bluuebunny/crossref_metadata_2025_split huggingface dataset:
Finding the shape of a local heterogeneity by means of a structural inversion with constraints
Authors: Ditmar Pavel 
Published: March 2003

ABSTRACTVarious aspects of structural inversion are considered. The aim of the inversion is limited to finding the shape of an isolated 2D homogeneous body, although the technique may be generalized to the case of interfaces with steep fragments, faults or overhangs. The unknown parameters are shifts of border points. The shift directions can be normal to the initial heterogeneity configuration or to another contour. Medium properties (seismic velocities, densities, etc.) within the heterogeneity are assumed to be known. The optimal shape is determined iteratively, a quadratic objective function being minimized at each iteration with the conjugate‐gradient method. Special attention is paid to preventing self‐intersections, for which purpose each unknown parameter is forced to lie within a certain predetermined interval. In order to achieve this, the classical conjugate‐gradient method has been modified accordingly. Three numerical examples are considered. These illustrate how the developed technique can be applied to different practical problems. The first example is devoted to monitoring an oil/steam interface by gravity gradiometry measurements. In the second example, a cross‐hole seismic experiment is simulated. It is shown that a structural inversion can restore the configuration of a local body much more accurately than traditional seismic tomography. In the third example, the shape of a salt dome is reconstructed by joint inversion of refracted traveltimes and gravity measurements. This example demonstrates how different kinds of data, used simultaneously in a structural inversion, can complement each other.
Successful application of ground‐penetrating radar in the exploration of gem tourmaline pegmatites of southern California
Authors: Patterson Jeffrey E. , Cook Frederick A. 
Published: March 2003

ABSTRACTApplication of ground‐penetrating radar has been successful in delineating gem‐bearing zones in the Himalaya pegmatite mine of the Mesa Grande district of southern California. The high frequency of the electromagnetic signal allows features as small as a few centimetres to be resolved within 1–2 m of the surface of a mine wall. Careful initial set‐up consisted of: (i) selection of antennae with sufficiently high central frequencies; (ii) recording with a short time of scan to reduce end‐of‐scan noise levels; (iii) choosing appropriate colour schemes to highlight extreme amplitude variations. Operation during data collection consisted of pre‐painting marking points on the mine face and air launching the signal to reduce false anomalies caused by rocking of the antenna on the rough surfaces. Data processing using the Hilbert transform provided images of the cavity geometry that were then used by the blasting captain for accurate placement of explosives. The instantaneous frequency plot was found to be effective in distinguishing air‐filled from clay‐filled pockets, and the instantaneous phase plot was helpful in selecting potential targets where the amplitude was less than the maximum range. When carefully used in conjunction with good knowledge of the geological conditions, the method promises to provide an important tool for mapping internal structures of pegmatites, thus assisting future mining activities.
Acquisition and processing of high‐resolution reflection seismic data from a survey within the complex terrain of the Bavarian Folded Molasse
Authors: Thomas R. , Bram K. , Fertig J. , Schwerd K. 
Published: March 2003

A high‐resolution reflection seismic survey was carried out in the southern part of the Bavarian Molasse Basin in 1998 and 1999. The survey aimed to investigate the near‐surface structure of the complicated transition from the unfolded Foreland Molasse to the Folded Molasse, and the Folded Molasse to the internally complicated thrust systems of the Helveticum, the Ultrahelveticum and the Rhenodanubian Flysch. The study is linked to the TRANSALP seismic project, and the results help to fill the gap between the surface and the upper 300–500 ms two‐way traveltime (TWT), typical of deep‐reflection seismic experiments. The environmental conditions encountered in the study area required that particular attention be paid to the acquisition parameters for the three seismic lines (each about 4 km long). The energy source was a small vibrator; the geophone spread, spacing and frequency range were adjusted to image reflectors, which were expected to dip steeply southwards.In general, the unprocessed field records did not show signals that could be attributed to specific reflectors. Individual trace processing considerably improved the data quality, taking into account the influence of the Quaternary cover and also the strong lateral velocity variations of the shallow subsurface. The effects of the various processing steps, such as muting, refraction statics, residual statics and velocity analysis, are discussed. To assess the NMO velocities, the qualitative analysis of the seismic energy in a common‐shotpoint gather offered advantages over an analysis in a common‐midpoint gather or in a stacked section, and proved to be very effective. As demonstrated along the Miesbach 9801 line, low‐velocity zones extend locally down to about 400 ms, adjacent to zones of extremely high velocities close to the surface, reflecting steeply dipping strata.Besides the Quaternary cover on top, the Miesbach 9801 and Miesbach 9802 lines exhibit many horizontal reflections, in places down as far as 1400 ms TWT, indicating the sedimentary sequences of the unfolded Foreland Molasse. The southern part of both lines is dominated by southward‐dipping reflection bands, indicating units of the Folded Molasse. The reflection pattern shown by the Miesbach 9901 line suggests that there is almost no Quaternary cover. Southward‐dipping elements reflect the internal structure of the Folded Molasse, whereas a rather diffuse reflection signature may be attributed to Rhenodanubian Flysch units.
Experimental validation of the wavefield transform of electromagnetic fields
Authors: Das Kaushik , Becker Alex , Lee Ki Ha 
Published: March 2003

The wavefield transform is a mathematical technique for transforming low‐frequency electromagnetic (EM) signals to a non‐diffusive wave domain. The ray approximation is valid in the transform space and this makes traveltime tomography for 3D mapping of the electrical conductivity distribution in the subsurface possible. The transform, however, imposes stringent frequency bandwidth and signal‐to‐noise ratio requirements on the data. Here we discuss a laboratory scale experiment designed to collect transform quality EM data, and to demonstrate the practical feasibility of transforming these data to the wavefield domain.We have used the scalable nature of EM fields to design a time‐domain experiment using graphite blocks to simulate realistic field conditions while leaving the time scale undisturbed. The spatial dimensions have been scaled down by a factor of a thousand by scaling conductivity up by a factor of a million. The graphite blocks have two holes drilled into them to carry out cross‐well and borehole‐to‐surface experiments. Steel sheets have been inserted between the blocks to simulate a conductive layer.Our experiments show that accurate EM data can be recorded on a laboratory scale model even when the scaling of some features, such as drill‐hole diameters, is not maintained. More importantly, the time‐domain EM data recorded in cross‐well and surface‐to‐borehole modes can be usefully and accurately transformed to the wavefield domain. The observed wavefield propagation delay is proportional to the direct distance between the transmitter and receiver in a homogeneous medium. In a layered medium, data accuracy is reduced and, hence, our results are not so conclusive. On the basis of the experimental results we conclude that the wavefield transform could constitute a valid approach to the interpretation of accurate, undistorted time‐domain data if further improvement in the transform can be realized.
Wavefield extrapolation and prestack depth migration in anelastic inhomogeneous media
Authors: Zhang Jianfeng , Wapenaar Kees 
Published: March 2003

ABSTRACTWavefield depth extrapolation and prestack depth migration in complex anelastic media are studied. Kjartansson's frequency‐independent Q law is used to describe the absorption of seismic energy. The macromodel used is analogous to the macromodel used for current migration schemes except that an additional frequency‐independent Q macromodel needs to be provided. Absorption in the forward one‐way propagator is introduced by assuming a complex phase velocity, and the inverse one‐way propagator is obtained using the reciprocity theorem for one‐way wavefields in dissipative media. The stability of the inverse propagator is achieved by limiting the angle of propagation of wavefields. A table‐driven explicit operator scheme for imaging complex 2D anelastic media is presented. High‐accuracy, short convolution operators are designed by the weighted least‐squares method, and two kinds of imaging conditions are proposed. Numerical examples of depth extrapolation in laterally varying media, the migration of a spatial impulse with dispersion as well as shot record depth migration demonstrate the potential of the proposed explicit forward operator, the explicit inverse operator and the prestack depth migration scheme, respectively.
Model estimates of methane emission from irrigated rice cultivation of China
Authors: HUANG YAO. , SASS RONALD L. , FISHER FRANK M. 
Published: March 2003

AbstractA model developed by the authors (Huang et al. 1998) was further validated against field measurements from various regions of the world and calibrated to estimate methane emission from irrigated rice cultivation of China. On the basis of available information on rice cultivated area, growth duration, grain yield, soil texture and temperature, methane emission from Chinese rice paddies was estimated for 28 rice cultivated provinces in mainland. The calculated daily methane emission rates, on a provincial scale, ranged from 0.15 to 0.86 g m–2 with an average of 0.32 g m–2. Five of the top six locations with higher daily methane emissions are located at a latitude between 28° and 31° N. A total amount of 9.66 Tg (1 Tg = 1012 g) CH4 per year, ranging from 7.19 to 13.62, was estimated to be released from Chinese rice paddy soils. Of the total, 45% is emitted from the single‐rice growing season, and 19% and 36% are from the early‐rice and the late‐rice growing seasons, respectively. Approximately 70% of the total is emitted in the region located at latitude between 25° and 32° N. The emissions from rice fields in Sichuan and Hunan Province were calculated to be 2.85 Tg y–1, accounting for ≈ 30% of the total. Comparisons of the estimated and the observed emission rates show that the estimates were, in general, close to the measurements at most locations.
Growth, loss, and vertical distribution of Pinus radiata fine roots growing at ambient and elevated CO2 concentration
Authors: Thomas Stephen M. , Whitehead David , Reid JefF. B. , Cook Freeman J. , Adams JohN. A. , Leckie AlaN. C. 
Published: March 2003

AbstractIncreased below‐ground carbon allocation in forest ecosystems is a likely consequence of rising atmospheric CO2 concentration. If this results in changes to fine root growth, turnover and distribution long‐term soil carbon cycling and storage could be altered.Bi‐weekly measurements were made to determine the dynamics and distribution of fine roots (< 1 mm diameter) for Pinus radiata trees growing at ambient (350 μmol mol–1) and elevated (650 μmol mol–1) CO2 concentration in large open‐top chambers. Measurements were made using minirhizotrons installed horizontally at depths of 0.1, 0.3, 0.5 and 0.9 m.During the first year, at a depth of 0.3 m, the increase in relative growth rate of roots occurred 6 weeks earlier in the elevated CO2 treatment and the maximum rate was reached 10 weeks earlier than for trees in the ambient treatment. After 2 years, cumulative fine root growth (Pt) was 36% greater for trees growing at elevated CO2 than at ambient CO2 concentration, although this difference was not significant. A model of root growth driven by daily soil temperature accounted for between 43 and 99% of root growth variability.Total root loss (Lt) was 9% in the ambient and 14% in the elevated CO2 treatment, although this difference was not significant. Root loss was greatest at 0.3 m. In the first year, 62% of fine roots grown between mid‐summer and late‐autumn disappeared within a year in the elevated CO2 treatment, but only 18% in the ambient CO2 treatment (P < 0.01). An exponential model relating Lt to time accounted for between 74 and 99% of the variability. Root cohort half‐lives were 951 d for the ambient and 333 d for the elevated treatment.Root length density decreased exponentially with depth in both treatments, but relatively more fine roots grown in the elevated CO2 treatment tended to occur deeper in the soil profile.
The impact of ultraviolet‐B radiation on the motility of the freshwater epipelic diatom Nitzschia lineariz
Authors: Moroz Adrienne L. , Ehrman James M. , Clair Thomas A. , Gordon Robert J. , Kaczmarska Irena 
Published: March 2003

AbstractDepletion of the stratospheric ozone layer has been of increased concern due to correlated increases in ambient ultraviolet radiation. Our research investigated the diel response of the motile behaviour of Nitzschia lineariz in both natural (freshwater stream) and experimental conditions. We classified the behaviour of individual diatom cells as gliding, immobile and oscillating. The experimental conditions were of simultaneous exposure either to net ambient solar radiation (control) or to ambient plus enhanced UV‐B levels (treatment). UV‐B for the control condition was filtered out using polyester filters; cells exposed to UV‐B were irradiated through cellulose diacetate filters. UVB‐313 levels were augmented to 33% (290 mW m–2) and 66% (365 mW m–2) above the local average ambient levels (220 mW m–2) for 10 h. The mobile behaviour of cells was examined and quantified every 2 consecutive hours from dawn to dusk in subsampled populations. The number of gliding cells in subsample populations declined on overcast days at both the 33 and 66% UV‐B enhancement levels, whereas the number of immobile and oscillating cells increased with increased UV‐B exposure. On sunny days, mobile behaviour was not affected at either enhancement level, which suggests that cellular repair mechanisms may be activated during sunny conditions.
Scaling net ecosystem CO2 exchange from the community to landscape‐level at a subarctic fen
Authors: Griffis Timothy J. , Rouse Wayne R. , Waddington J. M. 
Published: March 2003

SummaryLandscape‐ and community‐level CO2 measurements were made at a subarctic sedge fen near Churchill Manitoba during the 1997 growing season. Climatic conditions were warmer and drier than the 30‐y normal. Landscape‐scale micrometeorological measurements indicated that the wetland gained 49 g CO2 m−2 during the growing season. Chamber‐scale measurements from the main vegetation community types showed that small hummocks (Carex spp. sites) dominated the CO2 exchange, yielding an effective scaling factor of 70%. Scaled parameters of two algorithms describing photosynthesis and respiration for each community type show strong similarity to those derived at the landscape level. Scaling photosynthesis, respiration, and net ecosystem CO2 exchange from the community to landscape‐level over the season is within the maximum probable error of each methodological approach and helps substantiate the 1997 CO2 budget. We explore the equilibrium response of net ecosystem CO2 exchange of this fen to climatic change by examining the feedback of water table position on vegetation distribution and nitrogen availability. Based on the effective scaling factors computed for each community type, we hypothesize that a small decrease in mean water table position could nearly triple the net uptake of CO2 at this wetland.
CO2 stabilization, climate change and the terrestrial carbon sink
Authors: White Andrew , Cannell Melvin G. R. , Friend Andrew D. 
Published: March 2003

SummaryA nonequilibrium, dynamic, global vegetation model, Hybrid v4.1, with a subdaily timestep, was driven by increasing CO2 and transient climate output from the UK Hadley Centre GCM (HadCM2) with simulated daily and interannual variability. Three IPCC emission scenarios were used: (i) IS92a, giving 790 ppm CO2 by 2100, (ii) CO2 stabilization at 750 ppm by 2225, and (iii) CO2 stabilization at 550 ppm by 2150. Land use and future N deposition were not included. In the IS92a scenario, boreal and tropical lands warmed 4.5 °C by 2100 with rainfall decreased in parts of the tropics, where temperatures increased over 6 °C in some years and vapour pressure deficits (VPD) doubled. Stabilization at 750 ppm CO2 delayed these changes by about 100 years while stabilization at 550 ppm limited the rise in global land surface temperature to 2.5 °C and lessened the appearance of relatively hot, dry areas in the tropics.Present‐day global predictions were 645 PgC in vegetation, 1190 PgC in soils, a mean carbon residence time of 40 years, NPP 47 PgC y−1 and NEP (the terrestrial sink) about 1 PgC y−1, distributed at both high and tropical latitudes. With IS92a emissions, the high latitude sink increased to the year 2100, as forest NPP accelerated and forest vegetation carbon stocks increased. The tropics became a source of CO2 as forest dieback occurred in relatively hot, dry areas in 2060–2080. High VPDs and temperatures reduced NPP in tropical forests, primarily by reducing stomatal conductance and increasing maintenance respiration. Global NEP peaked at 3–4 PgC y−1 in 2020–2050 and then decreased abruptly to near zero by 2100 as the tropical source offset the high‐latitude sink. The pattern of change in NEP was similar with CO2 stabilization at 750 ppm, but was delayed by about 100 years and with a less abrupt collapse in global NEP. CO2 stabilization at 550 ppm prevented sustained tropical forest dieback and enabled recovery to occur in favourable years, while maintaining a similar time course of global NEP as occurred with 750 ppm stabilization. By lessening dieback, stabilization increased the fraction of carbon emissions taken up by the land. Comparable studies and other evidence are discussed: climate‐induced tropical forest dieback is considered a plausible risk of following an unmitigated emissions scenario.
Water use of two Mojave Desert shrubs under elevated CO2
Authors: Pataki D. E. , Huxman T. E. , Jordan D. N. , Zitzer S. F. , Coleman J. S. , Smith S. D. , Nowak R. S. , Seemann J. R. 
Published: March 2003

SummaryPlant responses to elevated atmospheric CO2 have been characterized generally by stomatal closure and enhanced growth rates. These responses are being increasingly incorporated into global climate models that quantify interactions between the biosphere and atmosphere, altering climate predictions from simpler physically based models. However, current information on CO2 responses has been gathered primarily from studies of crop and temperate forest species. In order to apply responses of vegetation to global predictions, CO2 responses in other commonly occurring biomes must be studied. A Free Air CO2 Enrichment (FACE) study is currently underway to examine plant responses to high CO2 in a natural, undisturbed Mojave Desert ecosystem in Nevada, USA. Here we present findings from this study, and its companion glasshouse experiment, demonstrating that field‐grown Ephedra nevadensis and glasshouse‐grown Larrea tridentata responded to high CO2 with reductions in the ratio of transpirational surface area to sapwood area (LSR) of 33% and 60%, respectively. Thus, leaf‐specific hydraulic conductivity increased and stomatal conductance remained constant or was increased under elevated CO2. Field‐grown Larrea did not show a reduced LSR under high CO2, and stomatal conductance was reduced in the high CO2 treatment, although the effect was apparent only under conditions of unusually high soil moisture. Both findings suggest that the common paradigm of 20–50% reductions in stomatal conductance under high CO2 may not be applicable to arid ecosystems under most conditions.
Leaf mineral concentrations of Erica arborea, Juniperus communis and Myrtus communis growing in the proximity of a natural CO2 spring
Authors: Peñuelas Josep , Filella Iolanda , Tognetti Roberto 
Published: March 2003

SummaryLeaf mineral concentrations of co‐occurring Erica arborea, Juniperus communis and Myrtus communis were measured at bimonthly intervals throughout a year in a natural CO2 spring and in a nearby control site with similar soil chemistry in a Mediterranean environment. There were different responses to the elevated [CO2] (c. 700 μL L−1) of the spring site plants depending on the element and the species. In the CO2 spring site K, Ca, Mg, Mn, Al, Fe, and Ti leaf concentrations and the ratio C/N showed significant greater values in at least one or two of the three species. Leaf S concentration were greater in all three species. Leaf concentrations of N, Sr, Co, and B were lower in at least one or two species, and those of C and Ba were lower in all the three studied species near the CO2 spring. P, Na, Zn, Si, Cu, Ni, Cr, Pb, Mo, V and Cd leaf concentrations and the specific leaf area (SLA, measured in Myrtus communis) did not show any consistent or significant pattern in response to the elevated [CO2] of the spring site. There was a slight trend towards maximum concentrations of most of these elements during autumn–winter and minimum values during the spring season, especially in Myrtus communis. Multivariate principal component analyses based on the leaf elemental concentrations clearly differentiated the two sites and the three species. Lower concentrations at the spring site were not the result of a dilution effect by increased structural or nonstructural carbon. In contrast to most experimental studies of CO2 enrichment, mainly conducted for short periods, several of these elements had greater concentrations in the CO2 spring site. Nutrient acclimation and possible causes including decreased nutrient export, increased nutrient uptake capacity, photosynthetic down‐regulation, Mediterranean water stress, and higher H2S concentration in the spring site are discussed.
Plant carbon balance, evolutionary innovation and extinction in land plants
Authors: Cowling Sharon A. 
Published: March 2003

SummaryThe plant fossil record was reviewed to highlight how consideration of plant carbon balance strengthens our understanding of various evolutionary innovation and extinction events. Following a brief physiological primer to carbon acquisition and allocation in C3‐plants, specific evolutionary events are discussed in connection with postulated carbon‐based mechanisms. Primary topics include: (i) the evolution of plants with the C4‐photosynthetic pathway; (ii) the surprising lack of plant extinctions during the Pleistocene (1.6 million years ago, Ma); (iii) the trend toward declining plant diversity and increasing rates of herbivory across the Palaeocene/Eocene transition (57–52 Ma); and (iv) megaherbivore extinctions at the end of the Pleistocene (10 thousand years ago, Ka). A framework is presented for testing hypotheses on the cause–effect relationships between global carbon cycling, plant carbon dynamics and the evolution of terrestrial ecosystems.