Depth to sources of magnetic anomalies

Abstract

The characteristics of several long magnetic total field intensity profiles have been determined. Only track lines which were nearly straight and longer than 2000 miles were considered. First, the centered dipole field total intensity was subtracted from the measured total intensity to obtain a real nondipole field. A smooth curve was then drawn through this nondipole field using a stiff spline curve. The distance between crossover points of the smooth field and the nondipole field was determined and a histogram of the results plotted. The results confirm the earlier results of Serson and Hannaford that most of the anomalies have a very short wavelength. Ninety-three per cent of the cases had crossover distances less than 60 nautical miles. The simple form of the smooth curves indicated a nearly sinusoidal departure from a dipole field having crossover points between 2100 and 5200 nautical miles. The most natural unforced explanation of the above results is that short-wavelength anomalies are due to crustal effects and the long-wavelength anomalies are due to causes within the core of the earth. The large gap between the short- and long-wavelength groupings supports the hypothesis that the mantle is a forbidden region for magnetic sources. This conclusion is illustrated by calculations based on simple models.