A comparison of aeromagnetic levelling techniques with an introduction to median levelling

Aeromagnetic data collected in areas with severe diurnal magnetic variations (auroral zones) are difficult to level. This paper describes levelling of an aeromagnetic survey where such conditions prevail, and where sophisticated levelling techniques are needed. Corrections based on piecewise low‐order polynomial functions are often used to minimize mis‐ties in aeromagnetic data. We review this technique and describe similar mis‐tie fitting methods based on low‐pass filter levelling, tensioned B‐spline levelling and median levelling. It is demonstrated that polynomial levelling, low‐pass filter levelling and tensioned B‐spline levelling depend on the careful editing of outlying mis‐ties to avoid the introduction of false anomalies. These three techniques are equally efficient at removing level errors. Median levelling also removes level errors efficiently, but it is more robust in the sense that mis‐tie editing is not required. This is due to the inherent noise‐removal capabilities of the median filter. After mis‐tie editing, the total field anomalies of the other three techniques closely resemble the unedited median‐levelled total field anomaly.