This thesis addresses deformations and gravity changes due to surface loads like the ocean tides, hydrology and glaciers. These phenomena are discussed in light of height and gravity observations collected by GPS and gravimeters of the FG5 and LaCoste & Romberg types.
A surface load is here defined as a mass resting at the surface of the Earth. Body loading due to the Earth tides is consequently outside the scope of this thesis. The analysis is further restricted to address elastic processes only, i.e. loading effects in phase with the time history of the load. Viscoelastic processes like glacial isostatic adjustment are not discussed in depth.
A significant part of this thesis addresses ocean tide loading (OTL). The phenomenon is theoretically discussed and observational results are provided. A suite of global OTL models was compared to gravity and GPS time series at coastal stations in Norway. It was found that global models are in phase with
the observations and only millimeter discrepancies exist between the magnitude of GPS observations and OTL models. When it comes to the magnitude of the gravity signals, best agreement was obtained by OTL corrections calculated from FES2004 and NAO99b. However, at several stations we observe periodic residuals of nearly 10 μgal amplitude. To reduce the weighted standard deviation of the gravity time series, an alternative method was developed for calculating OTL corrections. The method was based on locally observed ocean tides and a global OTL model for vertical displacement. Compared to global models, the alternative method reduced the RMS by up to 40 %.
The gravitational effect of hydrology was investigated in Trysil. Trysil is located inland Norway and our observations have revealed seasonal gravity changes of nearly 20 μgal. A hydrological model was developed from snow depth readings, well readings, and precipitation data. Compared to a three year long gravity time series, the model explained 64 % of the variation and reduced the amplitude of the seasonal signal strongly. More than 90 % of the gravity signal from the hydrology was formed by the snow cover within 200 m of the gravity laboratory.
The thesis also presents a high accuracy gravity network for Norway. The network includes 16 stations with gravity estimates accurate to 3-4 μgal. Compared to previously published values, this is an improvement of one order of magnitude. The gravity values will change by up to 1 μgal annually due to glacial isostatic adjustment.
Finally, attempts were made to use ground based relative gravity observations to measure the mass balance of a glacier. Preliminary results show that the method can resolve the mass balance within 10 % of the loss determined by conventional mass balance measurements. It still remains to fully validate the methodology in field
