Absolute gravity observations at principal New Zealand stations 2015

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Stagpoole, V.M.; Dando, N.; Caratori Tontini, F.; Black, J.; Amos, M. 2015 Absolute gravity observations at principal New Zealand stations 2015. Lower Hutt, N.Z.: GNS Science. GNS Science report 2015/46 iii, 48 p.

Abstract: From 26 January to 16 February 2015 absolute gravity observations were undertaken at three South Island and five North Island locations using the FG5/237 ballistic absolute gravimeter operated by Geoscience Australia. In the South Island observations were made at Godley Head (code AAJB) and Mt John (code MTJG) where earlier absolute gravity observations have been made, and at the IGSN71 site in Dunedin (code OUGG). In the North Island observations were made at the IGSN71 site in Wellington, WELLINGTON A (code WELG), the Measurement Standards Laboratory at Callaghan Innovation (code GCAG), new sites at Palmerston North (code MASG) and Tarawera (code TAAG), and at the Warkworth Radio Astronomy Observatory (code WARG). At the same time a relative gravity meter was used to measure the vertical gravity gradient at each site and to tie the absolute gravity observations to nearby New Zealand Primary Network sites and survey benchmarks that form part of the system of gravity reference marks around New Zealand. With the exception of the Tarawera gravity observation, each absolute gravity observation consisted of an average of 100 mass drops every hour for between 24 and 40 hours. The Tarawera site was found to be unstable and experienced a lot of ground noise, and hence observations consisted of 10 sets of 100 mass drops, each set repeated at 20 minute intervals. For all sites the uncertainty in gravity determined at the instrument measurement height (1.3 m) varies from 1.84 to 2.60 µGal. Absolute gravity values are determined for measurement height and also transferred to survey mark or floor level after making observations to calculate the vertical gravity gradient. Observed vertical gravity gradients varied from -2.765 to -3.878 µGal/cm. Sites with larger gradient observation errors have larger uncertainties at ground level. For observations at Godley Head and Mt John gravity values are also transferred to floor level and one metre above floor level using the vertical gravity gradients listed in Bilham (2000) to allow comparison with earlier observations. The absolute gravity values presented here form part of a time series, particularly at Godley Head and Mt John where earlier observations have been made. It is expected that future absolute gravity observations will also be made at the other localities occupied during the 2015 survey, particularly at the Warkworth Radio Astronomy Observatory where a purpose-built mark has been established near the VLBI (Very Large Baseline Interferometry) facility. (auth)