A comparison of predicted and observed ocean tidal loading displacements around the Puget Sound

Around coastlines and in shallow oceans, models of ocean tidal loading (OTL) are not highly accurate and can create sources of error in OTL analysis. OTL is tides moving ocean water that cause the surface of Earth to deform. In this study, forward-modelled predictions of OTL are compared to observations from Global Navigation Satellite System (GNSS) data to explore the elastic deformation response of Earth to OTL around the Puget Sound. Data from 75 stations were processed to yield position estimates at intervals of 5 minutes for a year. The OTL model used for comparison was the FES2014b ocean-tide model loading a spherically symmetric, non-rotating, elastic, and isotropic (SNREI) Earth model. The three tidal frequency bands used were the semidiurnal (M2), diurnal (O1), and fortnightly (Mf). The M2 tide is the largest and Mf the smallest. The model and observations have the largest residual displacements of 5 mm for the M2 tide, and the smallest 2 mm for the O1 tide. The particle motion residuals have strong spatial coherence in the M2 and O1 tide but are less coherent with the Mf tide. These residuals indicate that either the models of OTL and/or the observations of OTL have deficiencies. Model deficiencies are likely due, in part, to the FES2014b model not extending fully into and accounting for the complex morphology of the Puget Sound. Another element explored was quantifying the difference in observed OTL when a sidereal filter is applied to the GNSS time series. A sidereal filter is used to remove multipath errors that occur every sidereal day when GNSS satellite orbits repeat. The largest residual displacements were 1 mm in the Mf tidal band and the smallest 0.5 mm in the M2 band. These differences can alter the observations used in OTL analysis, which impacts model comparisons. Multipath errors can overlap in frequency space with tidal frequencies, so the use of a sidereal filter must be carefully considered before its application

State-of-the-art in studies of glacial isostatic adjustment for the British Isles: a literature review

Understanding the effects of glacial isostatic adjustment (GIA) of the British Isles is essential for the assessment of past and future sea-level trends. GIA has been extensively examined in the literature, employing different research methods and observational data types. Geological evidence from palaeo-shorelines and undisturbed sedimentary deposits has been used to reconstruct long-term relative sea-level change since the Last Glacial Maximum. This information derived from sea-level index points has been employed to inform empirical isobase models of the uplift in Scotland using trend surface and Gaussian trend surface analysis, as well as to calibrate more theory-driven GIA models that rely on Earth mantle rheology and ice sheet history. Furthermore, current short-term rates of GIA-induced crustal motion during the past few decades have been measured using different geodetic techniques, mainly continuous GPS (CGPS) and absolute gravimetry (AG). AG-measurements are generally employed to increase the accuracy of the CGPS estimates. Synthetic aperture radar interferometry (InSAR) looks promising as a relatively new technique to measure crustal uplift in the northern parts of Great Britain, where the GIA-induced vertical land deformation has its highest rate. This literature review provides an in-depth comparison and discussion of the development of these different research approaches

Secular changes in Earth's shape and surface mass loading

PhD ThesisThe changing distribution of surface mass (oceans, atmosphere, hydrology and cryosphere) causes detectable changes to the solid Earth’s shape on timescales from hours to millennia. Transient changes in Earth’s shape can be readily identified, but the tectonic plate movements and Glacial Isostatic Adjustment (GIA) will also influence the secular trends of Earth’s shape. To analyse secular trends in surface mass loading, these two confounding factors must be quantified. A suite of GPS-derived surface loading models, including both secular and transient terms is presented. Raw velocities are estimated from over 10 years of high quality combined global GPS position solutions, submitted as part of the first International GNSS Service (IGS) reprocessing campaign. A fiducial-free network approach is used with attention to estimating linear offsets and periodic signals. Consideration is given to realistic formal errors for station coordinates. A robust method is used for estimating horizontal and vertical linear velocities for all stations. Tests of the reprocessed data quality show that there is a dramatic improvement of the RMS of the weekly combined global network in comparison to the operational data used previously. The estimated Helmert transformations, when aligning the reprocessed frame to the IGS05 reference frame also show the stability and homogeneity of the new dataset. This permits a more precise estimate of individual station velocities, ~75% reduction to variability of Helmert parameters. Several a priori GIA models are applied to produce corresponding plate velocity estimates, leaving a range of computed residual surface displacements. Present-day surface mass loading is estimated from these residuals, using gravitationally consistent mass-conserving basis functions. GIA models are assumed to be error-free, so only nominal formal errors, with a white noise assumption, can be calculated, these will be adjusted to produce a realistic uncertainty value. Surface mass loading estimates show significant secular mass loss in Alaska and Greenland. The Greenland values (-140Gt/yr, 1999-2010) fall within published GRACE gravity mission values (-66 to -248Gt/yr, 2002-2009).Natural Environmental Research Council (NERC) studentship

Astrometry and geodesy with radio interferometry: experiments, models, results

Summarizes current status of radio interferometry at radio frequencies between Earth-based receivers, for astrometric and geodetic applications. Emphasizes theoretical models of VLBI observables that are required to extract results at the present accuracy levels of 1 cm and 1 nanoradian. Highlights the achievements of VLBI during the past two decades in reference frames, Earth orientation, atmospheric effects on microwave propagation, and relativity.Comment: 83 pages, 19 Postscript figures. To be published in Rev. Mod. Phys., Vol. 70, Oct. 199

Exploring Effects of GPS Processing on Atmospheric and Hydrologic Pressure-induced Crustal Responses

The Earth’s crust is in continuous motion from changes in fluid pressures associated with the redistribution of mass at the surface. These forces, known as surface mass loading, make up a significant amount of signal within GPS time series. This thesis is broken up into two projects exploring atmospheric and hydrologic pressure-induced crustal responses. The first project focuses on effects of GPS processing on corrections of atmospheric loading. We use data from over 1100 GPS stations within the Western US to investigate crustal displacements from atmospheric surface pressure variations. We find that modeling and removing atmospheric mass loading reduces root mean square (RMS) scatter of residual GPS time series by 16 % on average and up to 50 % for inland stations. We observe a trend of larger RMS reduction with increasing distance from the ocean, due to the inverted barometer effect. We then compare five sets of processed GPS data from three different processing centers (JPL, NGL, UNAVCO) and attempt to isolate possible causes for variations in the GPS displacements. The GPS products with the largest reductions in RMS scatter were generated using the more accurate, high resolution troposphere delays, with the UNAVCO data product providing the best retention of atmospheric mass loading (ATML) in the time series. The retention of ATML in the time series is affected by the temporal resolution of the tropospheric model used in initial processing of raw GPS signal. Mismodeling troposphere delays can lead to an inaccurate distance estimate between satellite and receiver, thereby limiting retention of atmospheric surface pressure-induced crustal displacements in the time series. As such, we recommend using high resolution tropospheric delays when possible. The second project focuses on isolating and quantifying hydrologic loading signal sources within GPS stations near the Columbia River along the Washington-Oregon border. We attempt to correlate seasonal river discharge with horizontal motions present within the GPS time series using particle motions ellipses. We also attempt correlation between sub-seasonal signals of displacement with changes in river discharge measured by USGS river gauges