ON THE MEASURMLNTS OF GRAVITY ON THE COURSES FROM SYOWA STATION TO 75°S AND TO COOK POINT

Abstract

Along the inland traverse route from Syowa Station to location C21 (F451, 74°57'S, 38°27'E) and along the traverce routeon the sea ice in Lutzow-Holm Bay toward the Cook Point, the gravity was measured by Worden Gravimeter in 1961 On the inland traverse, the intervals of the observation stations were 6 km in the range from the coast to point C6 (F158, 71°00'S, 41°00'E), and 12km in the range from C6 to C21 The number of the observation stations on the sea ice of the Lutzow-Holm Bay was seventeen Seven stations of them coincided with the points where the depth was sounded by plummet The error of the measurment seemed to be mainly caused by the abnormal drift of gravimeter Though the carrier (container) of the gravimeter was covered with sheets of foam rubber and set in a wooden box, which was set on a thick sheet of foam rubber and was mounted with rubber bands to the snow car, the gravimeter must have suffered frequent shocks when the snow car passed the group of sastrugi The mean drift of the gravimeter was 0 285mgal/day in the year. But it was smaller in the period of traverse, that is, 0 182 mgal/day in the autumn inland traverse, 0 247 mgal/day in the spring inland traverse and 0 082 mgal/day in the Cook Point traverse The negatue drift was noticed at some camp sites where we stayed for several days The mean error of the measurments estimated from all viewpoints was about 3 5 mgal The results obtained from the inland traverse route are listed in a table with the location ana the height of the observation station, the values of the standard gravity, the free air anomaly, the thickness of ice, the height of bed rock and the distance from Syowa Station along the route The thickness of ice was calculated under three assumptions (1) the Bouguer anomaly is zero, (2) the density of ice is 0 9 g/cm^3, (3) the density of bed rock is 2 67 g/cm^3. Since the seismic sounding by our party was a failure, the Bouguer anomaly could not be calculated But the traverse route of the third expedition party crosses our route, and a part of traverse route of the forth expedition party is very close to our route, then their results of seismic sounding can be utilized and the rough estimation of the Bouguer anomaly can be made At station F52, about 100 km from Syowa Station, the thickness of ice calculated from the gravity is approximately equal to that from the seismic sounding, that is, the Bouguer anomaly is about -6 mgal. But in the region of Oku-Shirase Daira (the head of the Shirase Glacier), the difference of ice thickness becomes 1000～1500 m, and the Bouguer anomaly is about -75～-110 mgal. If the Bouguer anomaly at the farther stations is taken as -100～-150 mgal, considering the value in the neighbourhood of Pole of Relative Inaccessibility or South Pole, the true thickness of ice will be 1300～2000 m smaller than that calculated from gravity The results obtained on sea ice in Lutzow-Holm Bay are listed in another table At some stations where the depth of sea is known, the Bouguer anomaly was calculated under the assumptions (1) the Bouguer anomaly is zero, (2) the density of sea water is 1 02 g/cm^3 (freezing point -1 5℃), (3) the density of rock 2 67 g/cm^3. As to the free air anomaly and the Bouguer anomaly, too, the vicinity of the tongue of the Shirase Glacier has the smallest value ⊿g_O=-78 2 mgal ⊿g_O"=-32 6 mgal and farther place has larger value This gradient continues to the offing of the Lutzow-Holm Bay where the maximum value of anomaly ⊿g_O=+79 mgal ⊿g_O"=+216 mgal has been observed in the second expedition. It must be noticed that the maximum of the free air anomaly in the neighbourhood of lat 68°S corresponds to the edge of the continental shelf, the minimum in the neighbourhood of lat 69 5°S corresponds to the depression of the bottom of sea and the maximum in the neighbourhood of lat. 71 7°S corresponds to the neighbourhood of the Yamato Mts. As to the Bouguer anomaly at the place in the range between the meridians 35°E and 40°E, the value of ⊿g_O" 280 mgal in the neighbourhood of 68°S decreases to about zero in the neighbourhood of 69°S, to the approximately estimated value -75～-110 mgal in the neighbourhood of 71°S and to the reasonabley assumed value -100～-150 mgal at the southern place of 72°S. In the neighbourhood of 68°S, the western place has smaller Bouguer anomaly and in the neighbourhood of 69°S, the western place has larger Bouguer anomaly. The elevation of Mohorovicic discontinuity M is calculated with the formulae presented by KORYAKIN, DEMENITSKAYA, and with the graph presented by WOOLLARD. KORYAKIN'S formula is M=M_O- (⊿g_O"/2_πG × density contrast) =35.3 - 0 0332 ⊿g_O", and DEMENITSKAYA'S formula is H=35 (1 - tanh 0 0037 ⊿g_O"). Where H is the thickness of the earth's crust. If H is taken as M, M will have the error by the depth of sea. For the value of M in the neighbourhood of Syowa Station, any of the above formulae would give about 35 km, but for the mean gradient of M in the range between 68°S and 69°S, the formula of KORYAKIN gives about 59 m/km, the curve of WOOLLARD gives about 119 m/km and the formula of DEMENITSKAYA gives aboat 186 m/km