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60 Abstract The Gas Technology Institute (GTI) is developing a scanning electromagnetic (EM) locator for metallic pipe. The scanning EM locator is based on an earlier GTI project that developed a metallic joint locator (MJL). The MJL technology was suc- cessfully demonstrated at several utilities and licensed by Sensit Technologies for commercialization. The EM locators currently in the marketplace require that the locator be moved with respect to the pipe in order to find the pipeâs location. The signal is presumably strongest when the locator is immediately above the pipe. The locating signal is injected into the pipe either by direct metallic connection or by induction. The MJL was originally developed to find bell and spigot joints on cast iron piping. These joints can represent a 30% change in the cross section of the pipe in localized areas. It was also discovered during testing that it detects much smaller features, such as service tees and repair clamps. The MJL places a signal on the target pipe by induction and reads the induced eddy currents with a pair of coils in a differential configuration. If the MJL is moved along a featureless pipe there is a null signal. The presence of any metallic appurte- nances on the pipe unbalances the differential coils and pro- duces a signal. From the discussion above, it can be seen that the MJL must be moved along the (assumed) centerline of the pipe to detect features. The addition of the rotating induction field for the scanning EM locator removes the necessity to have a priori knowledge of the pipe centerline. Not only the signal magnitude but also the angle from the cart centerline to the target can be captured. This also means that a featureless metal pipe will produce a signal as the induction field scans it once per rotation. This technique should also be able to resolve stacked metallic utilities by rolling the cart to one side of the stack to produce a view in which there is angular separation between the targets. Technology Synopsis and Key Performance Indicators Title: Scanning Electromagnetic Locator Using a Rotating Field Provider: Gas Technology Institute Targets: Metal, with greater sensitivity to ferrous metals. Depth range: Approximately 20 ft. Depth accuracy: Depth is inferred from multiple passes instead of being directly measured. Location accuracy: Location is centered by moving the loca- tor until a null is achieved. Application: The technology is cart mounted and provides a signal indicating the location of buried metallic objects rela- tive to the cart. The cart is moved about the target area to trace the buried facility. Basic principle: The instrument consists of three sets of coils mounted on a narrow, wheeled cart. A central coil gen- erates an EM signal that passes through the soil. The signal couples into buried metallic objects and is then picked up by two coils mounted at the extreme ends of the cart. The metallic object distorts the field, allowing it to be located between the two pickup coils by the null method. Addition- ally, the EM field is rotated perpendicular to the long axis of the cart to provide the angular direction of the metallic object with respect to the cart. This angular direction can be used to infer depth by making two parallel passes at the buried object. Limitations: The overall depth range is limited by the power of the EM signal radiated by the driven coil. The indication returned by the target is directly proportional to the size of the target; large diameter pipes give a greater indication. The A P P e n d I x F Technical Support Information for Scanning Electromagnetic Locator
61 accuracy of the null location is affected by the baseline of the instrument versus the depth of the target. Using the angular bearing can âsynthesizeâ a longer baseline by taking multiple, parallel passes at the same target. Additional notes: It will be possible to run the device within a buried, nonmetallic facility such as a storm drain. Given that the rotating field sweeps out 360°, it is possible to locate metallic facilities below, alongside, or above the one in which the device is run. The same is true of lowering the device into a vertical borehole. With some modification, the device could also detect the presence of radio frequency (RF) tags buried in the vicinity.
