Ocean tidal loading models assessment using 28 months of gravimetric tidal records in Dublin, Ireland

<p>In June of 2018 a project for the establishment of a modern permanent Absolute Gravity Network  on the island of Ireland was initiated by the National Mapping Agency of Ireland, Ordnance Survey Ireland (OSi) with the cooperation of  Institute of Geodesy and Cartography (IGiK), and Land and Property Services (LPS) in Northern Ireland. The project assumes conducting absolute gravity surveys of the network using  the A10 absolute gravimeter on approximately 60 stations homogenously distributed on the island of Ireland.</p><p>Data processing includes time variable corrections for body tides, barometric pressure, polar motion as well as ocean tidal loading. For Ireland the ocean tidal loading effect can reach peaks of between 400 nm/s<sup>2</sup> on the west coast and 200 nm/s<sup>2</sup> on the east coast. This effect is significant and up to now the authors are unaware of previous historical data or  tidal gravity records being performed in Ireland. Hence it was considered as a valid component of the overall Absolute Gravity Project to evaluate the current situation with ocean tidal loading effect in Ireland using gravimetric tidal records in order to validate available ocean tidal loading models.</p><p>In order to assure the most optimal use of ocean tidal model as well as minimize the errors of including ocean tidal correction in absolute gravity processing the LaCoste&Romberg model G spring gravimeter was installed at OSi headquarters in Phoenix Park, Dublin, Ireland. Over a continuous period of 28 months gravity record with more than 99% data completeness at near 2Hz sampling rate was conducted.</p><p>The project data was acquired through using a self-programmed Raspberry Pi computer allowing for automatic download and remote access to the data.</p><p>A set of CSR, DTU, EOT, FES, GOT, TPXO (ocean tide loading provider – Chalmers, http://holt.oso.chalmers.se/loading/) ocean tidal loading models were used in a joint analysis with the collected tidal record. Analysis included performing tidal adjustment of the gravity data in the ETERNA 3.40 (ET34-X-V73) as well as comparison of IAG (International Association of Geodesy) recommended model combinations with the collected data.</p><p>Recommendations by the project team as to which of the ocean tidal models is most suitable to be used in Ireland for the purpose of absolute gravity measurements were made.</p>

A comparison of predicted and observed ocean tidal loading in Alaska

SUMMARY We investigate the elastic and anelastic response of the crust and upper mantle across Alaska to mass loading by ocean tides. GPS-inferred surface displacements recorded by the Plate Boundary Observatory network are compared with predictions of deformation associated with the redistribution of ocean water due to the tides. We process more than 5 yr of GPS data from 131 stations using a kinematic precise point positioning algorithm and estimate tidal contributions using harmonic analysis. We also forward calculate load-induced surface displacements by convolving ocean-tide models with load Green’s functions derived from spherically symmetric Earth models. We make the comparisons for dominant tidal harmonics in three frequency bands: semidiurnal (M2), diurnal (O1) and fortnightly (Mf). Vector differences between predicted and observed ocean tidal loading (OTL) displacements are predominantly sub-mm in magnitude in all three frequency bands and spatial components across the network, with larger residuals of up to several mm in some coastal areas. Accounting for the effects of anelastic dispersion in the upper mantle using estimates of Q from standard Earth models reduces the residuals for the M2 harmonic by an average of 0.1–0.2 mm across the network and by more than 1 mm at some individual stations. For the relatively small Mf tide, the effects of anelastic dispersion (<0.03 mm) are undetectable within current measurement error. Incorporating a local ocean-tide model for the northeastern Pacific Ocean reduces the M2 vertical residuals by an average of 0.2 mm, with improvements of up to 5 mm at some coastal stations. Estimated RMS observational uncertainties in the vertical component for the M2 and O1 tides are approximately ±0.08 mm at the two-sigma level (±0.03 mm in the horizontal components), and ±0.21 mm for the Mf harmonic (±0.07 mm in the horizontal components). For the M2 harmonic, discrepancies between predicted and observed OTL displacements exceed observational uncertainties by about one order of magnitude. None of the ocean tide and Earth model combinations is found to reduce the M2 residuals below the observational uncertainty, and no single forward model provides a best fit to the observed displacements across all tidal harmonics and spatial components. For the O1 harmonic, discrepancies between predicted and observed displacements are generally several-fold larger than the observational uncertainties. For the Mf harmonic, the discrepancies are roughly within a factor of two of the observational uncertainties. We find that discrepancies between predicted and observed OTL displacements can be significantly reduced by removing a network-uniform tidal-harmonic displacement, and that the remaining discrepancies exhibit some regional-scale spatial coherency, particularly for the M2 harmonic. We suggest that the remaining discrepancies for the M2, O1 and Mf tides cannot be fully explained by measurement error and instead convey information about deficiencies in ocean-tide models and deviations from spherically symmetric Earth structure.

On the Influence of Diurnal and Subdiurnal Signals in the Normal Vector on Large Ring Laser Gyroscope Observations

The ring laser gyroscope (RLG) technique has been investigated for over 20 years as a potential complement to space geodetic techniques in measuring Earth rotation. However, RLGs are also sensitive to changes in their terrestrial orientation. Therefore in this paper, we review how the high-frequency band (i.e. signals shorter than 0.5 cycle per day) of the known phenomena causing site deformation contribute to the RLG observable, the Sagnac frequency. We study the impact of solid Earth tides, ocean tidal loading and non-tidal loading phenomena (atmospheric pressure loading and continental hydrosphere loading). Also, we evaluate the differences between available models of the phenomena and the importance of the Love numbers used in modeling the impact of solid Earth tides. Finally, we compare modeled variations in the instrument orientation with the ones observed with a tiltmeter. Our results prove that at the present accuracy of the RLG technique, solid Earth tides and ocean tidal loading effects have significant effect on RLG measurements, and continental hydrosphere loading can be actually neglected. Regarding the atmospheric loading model, its application might introduce some undesired signals. We also show that discrepancies arising from the use of different models can be neglected, and there is almost no impact arising from the use of different Love numbers. Finally, we discuss differences between data reduced with tiltmeter observations and these reduced with modeled signal, and potential causes of this discrepancies.

The Extraction of Ocean Tidal Loading from ASAR Differential Interferograms

The spatiotemporal crustal non-tectonic deformation caused by ocean tidal loading (OTL) can reach the centimeters scale in coastal land areas. The temporal variation of the site OTL displacements can be estimated by the global positioning system (GPS) technique, but its spatial variation needs to be further determined. In this paper, in order to analyze the spatial characteristics of the OTL displacements, we propose a multi-scale decomposition method based on signal spatial characteristics to derive the OTL displacements from differential interferometric synthetic aperture radar (D-InSAR) measurements. The method was tested using long-term advanced synthetic aperture radar (ASAR) data and GPS reference site data from the Los Angeles Basin in the United States, and we compared the results with the FES2014b tide model. The experimental results showed that the spatial function of the OTL displacements in an ASAR image can be represented as a higher-order polynomial function, and the spatial trends of the OTL displacements determined by the InSAR and the GPS techniques are basically consistent with the FES2014b tide model. The root-mean-square errors of the differences between the spatial OTL displacements of these two methods and the FES2014b tide model are less than 0.8 mm. The results indicate that the OTL displacement extracted from InSAR data can accurately reflect the spatial characteristics of the OTL effect, which will help to improve the spatial resolution and accuracy of the OTL displacement in coastal areas.

Gravity surveys using a mobile atom interferometer

Mobile gravimetry is important in metrology, navigation, geodesy, and geophysics. Atomic gravimeters could be among the most accurate mobile gravimeters but are currently constrained by being complex and fragile. Here, we demonstrate a mobile atomic gravimeter, measuring tidal gravity variations in the laboratory and surveying gravity in the field. The tidal gravity measurements achieve a sensitivity of 37 μGal/Hz (1 μGal = 10 nm/s2) and a long-term stability of better than 2 μGal, revealing ocean tidal loading effects and recording several distant earthquakes. We survey gravity in the Berkeley Hills with an uncertainty of around 0.04 mGal and determine the density of the subsurface rocks from the vertical gravity gradient. With simplicity and sensitivity, our instrument paves the way for bringing atomic gravimeters to field applications.

Assessment of continental hydrosphere loading using GNSS measurements

Presented paper is dedicated to problems of deformation of the Earth's crust as a response to the surface loading caused by continental waters. The aim of this study was to specify areas particularly vulnerable to studied deformation and to compare calculated and observed displacements. Information of the continental water volume was taken from the WaterGAP Global Hydrological Model. Calculated values of the deformations were verified with the results obtained with programs SPOTL and grat. Vertical deformations were almost 10 times higher than the deformation in the horizontal plane, for which reason later part of the paper focuses on the former. In order to check agreement of the calculated and observed deformation 23 stations of International GNSS Service (IGS) were selected and divided into three groups (inland, near the shoreline and islands). Before comparison outliers and discontinuities were removed from GNSS observations. Modelled and observed signals were centred. The analysed time series of the vertical displacements showed that only for the inland stations it is possible to effectively remove displacements caused by mass transfer in the hydrosphere. For stations located in the coastal regions or islands, it is necessary to consider additional movement effects resulting from indirect ocean tidal loading or atmosphere loading.