Stagpoole, V.M.; Dando, N. 2015 2015 repeat vertical gravity gradient observations at Callaghan - Lower Hutt (GCAG), Palmerston North - Massey (MASG) and Wellington A (WELG) to determine absolute gravity at benchmark level. Lower Hutt, N.Z.: GNS Science. GNS Science report 2015/64 24 p.
Abstract: This report describes the results of a series of gravity observations that supplement earlier vertical gravity gradient measurements made in conjunction with the 2015 absolute gravity survey described in Stagpoole et al. (2015). Gravity observations, using a relative gravity meter (Lacoste and Romberg D-37), were made at Callaghan – Lower Hutt (code GCAG), Palmerston North - Massey (code MASG) and Wellington A (code WELG) to check the vertical gravity gradient at these sites and to determine the meter calibration. The observations presented in this report are more accurate than the previous survey in 2015, and result in a recalculated vertical gravity gradient of -2.791 ± 0.015 µGal/cm (1σ standard deviation) at the Callaghan — Lower Hutt site. Vertical gravity gradient measurements at the Palmerston North — Massey site are improved to a 1σ standard deviation 7 µGal (previously 12 µGal) to give a recalculated gradient of -2.860 ± 0.052 µGal/cm. The vertical gravity gradient at Wellington A is re-determined as -2.945 ± .015 µGal/cm. Expanded uncertainty, based on both instrument set-up and observation accuracy and from long term local geophysical uncertainties outside the observation period is calculated for the Callaghan – Lower Hutt site at ± 11.09 µGal, (± 110.9 x 10-9 m/s2) with a confidence interval of approximately 95%. (auth)