TIDAL GRAVITY MEASUREMENTS IN AFRICA

Tidal variations of the gravity have been measured at 31 places in Africa and Madagascar with five carefully intercalibrated gravimeters. The minimum duration of the measurements was six months at each station. The main goal is to compare the results, corrected for the direct elastic solid earth tide effects, with the attraction and loading effects of the oceanic tides. The agreement with the oceanic corange-cotidal Schwiderski maps is fair at most stations, considering the noise level of the instruments (about 0.5 mugal on the M2 tide amplitude) but strong positive residues are observed in Afar and Somalia while strong negative residues are observed in West Africa. Such regional anomalies are observed on other continents. An interpretation needs further research. A practical result of this investigation is the possibility to accurately correct field gravity measurements for tidal effects.