Determination of the Differences in Gravity Values at Chiba (Japan), Singapore, Cape Town and Antarctica.

Abstract

Gravity values at Singapore, University of Malaya, and Cape Town, Trigonometrical Survey Office, were determined relative to Chiba, Geographical Survey Institute, by means of a G.S.I, pendulum apparatus. This work was an item of scientific observations carried out by the Second Japanese Antarctic Research Expedition in 1957-8. The differences in gravity values have been found to be as follows;g Singapore -g Chiba= -1709.3±0.45 milligal, g Cape Town -g Chiba = -142.8±0.45 milligal. In Singapore, a new gravity station for our pendulum observation was established at Geography Department, University of Malaya. Other gravity stations which have hitherto been established in Singapore, Raffles Museum, Kallang airport, etc. were re-occupied with a Worden gravimeter. The gravity value at the pendulum station was found to be 0.6 milligal greater than that at Raffles Museum and to be 0.1 milligal smaller than that at Kallang airport. The pendulum station of Cape Town is same as that occupied by A.L. HALES and D.I. GOUGH with a Cambridge pendulum apparatus for their measurement of gravity in South Africa in 1948 and also by W.E. BoNINI and G.P. WOOLLARD with a Gulf pendulum apparatus and two Worden gravimeters for their world gravity measurement. The difference in gravity values at Cape Town and Washington D.C., Coast and Geodetic Survey, g Cape Town -- .g Washington, which is calculated from results of two gravity ties, Cape TownTeddington tie and Teddington -Ottawa - Washington tie both with the Cambridge pendulum apparatus, is -471.8 milligals or -471.0 milligals; the former is original and the latter is adjusted value. On the other hand, another value of this difference obtained by combining the result of present tie with that of Washington -Chiba tie both by means of the G.S.I. pendulum apparatus is -472.6 milligals. A closure of 0.8 or 1.6 milligal has been yielded from the two values above mentioned. Unfortunately, we could not land "Syowa base", a Japanese Antarctic station, in our expedition. Gravity measurement, however, were carried out with the Worden gravimeter at nine stations on very closed pack-ice in Lutzow-Holm bay. Since observation was made on pack-ice, even though its thickness was over five meters, there was always observed a slight vertical movement. The effect of the movement on gravity value can be estimated to be ten milligals or less. The gravimeter used had stable and small drift which was comfirmed by continuous reading at Singapore and Cape Town. The drift rate adopted was + 0.20 milligal per day which was deduced from the closure at Cape Town after 88.1 days' voyage in Antarctica. Therefore, probable error due to inadequacy of the drift rate of the gravimeter also being taken into consideration, it is estimated that final accuracy of the measurement in this region might be ten milligals. The gravity value at all stations in Lutzow-Holm bay determined refering to Cape Town are larger than the normal value given be the international gravity formula, the magnitude of positive anomaly amounting to about +50 milligals in average . This amount of positive anomaly far exceeds the value of the estimated error in observation.