Extracting Geodetically Useful Information From Absolute Gravimetry at a Fundamental Geodetic Station Victoria Anne Smith Thesis submitted for the degree of Doctor of Philosophy of the University College London 2018 Department of Civil, Environmental and Geomatic Engineering University College London 1 Declaration I, Victoria Anne Smith, confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. 2 Abstract Measuring the acceleration due to gravity at the NERC Space Geodesy Facility (SGF), Herstmonceux, provides a complimentary geodetic technique to the long established satellite laser ranging and global navigation satellite system measurements. The gravimetry measurements at the SGF were added to conform to the gravity field objective of the Global Geodetic Observing System and the European Combined Geodetic Network. Since both SLR and GNSS measurements are used in the computation of the International Terrestrial Reference Frame any un-modelled movements in the ground stations are undesirable. Gravity measurements can be used in the identification of such signals. This thesis reports the establishment of the technique at the SGF and contains a description of the installation experiences, a maintenance guide for the instrument and a comprehensive analysis of the gravity measurements over the ten years of operation to date, from 2006- 2016. Environmentally driven influences on the gravity measurements are investigated. The precision of the measurements are shown to be seasonally dependent and of varying magnitude. The hydrological influence on the gravity data from groundwater variation is calculated to be approximately 3.14 µGal, determined from temporal measurement of groundwater depth and an estimation of soil properties. A maximum influence from soil moisture content is estimated resulting in an influence of less than a microgal, which confuses correlation studies between local tide gauge data and an intermittent periodic signal seen in the gravity data. The high frequency data taken at the SGF highlights bias corrections two explainable and one of unknown origin. The bias corrections, of magnitude +1.5, -2 and -7.33 µGal, are shown to be critical to the interpretation of the time series, and, simulated campaign style measurements, using one set of measurements on an annual basis, prove that the data would be easily misinterpreted if the bias offsets found are not applied. 3 Impact Statement Geodetic stations are currently under science-driven pressure to provide observations and analyses of sufficient quality to meet global efforts to realise an international terrestrial reference frame (ITRF) at mm-levels of accuracy and 0.1mm/yr stability. Many of the most accurate and prolific observatories are upgrading and expanding their capabilities with modern versions of the traditional techniques of satellite laser ranging (SLR), very long baseline interferometry (VLBI), global navigation satellite systems (GNSS) receivers and Doppler orbitography and radio-positioning integrated by satellite (DORIS) receivers. With the advent of the International Association of Geodesy initiative to encapsulate ongoing global geodetic efforts under the single banner of a Global Geodetic Observing System (GGOS), there is a general increase in the use, and demand for, gravity measurements at these fundamental stations. The accuracy demands of GGOS are driving improvements not only in the geodetic techniques but also in prompting the measurement and understanding of site-specific un-modelled ground movements which may contaminate and compromise efforts to determine accurate coordinates of each station that contributes to the determination of the ITRF. Recognising the GGOS initiative, the UK Space Geodesy Facility (SGF), Herstmonceux, took the opportunity to expand instrumentation to include an FG5 absolute gravimeter (AG); previous experience at Herstmonceux was based on two week-long deployments of an AG in collaboration with the National Oceanography Centre, during the 1990s. The work to integrate and begin to exploit the AG, reported in this Thesis, has therefore grown an entirely new capability for the SGF where the traditional space-geodetic techniques are now complimented by a permanently sited absolute gravimeter that is providing a unique dataset. The studies started in this work give a greater understanding of local tidal and hydrological loading effects on vertical crustal signals than previously known, which will eventually have a direct impact on the laser ranging and GNSS analyses. These results are likely to be applicable to other sites, if not directly, at least in highlighting potential shortcomings in existing models. 4 The SGF now provides the UK with a gravity reference site, which will be a ‘core’ station within the worldwide gravity reference frame currently under development, with the potential to provide key services such as; UK-wide monitoring of vertical signals in tide gauges, to the ability to provide overseas support to shipborne oceanic gravity surveys, by the provision of gravity reference stations. 5 Acknowledgements I have so much thanks to express, to family, friends, supervisors, colleagues & to my employer. I would like to wholeheartedly thank my principal supervisor Prof. Marek Ziebart, for offering the opportunity to undertake doctoral studies &, for having the faith, perseverance and enthusiasm which carried me though. I owe particular thanks to Dr Graham Appleby, the head of the Space Geodesy Facility, to whom I owe huge thanks for giving me the responsibility for the gravimeter & his endless support and encouragement. I know I taxed you both on more than one occasion. Thanks also to Dr. Simon Williams, for offering his expertise and knowledge & perhaps most of all for allowing me to use his processing software ‘gap’. In addition I need to thank my colleagues at the SGF for providing a great working environment & allowing me to drop out of SLR observing duties whilst writing up this thesis. I’d like to say thank you to the community of absolute gravimeter users, for being such helpful, lovely people. It is a pleasure to belong to this club. Special thanks to Dr. Derek van Westrum & Dr. Tim Niebauer who have made the instrumental learning curve easier & offering assistance when needed. To all of those who endured proof reading for me I am also very grateful; Dr Appleby, Prof. Ziebart, Dr van Westrum & my brother Nicholas Smith. To all of my friends who have been so wonderfully supportive & picked me up from dark times, I thank you. Three people who deserving a special mention are Dr. Peter Stacey, for the advice & support, my old school friend Alex Page, for being there to pick me up &, last but not least, my closest & dearest chum Clare Wilson, you are what I call fabulous. My thanks simply would not be complete without mention of my amazing family, my mother, brother, sister-in-law, nieces & nephew, & to my dogs Tomsk and Bungo. A simple thank you is inadequate. I love you all. I dedicate this thesis to my all of my family, those present and all of you who didn’t make it this far. Miss you Mum and Dad. 6 Table of Contents Title Page ................................................................................................................. 1 Declaration .............................................................................................................. 2 Abstract ................................................................................................................... 3 Impact Statement ..................................................................................................... 4 Acknowledgements ................................................................................................. 6 Table of Contents ..................................................................................................... 7 List of Figures ......................................................................................................... 14 List of Tables ........................................................................................................... 20 Glossary of Terms, Abbreviations and Acronyms ................................................... 21 Units ........................................................................................................................ 22 1. Introduction 1.1. Introduction ...................................................................................................... 23 1.2. A Brief History of Gravity Measurement ......................................................... 23 1.3. A Moment About Newton ................................................................................ 28 1.4. Geodetic Facilities ............................................................................................ 28 1.5. Absolute Gravity Measurement in Geodesy .................................................... 29 1.6. Outline of Thesis Structure ............................................................................... 30 2. The FG5 Absolute Gravimeter 2.1. Introduction ...................................................................................................... 33 2.2. Principles of the Design ................................................................................... 35 2.3. Principle Components of the FG5 .................................................................... 37 2.3.1. Dropping Chamber