Inversion of magnetic fields inside pipelines: Modeling, validations, and applications

Spherical detector is a novel and promising approach for pipeline inspections with high signal-to-noise ratio and low risk of blockage. As the magnetic signals are recorded in a rotating frame by the rolling spherical detector, it is difficult to directly deploy the raw periodic signals for magnetic inspections, such as pipeline orientation calculations and the identifications of girth welds, turning points, and other pipe features. Against this backdrop, a magnetic inversion approach for transforming the measured magnetic signals from the rotating sensor frame into the stationary pipe frame is developed and demonstrated both analytically and experimentally. By comparing the transformed values with that measured by a sensor moving on a straight route, the inversion approach is proven to hold for an arbitrary fixed rotation axis, with the average and maximum errors mostly less than 1 and 2 µT, respectively. Field experiments demonstrate that the magnetic inversion can expose and enhance hidden or imperceptible magnetic anomalies and pipeline turning points and improve pipeline route calculation accuracy via secondary alignments. The results will promote the spherical detector to play a greater role in pipeline inspections.