E. I. Robertson

The New Zealand Primary Gravity Network

Summary Between 1947 and 1955 the Department of Scientific and Industrial Research established the New Zealand Primary Gravity Network, a set of 437 control points for gravity observations in the North and South Islands of New Zealand. The gravity differences between nineteen stations were measured with the Cambridge pendulum apparatus, and between all stations with a North American gravity meter. Initially the network observations were reduced separately in the North and South Islands; the resulting gravity values constitute the New Zealand Provisional System, which has remained the basis for recording gravity results within New Zealand. Subsequent adjustment of the network, both to make it internally consistent and to bring it into agreement with the first order world network based on Potsdam, has resulted in a revised set of values of gravity, which constitute the New Zealand Potsdam System (1959), and which differ by an average of + 5·0 mgal from the New Zealand Provisional System values. Values of gr...

Gravity effects of volcanic islands

Abstract Gravity surveys of six volcanic islands in the Southern Cook Group (South-West Pacific Ocean) show that the radial anomaly gradient near the reef is much larger than the calculated effect for an island of uniform density. The gravity surveys also show that the difference between the modified Bouguer anomalies at the centre of each island and at the reef edge are much larger than the calculated effects for an island of uniform density. The observed values, however, are consistent with the calculated effects of an uncompensated island platform of density 2·35 g/cm3 which contains a core of density 2·87 g/cm3 and radius equal to the radius of the island at sea level. The gravity observations are incompatible with the theory of local isostatic compensation and therefore some form of regional compensation must apply. Gunns isobaric hypothesis (lithospheric thickness 50 km) would give rise to a maximum depression below each island of less than 0·1 km and hence would have negligible gravitational effec...

Gravity survey in the Cook Islands

Abstract The observed Bouguer gravity anomalies on the four volcanic islands (Mangaia, Mauke, Mitiaro, and Rarotonga) in the Southern Cook Group and Manihiki Atoll in the Northern Group are consistent with the calculated gravitational effects of uncompensated island platforms of average density 2·32 g/cm3 containing volcanic cores of average density 2·88 g/cm3. However, the anomalies on Rakahanga Atoll in the Northern Group are consistent with an uncompensated island platform having a uniform density of 2·52 g/cm3 and no core.

Geothermal heat flow through the soil at Wairakei

Abstract The variation of ground temperature with depth was measured between 1954 and 1961 at numerous locations in the Wairakei thermal area of New Zealand. Analysis of these data clearly shows that conduction is the dominant mechanism of heat transfer until the difference between the temperatures at the surface and at a depth of 1 metre reaches a value of about 25 deg C. For higher ground temperatures, heat transport by convection of a mixture of air and water vapour becomes progressively more important and the conductive effect becomes insignificant. These conclusions are used to present a simplified picture of the nature of the mechanisms in the transfer of heat to the surface in areas of steaming ground at Wairakei. The temperature data are also used to study the geothermal microclimate, to determine the thermal conductivity of the soil, and to derive relations between the temperatures at various depths used in practice for estimating the escape of natural heat from the Wairakei thermal area.

Additional gravity surveys in the Cook Islands

Abstract The observed Bouguer gravity anomalies on Atiu, Manuae, and Nassau Islands in the Cook Group are consistent with the calculated gravity effects of uncompensated island platforms of average density 2·38 g/cm3 containing volcanic cores of average density 2·85 g/cm3. Although insufficient gravity data are available for Aitutaki, Penrhyn, and Pukapuka, these islands also appear to have high density cores.

Bathymetry around isolated volcanic islands and atolls in the South Pacific Ocean

Abstract The results of bathymetric surveys around six isolated basaltic islands and atolls in Cook Group and three in the Tokelau group are presented. In all cases radial profiles approximate very closely to the equation y = yoe −cw where y is the elevation of the sea bedabove the mean level of the surrounding ocean floor (yo), x is the distance from the outer reef edge, and c is a constant. Using one km as the unit of length, the value of c for individual islands varies from 0.09 to 0.11, the average being 0.10. The use of this average value for a surrounding ocean depth of 4,500 m gives a maximumerror in the water depth of only 165m, and a sea bed slope near the reef of 24°

Bouguer anomaly map of Viti Levu, Fiji

Abstract The Bouguer anomaly map of Viti Levu in the Fiji Group consists essentially of a gravity “low” surrounded by gravity “highs”. The “low” can be explained partly by the effect of the younger rocks which have a specific gravity 0.4 g/cm3 less than the older rocks (Lower Tertiary), and partly by a relative thickening of the crust by about 10 km. The average crustal thickness is about 30 km which supports the geological evidence that the Fiji islands are continental.

Electrical resistivity and ground temperature at Scott Base, Antarctica

Abstract Electrical resistivities and ground temperatures were measured at Scott Base throughout 1959. These measurements showed that the resistivity of the permafrost near the surface varied by a factor of about 50 to 1 throughout the year; that an exponential relation exists between resistivity and temperature at least from −10°c to −30°c; and that the geothermal heat flux is about 40 times normal.

Gravity base stations in the south-west Pacific Ocean

Abstract Since 1950, gravity base stations have been established in the South-west Pacific Ocean area by G. P. Woollards group at the University of Wisconsin, P. Stahl of France, and the New Zealand Department of Scientific and Industrial Research. Full details, including free-air and Bouguer anomalies, are given for bases in the Auckland, Cook, Fiji, Kermadec, Macquarie, New Caledonia, Samoa, Society, Tokelau, and Tonga Islands.

Principal pendulum and airport gravity stations in New Zealand

Abstract Since 1947 the New Zealand Department of Scientific and Industrial Research, the University of Wisconsin group, and others have made numerous gravity observations at the airport and pendulum stations in the four main cities of New Zealand. These stations are frequently occupied by geophysicists en route to Antarctica, the Pacific Islands, or Australia. However, many of these are now inaccessible and have been replaced. This paper gives latitudes and longitudes, elevations, observed gravity values, and diagrams or descriptions of the present airport and pendulum stations in the Auckland, Wellington, Christchurch, and Dunedin areas, which also include the principal seaports