Physics and Chemistry of the Earth Part A Solid Earth and Geodesy

Published by Elsevier
Online ISSN: 1464-1895
Publications
Article
The earthquake we are dealing with occurred on December 28, 1908: because of the number of victims (about 60,000) and the extension of the destroyed area (6,000 km2), this earthquake with the epicentral MCS intensity XI may be considered the strongest event ever reported for Italy along with the 1693 eastern Sicily earthquake. The shock produced a large tsunami that caused severe damage and many victims. In all places the first sea movement was a withdrawal for a few minutes, followed by a flooding of the coast with at least three big waves. A post-event survey allowed to estimate flooding and run-up heights (more than 10 m in some places). In this work we perform some numerical simulations of the tsunami generation and propagation, taking into account different source faults: the model is based on the shallow water equations, solved numerically by means of a finiteelement method. The computational domain, covered by a mesh consisting of triangular elements, includes the Messina Straits and the sea facing the northeastern coast of Sicily and southern Calabria.
 
Article
The seismicity which affects Mt. Vesuvius is, at present, the only clear indicator of the volcano dynamics. In the last years, two periods of increased seismic activity occurred (August–October 1995 and March–May 1996). This seismicity was detected by the 10 analog stations of the Permanent Seismic Network as well as by up to 7 three-component temporary digital stations. A total number of about 600 events have been recorded, four of which showing magnitude M ≥ 3.0. The maximum magnitude earthquake (M = 3.4) was the strongest in the last fifty years and occurred on 25 April 1996. The use of three-component seismometers allowed us to obtain very reliable hypocentral locations. The focal volume of the two seismic crises does not exceed 5–6 km of depth below the crater area. Fault plane solutions of the most energetic events show focal planes oriented NW-SE and NE-SW, in agreement with the regional tectonic features, indicating that at present the seismicity of Mt. Vesuvius develops along pre-existing discontinuities. In addition, the occurrence of a fluid-driven source mechanism suggests a role played by the underground water on the seismic energy release. Shear wave splitting analyses confirmed the presence of an anisotropic volume related to a distribution of cracks and/or fractures parallely aligned to the main faults system of the volcano.
 
Article
The Miage Glacier is the biggest in the Italian side of the Mont Blanc and is located a few km west of Courmayeur. The Miage lake is a typical periglacial lake that is found on the right hydrographical side of the glacier. The glacier high dynamics produces frequent falls of huge ice blocks into the lake representing an important tourist draw. On August 9, 1996, after a long period of heavy rain, a big ice block, with estimated volume in the range 7,000–16,000 cubic meters, fell into the lake provoking an anomalous water wave that involved many tourists, causing some persons to be seriously wounded.The main goal of this work is to stress that for alpine glaciers a correct policy of natural hazard mitigation must be founded both on a good scientific understanding of the glacier dynamics and on rigorous measures of prevention, that in case of conflict must have anyhow priorities on touristic activities.
 
Article
Analyses of electric field observations are presented providing evidence of anomalous change occurrence during seismic swarms. We used mainly the special antenna using casing pipe of the deep borehole to sense the vertical electric field change. Anomalous signals are found in good correlation with the swarm activity in terms of duration and field strength on the bases of almost continuous observational record of more than six years. It was very easy to discriminate anomalous signals from normal state owing to sufficiently large of some 20. Anomalies were found in the whole frequency bands of dc, ULF and ELF/VLF bands with persistent occurrences in the dc and ULF bands. But anomalies in the ELF/VLF bands appeared occasionally at around the period of high seismic activities. The anomalies were detected only at the nearest site Hodaka from the swarm activity zone. A sharp spatial delectability division sensing particular seismic swarm activity at the site is attributed to the geological setting. Sources of the anomalous electric signals are suggested to be deep underground by comparing anomalous signals at different depth. The generation mechanism is indicated to be of hydrological origin through the elector-kinetic effects on the basis of whole available data around the site.
 
Article
Although the level of seismicity in the Eastern Pyrenees is currently moderate, with only one magnitude 5 earthquake recorded instrumentally, important earthquakes occurred there in the 15th century (1427–1428).This straightforward observation suggests that the area is currently in the interseismic regime between large earthquakes separated by a long recurrence interval. In this case, strain may be accumulating as crustal deformation which we can measure by repeated geodetic surveys.The geodetic network consists of 24 stations with a characteristic spacing of about 15 km. The monuments are concrete pillars anchored in bedrock outcrops chosen to span the traces of potentially active faults.The network has been observed twice, in 1992 and in 1994, using dual-frequency receivers. The difficulties encountered and the experience gained processing the campaigns using the Bernese and the GAMIT software are described. For multi-epoch comparisons and velocity field determination, a new mathematical model has been developed as part of the GeoTeX software at the Institut Cartogràfic de Catalunya (ICC). It yields results similar to those obtained by the GLOBK software. The lessons learned will allow us to improve the methodology and accuracy of future campaigns.
 
Article
We introduce a skeletization method based on the Voronoi diagram to determine local pore sizes in any porous medium. Using the skeleton of the pore space in a 3D image of the porous medium, a pore size value is assigned to each voxel and a reconstructed image of a spatialized local pore size distribution is created. The reconstructed image provides a means for calculating the global volume versus size pore distribution. It is also used to carry out fluid invasion simulation which take into account the connectivity of and constrictions in the pore network. As an example we simulate mercury intrusion in a 3D soil image.
 
Article
We present spatial distributions for pore path length and coordination number, pore throat size and nodal pore volume obtained for a 1.53 mm3 volume of 12.1% porosity Fontainebleau sandstone. The sandstone was imaged using Synchrotron X-Ray computed microtomography at 6 micron resolution. The spatial distributions were computed based upon three dimensional medial axis analysis of the void space in the image. We also present vesicle size and vesicle-vesicle contact surface area distributions for a 1.36 mm length of a 6.36 mm diameter core of basalt from a vesiculated lava flow imaged at 20 micron resolution.
 
Article
GPS total zenith delay (TZD) data have been assimilated into local area weather forecasting model using the Met Office's 3D variational assimilation system. The resulting analyses have been used as the initial conditions for a local area forecast model. Comparing model runs with and without GPS data assimilation shows that there is potential for GPS data to improve local area numerical weather forecasts. The expectation is that the largest forecast impact will be better seen in summer, non-dynamic weather conditions, rather than the winter cases presented here. The work shown here the current “state of play”; it is very much ongoing and so cannot represent definitive results. However it does show that GPS data can be assimilated into operational numerical weather prediction systems and shows some of the next steps that need to be taken.
 
Article
We present advances in the application of laser scanning confocal microscopy (LSCM) to image, reconstruct, and characterize statistically the microgeometry of porous geologic and engineering materials. We discuss technical and practical aspects of this imaging technique, including both its advantages and limitations. Confocal imaging can be used to optically section a material, with sub-micron resolution possible in the lateral and axial planes. The resultant volumetric image data, consisting of fluorescence intensities for typically ~50 million voxels in XYZ space, can be used to reconstruct the three-dimensional structure of the two-phase medium. We present several examples of this application, including studying pore geometry in sandstone, characterizing brittle failure processes in low-porosity rock deformed under triaxial loading conditions in the laboratory, and analyzing the microstructure of porous ceramic insulations. We then describe approaches to extract statistical microgeometric descriptions from volumetric image data, and present results derived from confocal volumetric data sets. Finally, we develop the use of confocal image data to automatically generate a three-dimensional mesh for numerical pore-scale flow simulations.
 
Article
Weathering rates based on temporal changes in a variety of rock properties were examined for four dated lava domes made of porous rhyolite. The lava domes were erupted at 1.1, 2.6, 20 and 40 ka BP. The time from each eruptive event to the present provide elapsed weathering times. The lava domes have many cooling joints formed immediately after eruption. Joint-bounded blocks have a high porosity, resulting in deep weathering. The blocks have almost uniform characteristics from top to bottom in one outcrop. The mineralogical, chemical, physical and mechanical properties of the four rhyolites were analysed. These results show that the chemical properties and physical properties, including specific surface area, change slowly in the early stage of weathering (0–20,000 years) and quickly in the later stage (20,000–40,000 years), while mechanical strength and bulk density or porosity of the rhyolites change rapidly in the early stage and slowly in the later stage. Mechanical strength reduces by 70–90% relative to fresh rock in 40,000 years, although chemical and physical properties change by only 1023̄0 %. It is concluded that different weathering properties exhibit different rates and patterns of change over 40,000 years, and that mechanical strength is the property most susceptible to weathering.
 
Article
COST Action 716 is in the area of meteorology and is run under supervision of the European Community. The primary objective of the COST action is “assessment of the operational potential on an international scale of the exploitation of a ground-based GPS system to provide near real-time observations for numerical weather prediction and climate applications”. The paper describes the nature of a COST action and gives a brief description of meteorological applications of ground-based networks of Global Positioning System (GPS) receivers. The content of the COST Action 716, its present status, and the plans for future work are described. The first two years of work has resulted in a common view in the geodetic and meteorological communities on how to proceed with the assessment of the ground-based GPS technique.
 
Article
MAGIC (Meteorological Applications of GPS Integrated Column Water Vapor Measurements in the Western Mediterranean) is a 3 year project financed in part by the European Commission for research on deriving and validating robust GPS integrated water vapor (IWV) and zenith tropospheric delay (ZTD) data sets and developing methods to assimilate the data into numerical weather prediction models (NWP) and test their impact. It was conceived independently from the COST 716 action, which seeks to coordinate research in the domain at an international scale, but addresses some of the same objectives. This has led to a productive cooperation between the two initiatives and their participants, and motivated the decision of MAGIC participants to provide research results as part of the COST demonstration system. Currently a database of 1.5 years of ZTD data are available on the MAGIC web site which has been validated through comparisons with radiosondes which gives differences with a standard deviation of 10 mm ZTD or the equivalent error in IWV of 1.6 kg/m2. NWP assimilation tests will be carried out in the final year of the project.
 
Article
We present a method for estimating the seismic intensity in terms of MMI or MSK scale using Fourier amplitude spectra of ground acceleration. The method implies that severity of earthquake ground motion is determined by spectral amplitudes in relatively narrow frequency band: so-called “representative frequencies”, at decreasing frequencies (from 7–8 Hz for small intensities to 0.7 – 1.0 Hz for MMI(MSK) = VIII–IX) with increasing intensity level. It is examined through estimation of probable intensity at a site using recordings of recent earthquakes in several seismic regions and prediction of intensity distribution patterns for some earthquakes. Seismic hazard maps, in terms of intensity levels based upon the proposed approach, should describe regional features of seismic waves excitation and propagation, as well as local ground conditions.
 
Article
Air-borne and satellite based altimetry are used to monitor the Greenland ice-cap. Since these measurements are related to fiducial sites at the coast, the robustness of the height differences depends on the stability of these reference points. To benefit from the accuracy of these methods on the centimeter level, station corrections regarding the Earth tides and the ocean tidal loading have to be applied. Models for global corrections esp. for the body tides are available and sufficient, but local corrections regarding the effect of the adjacent shelf area still have to be inferred from additional observations. Near the coast, ocean tidal loading causes additional vertical deformations in the order of 1 to 10 cm. Therefore, tidal gravity measurements were carried out at four fiducial sites around Greenland in order to provide corrections for the kinematic part of the coordinates of these sites. Starting in 1993 four stations were occupied on Greenland for a one year record each.The results show the expected strong tidal anomalies due to ocean tidal loading. The loading computations confirmed these observations, but it turned out that with global models only about 50 % of the observed effect can be explained. This means that at these stations a vertical deformation of up to ± 3.5 cm is not corrected applying these global models.
 
Article
In this paper we review the atmospheric modeling methods used in GPS data analysis. Due to the strong spatial inhomogeneity and temporal variability of atmospheric constituents, especially water vapor, accurate modeling of path delay in GPS signals is necessary for high-accuracy positioning (e.g., tectonics and sea-level change) and meteorological applications (climatology and weather forecasting). State-of-the-art path delay modeling consists primarily in parameter estimation. In this strategy, zenith path delays are estimated during the GPS data reduction. External correction is another common strategy, in which the wet path delay is measured by a remote sensing instrument (usually a microwave radiometer). However, the latter is not as generalized, and is rather used for specific field campaigns or local long term observations. Both strategies have led to quite similar coordinate accuracies (using daily GPS observations), at the level of 1–2 mm in the horizontal component and 5–10 mm in the vertical component. The external correction strategy is capable of achieving even higher accuracy under specific conditions. Recent models, including gradients in the parameter estimation strategy have only led to marginal improvement. A major limitation of both strategies seems to be the use of mapping functions for the hydrostatic path delay correction. With the parameter estimation strategy, this limitation applies also to wet path delay correction. The use of numerical weather prediction and analysis models, and/or spaceborne sounding instruments, is suggested for replacing mapping functions and possibly for performing directly the hydrostatic correction. New instruments, such as Raman lidars, might also be used for a more accurate external wet path delay correction in the presence of strong atmospheric inhomogeneity. Further work is still needed for achieving measurements of absolute water vapor distribution in the atmosphere for this purpose.
 
Article
The correction of the propagation delay due to the neutral atmosphere has been recognized as the major modeling error for most of the radiometric space techniques used in precise geodesy, affecting mainly the height component of position. For low elevation angles, many of the existing mapping functions produce large errors and are therefore not suitable for those applications demanding the highest precision.We have benchmarked five mapping functions acknowledged to be the most accurate of those currently available: those developed by the Harvard-Smithsonian Center for Astrophysics (CfA), Chalmers University of Technology (Ifadis), the Jet Propulsion Laboratory (Lanyi), the Massachusetts Institute of Technology (MTT), and Haystack Observatory (NMF) against ray traces of a one-year data set of radiosonde profiles from 50 stations. From a station-by-station analysis, none of the mapping functions revealed themselves to be superior for all elevation angles. For elevation angles above 15 degrees, Lanyi, MTT, and NMF yield identical mean biases and the best total error performances. At lower elevation angles, Ifadis and NMF are clearly superior. As regards the r.m.s. scatter about the mean, Ifadis performs the best for all elevation angles, followed closely by Lanyi.
 
Article
For specific applications such as permanent GPS network calibration and national leveling network surveying, a vertical accuracy of ∼1 mm for observing durations of a few hours to a few days at maximum in 10–100-km baselines would be required. To achieve a 1-mm accuracy in height determinations with differential-GPS measurements, path delay must be corrected with an accuracy of ∼0.3 mm. This level of accuracy is not achievable with standard GPS data analysis procedures. External correction from a water vapor remote sensing technique is therefore necessary. Microwave radiometers, which have been most extensively used for this purpose, solar spectrometers, DIAL and Raman lidars are considered in this paper. The principle and performance of these techniques is reviewed in the context of wet path delay retrieving. Namely, we evaluate the errors arising during the conversion of raw measurements to wet path delay, using retrieval coefficients or standard profiles. It is shown that changes in temperature profiles can produce errors of up to 1 cm in wet path delay with microwave radiometers. Similarly, mismodeled temperature profiles can produce errors of 2–3 mm in wet path delay with DIAL and Raman lidars. Raman lidar offers the possibility to retrieve the temperature profile from total air density. Assuming that absolute concentrations of water vapor and dry gases can be retrieved, the accuracy would be unbiased. In addition, Raman lidar would also allow for the correction of hydrostatic path delay without requiring the use of mapping functions. This might reduce the residual errors due to horizontal pressure and temperature gradients. This technique will therefore be investigated in more details in a future study.
 
Article
Turbulent transport of aerosols and droplets in a random velocity field with a finite correlation time is studied. We derived a mean-field equation and an equation for the second moment for a number density of aerosols. The finite correlation time of random velocity field results in the appearance of the high-order spatial derivatives in these equations. The finite correlation time and compressibility of the velocity field can cause a depletion of turbulent diffusion and a modification of an effective mean drift velocity. The coefficient of turbulent diffusion in the vertical direction can be depleted by 25 % due to the finite correlation time of a turbulent velocity field. The latter result is in compliance with the known anisotropy of the coefficient of turbulent diffusion in the atmosphere. The effective mean drift velocity is caused by a compressibility of particles velocity field and results in formation of large-scale inhomogeneities in spatial distribution of aerosols in the vicinity of the atmospheric temperature inversion. Results obtained by Saffman (1960) for the effect of molecular diffusivity in turbulent diffusion are generalized for the case of compressible and anisotropic random velocity field. A mechanism of formation of small-scale inhomogeneities in particles spatial distribution is also discussed. This mechanism is associated with an excitation of a small-scale instability of the second moment of number density of particles. The obtained results are important in the analysis of various atmospheric phenomena, e.g., atmospheric aerosols, droplets and smog formation.
 
Article
Important areas of the earth are still not covered by accurate gravity measurements. The gravity field may be determined by using different techniques but airborne gravity surveying is becoming the most powerful tool available today.One of the main problems in airborne gravity is the separation of the vertical accelerations acting on the airborne platform from the natural gravity signal. With the advances in DGPS techniques new prospects arise for gravity field recovery which are of great importance for geodesy, geophysics oceanography and satellite navigation. Furthermore, airborne gravimetric measurements depend not only on the determination of the position but also on the attitude of the aircraft. Inertial systems can provide attitude as well as information on short-term accelerations, which are more problematic for the gravimeter. A proper integration of these systems may allow a further improvement of the whole technique where the quality of both the accelerometers and the gyros is the key sensing element. In the scope of the MAST III Project AGMASCO, an airborne geoid mapping system was successfully implemented in different aeroplanes. The characteristics of the aeroplane and the flight parameters play a major role in airborne measurements.Within AGMASCO the airborne system was applied both in a close and an open ocean (Skagerrak, Fram Strait and Azores) areas. The system proved to be a powerful tool in a variety of conditions. The results obtained showed that an accuracy better than 2mGal over 5 to 6 kilometres can be achieved.This was proven by comparison of the airborne data with ground truth and satellite data. This accuracy makes the system interesting for use in various applications including geophysical exploitation.Different hardware installations were experienced and the methods validated. Recovery of the gravity values directly from measurements with the Lacoste & Romberg air/sea gravimeter and from measurements with the inertial sensors was analysed. The potential of these sensors to recover gravity and the experience gained within this project are reported here.
 
Article
In regolith-dominated terrains, the nature of contemporary processes and the surface distribution of regolith materials may be a poor guide to the character and history of regolith materials at depth. The nature of regolith materials at depth is often critical to unravelling the development of a landscape. Conventional mapping aids such as air photos, multispectral remote sensing and airborne radiometrics are not wholly adequate in this context, as they penetrate limited depths (<0.4 m). Airborne electromagnetics (AEM) on the other hand, has the potential to map regolith materials to considerable depths (>100m).The application of AEM to regolith mapping and its potential as a tool in geomorphology are illustrated by reference to an AEM survey flown at Lawlers in the Yilgarn Craton of Western Australia. At Lawlers, AEM identifies a palaeochannel that has no surface expression. It cannot be seen in images of the Landsat, airborne radiometric or airborne magnetic data. The disposition of this channel in the landscape, and in particular its association with ferruginous materials forming breakaways, suggest that inversion of relief has been a significant factor in the evolution of the Lawlers landscape.The AEM data at Lawlers have also been used to map the weathering front. The topography of the weathering front not only reflects the movement of water through the landscape in a general sense, but also reflects the influence of lithology and structure. Different lithologies are clearly weathering to different depths. Information on the nature of the weathering front is potentially an important constraint on models of groundwater flow, and by association, models of solute dispersion.
 
Article
In contrast to the loess-paleosol sequences of China and Central Europe, paleosols of the Halfway House and Gold Hill Steps loess sections in central Alaska show no enhancement in magnetic susceptibility. Evidence for maghemitization and ultrafine superparamagnetic minerals is found in the topsoil and in the lowest weathered loess of the sequence. These minerals are not detected in the three paleosols within the profiles. Rock-magnetic characteristics also indicate a smaller magnetic grain size in the loess with the lowest magnetic susceptibility values. This finding supports the interpretations of Begét et al. (1990) that magnetic concentration and corresponding susceptibility variations may be related to changes in wind intensity.
 
Article
Metapelitic xenoliths enclosed in the Crd-Grt-bearing lavas of the Neogene Volcanic Province of SE Spain retain evidence of partial melting and relevant information on the mechanisms and P-T conditions of crustal anatexis, preserved by rapid exhumation and cooling during eruption. Both at El Joyazo and Mazarrón, microstructures show that anatexis was accompanied by foliation development, implying that the xenoliths represent portions of a deforming crystalline basement, partially molten before being enclosed in the dacite.
 
Article
The main subject of the present paper is to show applications of quaternions in some general problems of geodesy and inertial navigation. Quaternionic descriptions of some generally employed geodetic reference frames connected with the earth and the sun are considered and quaternion form of relationships among them are presented. We present also different quaternion forms of the main kinematic equation of a rigid body motion in rotating reference frames. Using quaternions it is easy to see that such terms as tangential, Coriolis, and centripetal accelerations arise in kinematic equations very naturally. It is shown that mathematical modelling based on quaternions is a very elegant mathematical method which clarifies the essence of problems and is very useful for attacks of many complicated problems in different areas. Quaternions are convenient for computer modelling as far as they contain less parameters then usual matrices.
 
Article
Because of occurrence of ill-conditioning and outliers, use of direct Least Squares fit is now in decline, while robust M-estimators are currently attracting attention. We present here new algorithms based on the Spingarn Partial Inverse proximal decomposition method for L1 and Huber-M estimation that take into account both primal and dual aspects of the underlying optimization problem. The result is a family of highly parallel algorithms. Globally convergent, they are attractive for large scale problems as encountered in geodesy, especially in the field of Earth Orientation data analysis. The method is extended to handle box constrained problems. To obtain an efficient implementation, remedies are introduced to ensure efficiency in the case of models with less than full rank. Numerical results are discussed. Robust data pre-conditioning is shown to induce faster algorithm convergence. Practical implementation aspects are presented with application to series describing the Earth Rotation.
 
Article
Rock microfracturing significantly affects elastic and anelastic parameters as well as transport properties. Several rock models are proposed in literature relating rock microfractures and cracks to the effective velocities. However, the mechanisms involved in the attenuation of seismic waves still need further investigation in order to properly relate attenuation to fracturing. Laboratory measurements of ultrasonic wave propagation are performed on dry sandstones during increasing fracturing of the samples. Perpendicular to the loading axis the wave velocities decrease and attenuation increase. The observed decrease of wave velocity is a measure for the crack density of the newly formed cracks. Utilising crack densities and microstructural parameters the attenuation behaviour is interpreted in terms of the mechanisms friction and scattering. On the same rock samples the effect of fracturing on water transport is investigated. Measurements before and after the mechanical fracturing show a decrease of permeability in loading direction. The changes in seismic parameters and permeability are interpreted by different variations in rock microstructure. This should be considered when relating seismic and transport rock properties.
 
Sample references, facies and alteration features
Mean aperture and aperture probability for each type of ISGBP
Experimental conditions and results of permeability.
Image size and fraction surface area of each studied I:dcw
Article
Various experimental methods and image analysis procedures have been used to underscore the microporosity evolution of the Charroux-Civray tonalite. Throat pore effect is the dominant geometrical parameter, which controls permeability and determines the transport process. Indeed, in unaltered tonalite, flowing is not possible in spite of the fact that diffusion is made possible by a connected random network. Fluid pathways are located along the feldspar clusters joining quartz grains. Alteration effects are seen on the increased mean aperture of microcracks (feldspar dissolution). This paper raises the problem of the experimental limits in the study of very low permeable rocks.
 
Article
Assimilation of satellite altimeter data into ocean models offers the best prospect for testing the ability of ocean models to correctly represent near surface circulation and to extrapolate observations of that circulation in space and time. This will ultimately lead to real time prediction of ocean circulation and of sea surface temperature which will contribute to improving weather and climate forecasts. The relative scarcity of observations of the ocean, even when satellite data are considered, places special importance on the choice of numerical model in any oceanographic data assimilation system. The 1/4 degree OCCAM global ocean model used here represents the state of the art. This model has been combined with a new approach to assimilating altimeter data developed at Edinburgh, in which emphasis is placed on preserving sub-surface water mass properties. The basic altimeter assimilation scheme has been tested successfully in an idealised, ‘twin’ experiment. Further experiments are now underway assimilating TOPEX/POSEIDON altimeter data into the OCCAM model. Early results of a model run assimilating altimeter sea surface height data from 1993 are presented.
 
Article
Whilst satellite radar altimetry has been widely utilised over both ocean and ice surfaces for topographic mapping, applications over land have received relatively little attention. This is in part due to the complex nature of echoes returned from rapidly varying topographic land surfaces, which can cause an altimeter to generate erroneous range estimates. One approach to improving these data is to retrack using a single retracker, and construct a spatial average of heights obtained to give an estimate of mean orthometric height. This paper presents results obtained using an alternative approach: reprocessing returns at all levels of complexity through an expert system, which chooses one from a series of ten reprocessing algorithms based on an analysis of the return waveform shape. The selected algorithm then recalculates the range to surface, and hence derives an orthometric height. Utilising this approach with the geodetic mission dataset from ERS-1 has generated over 100 million height points with a near-global distribution. This paper presents selected results from this research using ERS-1 geodetic mission data together with ERS-1 and ERS-2 35 day data to demonstrate the accuracy to which orthometric heights can be determined, using global crossover analysis and comparison with ground truth. The paper illustrates applications of these data including validation and error correction of Digital Elevation Models, and discusses use and limitations of direct mapping with altimetry.
 
Article
During recent years altimetry from the two geodetic missions of GEOSAT and ERS-1 has enabled the derivation of high resolution near global gravity field from altimetry [Andersen and Knudsen, 1995, 1996; Sandwell and Smith, 1997].Altimetric gravity fields are unique in the sense that they provide global uniform gravity information with very high resolution, and these global marine gravity fields are registered on a two by two minute grid corresponding to 4 by 4 kilometres at the equator.In this presentation several coastal complications in deriving the marine gravity field from satellite altimetry will be investigated using the KMS98 gravity field. Comparison with other sources of gravity field information like airborne and marine gravity observations will be carried out and two fundamentally different test areas (Azores and Skagerak) will be studied to investigated the different role of these different sources of gravity information.
 
Article
Dynamo effect is considered in more general case than well known one when not only mean motion but also conducting components possess nonzero mean helicity. Dispersion equation for helical motions is studied, including inhomogeneous case. Criteria for development of instability are found. Interaction between large-scale internal wave and small-scale helical turbulence in plane Couette flow of fluid with statically stable uniform density gradient in gravitational field is studied basing on a set of equations for scalar and quasi-scalar wave fields and for helicity of turbulence. This system of equation describes acceleration of wave growth due to its backward action on turbulence, i.e. wave-turbulent instability. Chiral media are further considered and effects topologically close to helical ones are analyzed. It is shown that in such media ranges of scales exist, in which fluctuations of electric field can be anomalously amplified.
 
Article
Analog experiments of melt segregation and migration in lower crustal rocks have been conducted using paraffin wax. The wax has a mechanical planar anisotropy which reproduces the pervasive foliation of high-grade metamorphic rocks. The shortening of a layer of partially molten wax (melt fraction between 15 and 20%) results in the movement of a part of the liquid from the microscopic porosity of the wax to the outside of the layer in large accumulation sites. Four stages can be identified: (1) from the beginning of the shortening, melt segregates into dilatant foliation-parallel veins; (2) the development of a fold occurs with an increasing accumulation of liquid in the limbs; (3) strain localization and vein connection allows the nucleation of shear bands; (4) melt migration is channelled by the shear band toward external pockets. The first two stages involve melt percolation from kinematically controlled high-stress areas around growing veins. The third stage is associated with local attainment of a segregated melt critical concentration estimated at 14–15%. The last point involves both horizontal and upward migration of the melt. Melt segregation and migration are highly scale- and strain-dependent mechanisms.
 
Article
The application of two sets of methods for the characterization of rock pore systems is reported. Both are discussed in the context of other techniques described in the literature. (i) Impregnation with labelled polymethylmethacrylate (PMMA) and autoradiography or direct measurement of the tracer activity allows the investigation of the pattern of the spatial porosity distribution and quantitative measurement of mineral-specific, local porosities, porosity gradients as well as an assessment of hydraulic and diffusive transport properties. (ii) Quantitative information on surface areas within certain pore size ranges can be achieved by combination of mercury intrusion porosimetry with X-ray absorption computer-tomography. As examples of applications of these techniques, the effects of weathering, alteration, mechanical stress and large diameter coring on the pore network of rocks have been studied.
 
Article
An interdisciplinary approach is used to quantify partial melt fractions and to infer the origin and distribution (melt structure) of melts located in the crust beneath the Central Andes and the Tibetan plateau. In these areas field observations of Low Velocity Zones (LVZ) and High Conductivity Zones (HCZ), which are commonly attributed to partial melting, are used to quantify melt fractions. Additional information is obtained from νP/νS ratios, seismic attenuation data, and heat flow density and gravity anomalies. These data accompanied by thermal modelling suggest that melts of mainly crustal origin are interconnected through dykes and veins. Experimental results and model calculations indicate that the minimum fraction of melt necessary to describe the LVZs and HCZs in the Central Andes and the Tibetan plateau is approximately 20 vol.%, and the melt has a non-ideal interconnectivity.
 
Article
We investigate the seismic anisotropy signature of the continental rifting process. Several sources of anisotropy are considered: the lithospheric deformation, the asthenospheric flow, and the occurrence of oriented meltpockets in the asthenospheric mantle. Our results show that contrasted anisotropy patterns should be associated with the various conceptual models of rifting. Thus seismic anisotropy measurements may allow one to discriminate between these models. Anisotropy measurements in the Rio Grande, East-African and Rhine rifts suggest that these rifts formed by a transtensional deformation of the lithospheric mantle rather than by homogeneous extension of the lithosphere. Alignment of melt-lenses in the asthenospheric wedge may also account for a significant part of the seismic anisotropy recorded in the internal domains of these rifts.
 
Article
We present the results regarding a combined analysis of self-potential and acoustic emission time series, jointly monitored during 1996 by means of a remote station installed north of the town of Potenza, close to an active fault system in a seismic area of Southern Italy. The goal is to verify the existence of correlations between geoelectric and seismoacoustic signals and the local seismicity. Preliminary filtering procedures for the removal of meteoclimatic effects and noise fluctuations of anthropic origin were applied. Then, objective methods were used to discriminate anomalous patterns from background noise in electric and seismoacoustic time series. Finally, a deep analysis of the possible correlations between the self-potential and acoustic emission extreme events and local seismicity has been carried out. The extreme events occurred before the Md = 4.5 earthquake of April 3, 1996 seem to indicate a common physical mechanism likely referable to the dilatancy-diffusion-polarization model.
 
Article
Mt. Melbourne volcano (Northern Victoria Land-Antarctica) is part of the McMurdo Igneous complex, which spans a considerable time interval between 48 Ma to the Present. It is generally accepted that both the location and the magmatic products of the volcano are genetically linked to Ross Sea rifting and uplift of the Transantarctic Mountains (TAM) rift shoulder. Studies on pyroclastic falls suggest that an eruption likely occurred in the last few centuries. Presently the volcano has a low level of activity which has been highlighted and monitored by means of integrated geophysical networks. After a review of previous geophysical and geological findings at both regional and at a more local scale, in our study we display and interpret newly compiled magnetic and gravity images over the Mount Melbourne area to better constrain the tectonic framework of the region. We propose that the structural setting is dominated by major NWSE right-lateral strike-slip faults generating uplifted crustal blocks, namely the Deep Freeze Range block and more subsided N-S graben-like structures such as the one in which Mount Melbourne volcano itself appears to be located. Local seismic events could well be associated with the N-S faults within this graben. Gravity data is consistent with crustal thickening beneath the TAM and offshore it highlights together with heat flow data and seismic constraints a NNE trending pull- apart basin with extended crust also linked to the strike-slip tectonics of the region. Local gravity anomalies are also discussed.
 
Article
In this work we analyse the hourly geoelectrical time series recorded by means of three stations located in a seismically active area of Southern Apennine Chain (Italy): Tito station (January 1992 – December 1997), Tramutola station (January 1995 – December 1995) and Giuliano station (January 1996 – December 1997). The time dynamics of the geoelectrical time series was investigated to obtain information regarding: i) the stochastic or chaotic behaviour of the time series; ii) the presence of scaling laws in their power spectral density and the estimation of the power-law index; iii) the possible correlation between the temporal variation of the power-law index and the local seismic activity. Autoregressive models and the Lomb Periodogram method have been applied to get the most quantitative information about the time dynamics from geoelectrical signals. Firstly, the predictability of geoelectrical measurements has been investigated using autoregressive models. The procedure is based on two forecasting approaches: the global and the local autoregressive approximation. The comparison of the predictive skill of the two techniques is a strong test to discriminate between low-dimensional chaos and stochastic dynamics. Our findings suggest that the physical system governing the electrical phenomena in active seismic areas is characterized by a stochastic dynamics. In a second step we investigated the stochastic properties of the geolectrical signals searching for scaling laws in the power spectral density, identifying for it a power-law P(f) ∝ α, with the scaling exponent α being a typical fingerprint of fractional brownian processes. In this analysis we apply the Lomb Periodogram method, which allows to calculate the power spectral density for time series with missing data. Finally the possible correlation between the time fluctuations of power-law index and the local seismic sequences has been investigated. An interesting coseismic variation of the scaling exponent in power spectra of geoelectrical time series recorded at Tito station has been identified on April 3, 1996. During the same period significant anomalous patterns are indentified in geoelectrical time series measured at Giuliano and Tramutola stations.
 
Article
The linear and nonlinear elastic properties of granular media are analyzed within the context of effective medium theories, as well as with numerical molecular dynamic simulations, assuming the validity of the Hertz-Mindlin theory at the single contact level. There is a crucial distinction between force laws which are path independent, leading to a hyper-elastic effective medium theory, and those which are path dependent, for which the deformation history must be followed explicitly. The effective medium theories provide a reasonable description of existing experimental data, considered as a function of applied stress, but there are significant discrepancies. Numerical simulations resolve the question as to whether the problem lies with the treatment of the individual grain-grain contact or with the effective medium approximation (ema). We find that the problem lies principally with the latter: The bulk modulus is well-described by the ema but the shear modulus is not, principally because the ema does not correctly allow for the grains to relax from the affine motion assumed by the ema.
 
Article
Sino-Korean Craton (SKC) in eastern China is an important natural laboratory for studying temporal change to the lithosphere because there is the jutaxposition of Ordovician diamondiferous kimberlites, Mesozoic lamprophyre-basalt and Cenozoic tholeiite-alkali basalts in this craton. While diamond inclusions, xenoliths and mineral concentrates in kimberlites indicate a thick (180 km), cold and refractory lithospheric keel beneath the SKC prior to the Palaeozoic, basalt-borne xenoliths reveal the presence of thin (<80 km), hot and fertile lithosphere in the Cenozoic. This indicates the dramatic change in lithospheric architecture during the Phanerozoic. Geochemical characterization of late Jurassic to recent basalts further suggests that the lithospheric destruction started since the Jurassic, probably due to the loss of physical integrity of the craton as a result of the Triassic collision between North China and Yangtze blocks. The replacement of old lithospheric keel by “oceanic” mantle has been accomplished during the late Cretaceous. Coupled thermo-mechanical and chemical erosion within the lithosphere-asthenosphere interface is considered as an important mechanism to thin the lithosphere. The lithospheric thinning may proceed with gradual upward migration of the lithosphere-asthenosphere boundary. Alternatively, the lithospheric thinning could proceed in the way that the old lithospheric mantle was penetrated and then desegregated by hot mantle materials which rise along vertical lithospheric shear zones and spread like mushroom clouds.
 
Article
The NE Asian margin was the locus of an Andean-style subduction zone during most of the Mesozoic and Cenozoic. The Ukelayat flysch is a 10 to 15 km thick section of marine turbidites that were deposited in a 500 km long forearc basin along the outboard edge of this convergent margin. It was structurally overridden during the early Tertiary by an obducted island arc, the Olytorsky terrane. We present new fission-track (FT) grain ages from unreset detrital zircons. These data provide precise information about depositional age of the Ukelayat and the temporal evolution of the Asian margin continental arc, which provided much of the sediment for the Ukalayat. Because all sandstones contain a large fraction of first-cycle volcanic zircons (euhedral, colorless), we infer that the youngest component of the FT grain age distribution, designated P1, closely approximates the time of deposition. Twenty-seven samples have yielded P1 ages ranging from ~44 to 88 Ma, which indicates continuous magmatic activity in the arc from the Late Cretaceous to Middle Eocene. A second component (P2) records progressive exhumation of the basement to the Okhotsk-Chukotka arc. The difference between P1 and P2 ages indicates steady exhumation at ~200–400 m/Myr from ~90 to 44 Ma. This finding implies the removal of 9 to 18 km of rock in the most deeply exhumed parts of the source region. A change in exhumation rates at ~70 Ma may coincide with an eastward shift in the locus of volcanism from the Okhotsk-Chukotka arc to the younger Western Kamchatka-Koryak arc.
 
Article
Climatic change in SE Europe can be characterized by the term aridification, which means increasing semi-aridity, manifested in an increase of mean annual temperature and at the same time in a decrease in the yearly precipitation.The paper deals with research results obtained within the framework of the MEDALUS II project (funded by the Commission of the European Communities). The project had the following objectives: 1.(i) Assessment of the impact of global change on the climate of the investigated area, including possible future climates.2.(ii) Physical processes of aridification, including studies of groundwater level change, soil moisture profile dynamics, soil development, vegetation change and soil erosion.3.(iii) Land use change, involving research on present land use and suggestions for the future.Various methods were applied with respect to the different research objectives. 1.(i) Statistical analysis of climatic oscillations and computer runs of climatic scenarios,2.(ii) Analysis of ground water data, mapping and analysis of soils and vegetation, assessment of present and future soil, and3.(iii) Land capability assessment through ranking environmental conditions according to the demands of the most widely grown arable crops in Hungary.According to our results i) the average annual warming during the last 110 years was +0.0105 °C, and precipitation decreased by 0.917 mm/year; ii) a decline of −2 to −4 m in the annual mean groundwater level can be detected in the most sensitive areas, with gradual lowering of the water table in alkali ponds; complete desiccation of some of them severs the direct contact between groundwater and salt-affected soils, the solonchak soil dynamics cease, helophile and hygrophile plant associations disappear, and consequent changes in the soil erosion regime are likely to lead to disastrous erosion in the future; iii) the climatic changes induce a transformation in land use from arable crops to plantations, starting with orchards.
 
Article
In recent years a great effort has been made to quantitatively model major historical events in Europe, searching for models that are compatible with both seismic, hydrodynamic and sedimentological evidences. The possibility to do so, with the available modelling techniques, has been checked against instrumental tsunami and seismological data. Most of this effort has been made within the projects GITEC and GITEC TWO.The quantitavive data that can be gathered from historical sources are: extension of the source area, wave heights, first wave polarity and, only rarely, travel times and time intervals between main waves. With this data the accuracy of source studies is a function of both the model uncertainty (can tsunami waves be accurately modelled ? are seismic parameters compatible with tsunami source parameters ?) and the observation errors. These are large when we deal with historical documents hundreds of years old.Here we present the conclusions obtained from the study on the 1755 Lisbon earthquake and tsunami. The available historical data are reliable enough to allow a good source determination and, surprisingly, the results obtained differ from those obtained from previous macroseismic studies.The analysis of the results obtained in the tsunami modelling, checked against the macroseismic data, allow the identification of an important seismo and tsunamigenic area, close to the SW Portuguese coast, that must also be taken into consideration for the evaluation of seismic hazard.
 
Article
Tilt-strain measurements have been collected since 1977 by several underground stations in the Friuli seismic area (NE Italy) with the aim of detecting seismotectonic deformations. In the work, meteorological effects are initially considered as being able to obscure possible tectonic signals. Two middle-term anomalous signals (tilt and areal strain), not correlated with atmospheric changes, are successively selected for the analysis. They are the strongest and more evident signals recorded in the area after the 1976 destructive seismic events and also are the only two cases of epicentres very near to the recording stations (M = 4.1 and M = 3.9 at 1.8 and 2.9 km, respectively). Evaluations on the basis of micro-cracking and fault creep are carried out. Estimates based on trivial rheological models furnishes crustal viscosity values in agreement with those obtained in the analysis of the silent earthquakes recorded in the same area before the 1976–1977 seismic events. Our data support the hypothesis that prominent precursory signals can be detected only within a distance few times the dimensions of the source area.
 
Article
We compare tropospheric single path delays measured by the Global Positioning System (GPS) and calculated by a numerical weather prediction (NWP) model. We discuss the use of GPS slant path delays for NWP and the use of NWP models for better GPS data processing.We use the Bernese Software 4.2 to determine undifferenced slant path delays caused by the neutral atmosphere from undifferenced ground based GPS observations. We calculate the same delays using the global NWP model from the European Centre for Medium-Range Weather Forecasts (ECMWF) with ray tracing. The comparison indicates that the level of agreement (1 – 1.5 cm for the zenith total delay) depends on the amount of water vapor present in the atmosphere.
 
Article
The estimation of horizontal atmospheric gradients, in addition to zenith delays, is a strategy now commonly used in geodetic Global Positioning System (GPS) positioning. This strategy compensates for inhomogeneities in the atmospheric water vapor distribution above GPS sites, and has shown to increase the positioning precision, e.g. in geodynamic networks. While the zenith delay has been successfully related to the pressure at the GPS site and the water vapor above the site, the relation of the GPS estimated horizontal gradients to atmospheric quantities remains unclear. To get a better understanding of the nature of these gradients inferred by GPS, this study compares GPS tropospheric observations from the MAGIC permanent network on the NW side of the Mediterranean Sea with simulations based on the high resolution NWP model ALADIN (Météo France). To verify the model performance, we use meteorological measurements from the FETCH ship campaign in the Gulf of Lyon in March-April 1998. For this study, five stations of the MAGIC network close to the Golf of Lyon have been selected. Results from two periods, representing two different weather situations occurring within the FETCH observation campaign, are presented.
 
Article
Estimates of the atmospheric Integrated Water Vapor (IWV) for 30 GPS sites in Europe, covering the period from January 1997 until August 1999 are analyzed. For every month the diurnal cycle in the IWV is estimated by averaging the data referred to that particular hour for that month. The average peak-to-peak value of the diurnal cycle during the winter months is between 0.6 mm and 1.5 mm, while during the summer months the peak-to-peak value is between 0.8 mm and 3.2 mm. The GPS results at the Onsala Space Observatory are compared to those obtained from a co-located microwave radiometer and nearby (distance 37 km) radiosonde launches. We find that the estimated diurnal variations are comparable to the uncertainties in the measurements and the models used. However, a clear dependence between the peak-to-peak value and latitude of the observations is seen during the summer months. No such dependence is detected for the winter period.
 
Article
The purpose of this paper is to visualize atmospheric ozone data using a new knowledge representation method called Formal Concept Analysis. This method is based on the concept of concept and represents data in hierarchical line diagrams combining the logic of implications with statistical frequency analysis and hierarchical search methods.
 
Article
Mapping of the zenith hydrostatic delay at radio frequencies to elevations as low as 3° has been improved by making use of the in situ atmosphere state. This information might be obtained from a numerical weather model, thus providing better atmosphere calibration on a global scale. The new model has been evaluated by comparing the repeatability of the lengths of baselines measured by Very Long Baseline Interferometry for a limited set of high accuracy experiments from 1994 January. The improvement compared to using the Niell 1996 mapping function, though significant, is less than expected, indicating other sources of unmodeled error.
 
Article
I explore the statistics of integrated water-vapor (IWV) and the ability of GPS to provide information that reflects large-scale weather systems. I demonstrate that the power spectral density (PSD) is a much more robust estimator of IWV statistics than the structure function is for long time scales. I then use the PSD to investigate the IWV statistics of simulated atmospheres. I use a random walk to approximate the Kolmogorov behavior of local atmospheric turbulence. The atmospheres I simulate contain the turbulent behavior sumperimposed on systematic changes to the IWV that might be associated with the passage of fronts. I demonstrate that the PSD associated with such frontal systems might be quite similar to those for pure turbulent behavior. This result may explain why many previous studies that examined PSDs of IWV over a site concluded that the spectra was Kolmogorov. I also derive the first cross-power spectrum of IWV using GPS sites separated by ∼500 km. At this separation, the local turbulence may be assumed to have become decorrelated. I find that even this long wavelength signal consists of components at a wide range of temporal frequencies. The IWV cross power spectrum contains significant energy even at temporal frequencies of 2–4 cycles per hour.
 
Article
A new, high precision, high accuracy and high resolution gravimetric geoid of Australia has been produced using most updated data, theory and methodology. This paper presents a concise report of the new Australian geoid determination. Some aspects of the new geoid computation, such as data validation, geoid determination strategies and computational procedures, are described. The relative precision of the new geoid is better than 5 cm for average baseline length of 4km~40km and 18 cm for average baseline length of 120km when compared with three local GPS/levelling networks.
 
Top-cited authors
Yi-Gang Xu
  • Guangzhou Institute of Geochemistry Chinese Academy of Sciences
Ge Lin
  • Chinese Academy of Sciences
Feng Guo
  • Chinese Academy of Sciences
Yuejun Wang
  • Sun Yat-Sen University
Teng-fong Wong
  • The Chinese University of Hong Kong