302 research outputs found
Geophysical Well-Log Analysis In Characterizing The Hydrology Of Crystalline Rocks Of The Canadian Shield
A full suite of geophysical logs, including nuclear, electric, acoustic transit-time, acoustic waveform, and acoustic televiewer logs, and high-resolution flowmeter measurements have been used to investigate the lithologic and hydrologic properties of three igneous plutons located on the southern margin of the Canadian shield. Geophysical
logs were used to identify lithologic boundaries, determine the properties of unfractured
granitic or gabbroic rocks, interpret and calibrate the results of surface geophysical
surveys, and characterize permeable fracture zones that could serve as conduits
for fluid migration. Nuclear and acoustic transit-time logs provided good quantitative
correlation with changes in lithology. Electric logs yielded consistent qualitative
correlations, with lower resistivities associated with more mafic lithologies. Lithologic
contacts indentified on logs generally confirmed the results of surface electromagnetic,
seismic, and gravity surveys. All major fracture zones intersected by boreholes were
clearly indicated by the geophysical logs. Electric, epithermal-neutron, and acoustic
transit-time logs gave the most consistent indications of fracturing, but the lithologic
responses associated with some thin mafic intrusions were difficult to distinguish from
possible fractures, and some steeply-dipping fractures were not indicated by conventional
acoustic transit-time logs. Electric and neutron log response is attributed to
the effect of clay minearl alteration products in the vicinity of fractures. This alteration may be indirectly related to permebaility, but no direct relationship between
resistivity or neutron attenuation and permeability appears to exist. Tube-wave attenuation determined from acoustic waveform logs was related to the transmissivity
of equivalent infinite, plane fractures; these results agree qualitatively, and possibly
quantitatively with packer isolation and injection tests if the combined effects of differing scales of investigation and borehole enlargements in fracture zones are taken into account. Tube-wave attenuation in waveform logs also compares well with the permeability distributions determined from tube-wave generation in vertical seismic profiles. Comparison of conventional geophysical logs, acoustic televiewer images of the borehole wall, and fracture frequency distributions measured on core samples indicates
that many fractures are completely sealed and have no effect on log response, whereas
many more apparently sealed fractures have been slightly opened during drilling, and
do provide some log response. High resolution flowmeter meaurements of natural flow
in boreholes and comparison of packer isolation tests with log data indicate that a
relatively few individual fractures often provide a large proportion of fracture zone
transmissivity in the immediate vicinity of the borehole, and that the orientation of
these fractures may not coincide with fracture zone orientation. These results indicate
that the scale problem in relating borehole logs to regional configuration of fracture
flow systems may be the most important consideration in the appplication of geophysical
well logging to the characterization of ground water flow in crystalline rock
bodies
A quantitative assessment of the ecological value of sycamore maple habitats in the French Alps
La naturalité est un critère important pour l'évaluation de mesures conservatoires des écosystèmes. Au niveau local, une telle évaluation doit être basée sur des indicateurs objectifs et quantifiables sur le terrain. Dans cette étude, nous avons utilisé une méthode multicritères basée sur la différence entre Valeur Naturelle (NV) et Valeur Conservatoire (CV) pour quantifier la valeur écologique des érablaies de versant à érable sycomore (Acer pseudoplatanus L.) par comparaison avec les peuplements mixtes de hêtraie-sapinière-pessière avoisinants. En effet, les naturalistes ainsi que l'Union Européenne considèrent que les érablaies de versant ont une valeur de conservation et de naturalité élevée. Nos résultats montrent que les valeurs naturelle et de conservation sont significativement plus élevées pour l'érablaie que pour la forêt mixte avoisinante et que cette évaluation ne dépend pas de facteurs abiotiques tels que l'altitude ou l'exposition. En fait, la naturalité de structure et de composition des érablaies de versant sont plus fortes que celles des forêts mixtes et permettent de différencier les deux habitats en termes de valeur écologique. Les gestionnaires peuvent facilement utiliser cette méthode pour évaluer la valeur écologique de petits habitats en zone de montagne, ce qui permet d'établir des orientations sylvicoles pour une gestion conservatoire et proche de la nature. / Naturalness is an important criterion in nature conservation assessment. At the stand-level, such assessment must be based on objective and quantifiable indicators measurable in the field. In this study, we used a multi-criterion method based on the difference between a Natural Value(NV) and a Conservation Value (CV) to quantify the ecological value of sycamore maple patches compared to the surrounding mixed forests. Indeed, sycamore habitats are considered of high natural and conservation value both by naturalists and by European institutions. Our results showed that the natural and conservation values were significantly higher for the sycamore forests than for the surrounding mixed forests and that this assessment did not depend on abiotic factors such as elevation or aspect. Actually, naturalness of structure and composition in the sycamore habitats was higher than for mixed forests and allowed us to differentiate between the two habitats. Managers could easily use this method in order to assess the ecological value of small habitats in mountainous regions and to provide guidelines for close-to-nature and conservation-related silviculture.FORET DE MONTAGNE;ECOLOGIE FORESTIERE;EVALUATION;PROTECTION DE LA NATURE;ACER PSEUDOPLATANUS;METHODOLOGIE;HABITAT;VALEUR ECOLOGIQUE;NATURALITE;ALPES FRANCAISES;CHARTREUSE MASSIF;ACER PSEUDOPLATANUS;CHARTREUSE;INDICATORS;NATURALNESS;ECOLOGICAL VALUE
Habitable planets around the star Gl 581?
Radial velocity surveys are now able to detect terrestrial planets at
habitable distance from M-type stars. Recently, two planets with minimum masses
below 10 Earth masses were reported in a triple system around the M-type star
Gliese 581. Using results from atmospheric models and constraints from the
evolution of Venus and Mars, we assess the habitability of planets Gl 581c and
Gl 581d and we discuss the uncertainties affecting the habitable zone (HZ)
boundaries determination. We provide simplified formulae to estimate the HZ
limits that may be used to evaluate the astrobiological potential of
terrestrial exoplanets that will hopefully be discovered in the near future.
Planets Gl 581c and 'd' are near, but outside, what can be considered as the
conservative HZ. Planet 'c' receives 30% more energy from its star than Venus
from the Sun, with an increased radiative forcing caused by the spectral energy
distribution of Gl 581. Its habitability cannot however be positively ruled out
by theoretical models due to uncertainties affecting cloud properties.
Irradiation conditions of planet 'd' are comparable with those of early Mars.
Thanks to the warming effect of CO2-ice clouds planet 'd' might be a better
candidate for the first exoplanet known to be potentially habitable. A mixture
of various greenhouse gases could also maintain habitable conditions on this
planet.Comment: Astronomy and Astrophysics (2007) accepted for publicatio
Theoretical Models Relating Acoustic Tube-Wave Attenuation To Fracture Permeability - Reconciling Model Results With Field Data
Several recent investigations indicate that tube-wave amplitude attenuation in acoustic
full-waveform logs is correlated with permeability in fractured rocks. However, there
are significant differences between predictions based on theoretical models for tubewave
propagation and experimental waveform amplitude data. This investigation reviews
the results of existing theoretical models for tube-wave attenuation in fractured
rock and compares model predictions with acoustic full-waveform data where extensive
independent fracture-permeability data are available from straddle-packer permeability
tests. None of the tube-wave models presented in the literature predicts attenuation
at fracture apertures as small as those producing attenuation in the field; and most
models predict tube-wave reflections, which are rarely measured at frequencies greater
then 5 kHz. Even the unrealistic assumption that all of the tube-wave energy loss is
caused by viscous dissipation in fracture openings does not result in predicted apertures
being as small as those indicated by packer permeability measurements in most
situations.
On the basis of these results, it is concluded that plane-fracture models cannot
account for the measured tube-wave attenuation where natural fractures intersect fluidfilled boreholes. However, natural fractures are fundamentally different from plane
parallel passages. This difference appears to explain the small equivalent flow apertures
and lack of reflections associated with fractures in waveform-log data. Permeable
fracture openings modeled as irregular tubes embedded between asperities along the
fracture face are predicted to produce significant tube-wave attenuation when tube
diameters exceed 1.0 cm, but arrays of such tubes conduct fluid flow equivalent to that
through plane fractures less than 2 mm in effective flow aperture. Although the theory
predicts some reflection from simple cylindrical passages, scattering from irregular
distributions of natural fracture openings probably accounts for the infrequency with
which coherent tube-wave reflections occur in field data.Massachusetts Institute of Technology. Full Waveform Acoustic Logging ConsortiumUnited States. Dept. of Energy (Grant DE-FG02-86ER13636
Modeling Borehole Stoneley Wave Propagation Across Permeable In-Situ Fractures
The characterization of hydraulic transmissivity of permeable fracture reservoirs is a
very important task in the exploration of water resources and hydrocarbons. Previous
studies that model the permeable structure as a single fluid-filled fracture failed to
explain the observed significant Stoneley wave attenuation across the permeable structure.
In this paper, the structure is modeled as a permeable fracture zone and synthetic
Stoneley wave seismograms in the vicinity of the structure are calculated. The results
show that Stoneley waves can be strongly attenuated or even eliminated without significant
reflection, because of the dissipation of wave energy into the permeable zone.
Several field cases are also modeled and the theoretical results are compared with the
field data. It is shown that low- and medium-frequency Stoneley waves (1 kHz data from
Moodus, Conneticut, and 5 kHz data from Monitoba, Canada) are very sensitive to the
permeability of the fractures and can be used to assess permeability from in-situ logging
data, if the fracture porosity and zone thickness can be measured. At high frequencies,
however, Stoneley waves are not very sensitive to permeability but are mainly affected
by the sum of the fracture openings expressed as the product of fracture zone thickness
and porosity in the fracture zone. This finding is demonstrated by a logging data set
(Monitoba, Canada) obtained using high-frequency Stoneley waves at 34 kHz.United States. Dept. of Energy (Grant DE-FG0286ER13636)Massachusetts Institute of Technology. Full Waveform Acoustic Logging Consortiu
Laboratory Studies Of The Acoustic Properties Of Samples From The Salton Sea Scientific Drilling Project And Their Relation To Microstructure And Field Measurements
Compressional and shear wave velocities were measured at confining pressures up to
200 MPa for twelve core samples from the depth interval of 600 to 2600 m in the California State 2-14 borehole. Samples were selected to represent the various lithologies, including clean, heavily cemented sandstones, altered, impermeable claystones, and several intermediate siltstones. Velocities measured at ultrasonic frequencies in the laboratory correspond closely with velocities determined from acoustic waveform logs and vertical seismic profiles. The samples exhibit P-wave velocities around 3.5 km/sec at depths above 1250 m, but increase to nearly 5.0 km/sec at 1300 m in depth. Further increases with depth result in compressional wave velocity increasing to nearly 6.0 km/sec. These increases in velocities are related to systematic variations in lithology, microstructure and hydrothermal alteration of originally clay-rich sediments. Scanning electron microscope observations of core samples confirm that local core velocities are determined by the combined effects of pore size distributions, and the proportion of clays and alteration minerals such as epidote present in the form of pore fillings and veins.United States. Dept. of the Interior. Geological Survey (Grant 14-08-001A-0328)Elf-Aquitaine (Postdoctoral Fellowship
Raman excitation spectroscopy of carbon nanotubes: effects of pressure medium and pressure
Raman excitation and emission spectra for the radial breathing mode (RBM) are
reported, together with a preliminary analysis. From the position of the peaks
on the two-dimensional plot of excitation resonance energy against Raman shift,
the chiral indices (m, n) for each peak are identified. Peaks shift from their
positions in air when different pressure media are added - water, hexane,
sulphuric acid - and when the nanotubes are unbundled in water with surfactant
and sonication. The shift is about 2 - 3 cm-1 in RBM frequency, but
unexpectedly large in resonance energy, being spread over up to 100meV for a
given peak. This contrasts with the effect of pressure. The shift of the peaks
of semiconducting nanotubes in water under pressure is orthogonal to the shift
from air to water. This permits the separation of the effects of the pressure
medium and the pressure, and will enable the true pressure coefficients of the
RBM and the other Raman peaks for each (m, n) to be established unambiguously.Comment: 6 pages, 3 Figures, Proceedings of EHPRG 2011 (Paris
Active-distributed temperature sensing to continuously quantify vertical flow in boreholes
We show how a distributed borehole flowmeter can be created from armored Fiber Optic cables with the Active-Distributed Temperature Sensing (A-DTS) method. The principle is that in a flowing fluid, the difference in temperature between a heated and unheated cable is a function of the fluid velocity. We outline the physical basis of the methodology and report on the deployment of a prototype A-DTS flowmeter in a fractured rock aquifer. With this design, an increase in flow velocity from 0.01 to 0.3 m s−1 elicited a 2.5°C cooling effect. It is envisaged that with further development this method will have applications where point measurements of borehole vertical flow do not fully capture combined spatiotemporal dynamics
Trade‐offs between carbon stocks and biodiversity in European temperate forests
Abstract Policies to mitigate climate change and biodiversity loss often assume that protecting carbon-rich forests provides co-benefits in terms of biodiversity, due to the spatial congruence of carbon stocks and biodiversity at biogeographic scales. However, it remains unclear whether this holds at the scales relevant for management, and particularly large knowledge gaps exist for temperate forests and for taxa other than trees. We built a comprehensive dataset of Central European temperate forest structure and multi-taxonomic diversity (beetles, birds, bryophytes, fungi, lichens, and plants) across 352 plots. We used Boosted Regression Trees (BRTs) to assess the relationship between above-ground live carbon stocks and (a) taxon-specific richness, (b) a unified multidiversity index. We used Threshold Indicator Taxa ANalysis to explore individual species? responses to changing above-ground carbon stocks and to detect change-points in species composition along the carbon-stock gradient. Our results reveal an overall weak and highly variable relationship between richness and carbon stock at the stand scale, both for individual taxonomic groups and for multidiversity. Similarly, the proportion of win-win and trade-off species (i.e., species favored or disadvantaged by increasing carbon stock, respectively) varied substantially across taxa. Win-win species gradually replaced trade-off species with increasing carbon, without clear thresholds along the above-ground carbon gradient, suggesting that community-level surrogates (e.g., richness) might fail to detect critical changes in biodiversity. Collectively, our analyses highlight that leveraging co-benefits between carbon and biodiversity in temperate forest may require stand-scale management that prioritizes either biodiversity or carbon in order to maximize co-benefits at broader scales. Importantly, this contrasts with tropical forests, where climate and biodiversity objectives can be integrated at the stand scale, thus highlighting the need for context-specificity when managing for multiple objectives. Accounting for critical change-points of target taxa can help to deal with this specificity, by defining a safe operating space to manipulate carbon while avoiding biodiversity losses
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