Patent Publication Number: US-2019178844-A1

Title: Differential magnetic evaluation for pipeline inspection

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application claims the benefit of U.S. Provisional Application No. 62/596,731 filed on Dec. 8, 2017, entitled DIFFERENTIAL MAGNETIC EVALUATION FOR PIPELINE INSPECTION, which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Inspection of various piping systems and pipelines for defects, cracks, corrosion, wear and the like is important for maintaining the integrity of such systems to avoid potentially catastrophic consequences from failure of pipes during use. In some applications, the piping systems are used to transport hot and/or corrosive materials. Often, such piping systems are provided with an exterior layer of insulation or the like, which prevents visual inspection of the piping system and inhibits the use of conventional inspection systems that require direct access to pipes. For example, piping systems for transporting petroleum products or the like over large distances often include a thick layer of polymeric insulation and an outer metal sheathing. Such piping systems are extremely difficult and costly to effectively monitor for wear, corrosion, damage and similar defects. Other piping systems are difficult to access for other reasons. For example, piping systems and risers associated with off-shore drilling are substantially located underwater, and are therefore difficult to access. Such piping systems may also be coated or encased with a protective outer casing, for example a plastic, elastomeric or metal outer jacket. 
     Conventional state of the art pipe inspection systems typically use inspection probes called inline inspection pigs that are inserted directly into the pipe and travel along the pipe. An inspection pig may be self-propelled or may be carried through the pipe by the flow within the pipe. One obvious disadvantage of inspection pigs is that they require access to the interior of a pipe. For many pipe systems, accessing the pipe to insert the inspection pig can be problematic, as it typically requires shutting down the flow within the pipe, significant internal cleaning and some disassembly and/or use of an access port. Furthermore, many pipe systems operate at high internal pressure in order to quickly move material through the pipes. Inserting or removing an inspection pig from a pressurized piping system can be dangerous and can lead to fatal accidents if care is not taken. It would therefore be desirable to provide a pipe inspection system that may be used for inspecting the condition of the pipe even when the pipe is not easily accessible, is in use, and/or is covered with a protective covering. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a pipe inspection system arranged around a pipe. 
         FIG. 2  is a cross-sectional diagram of the pipeline inspection system arranged around the pipe. 
         FIG. 3  is a diagram that shows a magnetometer sensor pair positioned over a pipe that includes a defect. 
         FIG. 4 a    is a perspective view of the pipe inspection system arranged around a portion of the pipe. 
         FIG. 4 b    is a perspective view of the pipe inspection system without a cover to expose the magnetometer pairs arranged on a surface of the magnetometer ring. 
         FIG. 4 c    is an isometric cross-sectional view of the magnetometer ring that shows the magnetometer pairs distributed around the magnetometer ring. 
         FIG. 5  is a flow chart of a method of using the pipe inspection system to detect defects in the pipe. 
     
    
    
     DETAILED DESCRIPTION 
     An externally applied inspection system and method for non-invasive testing of piping or pipelines to detect the presence of defects is disclosed herein. The system includes a magnetometer ring with a plurality of magnetometer pairs that are circumferentially spaced about the magnetometer ring. Two excitation coils surround the pipe, and are mounted on either side of the magnetometer ring. Electric field generated in the excitation coils induce a magnetic field in the metal portions of a pipe being tested. The magnetic field is detected and measured by magnetometer pairs on the magnetometer ring. Each magnetometer pair includes two sensors that detect the strengths of the two magnetic fields. The system detects the presence of defects within the section of pipe by comparing the strengths of the magnetic fields measured by the two sensors within each magnetometer pair in the ring of magnetometer pairs 
     Various embodiments of the invention will now be described. The following description provides specific details for a thorough understanding and an enabling description of these embodiments. One skilled in the art will understand, however, that the invention may be practiced without many of these details. Additionally, some well-known structures or functions may not be shown or described in detail, so as to avoid unnecessarily obscuring the relevant description of the various embodiments. The terminology used in the description presented below is intended to be interpreted in its broadest reasonable manner, even though it is being used in conjunction with a detailed description of certain specific embodiments of the invention. 
       FIG. 1  depicts a perspective view of an inspection system  100  (the “system”) that is mounted to a pipe  102 . The inspection system  100  includes first and second excitation coils  110   a  and  110   b  that wrap around the outer surface of pipe  102 . A magnetometer ring  112  is disposed between the first and second excitation coils  110   a  and  110   b . A transportation assembly  106 , which includes multiple drive units  108  having rotating propulsion tracks, is mounted to excitation coils  110   a ,  110   b  and the magnetometer ring  112 . The transportation assembly provides motive force to move the inspection system along pipe sections. System  100  also includes a controller  104 , which provides control signals to the coils  110   a ,  110   b , and the transportation assembly  106  and receives data from the magnetometer ring  112 . As will be described in additional detail herein, signals applied to the excitation coil induce a magnetic field in the metal portions of pipe, which is detected and measured by the magnetometer ring. 
     The first and second excitation coils  110   a  and  110   b  are positioned around the exterior surface of pipe  102  at first and second axial positions, respectively. For convenience, the excitation coils  110   a  and  110   b  may each be formed from two semicircular halves that are configured to surround half of pipe  102 . The two semicircular halves may be attached to each other at the ends of the halves using a clamp or clamps  113 . The clamp (or clamps)  113  securely fasten the two halves of a given excitation coil to each other. However, the configuration depicted in  FIG. 1  is merely an example. In some embodiments, the two halves of a given excitation coil  110   a  and  110   b  may be connected to each other with a movable hinge on one end and a clamp (or clamps) on the other end, allowing the two halves of the given excitation coil to remain attached to each other when system  100  is being attached to pipe  102 . Each excitation coil  110   a ,  110   b  includes loops of conductive material, such as a copper wire, used to carry an electric current. The conductive material in each half of a given excitation coil may be connected to the conductive material in the other half with an electrical connector that is releasably engageable such that each coil  110   a ,  110   b  may be opened when inspection system  100  is attached to pipe  102 . When attached to the pipe, the electrical connector ensures that the coil halves are connected and current can flow the coil. 
     Controller  104  includes an alternating current (AC) power supply (not shown) that is connected to the first and second excitation coils  110   a  and  110   b , providing an alternating current to the two excitation coils in order to energize them. In some embodiments, the alternating current has a low frequency, such as a frequency less than 100 Hz, less than 10 Hz, or even less than 5 Hz. However, the optimal frequency range will depend on the particular geometry of the piping to be examined and one of ordinary skill in the art will be sufficiently able to identify a suitable frequency for a given piping section configuration. Additional frequencies and frequency patterns are disclosed in U.S. application Ser. No. 14/878,736, filed Oct. 8, 2015, entitled “EDDY CURRENT PIPELINE INSPECTION APPARATUS AND METHOD,” which is hereby incorporated by reference in its entirety. 
     Magnetometer ring  112  is arranged around the exterior surface of pipe  102  at a third axial position that is located between the first and second axial positions. The magnetometer ring may be located at a midpoint between the first and second excitation coils such that the distance between the third axial location and the first axial location is the same as the distance between the third axial location and the second axial location. The magnetometer ring  112  includes a plurality of magnetometer pairs (not shown) that are circumferentially spaced about the magnetometer ring  112  approximately adjacent to the exterior surface of the pipe  102 . The magnetometer ring  112  may also be formed from two semicircular halves that are attached to each other with a hinge, allowing the magnetometer ring to open when being placed around the pipe  102 . The use of a magnetometer ring with an inner circular opening ensures that the spacing and position of magnetometer pairs is accurately maintained. That is, a magnetometer pair may be spaced equidistance from the two neighboring magnetometer pairs based on the mounting location of the magnetometer pairs to the magnetometer ring. Moreover, each magnetometer pair may be accurately positioned adjacent to the exterior surface of the pipe  102  when the magnetometer ring is installed around the pipe. One or more clamps may also be used to secure the two semicircular halves of the magnetometer ring  112  to each other and electrical connectors may also be used to electrically connect circuitry in the two halves. As will be discussed in greater detail below, the electric field generated in the excitation coils  110   a  and  110   b  by the AC power induce a magnetic field in the metal portions of pipe  102 , which is detected and measured by the magnetometer pairs. 
     Controller  104  can function as a data acquisition and processing system for receiving and processing data. The controller  104  includes a data acquisition system that is operatively connected to the magnetometer pairs and the AC power supply. The data acquisition system monitors the application of the AC power to the first and second excitation coils  110   a  and  110   b  by the AC power supply, which stimulates the generation of magnetic fields in the pipe being tested. The data acquisition system also receives sensor data from the magnetometer pairs, which is used to evaluate and inspect the pipe  102  in the vicinity of the magnetometer pairs. The data acquisition system may include analog-to-digital converters to convert measured analog signals from the magnetometer pairs into digital representations of those signals for further processing. In some embodiments, the controller  104  may include a data processing system that is used to pre-process the sensor data. The data processing system may include one or more microprocessors, microcontrollers, field programmable gate arrays (FPGAs) and memory. Signals from the magnetometer pairs may be digitized, stored, and then processed to filter the signals (e.g., to remove noise), to add or subtract different signals from each other, or to compare signal values to one or more thresholds. The controller  104  may be operatively connected to a remote data processing system (e.g., a laptop) using either a wired or a wireless connection. After pre-processing the sensor data, controller  104  may transmit the processed sensor data to the remote data processing system for further processing. In other embodiments, the controller  104  may not include a data processing system and may transmit the sensor data to the remote data processing system without the controller  104  performing any pre-processing. 
     Controller  104  may also include a tracking system (not shown) that is used to sense the position and/or movement of the system  100  as the system moves along the length of the pipe  102 . In some embodiments, the tracking system may be a GPS module that may be used to collect position data of the system  100 . The tracking system may also include accelerometers to measure the motion and speed of system  100  as it moves along pipe  102 . The tracking system may also be configured to detect indicators attached to pipe  102  that can then be used to determine the location of the system  100 , as well as the time at which system  100  passes by the indicators. The position data may be stored on controller  104  or may be transmitted to a remote monitoring system to enable real-time tracking of system  100 . 
     Inspection system  100  may also include a transportation assembly  106  that is used to move inspection system  100  along the length of pipe  102 . Transportation assembly  106  includes moving means, such as multiple drive units  108  with rotating tracks or treads that contact the exterior surface of pipe  102  and propel the inspection system along the pipe  102 . As the transportation assembly moves the inspection system  100  along the length of the pipe  102 , the system  100  may pause at regular intervals to inspect specific parts of pipe  102  using excitation coils  110   a ,  110   b  and the magnetometers in magnetometer ring  112 . In other embodiments, the inspection may continuously move and inspect parts of pipe  102  using the excitation coils  110   a ,  110   b  and the magnetometers in the magnetometer ring without pausing at regular intervals. As the sensor data is collected with the magnetometers in magnetometer ring  112 , the sensor data is correlated with position data from the tracking system. By correlating the position data with the magnetometer data collected at each inspection point, a user of the inspection system  100  is able to accurately determine the location and position of a defect in pipe  102 . 
       FIG. 2  is a diagram that shows a cross section of inspection system  100  that is arranged around pipe  102 . Magnetometer ring  112 , which includes a plurality of magnetometers pairs  116  circumferentially spaced around ring  112 , is located between first and second excitation coils  110   a  and  110   b , spaced an axial distance L away from both excitation coils. The excitation coils are connected to an AC power supply (not shown) in the controller  104 . The AC power supply produces a first alternating current that is used to energize the first excitation coil  110   a  and a second alternating current that is used to energize the second excitation coil  110   b . In some embodiments, the first and second alternating currents are in phase with each other. If desired, the AC power supply may generate a single alternating current that is applied to both the first and second excitation coils such that the alternating currents are in phase with each other. In other embodiments, the AC power supply may generate a single alternating current, but the current is split and phase-shifted before being applied to the first and second excitation coils  110   a ,  110   b  such that the alternating currents received by first and second excitation coils are out of phase with each other. In alternative embodiments, the AC power supply may separately generate first and second alternating currents that are provided to the first and second excitation coils  110   a ,  110   b , respectively. These first and second alternating currents may be in phase or may be out of phase with each other. 
     During operation of the inspection system, excitation coils  110   a  and  110   b  are energized with an alternating current. Each excitation coil includes several loops of a conductive material, such as a copper wire, that receive the alternating current. As is well-known in the art, running a current through a wire generates a magnetic field around that wire. When an excitation coil is energized by an alternating current, it generates a magnetic field having a magnitude and direction that are dependent on the amount of current and the polarity of the alternating current passing through the excitation coil. The generated magnetic field propagates through pipe  102 , producing localized magnetization in the metal portions of the pipe  102 . The behavior of the magnetic field as it propagates through a conductive material is related to the physical properties of the material. For a material without any significant defects, such as cracks, corrosion, or pitting, the magnetic field propagates through the metal in a predictable manner, decreasing in magnitude as it moves further from the source of the magnetic field. However the magnetic field may deflect off of defects in the pipe, causing abnormalities in the propagation of the field. This phenomenon is often referred to as magnetic flux leakage because the magnetic field “leaks” from the conductive material. As the magnetic field propagates through the pipe  102 , magnetometers placed on (or near) the surface of pipe  102  can detect and measure the magnitude of the magnetic field in order to detect defects in the pipe. 
     The measured magnitude of the magnetic field generated by an excitation coil  110  at a given point on pipe  102  is dependent on the distance between the magnetometer and the excitation coil and the strength of the current flowing through the wires in the excitation coil. Increasing the distance between the given magnetometer and the excitation coil and/or decreasing the strength of the current decreases the magnitude of the magnetic field measured by the magnetometer. Conversely, decreasing the distance between the magnetometer and the excitation coil and/or increasing the strength of the current increases the magnitude of the magnetic field measured by the magnetometer. The measured direction of the magnetic field is dependent on the polarity of the electric current that is applied to the excitation coil. If the electric field has a positive polarity, the magnetic field points in one direction. If the electric field has a negative polarity, the magnetic field points in the opposite direction. For magnetic fields generated by alternating currents, the direction the magnetic field points will thereby switch directions as the alternating current changes polarity. 
     Depending on the alternating current applied to the two excitation coils, there are different locations in the pipe being tested where constructive and destructive interference occurs between the stimulated magnetic fields. If the two magnetic fields are in phase with each other, the magnetic fields may constructively interfere and the measured magnitude at a given point is equal to the sum of the magnitudes of the two fields at that point. If the two fields are out of phase with each other, the two magnetic fields may destructively interfere and the measured magnitude at a given point is equal to the difference between the magnitudes of the two fields at that point. In other words, if two magnetic fields have the same magnitude at a given point and are in phase with each other, the measured magnitude at that given point will be equal to twice the magnitude of one of the magnetic fields. However, if the two magnetic fields have the same magnitude at a given point but are 180° out of phase, the measured magnitude at that given point will be zero, as the two magnetic fields will destructively interfere and cancel each other out at that point. 
       FIG. 3  is a representative diagram that shows a magnetometer pair  116  positioned over a defect  118  formed in the pipe  102 . Magnetometer pair  116  includes a first magnetometer sensor (“sensor”)  120   a  and a second magnetometer sensor  120   b  that are able to detect and measure the strength of a magnetic field. Sensors  120   a  and  120   b  are radially positioned over respective first and second points on pipe  102  and are configured to measure the strength of the magnetic field at the first and second points. The sensors  120   a  and  120   b  in a given magnetometer pair  116  may be formed on a single circuit board and may be positioned at the same axial location around the pipe  102  (i.e., the magnetometer pairs are separated from one another in a radial direction around the pipe). The first and second magnetometer sensors  120   a  and  120   b  generate respective first and second sensor data based on the magnitude of measured magnetic fields in the pipe. The first and second sensor data from each magnetometer pair  116  is provided to the controller  104 , which calculates a difference between the first and second sensor data for each magnetometer pair  116 . When the magnetometer ring  112  is arranged around a portion of pipe  102  that has no defects (i.e., no cracks, corrosion, or pitting), the calculated difference between the first and second sensor data from a given magnetometer pair  116  will be low, indicating that the magnetic field measured by the first sensor  120   a  has a similar magnitude to the magnetic field measured by second sensor  120   b.    
     However, when the magnetometer pair  116  is located over a portion of the pipe  102  that does have defects, the calculated difference between the first and second sensor data for a given magnetometer pair  116  will be higher, indicating that the magnetic fields measured by the first sensor  120   a  has a different magnitude than the magnetic field measured by the second sensor  120   b . As previously mentioned, when a magnetic field interacts with a defect  118  in pipe  102 , the magnetic field is disturbed by the pipe defect. When a magnetometer sensor is positioned above (or near) the defect, the change in the magnetic field caused by the magnetic flux leakage is detectable by the sensor. If a given magnetometer pair  116  is arranged near (or at least partially overlaps with) a portion of the pipe  102  that has a defect, one sensor  120   a  in the magnetometer pair  116  may detect a magnetic field that is affected by the defect while the other sensor  120   b  may detect a magnetic field that is not affected by the defect. In this scenario, the sensor data generated by the two sensors  120   a ,  120   b  will be different and the controller  104  can calculate a difference between the two sensor data. If the calculated difference is higher than a threshold, the controller  104  interprets such difference as evidence that a defect is located near the magnetometer pair  116 . 
     In some embodiments, first and second sensors  120   a  and  120   b  may be vector magnetometer sensors, and more particularly fluxgate magnetometer sensors. In other embodiments, other types of magnetic field detectors may alternatively be used. For example, in some embodiments, sensors  120   a  and  120   b  may be magnetoresistive magnetometer sensors (e.g., giant magnetoresistive or anisotropic magnetoresistive magnetometer sensors). 
     Magnetometer sensors may have a direction of sensitivity along which the sensor is most sensitive to a magnetic field. As shown by the dashed lines interposed between the sensors  120  and the pipe  102  in  FIG. 3 , the first and second sensors  120   a  and  120   b  may have directions of sensitivity that are oriented towards the pipe  102 . Preferably, the first and second sensors  120   a  and  120   b  are aligned so that each magnetometer pair  116  has a direction of sensitivity that is oriented towards the central axis of the pipe  102 . In other words, the first sensor  120   a  is most sensitive to changes in the magnetic field that occur between the first sensor  120   a  and the central axis of the pipe  102  and the second sensor  120   b  is most sensitive to changes in the magnetic field that occur between the second sensor  120   b  and the central axis of the pipe  102 . Depending on the size of the pipe being tested, and the number of magnetometer pairs, that means that the first sensor and the second sensor are aligned at slight angles (i.e., not parallel) to one another. 
     In the example shown in  FIG. 3 , defect  118  is located between the first sensor  120   a  and the central axis of the pipe  102 , and therefore lies in the direction of sensitivity of the first sensor  120   a , but defect  118  is not located between the second sensor  120   b  and the central axis of the pipe  102 , and therefore does not lie in the direction of sensitivity of the second sensor  120   b . During operation of inspection system  100 , magnetic fields induced in the pipe  102  may interact with defect  118 , changing the magnitude and/or direction of the magnetic field. Because defect  118  lies in the direction of sensitivity of first sensor  120   a  but not in the direction of sensitivity of second sensor  120   b , the first sensor  120   a  will be more likely to detect the presence of defect  118  (by the perturbation of the magnetic field by the defect) than the second sensor  120   b . In this scenario, the sensor data generated by the first sensor  120   a  will differ from the sensor data generated by the second sensors  120   b . By calculating the difference in generated signals and comparing that difference with certain detection thresholds, the controller  104  can determine that a defect is present at the specific axial and radial location under first and second sensors  120   a  and  120   b.    
     First and second sensors  120   a  and  120   b  are arranged at the same axial location along the pipe  102  but are located at different radial positions around the pipe. In order to ensure that the respective directions of sensitivity for the two sensors are oriented towards the central axis of the pipe  102 , the two sensors are rotated relative to each other. In particular, the first and second sensors  120   a  and  120   b  are preferably rotated relative to each other such that their respective directions of sensitivity intersect at the central axis of the pipe  102 . The amount that the first and second sensors  120   a  and  120   b  in a given magnetometer pair  116  are rotated relative to each other is dependent on the number of sensor pairs mounted on the magnetometer ring  112  (impacting the distance between the two sensors  120   a ,  120   b ) and the size of the pipe that the inspection system  100  is designed to inspect (determining the distance between the central axis of the pipe and the magnetometer pair  116 ). In the embodiment shown in  FIG. 3 , the first and second sensors  120   a  and  120   b  are one of 48 designed for measuring signals from a 24-inch diameter pipe. In that configuration, the respective directions of sensor sensitivity, are rotated by about 4° relative to each other. However, this is only an example. In other embodiments, the two sensors may be rotated by less than 4°, between 4° and 10°, or by more than 10°. In alternative embodiments, the two sensors  120   a ,  120   b  may be rotated by some other amount to each other. For example, first and second sensors  120   a  and  120   b  may perfectly parallel to each other. In general, the first and second sensors  120   a  and  120   b  may be oriented by any desired amount relative to each other. 
       FIG. 4 a    is a perspective view of inspection system  100  positioned around a portion of pipe  112 . Magnetometer ring  112  is arranged between first and second excitation coils  110   a  and  110   b  and encircles pipe  102 . A cover  122  of the magnetometer ring  112  encloses the magnetometer pairs  116  and shields the magnetometer pairs from the external environment.  FIG. 4 b    is a perspective view of inspection system  100  without cover  122 , revealing the ring of magnetometer pairs  116  that are attached to the magnetometer ring  112 . The magnetometer pairs  116  are arranged around the circumference of magnetometer ring  112 . As shown in  FIG. 4C , which is a perspective view of magnetometer ring  112 , magnetometer ring  112  may include 48 magnetometer pairs  116  that are mounted on a surface of magnetometer ring  112  and arranged along an inner edge of the ring. Each magnetometer pair  116  includes first and second sensors, as previously described, and each magnetometer pair may be rotated by 7.5° relative to a neighboring magnetometer pair to ensure that each pair  116  is oriented towards the center of pipe  102 . It will be appreciated that the depicted magnetometer ring  112  with 48 magnetometer pairs  116  is merely an example, and that the magnetometer ring may contain more or less pairs depending on the size of the pipe to be tested and the desired sensitivity. For example, magnetometer ring  112  may include less than 16, 16-24, less than 36, 36-60, or more than 60 magnetometer pairs  116 . In general, magnetometer ring  112  may include any desired number of magnetometer pairs  116  that are arranged and evenly spaced around a surface of magnetometer ring  112 . Each magnetometer pair  116  is typically rotated relative to the neighboring magnetometer pairs  116  to ensure that each magnetometer pair is oriented towards the center of pipe  102 . 
       FIG. 5  is a flow chart of a method  500  of using inspection system  100  to detect a defect in a section of pipe. At step  501 , the excitation coils and the magnetometer ring are placed around the exterior surface of a pipe under test. The first and second excitation coils are located proximate to a section of pipe at a first and second axial locations, and the magnetometer ring is located proximate the section of pipe at a third axial location that is at or near the midpoint of the distance between the first and second axial locations. The magnetometer ring may include any desired number of magnetometer pairs that are disposed around an interior surface of the magnetometer ring and are evenly spaced apart. In a preferred embodiment, the magnetometer ring may include 48 magnetometer pairs that each include a respective first and second sensors. As previously mentioned, the excitation coils and magnetometer ring may be releasably engageable around the exterior surface of the pipe using hinge and clamp mechanisms. In some alternative embodiments, the excitation coils may each be formed from two semi-circular halves that are separately arranged around the exterior surface of the pipe and clamping mechanisms are used to secure the two halves to each other. 
     At step  505 , the first and second excitation coils are energized with an alternating current produced by a power source associated with the inspection system. The alternating current may have a frequency of less than 100 Hz, less than 10 Hz, or less than 5 Hz. The energized excitation coils induce an alternating magnetic field in the adjacent section of piping. 
     At step  510 , the induced magnetic field is measured using the first and second sensors in the magnetometer pairs that are distributed around the magnetometer ring. The sensors are sensitive to the strength of the magnetic field at points in the pipe wall that are immediately adjacent to the sensors. 
     At step  515 , the inspection system records first and second sensor signals from the first and second sensors in each magnetometer pair of the magnetometer ring. The signal values recorded by each magnetometer mounted on the magnetometer ring is stored in volatile or non-volatile memory of the inspection system for subsequent processing or review. 
     At step  520  the system selects a given one of magnetometer pairs in the magnetometer ring for analysis and at step  525  the system calculates a difference value for the selected magnetometer pair by determining the difference between the recorded first and second sensor signals of the pair. Because the first and second sensors in each magnetometer pair are disposed over the surface of the pipe at different radial locations, the magnetic field measured by the first and second sensors may be different, so the recorded sensor signals may also be different. 
     At step  530 , the system compares the calculated difference value for the selected magnetometer pair to a predetermined threshold difference value and determines if the calculated difference is greater than the predetermined threshold. The predetermined threshold difference value is dependent on the size and composition of the pipe under analysis and is indicative of the presence of a defect. The threshold may be configured by a system operator when the inspection system is installed on the pipe or may be a dynamically determined by taking an average reading from all sensors on a pipe section with no known defects and applying a fixed percentage (e.g., 70%) to that average reading. If the calculated difference is greater than the predetermined threshold difference value, the method proceeds to step  535 . If the calculated difference is less than or equal to the predetermined threshold difference value, the method proceeds to step  540 . 
     At step  535 , the system stores axial and radial position data of the selected magnetometer pair and indicates that a defect is likely present at the position. At step  540 , the system stores axial and radial position data of the selected magnetometer and indicates that a defect is likely not present at the position. After storing the position data, the method proceeds to step  545 . 
     At step  545 , the system determines if there are additional magnetometer pairs that have yet to be analyzed at the axial location. If there are additional magnetometer pairs, the method returns back to step  520  to perform additional analysis of the remaining magnetometer pairs. If there are not any additional magnetometer pairs, the method proceeds to step  550 . 
     At step  550 , the system uses a transportation assembly to move the entire inspection system to a new section of the pipe. The inspection system is moved only so far as to ensure that all portions of underlying pipe are scanned by inspection system. After moving the inspection system, processing continues to step  505  and the inspection system scans the new section of pipe for the presence of defects. 
     As previously described, a power supply in controller  104  generates an alternating current with a low frequency. This alternating current, which has a frequency of less than 100 Hz, less than 10 Hz, or less than 5 Hz, generates a magnetic field in pipe  102  that interacts directly with defects in pipe  102 . The direct interaction between the magnetic field and defects in the pipe  102  causes changes in the magnetic field due to magnetic flux leakage, which are detectable by sensors  120  in magnetometer ring  112 . In other embodiments, the power supply in controller  104  may generate an alternating current with a frequency that is greater than or equal to 100 Hz, such as 150 Hz, 200 Hz, or 250 Hz. At these frequencies, sensors  120  may detect magnetic fields induced by eddy currents in pipe  102 . Running an alternating current with a higher frequency through the excitation coils  110   a ,  110   b  induces a changing primary magnetic field disposed generally axially along the pipe  102 . This primary magnetic field induces eddy currents within the pipe  102 . Eddy currents then induce a secondary magnetic field that has an opposite direction from the primary magnetic field. The induced eddy currents within the pipe  102  will be impacted by defects or other anomalies in the pipe  102 , such as cracks, corrosion, pitting, or the like. Changes in the eddy currents will in turn cause corresponding changes in the secondary magnetic fields induced by the eddy currents. The changes to the secondary magnetic fields are detectable by sensors  120 . 
     In general, the source of the magnetic fields detected and measured by sensors  120  in magnetometer ring  112  is dependent on the frequency of the alternating current provided to the first and second excitation coils  110   a  and  110   b . At lower frequencies, eddy currents generated by the magnetic field induced by excitation coils are not strong enough to generate secondary magnetic fields that are detectable by the sensors  120 . At higher frequencies, the eddy currents generate secondary magnetic fields that overwhelm the effects of magnetic flux leakage on primary magnetic fields. At frequencies of about 100 Hz, sensors  120  in magnetometer ring  112  may detect defects in the pipe  102  due to the hybridized effects of magnetic flux leakage affecting primary magnetic fields in pipe  102  and secondary magnetic fields that are generated by eddy currents, which are affected by defects in the pipe  102 . 
     The depth of the induced eddy currents in the pipe  102  is dependent on the frequency of the alternating current applied to the excitation coils  110   a ,  110   b . For example, a higher frequency current applied to the excitation coils  110   a ,  110   b  may induce a relatively shallow eddy current (i.e., closer to the outer surface of the pipe  102 ). A lower frequency current applied to the excitation coils  110   a ,  110   b  may in turn induce a relatively deeper eddy current (i.e., closer to the inner surface of the pipe  102 ). The depth of the eddy current will depend on the characteristics of the AC signal applied to the excitation coil, the configuration of the coil, and the dimensions and materials of the pipe itself. By utilizing varying frequencies to produce eddy currents of varying depth, the depth of a defect can be detected in addition to detecting its axial location. For example, a scan including multiple frequencies applied sequentially can indicate both the axial location of the defect as well as the depth of the defect. 
     However, using multiple frequencies that are applied sequentially can be time-consuming. To expedite the process, a synthesized multi-frequency waveform can be applied to the excitation coils  110   a ,  110   b  in order to measure a range of depths simultaneously. For example, a multi-frequency waveform may include peaks at 100 Hz, 150 Hz, 200 Hz, and 250 Hz, which are each sensitive to defects at different depths of the pipe. The excitation coils generate four eddy currents in pipe  102 , each corresponding to a specific frequency peak in the multi-frequency waveform of the primary magnetic field and each eddy current is only sensitive defects present at the same depth as the given eddy current. If a defect is located at a depth that corresponds to an eddy current induced by the 200 Hz peak of the multi-frequency waveform, as an example, then only the secondary magnetic field generated by the 200 Hz eddy current will be affected by the defect. Sensors  120  in magnetometer ring  112  may be able measure and detect both the frequency and the magnitude of the secondary magnetic field and sensor data that is transferred to controller  104  may include both frequency and magnitude data. By utilizing both the frequency data in conjunction with the magnitude data, the controller may be able to calculate the depth of a defect within the pipe, in addition to the axial and radial location of the defect. 
     As shown in  FIG. 1 , the inspection system  100  includes first and second excitation coils  110   a  and  110   b . However, this is merely an example. In other embodiments, the inspection system  100  may include only a single excitation coil  110 . During operation of the inspection system  100 , a current may be generated by a power supply in controller  104 . The current is provided to the single excitation coil  110 , which generates a magnetic field in the pipe  102  using the provided current. Sensors  120  in magnetometer ring  112  detect and measure the magnetic field generated by the single excitation coil and provide sensor data to the controller  104  based on the strength of the magnetic field. 
     In some embodiments, magnetic fields in pipe  102  are induced by excitation coils  110   a  and  110   b  when a current is run through conductive material in the excitation coils  110   a  and  110   b . In other embodiments, however, the magnetic fields may be induced in pipe  102  using other magnetic means. For example, in some embodiments, inspection system  100  may include permanent magnets, such as rare earth magnets, that are used to induce magnetic fields in pipe  102 . In other embodiments, the power supply may generate a direct current that is provided to the excitation coils  110   a ,  110   b  instead of an alternating current to produce a static magnetic field. In general, the system  100  may induce a magnetic field in the pipe  102  using any desired mechanism. 
     From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the scope of the invention. Accordingly, the invention is not limited except as by the appended claims.