bluuebunny/crossref_metadata_2025_split huggingface dataset:
Magnetic field transforms with low sensitivity to the direction of source magnetization and high centricity
Authors: Stavrev Petar , Gerovska Daniela 
Published: March 2003

Magnetic data interpretation faces difficulties due to the various shapes of magnetic anomalies and the positions of their extrema with respect to the causative bodies for different directions of the source magnetization. The well‐known transforms — reduction to the pole, pseudogravity field, and analytic signal (total gradient) — help in reducing the problem. Another way to achieve the required effect is the transformation of magnetic data, ΔT or Z, into values of the anomalous magnetic intensity T. In this respect, we have found some transforms based on differential operators such as the gradient of T and its modulus R = |∇T|, the Laplacian L = ∇2T, the product T ∇2T and its square root Q, and the Laplacian ∇2(T2) and its square root E, to be useful. They are slightly sensitive to the magnetization orientation and their extrema occur above the sources.For a 2D anomaly of a homogeneous causative body, the proposed transforms do not depend on the inclination of magnetization. In the 3D case, such independence does not exist even for the elementary field of a point dipole. The influence of the magnetization direction is estimated by an integral coefficient of sensitivity. This coefficient takes values of up to 2.0 for ΔT or Z anomalies, while their transforms T, R, E, Q and L have values of less than 0.28, 0.29, 0.24, 0.16 and 0.07, respectively, i.e. on average, 10 times less. The estimation of the centricity is carried out using the relative deviation of the principal extremum of the anomaly or its transforms from the epicentre of the model body at a depth equal to 100 units. For a ΔT anomaly this deviation is up to 67%; for the L transform it is less than 8%; for Q, E, R and T it is less than 10%, 15%, 20% and 25%, respectively.The proposed transforms take only non‐negative values. With respect to their shape, the peripheral magnetic extrema are removed, the anomalous configuration is simplified and the resolution of complicated interference patterns is improved. Their calculation does not require additional data for the direction of magnetization, which is an essential advantage over the reduction‐to‐the‐pole and pseudogravity‐field transforms. A joint analysis of the measured field and its transforms T, E and L offers possibilities for more confident separation of the anomalous effects and direct correlation to their sources. The model tests performed and the 3D field applications to real magnetic data confirm the useful properties of the transforms suggested here.
Poisson's ratio at high pore pressure
Authors: Carcione José M. , Cavallini Fabio 
Published: March 2003

Laboratory investigations suggest that a precise relationship exists between Poisson's ratio, pore pressure and fluid type. Values of Poisson's ratio for dry samples are significantly smaller than those for fluid‐saturated samples. The values are anomalously high for high pore pressure, with the possibility of differentiating between gas‐saturated, brine‐saturated and oil‐saturated porous rocks.The present study considers two overpressure models, based on oil/gas conversion and disequilibrium compaction, to obtain Poisson's ratio versus differential pressure (confining pressure minus pore pressure). The model results are in good agreement with experiments. Poisson's ratio is approximately constant at high differential pressures and increases (decreases) for saturated (dry) rocks at low differential pressures. Fluid type can be determined at all differential pressures from Poisson's ratio. The analysis is extended to the anisotropic case by computing the three Poisson's ratios of a transversely isotropic rock versus differential pressure. While one of them is practically independent of effective pressure, the others increase with increasing pore pressure. Experiments performed on cores under different pressure conditions, and calibration of the models with these data, provide a tool for inverting pore pressure from seismic data.
Fluid‐dependent shear‐wave splitting in fractured media
Authors: Sayers Colin M. 
Published: March 2003

Azimuthal anisotropy in rocks can result from the presence of one or more sets of partially aligned fractures with orientations determined by the stress history of the rock. A shear wave propagating in an azimuthally anisotropic medium splits into two components with different polarizations if the source polarization is not aligned with the principal axes of the medium. For vertical propagation of shear waves in a horizontally layered medium containing vertical fractures, the shear‐wave splitting depends on the shear compliance of the fractures, but is independent of their normal compliance. If the fractures are not perfectly vertical, the shear‐wave splitting also depends on the normal compliance of the fractures. The normal compliance of a fluid‐filled fracture decreases with increasing fluid bulk modulus. For dipping fractures, this results in a decrease in shear‐wave splitting and an increase in shear‐wave velocity with increasing fluid bulk modulus. The sensitivity of the shear‐wave splitting to fluid bulk modulus depends on the interconnectivity of the fracture network, the permeability of the background medium and on whether the fracture is fully or partially saturated.
Analysis of the resolution function in seismic prestack depth imaging
Authors: Gelius L.‐J. , Lecomte I. , Tabti H. 
Published: March 2003

We consider the problem of estimating subsurface quantities such as velocity or reflectivity from seismic measurements. Because of a limited aperture and band‐limited signals, the output from a seismic prestack reconstruction method is a distorted or blurred image. This distortion can be computed using the concept of resolution function, which is a quantity readily accessible in the Fourier space of the model. The key parameter is the scattering wavenumber, which at a particular image point is defined by the incident and scattered ray directions in a given background model. Any location in any background model can be considered. In general, the resolution function will depend on the following four quantities: the background velocity model, the frequency bandwidth, the wavefield type and the acquisition geometry.We first establish the resolution function for a general scattering model assuming local reaction. We then adapt this result for two well‐known scattering models: Born and Kirchhoff. For each of these approximations the corresponding resolution function is derived and discussed. Finally, by employing a simple synthetic data example we demonstrate the ability of the resolution function to predict the image distortions.
Prediction of reproductive success and failure in lesser snow geese based on early season climatic variables
Authors: Skinner W. R. , Jefferies R. L. , Carleton T. J. , Abraham R. F. Rockwell&dagger K. F. 
Published: March 2003

AbstractThe North American mid‐continent population of lesser snow geese (Anser caerulescens caerulescens) breeds in coastal areas of the Hudson Bay region. Breeding success is highly variable, particularly during recent decades. The availability of long‐term data sets of weather and the breeding success of geese allowed us to determine whether climatic variables in spring and early summer (May–June) are reliable predictors of different attributes of the reproductive biology of snow geese. A large region of strong anomalous cooling in north‐eastern North America has been the dominant anomalous climatic feature since the mid‐1970s. The cooling which becomes established during winter persists into spring and early summer when migration, nesting and hatching of geese are occurring. Redundancy analysis (RDA) of the data sets was made to identify dominant correlations and regression relationships between climatic and goose variables. Individual goose response variables were further explored with stepwise multiple regression and bipartial regression.96.7% of year‐to‐year variance in the goose data was explained by the selected climatic data. The first four orthogonal axes out of seven possible axes explained 92.2% of the total variance. Date of last snow on the ground and mean daily temperature from 6 to 20 May formed the lowest and highest predictor scores, respectively. Initiation date and hatching date at the low end and total clutch size and clutch size at hatch at the high end were associated with these extremes, particularly in certain years. Days of freezing rain in May and total rainfall were correlated with nest failure. Bivariate correlation/regression showed that the most parsimonious model for nest initiation day was based on four climatic predictors, for hatching day four predictors, and for clutch size at hatch, nine predictors.Both the multiple regression analyses and the redundancy analyses confirm the high degree of predictability of goose reproductive variables from selected climatic variables. As discussed, the correlations reflect both direct and indirect effects of climate on the reproductive biology of geese. The correlations are strongest in the early season and weaken by early summer.
Elevated CO2 concentrations and stomatal density: observations from 17 plant species growing in a CO2 spring in central Italy
Authors: Bettarini Isabella , Vaccari Francesco P. , Miglietta Franco 
Published: March 2003

AbstractStomatal density (SD) and stomatal conductance (gs) can be affected by an increase of atmospheric CO2 concentration. This study was conducted on 17 species growing in a naturally enriched CO2 spring and belonging to three plant communities. Stomatal conductance, stomatal density and stomatal index (SI) of plants from the spring, which were assumed to have been exposed for generations to elevated [CO2], and of plants of the same species collected in a nearby control site, were compared. Stomatal conductance was significantly lower in most of the species collected in the CO2 spring and this indicated that CO2 effects on gs are not of a transitory nature but persist in the long term and through plant generations. Such a decrease was, however, not associated with changes in the anatomy of leaves: SD was unaffected in the majority of species (the decrease was only significant in three out of the 17 species examined), and also SI values did not vary between the two sites with the exception of two species that showed increased SI in plants grown in the CO2‐enriched area. These results did not support the hypothesis that long‐term exposure to elevated [CO2] may cause adaptive modification in stomatal number and in their distribution.
Nonlinearity of photosynthetic responses to growth in rising atmospheric CO2: an experimental and modelling study
Authors: Luo YiqI. , Sims Daniel A. , Griffin Kevin L. 
Published: March 2003

AbstractNonlinear responses of photosynthesis to the CO2 concentration at which plants were grown (Cg) have been often reported in the literature. This study was designed to develop mechanistic understanding of the nonlinear responses with both experimental and modelling approaches. Soybean (Glycine max) was grown in five levels of Cg (280, 350, 525, 700, 1000 ppm) with either a high or low rate of nitrogen fertilization. When the rate of nitrogen fertilization was high, the photosynthetic rate measured at Cg was highest in plants from the 700 ppm CO2 treatment. When the rate of nitrogen fertilization was low, little variation was observed in the photosynthetic rates of plants from the different treatments measured at their respective Cg. Measurements of CO2‐induced changes in mass‐based leaf nitrogen concentration (nm, an index of changes in biochemical processes) and leaf mass per unit area (h, an index of morphological properties) were used in a model and indicate that the nonlinearity of photosynthetic responses to Cg is largely determined by relative changes in photosynthetic sensitivity, biochemical downregulation, and morphological upregulation. In order to further understand the nonlinear responses, we compiled data from the literature on CO2‐induced changes in nm and h. These compiled data indicate that h generally increases and nm usually decreases with increasing Cg, but that the trajectories and magnitudes of the changes in h and nm vary with species and growth environments. Integration of these variables (nm and h) into a biochemically based model of photosynthesis enabled us to predict diverse responses of photosynthesis to Cg. Thus a general mechanism is suggested for the highly variable, nonlinear responses of photosynthesis to Cg reported in the literature.
Evidence from rain‐use efficiencies does not indicate extensive Sahelian desertification
Authors: Prince S. D. , De Colstoun E. Brown , Kravitz L. L. 
Published: March 2003

AbstractDesertification is regarded as one of the major global environmental problems of the 20th century and the African Sahel is often quoted as the most seriously affected region. Previous attempts to map the occurrence and severity of desertification in the Sahel have been unsatisfactory, mainly because of the lack of any readily measured, objective indicators. We explore here the properties of the ratio of net primary production (NPP) to precipitation – the rain‐use efficiency (RUE) – calculated from remotely sensed vegetation indices and rain gauge data. Negative deviations from the normal range of RUE values are shown to be an indicator of desertification. Observations of NPP of the entire Sahel were possible using satellite platforms for the period 1982–90, including the 1984 drought. The results suggest that NPP was remarkably resilient, a fact that was reflected in only little variation in the RUE during the period of study. Thus, in much of the region, NPP seems to be in step with rainfall, recovering rapidly following drought and not supporting the fears of widespread, subcontinental scale desertification taking place in the 9‐year period that is studied. In fact the results show a small but systematic increase in RUE for the Sahel as a whole from 1982 to 1990, although some areas contained within the region did have persistently low values.
Effect of climate change on mast‐seeding species: frequency of mass flowering and escape from specialist insect seed predators
Authors: MCKONE MARK. J. , KELLY DAVE. , LEE WILLIAM G. 
Published: March 2003

AbstractGlobal surface temperatures are expected to increase by several degrees in the next century, with potentially large but poorly understood impacts on ecological interactions. Here we propose potential effects of increased temperatures on ecologically dominant New Zealand grasses (Chionochloa spp.) that mass flower and mast seed. Twenty‐two years’ data from five masting Chionochloa species in New Zealand showed that the cue for heavy flowering was unusually high temperature in the summer of the year before flowering. Attack by predispersal insect seed predators was much reduced in mast years, apparently because predator populations were satiated. Increased temperatures would greatly decrease interannual variation in Chionochloa flowering, allowing seed predator populations to increase and potentially to devastate the seed crop annually. Similar responses are likely in masting species worldwide. This previously unrecognized effect of global warming could have widespread impacts on temperate ecosystems.
Litter decomposition rates in Canadian forests
Authors: Moore T. R. , Trofymow J. A. , Taylor B. , Prescott C. , Camiré C. , Duschene L. , Fyles J. , Kozak L. , Kranabetter M. , Morrison I. , Siltanen M. , Smith S. , Titus B. , Visser S. , Wein R. , Zoltai S. 
Published: March 2003

AbstractThe effect of litter quality and climate on the rate of decomposition of plant tissues was examined by the measurement of mass remaining after 3 years’ exposure of 11 litter types placed at 18 forest sites across Canada. Amongst sites, mass remaining was strongly related to mean annual temperature and precipitation and amongst litter types the ratio of Klason lignin to nitrogen in the initial tissue was the most important litter quality variable. When combined into a multiple regression, mean annual temperature, mean annual precipitation and Klason lignin:nitrogen ratio explained 73% of the variance in mass remaining for all sites and tissues. Using three doubled CO2 GCM climate change scenarios for four Canadian regions, these relationships were used to predict increases in decomposition rate of 4–7% of contemporary rates (based on mass remaining after 3 years), because of increased temperature and precipitation. This increase may be partially offset by evidence that plants growing under elevated atmospheric CO2 concentrations produce litter with high lignin:nitrogen ratios which slows the rate of decomposition, but this change will be small compared to the increased rate of decomposition derived from climatic changes.
Hydrology, water availability and tundra ecosystem function in a changing climate: the need for a closer integration of ideas?
Authors: Hodkinson I. D. , Webb N. R. , Bale J. S. , Block W. 
Published: March 2003

AbstractHydrologists and ecologists studying tundra ecosystems have worked largely independently, with little cross‐fertilization between disciplines. Their disciplines are, however, inextricably linked by a need to understand the dynamics and significance of the common substance water, in its liquid, solid and gaseous state within tundra environments. The impacts of predicted long‐term changes in climate have particularly important consequences for the functioning of tundra systems and there is a pressing need to initiate studies that integrate hydrological and ecological methodologies and concepts. Our paper attempts to summarize existing information on the role of water within tundra ecosystems, to emphasize the fundamental links between the biotic and the physico/chemical environments and to suggest how a closer integration of ideas might be achieved. Given the breadth of the subject matter the paper is intended to be illustrative rather than comprehensive. The paper examines the physical impacts of water in its various states on the tundra environment, emphasizing in particular the causes of spatial variation in water availability to living organisms. The significance of water is discussed for a range of organism groups, including plants, invertebrates and microorganisms and its pivotal role in ecosystem function and disturbance stressed. The need to develop integrated hydological/ecological models for tundra systems on different spatial scales is emphasized.
Seasonal changes in the response of winter wheat to elevated atmospheric CO2 concentration grown in Open‐Top Chambers and field tracking enclosures
Authors: Dijkstra Paul , Schapendonk AD. H. M. C. , Groenwold KO. , Jansen Marinus , Van De Geijn Siebe C. 
Published: March 2003

SummaryWinter wheat was grown at ambient and elevated (ambient plus 350 μL L–1) CO2 concentrations in open top chambers and in field‐tracking sun‐lit climatized enclosures (elevated is 718 μL L–1). There was no significant effect of CO2 concentration on sheath, leaf and root biomass and leaf area in the early spring (January to April). 24‐h canopy CO2 exchange rate (CCER) was not significantly affected either. However, elevated CO2 concentration increased CCER at midday, decreased evapotranspiration rate and increased instantaneous water‐use‐efficiency during early spring. Leaf, sheath and root nitrogen concentration per unit dry weight decreased and nonstructural carbohydrate concentration increased under elevated CO2, and N‐uptake per unit ground area decreased significantly (– 22%) towards the end of this period.These results contrast with results from the final harvest, when grain yield and biomass were increased by 19% under elevated CO2. N concentration per dry weight was reduced by 5%, but N‐uptake per unit ground area was significantly higher (+ 11%) for the elevated CO2 treatment. 24‐h and midday‐CCER increased significantly more in late spring (period of 21 April to 30 May) (respectively by + 40% and 53%) than in the early spring (respectively 5% and 19%) in response to elevated CO2. Midday evapotranspiration rate was reduced less by elevated CO2 in the late spring (– 13%) than in early spring (– 21%). The CO2 response of midday and 24‐h CCER decreased again (+ 27% and + 23% resp.) towards the end of the growing season.We conclude that the low response to CO2 concentration during the early spring was associated with a growth‐restriction, caused by low temperature and irradiance levels. The reduction of nitrogen concentration, the increase of nonstructural carbohydrate, and the lower evapotranspiration indicated that CO2 did have an effect towards the end of early spring, but not on biomass accumulation. Regression analysis showed that both irradiance and temperature affected the response to CO2.
Elevated UV‐B radiation effects on experimental grassland communities
Authors: Norton Lisa. R. , Mcleod Andrew R. , Greenslade Peter D. , Firbank Les. G. , Watkinson Andrew R. 
Published: March 2003

SummaryExperimental grassland communities (turves) were exposed to supplemental levels of UV‐B radiation (280–315 nm) at an outdoor facility, under treatment arrays of cellulose diacetate‐filtered fluorescent lamps which also produce UV‐A radiation (315–400 nm). Control treatments consisted of arrays of polyester‐filtered lamps, which allowed for exposure to UV‐A radiation alone, and arrays of unenergized lamps allowing for exposure to ambient levels of solar radiation.
Carbon partitioning and rhizosphere C‐flow in Lolium perenne as affected by CO2 concentration, irradiance and below‐ground conditions
Authors: Paterson EriC. , Hodge Angela , Thornton Barry , Millard Peter , Killham KeN. 
Published: March 2003

SummaryPlant responses to increasing atmospheric CO2 concentrations have received considerable interest. However, major uncertainties in relation to interactive effects of CO2 with above‐ and below‐ground conditions remain. This microcosm study investigated the impacts of CO2 concentration on plant growth, dry matter partitioning and rhizodeposition as affected by: (i) photon flux density (PFD), and (ii) growth matrix. Plants were grown in a sandy loam soil for 28 d under two photon flux densities: 350 (low PFD) and 1000 μmol m–2 s–1 (high PFD) and two CO2 concentrations: 450 (low CO2) and 720 μmol mol–1 (high CO2). Partitioning of recent assimilate amongst plant and rhizosphere C‐pools was determined by use of 14CO2 pulse‐labelling. In treatments with high PFD and/or high CO2, significant (P < 0.05) increases in dry matter production were found in comparison with the low PFD/low CO2 treatment. In addition, significant (P < 0.05) reductions in shoot %N and SLA were found in treatments imposing high PFD and/or high CO2. Root weight ratio (RWR) was unaffected by CO2 concentration, however, partitioning of 14C to below ground pools was significantly (P < 0.05) increased. In a separate study, L. perenne was grown for 28 d in microcosms percolated with nutrient solution, in either a sterile sand matrix or nonsterile soil, under high or low CO2. Dry matter production was significantly (P < 0.01) increased for both sand and soil grown seedlings. Dry matter partitioning was affected by matrix type. 14C‐allocation below ground was increased for sand grown plants. Rhizodeposition was affected by CO2 concentration for growth in each matrix, but was increased for plants grown in the soil matrix, and decreased for those in sand. The results illustrate that plant responses to CO2 are potentially affected by (i) PFD, and (ii) by feedbacks from the growth matrix. Such feedbacks are discussed in relation to soil nutrient status and interactions with the rhizosphere microbial biomass.