Cross-over analysis of Marine magnetic anomalies

Using cross-over analysis the reduction of total magnetic intensity values to magnetic anomalies was studied in an area with ship-tracks which have been made over a period of several years. Under these circumstances the secular variation becomes important. At first we used the old IGRF 1975. This reference field does not correct the secular variation accurately. In the period 1969–1980 the mean values of the differences of the magnetic anomalies at crossing rise to more than 150 nT. The recently adopted reference fields DGRF 1965, DGRF 1970, DGRF 1975 and PGRF 1975 correct the secular variation much better. The mean values now only rise to about 50 nT over the same period. A reduction using a regional determined by filtering along the track removes the secular variation completely. The latter reduction has the disadvantage that the standard deviations of the mean values at the intersections increase, because the regional depends on the orientation of the track line. Calculation of average secular variation in subareas of 4×4 degrees yields a decrease from the northeast to the southwest.