Transverse Continental Margin Fracture Zone off Sierra Leone

WE report here the results of a seaborne magnetic survey, using a Varian proton precession magnetometer, conducted on the continental shelf and upper slope off southern Sierra Leone (Fig. 1). During the survey period no magnetic storms were recorded but the data were corrected for daily variations of the Earth's field at M'bour, Senegal, which showed a peculiar periodicity with peaks around the noon hour averaging 50 gamma. The night hours were characterized by roughly constant values. The corrected total intensity data were plotted on the adjusted ship's track lines at 3 min spacings and an anomaly map was prepared, utilizing the AWC-70 reference field.