Abstract:
An in-line inspection tool is disclosed for inspecting the wall of a pipeline while traveling therethrough. The in-line inspection tool may include a transmitter, a signal generator, one or more receivers, and a decoder. The signal generator may generate a pseudorandom signal, generate an inspection signal, and drive the transmitter with a convolution of the pseudorandom signal and the inspection signal. The transmitter may transmit the convoluted signal to the wall of the pipeline. One or more receivers may receive from the wall of the pipeline a received signal comprising at least one of the convoluted signal and a reflection of the convoluted signal. The decoder may identify the inspection signal within the received signal by cross correlating the received signal and the pseudorandom signal.

Description:
RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/087,537 filed Aug. 8, 2008, which is hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates to pipeline inspection tools, and more particularly to apparatus and methods for data collection and analysis for an in-line inspection tool. 
         [0004]    2. Background of the Invention 
         [0005]    Oil, petroleum products, natural gas, hazardous liquids, and the like are often transported using pipelines. The majority of these pipelines are constructed from steel pipe. Once installed, a pipeline will inevitably corrode or otherwise degrade. Proper pipeline management requires identification, monitoring, and repair of defects and vulnerabilities of the pipeline. For example, information collected about the condition of a pipeline may be used to determine safe operating pressures, facilitate repair, schedule replacement, and the like. 
         [0006]    Typical defects of a pipeline may include corrosion, gouges, dents, and the like. Corrosion may cause pitting or general wall loss, thereby lowering the maximum operating pressure of the pipeline. Vulnerabilities may also include curvature and bending anomalies and combined stress and chemical or biological action such as stress corrosion cracking. Without detection and preemptive action, all such defects and vulnerabilities may lead to pipeline failure. 
         [0007]    Information on the condition of a pipeline is often collected using an in-line inspection tool. Ferromagnetic pipelines can be inspected for defects including cracks along the axis of a pipe by a limited number of technologies. These technologies include magnetic flux leakage (MFL), ultrasonic inspection (UT), eddy current inspection, and, in certain limited applications, electromagnetic acoustic transducers (EMAT). 
         [0008]    Many in-line inspection tools use a pulsed signal source and sensors to collect information about a pipeline as they travel therethrough. In the past, in-line inspection tools have used technologies such as ultrasonic transducer (UT) inspection, eddy current inspection, electromagnetic acoustic transducer (EMAT) inspection and other technologies involving a transmitted and received energy pulse. Such technologies have been limited by low relative signal strength and low signal to noise ratio, further compromised by the noisy background of the pipeline environment. 
         [0009]    While some technologies are more adversely affected by these factors than other are, all such techniques can be improved with better signal detection and recognition. What is needed is a better device and method for the generation and reception of pulsed signals for the various inspection technologies. 
       SUMMARY 
       [0010]    In-line inspection (ILI) tools for inspecting pipelines utilize technologies that generate signals with low signal strength and consequent low signal-to-noise ratios. These technologies may include, but are not limited to, ultrasonic inspection (UT), eddy current inspection (ECI), electromagnetic acoustic transducer (EMAT) inspection using both Lohrentz force and magnetostrictive technologies, and some forms of magnetic flux leakage (MFL) inspection. 
         [0011]    The prevailing approach to producing an acceptable signal-to-noise ratio is to increase the strength of the incident signal and the sensitivity of the sensing device. Some systems use other techniques in addition, but existing in-line inspection tools produce workable devices by maximizing their signal-to-noise ratios. In many instances, this approach produces devices with marginal detection capability. Moreover, this approach certainly limits the applicability to pipeline inspection of technologies that are in use in other fields. In view of the foregoing, certain embodiments in accordance with the present invention provide an alternative to attempts at improving the signal-to-noise ratio as a means of detecting inspection signals. 
         [0012]    In selected embodiments, systems in accordance with the present invention may combine or convolute pseudorandom binary sequences (PRBS) or pseudorandom noise into the transmitted inspection signal. By so doing, signals with low strength or signals that are obscured by the background noise become visible. Even when signal-to-noise ratios are low, even when they may be less than one, the signal received in response to an anomaly such as a crack or corrosion may still be detected and analyzed. Cross correlation of PRBS convoluted signals detected in noisy environments supports suppression of the background noise, producing a visible signal even when signal-to-noise ratios may be comparable to, or less than, background levels. 
         [0013]    Additionally, for maximum effectiveness, special mounting may be required to position a sensing device in proximity to the pipe wall. Selected embodiments in accordance with the present invention may include a mounting system maintaining the inspection system in contact with the pipe wall as the in-line inspection tool travels inside the pipe being inspected. 
         [0014]    Accordingly, selected embodiments in accordance with the present invention provide apparatus and methods for developing a PRBS signal, convoluting that signal with an incident inspection signal, transmitting the convoluted signal through the pipe wall where it may be affected by pipeline anomalies, detecting the affected convoluted signal, and cross correlating or decoding the signal to suppress background noise and produce a signal that may be used to characterize anomalies. 
         [0015]    Certain embodiments of the present invention may be discussed or illustrated in the context of an in-line inspection tool using EMAT technologies. However, concepts in accordance with the present invention are not limited to EMAT technologies and may be applicable to other in-line inspection technologies. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The foregoing features of the present invention will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only typical embodiments of the invention and are, therefore, not to be considered limiting of its scope, the invention will be described with additional specificity and detail through use of the accompanying drawings in which: 
           [0017]      FIG. 1  is an elevation view of one embodiment of an in-line inspection tool in accordance with the present invention; 
           [0018]      FIG. 2  is a perspective view of one embodiment of an inspection assembly with an EMAT inspection device and linkages positioned with respect to one another as they would be when installed on an in-line inspection tool in accordance with the present invention; 
           [0019]      FIG. 3  is a perspective view of a magnetostrictive EMAT inspection device positioned to produce a horizontal shear inspection wave guided circumferentially within the wall of the pipe being inspected in accordance with the present invention; 
           [0020]      FIG. 4  is a schematic diagram illustrating selected embodiments of elements of coils of an EMAT in-line inspection tool showing their positions relative to one another and to signals generated and/or received thereby in accordance with the present invention; 
           [0021]      FIG. 5  is an illustration showing one embodiment of a pseudorandom binary sequence (PRBS) and the Delta function autocorrelation thereof: 
           [0022]      FIG. 6  an illustration showing another embodiment of a pseudorandom binary sequence and the Delta function autocorrelation thereof; 
           [0023]      FIG. 7  is an illustration showing from top to bottom a first PRBS, a time delayed version of the first PRBS, an autocorrelation of the first PRBS and the time delayed version, a typical incident pulse as may be generated by an in-line inspection tool in accordance with the present invention, a convolution of the first PRBS and the typical incident pulse delayed by time, and a cross correlation of the time delayed convolution and the original version of the PRBS; 
           [0024]      FIG. 8  is an illustration showing from top to bottom an inspection pulse obscured in typical background noise as may be found in a pipeline environment, a convolution of an inspection pulse and a PRBS together with the background noise, and a cross correlation of the convoluted pulse in the background noise and the original PRBS; and 
           [0025]      FIG. 9  is a schematic block diagram of one embodiment of a PRBS-based system for generating and receiving an inspection pulse in a pipe wall in accordance with the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    It will be readily understood that the components of the present invention, as generally described and illustrated in the drawings herein, could be arranged and designed in a wide variety of different configurations. Thus, the following more detailed description of the embodiments of the system and method of the present invention, as represented in the drawings, is not intended to limit the scope of the invention as claimed, but is merely representative of various embodiments of the invention. The illustrated embodiments of the invention will be best understood by reference to the drawings, wherein like parts are designated by like numerals throughout. 
         [0027]    Referring to  FIG. 1 , an in-line inspection tool  10  or vehicle  10  in accordance with the present invention may comprise various components including one or more inspection assemblies  12 , canisters  14 , driving cups  16 , couplers  18 , position sensors  20 , and the like. Depending on the configuration of the in-line inspection tool  10  and the size of the pipeline to be inspected, the arrangement and number of components (e.g., the number of canisters  14 ) may vary. 
         [0028]    Canisters  14  may house equipment such as one or more processors, memory devices, and batteries. The driving cups  16  may center the tool  10  within the pipeline and enable fluid traveling within a pipeline to engage the tool  10 , thereby pushing the tool  10  through the pipeline. In selected embodiments, driving cups  16  may be formed of a somewhat flexible polyurethane or similar material. Couplers  18  may support bending of the tool  10 , enabling the tool  10  to accommodate bends in the pipeline. Like the driving cups  16 , in selected embodiments the couplers  18  may be formed of somewhat flexible polyurethane or similar material or a mechanical pivoting device. 
         [0029]    An in-line inspection tool  10  may extend in a longitudinal direction  22  from a head end  24  to a tail end  26 . The various components  12 ,  14 ,  16 ,  18 ,  20  of an in-line inspection tool  10  may be arranged in series. For example, in the illustrated embodiment, the head end  24  of a tool  10  may comprise a head section  28  comprising one or more driving cups  16 . Following the head section  28  may be a primary sensor suite  30 . A coupler  18   a  may extend to connect the head section  28  to the primary sensor suite  30 . 
         [0030]    In selected embodiments, an in-line inspection tool  10  in accordance with the present invention may include one or more inspection assemblies  12  comprising one or more signal sources, sensors, or combinations thereof positioned so as to travel along the interior of a pipe wall being inspected. Such signal sources and sensors may generate and receive a wide variety of stress waves propagating in any of many directions. While certain embodiments of the present invention may be discussed or illustrated in the context of an in-line inspection tool using EMAT technologies generating magnetostrictive stress waves, is should be understood that concepts in accordance with the present invention are not limited to EMAT technologies and may be applicable to other in-line inspection technologies. 
         [0031]    For example, suitable signal sources, sensors, or combinations thereof may include one or more piezoelectric ultrasonic transducers of normal incidence, piezoelectric ultrasonic transducers of oblique incidence, and piezoelectric ultrasonic transducers operating as a phased array. Other suitable signal sources, sensors, or combinations thereof may include one or more pulsed eddy current inspection devices, remote field eddy current devices, and magnetic flux leakage inspection devices. 
         [0032]    Following the primary sensor suite  30  may be a first canister  14   a.  In one embodiment, the first canister  14   a  may house the hardware providing the processing and memory storage for the in-line inspection tool  10 . A coupler  18   b  may extend to connect the primary sensor suite  30  to the first canister  14   a.    
         [0033]    The first canister  14   a  may be followed by another driving cup  16 . A coupler  18   c  may engage a first canister  14   a  and extend rearward to engage a second canister  14   b.  In one embodiment, the second canister  14   b  may house batteries providing the power for the in-line inspection tool  10 . In selected embodiments, a driving cup  16  may connect to the second canister  14   b.  One or more position sensors  20  may then engage the second canister  14   b,  driving cup  16 , or some combination thereof to form the tail end  26  of the in-line inspection tool  10 . In one embodiment, the position sensors  20  may comprise one or more odometers  20  positioned to roll along the interior surface of the pipeline and measure the distance traveled by the in-line inspection tool  10 . 
         [0034]    Referring to  FIG. 2 , in selected embodiments, a primary sensor suite  30  may include inspection assemblies  12  distributed circumferentially (i.e., in a circumferential direction  38 ) about a central axis  40  of an in-line inspection tool  10 . Each inspection assembly  12  may include a back bar  42  for supporting the various components of the inspection assembly  12 . A linkage mount  44  may be positioned at each end of a back bar  42 . Linkages  45  may engage an inspection assembly  12  via the linkage mounts  44  and extend therefrom to connect the inspection assembly  12  to an interior cylinder (not shown) forming the back bone of the primary sensor suite  30 . Suitable linkages  45  are disclosed in U.S. patent application Ser. No. 12/478,137 filed Jun. 4, 2009, which is hereby incorporated by reference. Accordingly, inspection assemblies  12  may partially or fully encircle the interior cylinder. 
         [0035]    In embodiments utilizing magnetostrictive EMAT inspection, a magnetic flux field may be introduced into the wall of a pipe by magnets  48 . One or more coils  54  may generate and receive or both generate and receive a magnetostrictive stress wave so that anomalies within the wall of the pipe being inspected may be detected. 
         [0036]    In selected embodiments utilizing EMAT inspection, a back bar  42  may support one or more magnets  48 . In certain embodiments, an inspection assembly  12  may include a magnet mount  46  providing an interface between a back bar  42  and a magnet  48 . The magnet mount  46  may protect the magnet  48 . The magnet mount  46  may also assist in transferring flux into the wall of the pipe being inspected. 
         [0037]    In selected embodiments, a back bar  42  may support two magnets  48 , one magnet  48  proximate each end thereof. A sensor mount  50  may secure to the back bar  42  at a location between the two magnets  48 . The sensor mount  50  may connect a sensor housing  52  to the back bar  42 . 
         [0038]    A sensor mount  50  may comprise flexible material. Accordingly, a sensor mount  50  may permit relative motion between a sensor housing  52  and a back bar  42 . Constraints such as the magnet mount  46  may be positioned proximate a sensor mount  50  to control or limit certain motion of the sensor mount  50  and sensor housing  52  with respect to the back bar  42 . In selected embodiments, a constraint  46  may prevent the sensor housing from contacting or being crushed by the back bar  42 . Accordingly, a constraint  46  may provide an additional control over the motion of a sensor housing  52  with respect to a back bar  42 . 
         [0039]    Inspection assemblies  12  may move with respect to the interior cylinder or main body of an in-line inspection tool  10 . For example, inspection assemblies  12  may move in a radial direction  56  with respect to the rest of an in-line inspection tool  10 . This freedom of motion may accommodate changes in the pipe being inspected. For example, features such as bends, constrictions, changes in the thickness of the wall of the pipe, circumferential welds, dents, and damaged pipe walls may all affect the interior diameter of a pipeline. Movement of an inspection assembly  12  may permit sensor housings  52  to closely track the interior surface of a pipeline in spite of changes in the interior diameter thereof. 
         [0040]    Referring to  FIG. 3 , in certain embodiments, an inspection assembly  12  comprising a magnetostrictive EMAT may comprise a transmitting coil  54  (e.g., a meander line coil  54 ) that may be placed with its long axis parallel to the axis  62  of the pipe  60  being inspected. A magnetic field  64  may be introduced into the pipe wall by the magnets  48  of the inspection assembly  12  such that the field  64  is directed parallel to the long axis of the transmitting coil  54  and along the axis  61  of the pipe  60 . When the transmitting coil  54  is activated by an alternating current pulse, a magnetostrictive stress wave  66  (e.g., a horizontal shear wave  66 ) may be emitted perpendicular to the axis of the coil  54  in the circumferential direction  63  of the pipe  60 . 
         [0041]    Referring to  FIG. 4 , in selected embodiments, once produced the horizontal shear wave  66  may be guided by the surfaces of the pipe wall to travel around the circumference of the pipe  60 . In so doing, the wave  66  may encounter an obstruction such as a defect  68  in the pipe wall. A defect  68  may produce a reflected pulse  70  that can be received at or near the location of the transmitting coil  54  in a pulse-echo mode. A defect  68  may also alter the portion  72  of the wave  66  that passes the defect  68 . 
         [0042]    The altered wave  72  may be received by a receiving coil  74  located past the defect  68  in a pitch-catch mode. This receiving coil  74  may be located anywhere up to 360° around the pipe  60  in the circumferential direction  63  where it would overlay the transmitting coil  54 . At 360° the receiving coil  74  may be the transmitting coil  54  itself in a non-transmitting mode. The receiving coil  74  may be placed even slightly more than 360° around the circumference of the pipe  60 . Accordingly, the entire circumference of the pipe  60  may be inspected by one or more such devices  54 ,  74 . 
         [0043]    Embodiments in accordance with the present invention may include any suitable combination of transmitter and receiver including, without limitation, a transmitting coil  54  used as both a transmitter and receiver in a pulse-echo mode; a transmitting coil  54  with a separate, comparable receiving coil  74   a  placed within the magnetic bias field  64  or in a similar bias field slightly ahead of the transmitting coil  54  in a pulse-echo mode; a transmitting coil  54  with a comparable receiving coil  74  in a comparable magnetic bias field placed beyond the location of potential axially oriented defects  68  in a pitch-catch mode; a transmitting coil  54  used as a transmitter and receiver in a pitch-catch mode for guided waves traveling 360° completely around the circumference of the pipe; or a transmitting coil  54  with a separate comparable receiving coil  74   a  placed within the magnetic bias field  64  or in a similar bias field slightly removed from the transmitting coil  54  in a pitch-catch mode for guided waves  66  traveling just under or just over 360° around the circumference of the pipe  66 . 
         [0044]    Full coverage of the pipe cross section may be enhanced by using one inspection assembly  12  as described or by two or more of the described inspection assemblies  12  offset from one another along the pipe axis  62  with their transmitting coils  54  offset from one another circumferentially as well. In one embodiment, an in-line inspection tool  10  in accordance with the present invention may include three inspection assemblies  12  operating in accordance with the foregoing. The three inspection assemblies  12  positioned equidistant from one another in the circumferential direction  38  around the tool  10 . 
         [0045]    In embodiments where the direction of the magnetic field  64 , the direction of the axis of the transmitting coil  54 , or the configuration of the receiving coil  54  may be different from the embodiment illustrated, other stress waves may also be emitted. To isolate the waves used for inspection, waves extraneous to the inspection may be removed by frequency filtering, time gating, digital signal processing or other techniques consistent with the nature of the initiating pulse. 
         [0046]    Referring to  FIG. 5 , the pipeline environment is noisy. Accordingly, low level signals propagating through a pipeline environment may be partially or entirely obscured by ambient background noise from pipeline activity, movement an-line tool through the pipeline, etc. In the past, to have operated in the noisy pipeline environment, an in-line inspection tool using EMAT would have required large, high power equipment to produce a signal strong enough to be visible above ambient pipeline background. However, such equipment is incompatible with the confined spaces and limited power of in-line inspection tools. As a result, few EMAT in-line inspection tools exist in the market today. 
         [0047]    To overcome the noisy pipeline environment, selected embodiments in accordance with the present invention combine low level inspection signals with a pseudorandom binary sequence (PRBS) to improve the tool&#39;s  10  ability to detection the inspection signals. That is, the pseudorandom binary sequence may be used to separate the inspection signal from ambient background signal or ambient noise. By so doing, technologies such as EMAT become practical because low level signals may be generated by smaller transducers that consume much less power. 
         [0048]    A PRBS may comprise a sequence of analytically manageable binary signals formulated to represent a period of random noise that typically would not be analytically manageable. Correlation is a mathematically defined term that essentially measures how well a function will match another function. If the functions are identical, correlation will have a value of 1, which means that a period of the signal will exactly match the same period of the identical function when the two periods overlay one another. A PRBS is formulated as a periodic function with continual repetition of the period. 
         [0049]    Autocorrelation is the correlation of a function with itself. Cross correlation is the correlation of one function with another function. Each of these terms is also mathematically defined. A true random number sequence will auto correlate perfectly with itself to produce a delta function, or a function with a value of one where the function overlays itself once each period and a value of zero everywhere else. 
         [0050]    A basic PRBS is illustrated in  FIG. 5   a.  The sequence consists of binary values of 1 or −1 and is periodic from a beginning point  80  to an ending point  81 . As shown in  FIG. 5   b,  autocorrelation of the basic PRBA of  FIG. 5   a  produces an approximation of a delta function. Peaks  82  and  83  in the auto correction function appear at the points where the basic PRBA auto correlates with itself, separated by one period. Periods may be represented in time. 
         [0051]    Referring to  FIG. 6 , different PRBSs can operate at different frequencies (e.g., average frequencies). For example, the PRBS of  FIG. 6   a  has a significantly higher frequency than does the basic PRBS of  FIG. 5   a.    FIG. 6   b  illustrates the autocorrelation function corresponding to the PRBS of  FIG. 6   a.  This autocorrelation function demonstrates that increased signal frequency produces an improved approximation of the delta function with sharper nodes as shown by the resulting peaks  84  and  85 . 
         [0052]    Referring to  FIG. 7 , a working model of a PRBS is illustrated. A first PRBS is shown in  FIG. 7   a.  A time delayed version of the first PRBS is shown in  FIG. 7   b.  The autocorrelation function of the first PRBS with the delayed version of itself is shown in  FIG. 7   c.  An inspection pulse  90  (e.g., wave  66 ) that may be generated by an inspection assembly  12  (e.g., a magnetostrictive EMAT  12 ) is shown in  FIG. 7   d.  This pulse  90  may have a low power and would be difficult to identify in a noisy background. To improve its detectability, the pulse  90  is combined or convoluted with the PRBS (i.e., the signal of  FIG. 7   a ). The resulting convolution as delayed by travel time is shown in  FIG. 7   e.  When the convoluted signal of  FIG. 7   e  is cross correlated with the reference PRBS of  FIG. 7   a,  the result is the correlation function of  FIG. 7   f,  which reveals the pulse  90  with a time delay equal to the delay of the delayed version of the first PRBS. 
         [0053]    In operation, the convoluted signal (e.g., the signal of  FIG. 7   e ) may be transmitted by an inspection assembly  12  (e.g., EMAT  12 ). As the convoluted signal travels through the wall of the pipe  60  being inspected, it may be affected by defects or other anomalies in the wall. On reception by a receiver, the convoluted signal may be cross correlated with the reference PRBS. The resulting cross correlation function  7   f  may then reveal the inspection signal or pulse  90 . 
         [0054]    In operation, the time delay before an inspection wave or pulse  90  may be equal to the travel time of the signal  90  from transmission to reception. Knowing the speed of travel of the stress wave  90  in the pipe wall material allows calculation of the distance between a transmitter/receiver and a defect  68 . 
         [0055]    In a noisy pipeline environment, the pulse  90  of  FIG. 7   d  would be totally obscured. However, using the apparatus and methods in accordance with the present invention, an inspection pulse  90  may be identified despite the background noise. For example,  FIG. 8   a  shows the pulse  90  of  FIG. 7   d  in background noise from the pipeline environment and the pulse is entirely obscured. As used in the invention,  FIG. 8   b  shows the transmitted signal of  FIG. 7   e  (which is a convolution of the pulse and the PRBS delayed by travel time) in the noisy background. Thus, the signal shown in  FIG. 8   b  may be comparable to the signal received by a receiving coil  74  in accordance with the present invention. 
         [0056]    When the signal of  FIG. 8   b  is cross correlated with the original PRBS of  FIG. 7   a  the function of  FIG. 8   c  is obtained. The function clearly shows the inspection pulse  100  somewhat modified by the effects of travel and delayed by travel time. It clearly evidences the shape of the original pulse  90 . The random noise in the environment is uncorrelated and drops out. Accordingly, anomalies  68  in the pipe wall may be thus identified and located. Characteristics of the anomalies  68  may be determined by alteration of the pulse shape as it is reflected by or partially passes by the anomaly  68 . 
         [0057]    Referring to  FIG. 9 , one embodiment of an in-line inspection tool  10  in accordance with the present invention may include a signal generator  110 , transmitter  112 , receiver  114 , decoder  116 , and other components as desired or necessary. A signal generator  110  may be formed using software, hardware, or some combination thereof. In operation, a signal generator  110  may generate a PRBS and convolute the PRBS with an input pulse to pass a coded pulse to a transmitter  112  (e.g., EMAT). In certain embodiments, the signal driving the transmitter  112  thus may be the convolution output from the signal generator  110 . 
         [0058]    The transmitter  112  may induce a signal  118  into the pipe wall. In embodiments where the transmitter  112  comprises an EMAT, the inherent characteristics of EMAT technology may result in a relatively low strength signal  118  in the pipe wall. As the convoluted signal  118  travels through the pipe wall, it may encounter defects in the pipeline. These defects may produce reflections received by a receiver  114 . A decoder  116  may then correlate the signal received by the receiver  114  with a reference pseudorandom signal. In this manner, the weak inspection signal may be identified despite low signal strength and noise in the environment, even when the signal-to-noise ratio is relatively low and may even be less than one. 
         [0059]    In-line inspection tools  10  based on technologies other than EMAT are hindered by the same noise effects. While the present invention may be described herein in the context of a magnetostrictive EMAT in-line inspection tool  10 , the present invention may be used in conjunction with other sensing technologies. For example, the present invention may be applied to in-line inspection tools utilizing ultrasonic inspection (UT), eddy current inspection, Lohrentz Force electromagnetic acoustic transducer (EMAT) inspection, and the like. 
         [0060]    The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects only as illustrative, and not restrictive. The scope of the invention is, therefore, indicated by the appended claims, rather than by the foregoing description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope. 
         [0061]    What is claimed and desired to be secured by United States Letters Patent is: