Analysis and prediction of ocean tides by the computer program VAV

The paper presents various results from the application of VAV tidal analysis program on a 36 years (7/01/1945-31/12/1980) series of ocean data from the Belgian port Ostend. The program VAV was originally developed for the processing of Earth tide data. Now, it has been supplied with specific options for the analysis of ocean tide data. One of the new options is the determination of the shallow water tides. VAV is also able to make ocean tide predictions, as well as to investigate the mean sea level and its long-term trend. Generally, useful properties of VAV are a correct application of the Method of the Least Squares, taking into account the colored character of the noise, as well as its ability to process data with gaps, without any interpolation

Analysis of co-located measurements made with a LaCoste&Romberg Graviton-EG gravimeter and two superconducting gravimeters at Strasbourg (France) and Yebes (Spain)

Two experiments of intercomparison between the LaCoste&Romberg Graviton-EG1194 spring gravimeter and the superconducting gravimeters SG-026 and OSG-064, operating respectively at J9-Strasbourg (France) and CDT-Yebes (Spain), were analyzed. The main objective was to check the instrumental response of the spring meter, both in amplitude and phase as well as its time stability. A general conclusion is that normalization factors have been obtained with a similar ratio for main diurnal constituent O1 at both observing sites. The accuracy of scale factors was determined at the level of 0.03 % (at J9-Strasbourg) and 0.1 % (at CDT-Yebes). For the semidiurnal constituent M2, slight differences were found at J9-Strasbourg, which would require further investigations. Site effects were also demonstrated by considering the different response of the spring gravimeter to tilts and atmospheric pressure variations at both sites

Global analysis of the GGP superconducting gravimeters network for the estimation of the pole tide gravimetric amplitude factor

The tidal records of superconducting gravimeters (SG) in nine stations are analyzed, in order to determine the gravity variation due to the polar motion. In a first step the tidal constituents are estimated and subtracted from the original data, together with the estimated atmospheric pressure effects, by the computer program VAV. The data so obtained are submitted to a regression analysis by a specially developed program POLAR, whose aim is the estimation of the gravimetric amplitude factor dCH of the pole tide and its time shift with respect to the theoretical gravity signal due to the polar motion. The procedure includes an optimization of various parameters of the regression model, as well as a detection and elimination of the anomalous portions of the records. The analysis has been first applied separately on each one of the nine series. Most of the stations provided a dCH factor between 1.17 and 1.19 with mean square deviation close to 1%. Further all series have been submitted to a global analysis through which a common value of the dCH factor has been estimated. No significant global time shift has been found. The global adjustment values are dCH = 1. 1816 +/- 0.0047 or 1. 1797 +/- 0.0047, depending on the way the time shift is introduced, while the simple arithmetic mean of the stations is 1. 1788 +/- 0.0040. This result differs considerably from the values predicted by Earth response models, e.g. from delta = 1. 158 obtained for the annual period through the non-hydrostatic anelastic model, usually called DDW99. The discrepancy is due to the indirect effects of the ocean tides. A preliminary correction scheme based on an equilibrium ocean pole tide is indeed reducing the globally adjusted dCH factor to 1.1612 or 1.1593 and the arithmetic mean to 1.1605

A new ocean tide loading model in the Canary Islands region

A new high-resolution (1/12◦ ×l/12◦) regional ocean tide model for Canary Islands region (Spain), by assimilating TOPEX/Poseidon altimetry data and tide gauge measurements into a hydrodynamic model, is presented. This regional ocean tide model is also refined along all the coastlines in the Canary region, using automatic grid discretization and bilinear interpolation. The new ocean model obtained reveals differences in some areas when we compare it with global models. The results confirm that data assimilation for high resolution models improves the ocean tide estimation in complex areas as the Canarian Archipelago. Gravity tide measurements, which are available in two islands of the Canarian Archipelago, have been used to test the ocean tide model. In addition, a comparison of nine global ocean tide models, supplemented with the regional model, is done for the M2 and O1 tidal constituents. The tidal gravity residues reveal that, for the M2 wave, there exists a dependence of the global ocean tide model considered. In general, the agreement of the nine ocean models is rather similar, although TPXO.2 and SCHW displays the most discrepant results. Among the ocean tide models, which are in close agreement at both places for M2 and O1 tidal waves, no one of them give better results than other

Comparison of the performances of different spring and superconducting gravimeters and STS-2 seismometer at the Gravimetric Observatory of Strasbourg, France

Since 1973, the Gravimetric Observatory of Strasbourg (France) is located in an old fort named J9 and has been the place for various gravity experiments. We present a comparison of the noise levels of various instruments that are or were continuously recording at J9, including the LaCoste&Romberg Earth-Tide Meter ET-5 (1973???1985), the GWR Superconducting Gravimeter TT-T005 (1987???1996), the Superconducting Gravimeter C026 (since 1996), the STS-2 seismometer (since 2010) and the LaCoste&Romberg ET-11 (continuously since October 2010). Besides these instruments, the J9 Observatory has hosted temporary gravity experiments with the Micro-g LaCoste Inc. gPhone-054 (May???December 2008 and May???September 2009) and the Micro-g LaCoste Inc. Graviton-EG1194 (June???October 2011). We include also in the comparison the absolute gravimeter Micro-g FG5 #206 which is regularly performing absolute gravity measurements at J9 since 1997 and a spring gravimeter Scintrex CG5 which recorded at J9 between March 2009 and February 2010. We present the performances of these various instruments in terms of noise levels using a standardized procedure based on the computation of the residual power spectral densities over a quiet time period. The different responses to atmospheric pressure changes of all the instruments are also investigated. A final part is devoted to the instrumental self-noise of the SG C026, STS-2 and L&R ET-11 using the three channel correlation analysis method applied to 1-Hz data

The crustal structure of El Hierro (Canary Islands) from 3-D gravity inversion

El Hierro is the youngest and westernmost island of the Canarian Archipelago. Due to its singular and interesting characteristics, with extensive landsliding, studying its crustal structure can provide very useful information about its origin and evolution. With this aim, we use the gravity inversion methodology obtaining a crustal model of mass distribution. We performed a gravity survey of the entire island, completing the gravity anomaly map of this area with marine data from the United States Geological Survey. The calculated Bouguer gravity anomaly map was studied using a covariance analysis and the least squares technique. To obtain crustal information from these data, we used a 3D gravity inversion based on genetic algorithms (GA). Thus, our inversion technique aims to determine the geometry of the sources of the observed gravity field, upon a prismatic partition of the subsoil volume, and adopting a priori density contrast values. The results of this gravity study of El Hierro Island show the correlation of several volcanic structures with the distribution of the gravity field sources. The characteristic triple rift system of the island is associated with low-density areas, and the older volcanic stages with high-density structures. We also found differences among the structures related to the several landslides, which helped the island takes shape

Structural interpretation of El Hierro (Canary Islands) rifts system from gravity inversion modelling

Recent volcanism in El Hierro Island is mostly concentrated along three elongated and narrow zones which converge at the center of the island. These zones with extensive volcanism have been identified as rift zones. The presence of similar structures is common in many volcanic oceanic islands, so understanding their origin, dynamics and structure is important to conduct hazard assessment in such environments. There is still not consensus on the origin of the El Hierro rift zones, having been associated with mantle uplift or interpreted as resulting from gravitational spreading and flank instability. To further understand the internal structure and origin of the El Hierro rift systems, starting from the previous gravity studies, we developed a new 3D gravity inversion model for its shallower layers, gathering a detailed picture of this part of the island, which has permitted a new interpretation about these rifts. Previous models already identified a main central magma accumulation zone and several shallower high density bodies. The new model allows a better resolution of the pathways that connect both levels and the surface. Our results do not point to any correspondence between the upper parts of these pathways and the rift identified at the surface. Non-clear evidence of progression toward deeper parts into the volcanic system is shown, so we interpret them as very shallow structures, probably originated by local extensional stresses derived from gravitational loading and flank instability, which are used to facilitate the lateral transport of magma when it arrives close to the surface

Exploring deformation scenarios in Timanfaya volcanic area (Lanzarote, Canary Islands) from GNSS and ground based geodetic observations

We report on a detailed geodetic continuous monitoring in Timanfaya volcanic area (TVA), where the most intense geothermal anomalies of Lanzarote Island are located. We analyze about three years of GNSS data collected on a small network of five permanent stations, one of which at TVA, deployed on the island, and nearly 20 years of tiltmeter and strainmeter records acquired at Los Camelleros site settled in the facilities of the Geodynamics Laboratory of Lanzarote within TVA. This study is intended to contribute to understanding the active tectonics on Lanzarote Island and its origin, mainly in TVA. After characterizing and filtering out the seasonal periodicities related to “non-tectonic” sources from the geodetic records, a tentative ground deformation field is reconstructed through the analysis of both tilt, strain records and the time evolution of the baselines ranging the GNSS stations. The joint interpretation of the collected geodetic data show that the area of the strongest geothermal anomaly in TVA is currently undergoing a SE trending relative displacement at a rate of about 3 mm/year. This area even experiences a significant subsidence with a maximum rate of about 6 mm/year. Moreover, we examine the possible relation between the observed deformations and atmospheric effects by modelling the response functions of temperature and rain recorded in the laboratory. Finally, from the retrieval of the deformation patterns and the joint analysis of geodetic and environmental observations, we propose a qualitative model of the interplaying role between the hydrological systems and the geothermal anomalies. Namely, we explain the detected time correlation between rainfall and ground deformation because of the enhancement of the thermal transfer from the underground heat source driven by the infiltration of meteoric water

Tilt observations in the normal mode frequency band at the Geodynamic Observatory Cueva de los Verdes, Lanzarote

We have conducted observations with the aid of a seismo-tiltmeter station, which is based on the Ostrovsky pendulum and installed at the Geodynamic Observatory Cueva de los Verdes at Lanzarote Island since 1995. In this station the signal is separated into two frequency bands – tidal tilts (from 0 to 5 mHz) and ground oscillations in the frequency range of free Earth’s normal modes (from 0.2 to 5 mHz). The later band, called accelerometer channel, has additional amplification. We analyzed the background records in the frequency range of Earth’s free oscillations from August 2000 to September 2001, as well as, Earth’s normal modes after strong earthquakes. We found several distinctive persistent peaks in the spectra of background oscillations. Both amplitudes of distinguished peaks and noises have seasonal variations. We found that spectra of background oscillations are different in the frequency interval between 1.4 and 2.5 mHz for North- South and East-West components