Abstract
Magnetic anomalies are formed by the superposition of earth's magnetic field and the induced field of an underground ferromagnetic pipeline. To analyze the influence of all factors on the detected magnetic anomalies, a forward model is established in this paper. A numerical integration method, magnetic dipole reconstruction (MDR), for magnetic anomaly forward modeling is proposed, and a series of calculations is performed between model parameters of the pipeline, the geomagnetic field, the measuring trace, and total magnetic anomaly (TMA). The influence of model parameters on the shape, amplitude, and magnetic width is then analyzed. Results show that the shape of TMA curve is primarily influenced by the magnetic inclination and relative azimuth of the pipeline. The amplitude is linearly related to geomagnetic intensity and pipeline parameters including diameter, thickness, and susceptibility, and decreases in inverse proportion to increase in distance between the pipeline's axis and measurement plane; it decreases in a cosine manner with decrease in inclination and relative azimuth. The magnetic width is less affected by the geomagnetic intensity and pipeline parameters, but does increase with increase in distance between the pipeline's axis and measurement plane, and decreases with decreasing inclination and relative azimuth. The MDR method effectively simulates the distributions of TMA caused by various models of pseudo-2D underground ferrous pipeline. Magnetic anomaly detection (MAD) does not effectively estimate existence or orientation of pipeline that are deeply-buried, slender or with axes parallel to the earth's magnetic field in the low magnetic latitude area. Rough measurement of magnetic anomaly is necessary before the orientation of the underground pipeline is identified, followed by exact measurements along the perpendicular direction of the axis of pipeline to accurately locate underground pipeline.
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