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Title: Environmental and instrumental effects on high precision gravimetry : a case study in Britain
Author: Hopewell, Hugh
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1999
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This thesis investigates the removal of environmental and instrumental effects from high precision gravity records and applies the findings to measurements made in Britain. Ocean loading calculations have been developed by manipulating ocean tide model grids to be of variable size. This has improved the fit to irregular coastlines and the approximation of grid cell masses to point mass loads. Ocean loading is calculated for 12 components at the 69 sites of the British Precise Gravity Network (BPGN). The new ocean loading correction amplitude for Newlyn has increased from 16 to 23 μgal; the set standard deviation of a 24 hour FG5 absolute gravity data set reduces from 4.3 to 1.8 μgal. Adjustment of the BPGN with the new correction reduces the mean site standard error from 3.32 ± 0.60 μgal to 2.96 ± 0.46 μgal. FG5-103 Bidston measurements show a seasonal change of 8 μgal. Water table data after cor­rection for air pressure show a correlation with gravity, having a lag of 3 months. Using 6 months of continuous LaCoste & Romberg (LCR) Earth-tide gravimeter data, the air pressure admittance at Bidston varies seasonally from 0.35 to 0.50 μgal mbar-1. Frequency dependent admittances correcting for instrumental humidity effects and environmental air pressure and rainfall effects removed structure in the data. Admittances derived from LCR Earth-tide data for air pressure and rainfall do not improve the absolute data. Using synthetic data, the frequencies of damped sinusoids found from FG5 residuals are dif­ferent to those originally present. Noise found at 9.5 Hz causes gravity changes of < 1 μgal when fitted to the equation of motion with real data. However, gravity changes by 6.3 μgal upon removal of its cause. Fitting frequencies from 4 -13 Hz to this data creates changes in the order of 10 μgal. Existing noise correction algorithms may miss errors of this size.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available