A field test of imaging properties of rotational invariants of the magnetotelluric impedance tensor   总被引：1，自引：0，他引：1  

László Szarka  Antal Ádám  Michel Menvielle 《Geophysical Prospecting》2005,53(3):325-334

A part of the Békés Basin (an extensional sub‐basin of the Pannonian Basin, where the basement under thick Pannonian sediments is well known from deep boreholes and from seismic measurements, and where many magnetotelluric (MT) soundings have been carried out for frequencies ranging from 1 to 10^?3 Hz) was selected as a test area to assess the imaging performances of various apparent‐resistivity definitions computed with rotational invariants of either the real part of the complex impedance tensor, or its imaginary part, or both. A comparison (based on earlier 3D numerical studies) has been made between the magnetotelluric images obtained in this way and the depths to the high‐resistivity basement, as known from boreholes and seismic investigations. The correlation coefficient between the series of basement depth values at 39 MT sites and the apparent‐resistivity values was found to be stronger and high correlation appeared at a shorter period when it was computed with apparent resistivities based on the real tensor rather than with apparent resistivities based on the imaginary tensor. In the light of our studies, ρ_{Re Z} and the impedance phase seem to be more informative than any other combination of magnetotelluric interpretation parameters.  相似文献   

3D modelling and sensitivity in DC resistivity using charge density     

Olivier Boulanger  Michel Chouteau 《Geophysical Prospecting》2005,53(4):579-617

A three‐dimensional (3D) electrical resistivity modelling code is developed to interpret surface and subsurface data. Based on the integral equation, it calculates the charge density caused by conductivity gradients at each interface of the mesh, allowing the estimation of the potential everywhere without the need to interpolate between nodes. Modelling generates a huge matrix, made up of Green's functions, which is stored by using the method of pyramidal compression. The potential is compared with the analytical and the numerical solutions obtained by finite‐difference codes for two models: the two‐layer case and the vertical contact case. The integral method is more accurate around the source point and at the limits of the domain for the potential calculation using a pole‐pole array. A technique is proposed to calculate the sensitivity (Jacobian) and Hessian matrices in 3D. The sensitivity is based on the derivative with respect to the block conductivity of the potential computed using the integral equation; it is only necessary to compute the electrical field at the source location. A direct extension of this technique allows the determination of the second derivatives. The technique is compared with the analytical solutions and with the calculation of the sensitivity according to the method using the inner product of the current densities calculated at the source and receiver points. Results are very accurate when the Green's function that includes the source image is used. The calculation of the three components of the electric field on the interfaces of the mesh is carried out simultaneously and quickly, using matrix compression.  相似文献   

Modelling of Seepage Water Composition from Experiments with an Acid Soil and a Calcareous Sediment     

Kerstin Michel  Bernard Ludwig 《洁净——土壤、空气、水》2005,33(6):595-604

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Increase of organochlorines and mercury levels in common guillemots Uria aalge during winter in the southern North Sea     

Claude R. Joiris  German Tapia  Ludo Holsbeek 《Marine pollution bulletin》1997,34(12):1049-1057

Beached seabirds, mainly common guillemots Uria aalge, were collected on the Belgian coast during winter from 1990 to 1995. Concentrations of total and organic mercury, and of organochlorines (PCBs and pesticides) were determined in muscle, liver and kidney. They were high compared to summer data (up to one order of magnitude), and increased during winter. This increase is not due to changes of total body weight nor polar lipid content, and thus reflects an actual increase of the seabirds' contamination while wintering in the southern North Sea. The observed annual cycle can be understood by assuming differences in prey contamination: higher during winter in the southern North Sea ecosystem than during summer in the Atlantic water ecosystem.  相似文献   

The influence of H2O-H2 fluids and redox conditions on melting temperatures in the haplogranite system     

B. C. Schmidt  François Holtz  Bruno Scaillet  Michel Pichavant 《Contributions to Mineralogy and Petrology》1997,126(4):386-400

Solidus temperatures of quartz–alkali feldspar assemblages in the haplogranite system (Qz-Ab-Or) and subsystems in the presence of H₂O-H₂ fluids have been determined at 1, 2, 5 and 8 kbar vapour pressure to constrain the effects of redox conditions on phase relations in quartzofeldspathic assemblages. The hydrogen fugacity (f _H2) in the fluid phase has been controlled using the Shaw membrane technique for moderately reducing conditions (f _H2 < 60 bars) at 1 and 2 kbar total pressure. Solid oxygen buffer assemblages in double capsule experiments have been used to obtain more reducing conditions at 1 and 2 kbar and for all investigations at 5 and 8 kbar. The systems Qz-Or-H₂O-H₂ and Qz-Ab-H₂O-H₂ have only been investigated at moderately reducing conditions (1 and 5 kbar) and the system Qz-Ab-Or-H₂O-H₂ has been investigated at redox conditions down to IW (1 to 8 kbar). The results obtained for the water saturated solidi are in good agreement with those of previous studies. At a given pressure, the solidus temperature is found to be constant (within the experimental precision of ± 5°C) in the f _H2 range of 0–75 bars. At higher f _H2, generated by the oxygen buffers FeO-Fe₃O₄ (WM) and Fe-FeO (IW), the solidus temperatures increase with increasing H₂ content in the vapour phase. The solidus curves obtained at 2 and 5 kbar have similar shapes to those determined for the same quartz - alkali feldspar assemblages with H₂O-CO₂- or H₂O-N₂-bearing systems. This suggests that H₂ has the behaviour of an inert diluent of the fluid phase and that H₂ solubility in aluminosilicate melts is very low. The application of the results to geological relevant conditions [HM (hematite-magnetite) > f _O2 > WM] shows that increasing f _H2 produces a slight increase of the solidus temperatures (up to 30 °C) of quartz–alkali feldspar assemblages in the presence of H₂O-H₂ fluids between 1 and 5 kbar total pressure. Received: 4 March 1996 / Accepted: 22 August 1996  相似文献   

Two-stage prograde and retrograde Variscan metamorphism of glaucophane-eclogites, blueschists and greenschists from Ile de Groix (Brittany, France)     

Bernhard Schulz  Claude Triboulet  Claude Audren  Hans-Rudolf Pfeifer  Albert Gilg 《International Journal of Earth Sciences》2001,90(4):871-889

On Ile de Groix, Variscan metamorphic former tholeiitic and alkaline basalts occur as glaucophane-eclogites, blueschists and greenschists in isolated lenses and layers within metapelites. Whole-rock '¹⁸O_SMOW values of the metabasites show limited variations (10.4-12.0‰) and no systematic differences among rock types and metamorphic grades. This provides no argument for large-scale blueschist-to-greenschist transformation driven by infiltration of externally derived fluids. Metamorphic mineralogical changes should have been triggered by internal fluids. Element variations in interlayered blue- and greenschists can be attributed to magmatic fractionation. Assemblages with garnet, clinopyroxene and glaucophane of a high-pressure/low-temperature (HP-LT) metamorphism M1, and NaCa-amphiboles (barroisite, magnesiohornblende, actinolite) of a medium-pressure/medium-temperature metamorphism M2 crystallized during deformation D1. Detailed core-rim zonation profiles display increasing and then decreasing Al^IV in glaucophane of M1. NaCa-amphiboles of M2, mantling glaucophane and crystallized in porphyroblasts, show first increasing, then decreasing, Al^IV and Al^VI. Empirically calibrated thermobarometers allowed P-T path reconstructions. In glaucophane-eclogites of a metamorphic zone I, a prograde evolution to M1 peak conditions at 400-500°C/10-12 kbar was followed by a retrograde P-T path within the glaucophane stability field. The subsequent M2 evolution was again prograde up to >600°C at 8 kbar and then retrograde. Similarly, in metamorphic zones II and III, prograde and retrograde paths of M1 and M2 at lower maximal temperatures and pressures exist. The almost complete metamorphic cycle during M2 signalizes that the HP-LT rocks escaped from an early erosion by a moderate second burial event and explains the long-lasting slow uplift with low average cooling rates.  相似文献   

Secondary Ca–Al silicates in plutonic rocks: implications for their cooling history     

Regina Freiberger  Lutz Hecht  Michel Cuney  Giulio Morteani 《Contributions to Mineralogy and Petrology》2001,141(4):415-429

Secondary Ca-Al silicates are used to constrain the P-T-x conditions of the very early post-magmatic stage of the intermediate to basic Hercynian plutonic complexes of Charroux-Civray (NW Massif Central, France) and Fichtelgebirge (NE Bavaria, Germany). The secondary Ca-Al silicates hydrogarnet, prehnite, pumpellyite, epidote and laumontite form lenses within unaltered or only slightly chloritized biotite. Hydrogarnet as the first occurring Ca-Al silicate phase crystallizes at temperatures above 340 °C. The common paragenesis prehnite + pumpellyite post-dates hydrogarnet and indicates rather narrow ranges of temperature (200-280 °C) and pressure (2-3 kbar). Laumontite is formed at the end of Ca-Al silicate crystallization (180-260 °C, 1-3 kbar), mostly in small fractures in association with prehnite and adularia. The observed crystallization sequence of the Ca-Al silicates and their stabilities define a retrograde alteration path for the plutonic rocks. The Ca-Al silicate assemblage results from an early pervasive alteration of the plutonic rocks by low X_CO2 fluids during post-magmatic cooling. Subsolidus cooling starts at about 4 kbar at solidus temperatures as indicated by magmatic epidote stability, hornblende barometry and fluid inclusion data, and continues under slightly decreasing pressure (uplift) down to 2-3 kbar at 200-280 °C (prehnite-pumpellyite paragenesis). This shows that Ca-Al silicate assemblages may be a unique tool to constrain the P-T conditions of the subsolidus cooling of intermediate to basic plutonic bodies.  相似文献   

Brood patch temperature during provocation of common eiders in Ny-Ålesund, Svalbard     

Francois Criscuolo  Michel Gauthier-Clerc  Yvon Le  Maho Geir W. Gabrielsen 《Polar research》2001,20(1):115-118

In this short note we describe the behaviour and body changes of three incubating female common eiders ( Somateria mollissima ) during provocation made by humans approaching the nest. The study site was near the settlement of Ny-Ålesund, Svalbard. Temperture transmitters were implanted subcutaneously at the brood patch and data recorded using a VHF receiver. We found that the female experiment exhibited a passive defence response ("freezing"), accompanied by a significant drop in brood patch temperature (0.6 °C) during provocation; this temperature drop lasted for 5 minutes. These accord with other studies of the physiological changes which the passive defence response in birds and other animals.  相似文献   

Observations of the E-region horizontal winds in the auroral zone and at mid-latitudes by a ground-based interferometer     

Vincent Fauliot  Gérard Thuillier  Michel Hersé 《Annales Geophysicae》1995,13(11):1172-1186

The MICADO instrument, consisting of a Michelson interferometer, has observed winds and temperatures during three winter campaigns in the auroral zone, and during 2 years at the Observatoire de Haute-Provence. The instrument observed the O(¹S) oxygen emission line. Emission from this line originates from both the E- and F-regions. A method to separate the contribution from these two regions is presented for cases when the thermospheric component is comparable to that for the mesosphere. For the auroral latitudes, a mean model of the meridional and zonal neutral wind components as a function of magnetic activity and time is presented and compared to predictions from recent empirical models. For the mid-latitudes, several properties of the semi-diurnal tides are shown and compared to radar observations and predictions from recent theoretical models.  相似文献   

Calculation of Low-Range Flow Velocities in Fractured Carbonate Media from Borehole Hydrochemical Logging Data Comparison with Thermometric Results     

Pascal Bidaux  Claude Drogue 《Ground water》1993,31(1):19-26

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