B. Ducarme

Effect of the atmospheric pressure on surface displacements

SummaryAtmospheric pressure variations with periods of some days and months can be considered as loading functions on the Earths surface and can induce quasi-periodic surface deformations. The influence of such surface displacements is calculated by performing a convolution sum between the mass loading Greens functions and the local and regional barometric pressure data (geographically distribution in a 1° × 1° grid system extending to more than 1000km). The results for 5 stations in Europe show that the average values reach about 22.9–30.2mm depending on the ocean response: the inverted or non-inverted barometer ocean model. The corresponding admittances are 0.576–0.758mm/mbar respectively. The horizontal displacements are not negligible but always smaller. The magnitudes are about 2–3mm for East-West component and 0.5–1.0mm for North-South component.The results of the dependence on the lateral extension of the pressure load show that the admittance for radial displacement varies from 0.250mm/mbar for a column load of 100km radius to 0.539mm/mbar for a column load of more than 1000km extension. It means that the main contribution of the loads comes from the horizontal distribution of the air pressure in a broad region.The time dependent effects of the atmospheric pressure are also computed with the two-coefficient correction equations provided by Rabbel & Zschau (1985) using ground pressure data in a 1.125° × 1.125° grid system. The computations demonstrate that the results are in good agreement with those obtained with a convolution sum. It shows that this method can provide us with a good approximation for vertical displacement caused by the deformation of the Earth.

Tidal loading along a profile Europe-East Africa-South Asia-Australia and the Pacific Ocean

Abstract A knowledge of the vertical component of the oceanic tidal load to a precision of at least one microgal is essential for the geophysical exploitation of the high-precision absolute and differential gravity measurements which are being made at ground level and in deep boreholes. On the other hand the ocean load and attraction signal contained in Earth tide gravity measurements can be extracted with a precision which is sufficient to characterize the behaviour of the oceanic tides in different basins and this provides a check of the validity of the presently proposed cotidal maps. The tidal gravity profiles made since 1971 from Europe to Polynesia, through East Africa, Asia and Australia, with correctly intercalibrated gravimeters, comprise information from 91 tidal gravity stations which is used in this paper with this goal in mind. A discussion of all possible sources of error is presented which shows that at the level of 0.5 μgal the observed effects cannot be ascribed to computational or instrumental errors. Cotidal maps which generate computed loads in agreement with the Earth tide gravity measurements over a sufficiently broad area can be used with confidence as a working standard to apply tidal corrections to high-precision measurements made by using new techniques in geodesy, geophysics and geodynamics, satellite altimetry, very long baseline interferometry, Moon and satellite laser ranging and absolute gravity. The recent cotidal maps calculated by Schwiderski for satellite altimetry reductions agree very well with land-based gravimeter observations of the diurnal components of the tides (O1, K1 and P1 waves) but his semi-diurnal component maps (M2, S2 and N2 waves) strangely appear less satisfactory in some large areas. The maps of Hendershott and Parke give good results in several large areas but not everywhere. More detailed investigations are needed not only for several coastal stations but mainly in the Himalayas.

Experimental earth tidal models in considering nearly diurnal free wobble of the Earth＇s liquid core

Based on the 28 series of the high precision and high minute sampling tidal gravity observations at 20 stations in Global Geodynamics Project (GGP) network, the resonant parameters of the Earths nearly diurnal free wobble (including the eigenperiods, resonant strengths and quality factots) are precisely determined. The discrepancy of the eigenperiod between observed and theoretical values is studied, the important conclusion that the real dynamic ellipticity of the liquid core is about 5% larger than the one under the static equilibrium assumption is approved by using our gravity technique. The experimental Earths tidal gravity models with considering the nearly diurnal free wobble of the Earths liquid core are constructed in this study. The numerical results show that the difference among three experimental models is less than 0.1%, and the largest discrepancy compared to those widely used nowdays given by Dehant (1999) and Mathews (2001) is only about 0.4%. It can provide with the most recent real experimental tidal gravity models for the global study of the Earths tides, geodesy and space techniques and so on.

The response of the Earth to tidal body forces described by second-and third-degree spherical harmonics as derived from a 12 year series of measurements with the superconducting gravimeter GWR/T3 in Brussels

Abstract A 12 year tidal gravity registration performed in Brussels with a GWR superconducting gravimeter allows us to separate six tidal components derived from the third-degree lunar potential from those derived from the classical second-degree luni-solar potential (39 components) with which they differ in frequency by the lunar perigee period (8.847 year). Four ter diurnal waves are also separated. Such a separation appears necessary as the results show a non-negligible residue on the third-degree frequency wave M 1 (lunar day period) owing to an amplification of the corresponding oceanic component in the North Atlantic ocean. The other third-degree components also differ slightly from the theoretical model amplitudes. A correct determination of the free core nutation resonance acting on the ψ 1 diurnal solar wave is discussed in relation to atmospheric pressure gravitational effects and climatological disturbances. The zonal fortnightly tide Mf is also deduced with precision but its comparison with the theoretical model raises a problem.

One Year of Registration with the C021 Cryogenic Gravimeter at Station Membach (Belgium)

The results of one year registration of the C021 superconducting gravimeter at Membach (Belgium) are presented. Emphasis is placed on different methods to calibrate the gravimeter. The Earth tide analysis reveals the high quality of the data; the agreement with the Wahr-Dehant-Wang model is excellent. The long-term gravity changes observed with the C021 superconducting gravimeter are coherent with those measured with the FG5-102 absolute gravity meter. We prove for the first time that the long-term signal of the C021 cryogenic gravimeter is mainly of geophysical origin, and that the drift is within the range of 1 microgal.

Comprehensive comparison and analysis of the tidal gravity observations obtained with superconducting gravimeters at stations in China, Belgium and France

The long-term tidal gravity observations of about 27 a obtained with superconducting gravimeters (SG) at stations in Wuhan (China), Brussels (Belgium) and Strasbourg (France) are analyzed comprehensively. The quality factors of these observations at various stations are obtained and the tidal parameters are determined accurately. The efficiency when using various procedures on improving accuracy of the tidal parameters is discussed. The loading corrections on tidal parameters are carried out based on the global ocean tidal models. The correlations between the observed residuals and air pressure change are studied, the atmospheric gravity admittances in both temporal and frequency domains are determined, and the possible reasons of inducing the discrepancy between the observed tidal parameters and those in the standard tidal models are studied.

Transcontinental tidal transect: European Atlantic coast-Southern Siberia-Russian Pacific coast

The paper presents results of measurements with digital tidal LaCoste-Romberg gravimeters on the European Atlantic coast-Southern Siberia-Russian Pacific coast transect in 1995–2005. The transect includes four West European (Chizé, Ménesplet, Mordelles, and Wikle), two South Siberian (Klyuchi and Talaya), and two Far Eastern (Zabakalskoe and Yuzhno-Sakhalinsk) stations. Gravimetric measurements at the Talaya station (SW Baikal rift zone) are supplemented by long-term laser extensometer observations. The position of the stations within the rectangle (45°–55°N, 0.4°–142°E) allows one to assess existing tidal strain models (WD93 and DDW99) and various ocean tide models (SCW80, CSR3, FES95, ORI96, CSR4, FES02, GOT00, NAO99, and TPX06). Data of intracontinental stations (with a small ocean effect at distances of 2000–3000 km) agree well with the DDW99 tidal strain model (with regard to the mantle viscosity). The uncertainty of digital tidal gravity measurements is 0.25%. Results of laser extensometer measurements are at the same accuracy level. Then, the Love and Shida numbers calculated at midlatitudes of the intracontinental zone of Eurasia from combined data are h = 0.6077 ± 0.0008, k = 0.3014 ± 0.0001, and l = 0.0839 ± 0.0001. The analysis of results of Pacific and Atlantic stations located at distances of 30–300 km from the ocean showed that the FES02, CSR4, GOT00, NAO99, and TPX06 ocean tide models are preferable.

Tidal Gravity Measurements in Latin-America

For high precision Geodesy at the end of this century, a precise knowledge of the tidal deformations of the earth’s surface is essential in particular to carefully correct gravity and distance measurements. Absolute and field differential gravity determinations have now about the same precision, that is 10 microgals (1 μgal = 10-8 m s-2) or better in some cases.

Interactions between Oceanic and Gravity Tides, as Analysed from World-Wide Earth Tide Observations and Ocean Models

AbstractThe problem of interactions between earth tides and oceanic tides is rather complex as it involves effects of newtonian attraction, loading and associated change of earth potential, tangential pressure and friction on the moving ocean floor which are not always easy to evaluate, principally for coastal or island stations.This paper takes advantage of two facts:(1)By the end of 1983 the International Center of Earth Tides has collected and evaluated a considerable amount of data from 223 stations including those of the Trans World Profiles developed by the same group of authors (102 stations). This ensures, for the first time, a World wide distribution including the tropical areas and the southern hemisphere.(2)In 1978–80, new oceanic cotidal maps of high quality, established by E.W. Schwiderski, became available. We have calculated, for the eight principal tidal waves, the correlations between the observed gravity variations and those resulting from a calculation based upon the Schwiderski maps. This correlation is highly significant.At the level of accuracy of the best transportable gravimeters the agreement is perfect except at a few places where effects of lateral heterogeneities in the mantle can perhaps be suspected.These cotidal maps can therefore be safely used as working standards for other geodetic and geophysical applications.

Tidal gravity measurements in Africa

Abstract Tidal variations of the gravity have been measured at 31 places in Africa and Madagascar with five carefully intercalibrated gravimeters. The minimum duration of the measurements was six months at each station. The main goal is to compare the results, corrected for the direct elastic solid earth tide effects, with the attraction and loading effects of the oceanic tides. The agreement with the oceanic corange-cotidal Schwiderski maps is fair at most stations, considering the noise level of the instruments (about 0.5 μgal on the M2 tide amplitude) but strong positive residues are observed in Afar and Somalia while strong negative residues are observed in West Africa. Such regional anomalies are observed on other continents. An interpretation needs further research. A practical result of this investigation is the possibility to accurately correct field gravity measurements for tidal effects.