Plumb line deflection near the North Pole

In April 1969 the deflection of the plumb line in the vicinity of the Lomonosov ridge near the north pole was determined from a drifting ice station by measuring the displacement between two drift paths, one determined astronomically and the other determined by means of a Transit Satellite receiver. In addition, the fine structure of the drift path relative to the ocean floor was obtained from acoustic measurements. The mean plumb line deflection at 89°40′N, 77°W was found to be 9 arc sec in the direction of 34°E, pointing away from the Lomonosov ridge. Computations were made concerning the perturbation of the gravity vector on the basis of a hypothetical model of the Lomonosov ridge. This model involves a density structure consistent with a continental origin of the ridge and shows that deflection gradients of up to 0.7 arc sec/km and horizontal gravity gradients of up to 3.4 mGal/km can occur. The measured plumb line deflection and gravity observations carried out in the vicinity of the ridge are consistent with the model, but because of the scarcity of observations and the very incomplete bathymetric data, no final conclusions on the origin of the Lomonosov ridge can be made. Continuous plumb line deflection measurements are ideally suited to be carried out from a drifting sea ice station where they will complement the gravity measurements in an area where regional gravity observations are scarce.