Some critical factors for engineering and environmental microgravity investigations

The gravity method is one of the geophysical tools used for engineering and environmental investigations where the detection of cavities, karst phenomena, subsoil irregularities, or landfills is essential. In many cases, deep or small-scale heterogeneities generating low-amplitude anomalies have to be detected and the reliability of further interpretation requires highly accurate measurements, carefully corrected for any quantifiable disturbing effects. The purpose of this study is to investigate the factors likely to limit measurement quality and how to make improvements.Calibrations of a Scintrex gravimeter were made between French relative and absolute base stations, and the relative uncertainties on the calibration factors were estimated for these links. Ranging from 10⁻³, for calibration on an old gravity net, to 10⁻⁴, for a high amplitude absolute base line, this accuracy will be generally sufficient for microgravity surveys.Continuous gravity recordings of Scintrex gravimeters, installed at the same stable site, enabled the estimation of the stability and accuracy of the instruments and revealed that some of the time variations of g measurements, such as instrumental drift, tidal effects and seismic noise, are not entirely removed by standard processing procedures. The accuracy of corrected gravity measurements is mainly limited by inadequate corrections of tidal effects and by a poor estimation of ocean loading effects. In comparison with residual defaults in tidal corrections, instrumental and seismic noises are taken more properly into account by statistical data processing.In field operation, residual tidal effects are generally integrated into an experimental terrain drift estimated on the basis of frequent repeated measurements. A differential gravity approach, based on a fixed gravimeter reference whose recordings are used to correct measurements made with a mobile gravimeter, has also been investigated at a test site. Compared to standard processing, this method can help improve repeatability of gravity measurements.Microgravity surveys in the urban environment require effective and accurate consideration of the effects of infrastructures, nearby buildings and basements, as well as those of topography, in the vicinity of a gravity station. Correction procedures, applied at the same experimental site, where gravity points are located close to buildings, walls and basement slope, appear to have almost totally eliminated these disturbances.     

The Lac Cornu retrograded eclogites (Aiguilles Rouges massif,Western Alps,France): evidence of crustal origin and metasomatic alteration

Metabasic rocks interbedded in amphibolite facies supracrustal gneisses outcrop around Lac Carnu in the Aiguilles Rouges massif (Western Alps). The cores of the thickest boudinaged lenses are made up of eclogitic amphibolites grading outwards into amphibolites. The common assemblage is unzoned garnet + symplectitic clinopyroxebe + hornblende + plagioclase. In a slightly amphibolitized sample, minimum P and T conditions of equilibrium between garnet and omphacite inclusions are 780°C and 11 Kb. A polymetamorphic pre-Alpine evolution of the massif is thus demonstrated. Eclogitic amphibolites show variations in major element composition similar to the Skaergaard evolution in the Al₂O₃ ? FeO + Fe₂O₃ ? MgO triangle. Variation diagrams in which Zr is taken as differentiation index also indicate magmatic trends for Mn, Ti, P, Ni, Co, Y, V and Cr, Ca, K, Na, Sr, Rb and Si were mobile during the evolution of the rocks. The igneous trend can be described by a quantitative model of fractional crystallization in which a noritic assemblage separated in the initial stage. This provides evidence of crustal P, T conditions of differentiation for the original materoal and in situ evolution for the eclogites. A second group of amphibolites shows banded structure and transitional terms with the surrounding gneisses. A volcano-sedimentary origin is suggested though the process cannot be modelled. The rocks may result from complex interactions of magnetic, metasomatic and sedimentary processes. It is concluded that Lac Cornu metabasites were originally continental tholeiites, though several geochemical criteria tend to indicate an oceanic origin. This casts some doubt on the validity of these criteria, when applied to metamorphic rocks.     

Turbulent Dissipation Rate In The Boundary Layer Via Uhf Wind Profiler Doppler Spectral Width Measurements

The study is focused on the retrieval and validation of the turbulent kinetic energy dissipation rate from spectral width measurements made by a UHF wind profiler in the convective atmospheric boundary layer. The possibility to deduce , which is one of the most important parameters for boundary-layer study and monitoring, from Doppler radar spectral width measurements has a firm theoretical basis established by numerous earlier works. However, the major drawback of this approach lies in various meteorological and instrumental sources of non-turbulent spectral width broadening which have to be recognised and accounted for. In the first part of the study, the theoretical background of the retrieval is detailed and all possible causes of spectral broadening are listed and evaluated. In the second part, the method is applied to four days of UHF diurnal boundary-layer observations, collected during the TRAC-98 experiment, for which in situ aircraft measurements were available. Comparison between radar-retrieved and in situ aircraft measurements yields a fairly good agreement with a linear correlation coefficient of about 0.9 and a residual bias less than 2 × 10^-4 m² s^-3. The analysis of derived from vertical and off-zenith observation leads to the recommendation, that in the boundary layer, where the wind is usually moderate, data collected by off-zenith beams should be used. Indeed, the measurement of the vertical spectral width, less affected by the large-scale broadening factor, can still be altered by the ground clutter removal.     

Les rhyolites hyperalcalines (pantellérites) du lac Tchad. Composition et signification tectonomagmatiqueThe peralkaline rhyolites (pantellerites) of the Lake Chad. Composition and tectonomagmatic significance

Remnants of rhyolite lava-dome and of alkaline microgranite extrusions emerge, as inselbergs, in the alluvial plain, south of Lake Chad. The peralkaline, arfvedsonite- and acmite-bearing rhyolites are determined as pantellerites. They can be related to a tectonomagmatic stage, dated of the Late Cretaceous, of the western central Africa rift system. They may constitute the NNE extension of a N50° to 30° elongated succession of small alkaline complexes, from the Gulf of Guinea to the North-Cameroon, which initiated the magmatic activity of the Cameroon Line. To cite this article: J.-P. Vicat et al., C. R. Geoscience 334 (2002) 885–891.     

Effets de la vitesse d'écoulement sur le transport et la cinétique de dépôt de particules en suspension en milieu poreux saturé

We study in this paper the transport and deposition of suspended particles in saturated porous media. Some chromatographic short-pulse tests were performed in a laboratory column with a tracer and two types of suspended particles. A mathematical model was used to determine the hydrodispersive parameters and deposition rates from the breakthrough curves. The results show that the fitted parameters obtained vary with the flow rate and the characteristics of the suspended particles. We also observe, at high flow rates, that particles have a migration velocity higher than that of the conservative tracer. To cite this article: A. Benamar et al., C. R. Geoscience 337 (2005).     

Ash layers in deep-sea sediments as tracers of arc volcanic activity: Japan and Central America as case studies

Abstract Arc volcanic activity on opposite sides of the Pacific Ocean (Japan and Central America) has been investigated by examining the number of volcanic ash layers recorded in Neogene and Quaternary deep-sea sediments. The data suggest that ash layers counted in deep-sea sediments may provide a reliable record of arc volcanism. The study is based on a quantitative analysis of arc volcanic activity using cores collected on DSDP (Deep-Sea Drilling Project) and ODP (Ocean Drilling Program) legs. Five distinct parameters which might affect ash distribution in marine sediments were reviewed: nature of the eruption, wind influence, settling conditions, diagenesis, and plate motion. Of these five, past atmospheric circulation was the most significant. The main constraint on the analysis is that temporal scattering of ash is not directly related to wind pattern variations. Results of this analysis are correlated with dating of terrestrial volcanic sequences. Although marine tephra records for individual regions reveal minor differences in the episodes of volcanic activity, a general correlation exists between activity of arc volcanism in Japan and in Central America. Two important pulses of arc volcanism occurred during Middle Miocene times (18–13 Ma) and Plio-Quaternary times (5–0 Ma). These episodes of intense volcanism are separated by a well recorded quiescent period during Late Miocene times. These correlating episodes of the volcanic record indicate a direct link between arc volcanism and the global tectonic evolution of the Pacific ocean margins.     

Radiative surface temperature and convective flux calculation over crop canopies

The analysis presented in this paper aims at a better understanding of the potential role of radiative temperature, as measured by a radiometer over crops, in sensible heat flux calculation. Defining radiative temperature as the mean temperature of the surfaces viewed by the radiometer (leaves and soil surface) and assuming that an Ohm's law type formula can be used to express sensible heat flux as a function of the difference between air temperature and radiative temperature, the aerodynamic resistance which divides this temperature difference has been analytically defined. The parameters which appear in the resistance expression depend essentially on wind velocity and canopy structure but also on the inclination angle of the radiometer. Finally an experimental verification is presented with data obtained over a potato crop.