed arrow. Thus, by driving RF current through the coupling winding  34  in the presence of the magnetic field  38 , the resulting forces on the object  40  thereby create acoustic waves within the object  40 . In a receive mode of the transducer  22  operation the coupling winding  34  is connected to a receiver (not shown in  FIG. 2 ). 
         [0019]      FIG. 2   b , shown in side cross-sectional view similar to  FIG. 2   a , represents an alternative mode of operation of the transducer  22 . In this mode the pulsed current source  29  selectively provides pulsed current to the coil  28  in a direction that reverses the core magnetization polarity opposite from that of the mode of  FIG. 2   a . Due to substantial magnetic hysteresis of the magnetic material of the core  26 , the core remains magnetized after the pulse of current. The opposite polarity is shown by the direction of the arrow  27   a , which points in the direction opposite that of arrow  27 . This reverse in polarity causes the core  26 /winding  28  combination to produce a magnetic field having a polarity opposite of the magnetic field produced by the permanent magnet  32 . Interacting two oppositely polarized magnetic fields (or introducing a canceling magnetic field to another magnetic field) cancels both fields. 
         [0020]    As such, there is no resulting magnetic field extending into the body of the object  40 . The signal recording taken while generating the compensated magnetic field is subtracted from the signal recording taken with the full magnetic field to obtain a clean signal. The cross-talk elimination as described above can be achieved by canceling the static magnetic field of one of the receiving or transmitting transducer. 
         [0021]      FIG. 3  provides an alternate embodiment shown in a side cross-sectional view. In this embodiment the transducer  42  comprises a core  44  that has a substantially U-shaped cross section. As shown, a winding  46  is wrapped around the longitudinal portion of the core  44 . The double-headed arrow represents electrical communication between the electrical current source  47  and the winding  46 . A coupling coil  52  disposed between the core  44  and the object  40   a  is shown in cross-sectional view and comprises an electrically conductive elongated wire  54  arranged in a typical winding pattern. With respect to the present disclosure, the winding pattern of the wire  54  can be any pattern useful for the coupling of the transducer with the object  40   a  for creating the requisite acoustic waves. Electrically coupled with the coil  52  is a current source  53 , the coupling is shown by virtue of the double-headed arrow. As with the transducers of  FIG. 2   a  energizing the coil  46  with the electrical current source  47  results in a resulting magnetic field that extends into the body of the object  40   a . This magnetic field in the object  40   a  is illustrated by the series of curved lines  56 . This magnetic field in combination with eddy currents induced in the object  40   a  as a response to the magnetic field of the energizing the coil  52  in turn produces the acoustic waves within the body of the object  40   a.    
         [0022]    In this embodiment the current source  47  is selectable to turn the supplied current to an on and off manner thereby eliminating the magnetic field  56 . By synchronizing elimination of the magnetic field, along with the acquisition phase, the artifacts due to the cross-talk between an acoustic transmitter and an acoustic receiver can be eliminated. 
         [0023]    With reference now to  FIG. 4  one embodiment of a wellbore interrogation system in accordance with the present disclosure is shown in a side view. In this embodiment a downhole tool  72  is shown disposed within a wellbore via wireline  74 . Transducers  76  are provided on the surface of the downhole tool  72 . In this embodiment the transducers  76  may comprise an EMAT configuration and may be a combination of transmitters as well as receivers. Additionally, when disposed in a cased hole, the downhole tool  72  is useful for determining information regarding the casing and the casing bonding. Optionally, the transducers  76  can be used to obtain information regarding the formation surrounding the wellbore. 
         [0024]    In this embodiment a surface truck  78 , disposed at the surface, is used for controlling and operating the insertion and retrieval of the downhole tool  72 . Optionally an information handling system (IHS) may be used in conjunction with the surface truck  78  for acquisition, recordation, as well as analysis of any acoustical or other retrieved signal data obtained by use of the transducer  76 . 
         [0025]    The present invention described herein, therefore, is well adapted to carry out the objects and attain the ends and advantages mentioned, as well as others inherent therein. While a presently preferred embodiment of the invention has been given for purposes of disclosure, numerous changes exist in the details of procedures for accomplishing the desired results. The current source used for provided electrical power to the embodiments discussed may be disposed with the device, such as within a wellbore, or away from the device and coupled with a conductive member, such as a wire. This and other similar modifications will readily suggest themselves to those skilled in the art, and are intended to be encompassed within the spirit of the present invention disclosed herein and the scope of the appended claims.