Tracking the isostatic and eustatic signals in an end Ordovician–early Silurian deglacial sedimentary record (Algeria – Libya border)     

Flavia Girard  Rmy Deschamps  Jean‐Franois Ghienne  Stphane Rouss  Jean‐Loup Rubino 《Sedimentology》2019,66(3):1042-1066

To elucidate the signature of isostatic and eustatic signals during a deglaciation period in pre‐Pleistocene times is made difficult because very little dating can be done, and also because glacial erosion surfaces, subaerial unconformities and subsequent regressive or transgressive marine ravinement surfaces tend to amalgamate or erode the deglacial deposits. How and in what way can the rebound be interpreted from the stratigraphic record? This study proposes to examine deglacial deposits from Late‐Ordovician to Silurian outcrops at the Algeria–Libya border, in order to define the glacio–isostatic rebound and relative sea‐level changes during a deglaciation period. The studied succession developed at the edge and over a positive palaeo‐relief inherited from a prograding proglacial delta that forms a depocentre of glaciogenic deposits. The succession is divided into five subzones, which depend on the topography of this depocentre. Six facies associations were determined: restricted marine (Facies Association 1); tidal channels (Facies Association 2); tidal sand dunes (Facies Association 3); foreshore to upper shoreface (Facies Association 4); lower shoreface (Facies Association 5); and offshore shales (Facies Association 6). Stratigraphic correlations over the subzones support the understanding of the depositional chronology and associated sea‐level changes. Deepest marine domains record a forced regression of 40 m of sea‐level fall resulting from an uplift caused by a glacio‐isostatic rebound that outpaces the early transgression. The rebound is interpreted to result in a multi‐type surface, which is interpreted as a regressive surface of marine erosion in initially marine domains and as a subaerial unconformity surface in an initially subaerial domain. The transgressive deposits have developed above this surface, during the progressive flooding of the palaeo‐relief. Sedimentology and high‐resolution sequence stratigraphy allowed the delineation of a deglacial sequence and associated sea‐level changes curve for the studied succession. Estimates suggest a relatively short (<10 kyr) duration for the glacio‐isostatic uplift and a subsequent longer duration transgression (4 to 5 Myr).  相似文献   

Modeling coupled erosion and filtration of fine particles in granular media     

Yang  Jie  Yin  Zhen-Yu  Laouafa  Farid  Hicher  Pierre-Yves 《Acta Geotechnica》2019,14(6):1615-1627

One of the major causes of instability in geotechnical structures such as dikes or earth dams is the phenomenon of suffusion including detachment, transport and filtration of fine particles by water flow. Current methods fail to capture all these aspects. This paper suggests a new modeling approach under the framework of the porous continuous medium theory. The detachment and transport of the fine particles are described by a mass exchange model between the solid and the fluid phases. The filtration is incorporated to simulate the filling of the inter-grain voids created by the migration of the fluidized fine particles with the seepage flow, and thus, the self-filtration is coupled with the erosion process. The model is solved numerically using a finite difference method restricted to one-dimensional (1-D) flows normal to the free surface. The applicability of the model to capture the main features of both erosion and filtration during the suffusion process has been validated by simulating 1-D internal erosion tests and by comparing the numerical with the experimental results. Furthermore, the influence of the coupling between erosion and filtration has been highlighted, including the development of material heterogeneity induced by the combination of erosion and filtration.

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Structural mapping in basin-and-range-like geology by electromagnetic methods: a powerful aid to seismic     

Pierre-Yves Galibert  Roger Guerin  Pierre Andrieux 《Geophysical Prospecting》1996,44(6):1019-1040

A case history is presented where electromagnetic (EM) methods were applied as a complement to seismic, for structural mapping in basin-and-range-like geology: 366 five-component magnetotelluric (MT) soundings were carried out together with 331 transient soundings (TDEM) along seismic lines. Due to high structural complexity, seismic shows a number of limitations. For the same reasons, MT is highly perturbed and three specific interpretation techniques were comprehensively applied: 1. a classical correction of static effect using TDEM sounding, to determine the high-frequency nondistorted apparent resistivities and thus the corrected tensor; 2. a so-called regional correction based upon the same concept as the static effect, to transform distorted resistivity curves due to the horst/graben situation into plausible 1D curves, through the use of nomograms built for 2D H-polarization situations; 3. a stripping technique which made it possible to map areas where a deep conductive Mesozoic shale was present below carbonates, at a depth of 3 km. After the best MT interpretation was obtained along each line, it was integrated with seismic and with the results from two boreholes. A crude empirical law relating resistivity and acoustic velocity was established and the MT horizons were plotted on the two-way traveltime seismic sections. The final integrated cross-sections obtained are undoubtedly of greater use to the explorationist than the initial seismic sections alone and two wells were accurately predicted.  相似文献   

An algorithm for remote sensing of water color from space   总被引：3，自引：0，他引：3  

M. Viollier  D. Tanré  P. Y. Deschamps 《Boundary-Layer Meteorology》1980,18(3):247-267

The ocean color algorithm proposed in this paper takes into account the effects of Rayleigh and aerosol scattering. The inherent reflectance and the diffuse transmittance of the Rayleigh atmosphere are expressed as functions of optical thickness and satellite measurement geometry with the aid of simple and accurate formulas. In the case of a turbid atmosphere, from which the aerosol optical thickness is unknown, the aerosol contribution is estimated with the aid of a measurement in a channel where the ocean is a blackbody (in the red or near infrared). If the relationship between the ocean color and the chlorophyll-like pigment concentration is assumed to be known at sea level, it is shown that the chlorophyll-like pigment concentration at an open ocean site can be determined from space to within a factor of 1.5 to 3 (uncertainty equal to 0.2 to 0.5 log interval), depending on the atmospheric turbidity.  相似文献   

Prevention of speleothem rupture during nearby construction     

Corinne Lacave  Martin Koller  Urs Eichenberger  Pierre-Yves Jeannin 《Environmental Geology》2003,43(8):892-900

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Temperature distribution in karst systems: the role of air and water fluxes   总被引：3，自引：0，他引：3  

Marc Luetscher  Pierre-Yves Jeannin 《地学学报》2004,16(6):344-350

A better understanding of heat fluxes and temperature distribution in continental rocks is of great importance for many engineering aspects (tunnelling, mining, geothermal research, etc.). This paper aims at providing a conceptual model of temperature distribution in karst environments which display thermal ‘anomalies’ as compared with other rocks. In temperate regions, water circulation is usually high enough to ‘drain‐out’ completely the geothermal heat flux at the bottom of karst systems (phreatic zone). A theoretical approach based on temperature measurements carried out in deep caves and boreholes demonstrates, however, that air circulation can largely dominate water infiltration in the karst vadose zone, which can be as thick as 2000 m. Consequently, temperature gradients within this zone are similar to the lapse rate of humid air (～0.5 °C 100 m^?1). Yet, this value depends on the regional climatic context and might present some significant variations.  相似文献   

Seismic Microzonation: A Comparison between Geotechnical and Seismological Approaches in Pointe-à-Pitre (French West Indies)   总被引：1，自引：0，他引：1  

Benoit LeBrun  Anne-Marie Duval  Pierre-Yves Bard  Olivier Monge  Myriam Bour  Sylvain Vidal  Hubert Fabriol 《Bulletin of Earthquake Engineering》2004,2(1):27-50

Pointe-à-Pitre, the main city of Guadeloupe in the French West Indies, has on several occasions been partially destroyed by major historical earthquakes. Moreover, a post-seismic assessment of the damage from the 1985 Montserrat earthquake indicates that the town is prone to site effects. Consequently, from 1996 to 1998, BRGM conducted a seismic microzonation study based on geotechnical and geological data. At the same time, three seismological studies were being conducted – two based on earthquake recordings using a time-series analysis and the classical spectral ratio (CSR) method (CETE/LCPC and BRGM), and the third based on noise measurement at 400 points using the horizontal-to-vertical noise ratio (HVNR) method (CETE/LCPC). The objective of this paper is not to carry out a new microzonation study by taking into account all the results, but rather to show in what respects the results of these different methods are in agreement or not. A comparison of the results of the seismological studies with the geotechnical microzonation shows that they are in fairly good agreement, albeit with some discrepancies. The results indicate that the seismological methods and the geotechnical data are highly complementary and should be used together in compiling seismic transfer-function microzonation maps. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Inversion of the attenuation data of free oscillations of the Earth (fundamental and first higher modes)   总被引：1，自引：0，他引：1  

A. Deschamps 《Geophysical Journal International》1977,50(3):699-722

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Retrieval of chlorophyll-a concentration via linear combination of ADEOS-II Global Imager data     

Robert Frouin  Pierre-Yves Deschamps  Lydwine Gross-Colzy  Hiroshi Murakami  Takashi Y. Nakajima 《Journal of Oceanography》2006,62(3):331-337

Top-of-atmosphere reflectance measured above the ocean in the visible and near infrared, after correction for molecular scattering, may be linearly combined to retrieve surface chlorophyll-a abundance directly, without explicit correction for aerosol scattering and absorption. The coefficients of the linear combination minimize the perturbing effects, which are modeled by a polynomial, and they do not depend on geometry. The technique has been developed for Global Imager (GLI) spectral bands centered at 443, 565, 667, and 866 nm, but it is applicable to other sets of spectral bands. Theoretical performance is evaluated from radiation-transfer simulations for a wide range of geophysical and angular conditions. Using a polynomial with exponents of −2, −1, and 0 to determine the coefficients, the residual influence of the atmosphere on the linear combination is within ±0.001 in most cases, allowing chlorophyll-a abundance to be retrieved with a root-mean-squared (RMS) error of 8.4% in the range 0.03–3 mgm⁻³. Application of the method to simulated GLI imagery shows that estimated and actual chlorophyll-a abundance are in agreement, with an average RMS difference of 32.1% and an average bias of −2.2% (slightly lower estimated values). The advantage of the method resides in its simplicity, flexibility, and rapidity of execution. Knowledge of aerosol amount and type is avoided. There is no need for look-up tables of aerosol optical properties. Accuracy is adequate, but depends on the polynomial representation of the perturbing effects and on the bio-optical model selected to relate the linear combination to chlorophyll-a abundance. The sensitivity of the linear combination to chlorophyll-a abundance can be optimized, and the method can be extended to the retrieval of other bio-optical variables.  相似文献   

Spatial coherence of seismic wave fields across an alluvial valley (weak motion)     

Judith Riepl  Carlos Sousa Oliveira  Pierre-Yves Bard 《Journal of Seismology》1997,1(3):253-268

The loss of coherence of seismic waves, even over small distances, can become significant and consequently important for engineering applications. This coherence loss depends on several parameters (frequency, inter-station distance), but their dependencies are not well understood yet. We show the results of a detailed analysis of the spatial variability of the coherence between two signals. This analysis is based on a simple model which is an exponential function of inter-station distance and frequency. Data from a temporary network of 36 weak-motion instruments installed at the EURO-SEISTEST site, north-east of Thessaloniki (Greece), is used in this work. The data offers the unique possibility to study the loss of coherence over a wide range of distances, as the inter-station distances are between 8 m and 5488 m. We test the influence on the coherence of the length of the time-window of the signals, of the type of the waves present in the time-window and of the common energy of the signals. We also show that, at least at this particular site, the loss of coherence with distance is probably marked by a "cross over" distance, distinguishing two different ranges: one for inter-station distances up to 100 m and the other above 100 m. Finally, we find that the coherence determined from noise recordings behaves in a rather similar way to the one determined from coda-waves and more stationary (longer) signals. Therefore, noise can be useful for a rough, but quick estimation of the loss of coherence, at least for inter-station distances larger than 100 m.  相似文献