bluuebunny/crossref_metadata_2025_split huggingface dataset:
Effect of mining subsidence on seismic velocity monitored by a repeated reflection profilel1
Authors: Al‐Rawahy S.Y.S. , Goulty N.R. 
Published: April 2006

AbstractWe have monitored changes in seismic velocity due to longwall coal‐mining in the Selby coalfield, Yorkshire, England by. ten repeated surveys of a surface seismic reflection profile. The direction of face advance in the Barnsley Seam, at 550 m depth, was parallel to the orientation of the profile. The traveltime of a strong reflection event from an anhydrite bed at 150 m depth was measured after processing the data with standard techniques. As the face advanced, the traveltime increased by about 4% overall. In detail, the progressive increase in traveltime correlates well with empirical calculations of differential subsidence between the surface and the anhydrite. However, the magnitude of the change must principally be accounted for by a decrease in seismic velocity, which we attribute to a reduction in the vertical effective stress.
Ultrasonic wave propagation in dry and brine‐saturated sandstones as a function of effective stress: laboratory measurements and modelling1
Authors: Tao G. , King M.S. , Nabi‐Bidhendi M. 
Published: April 2006

AbstractCompressional and shear‐wave velocities have been measured and a novel approach using digital processing employed to study wave attenuation for brine‐ and gas‐ saturated sandstones, over a range of effective stresses from 5 to 60 MPa. Also measured were the complex conductivity in the brine‐saturated state and permeability in the gas‐saturated state over the same range of stresses as for the velocity measurements. Broadband ultrasonic pulses of P‐ and orthogonally polarized S‐waves in the frequency range 0.3–0.8 MHz are transmitted through the specimen to be characterized for comparison with a reference (aluminium) having low attenuation. The attenuation is calculated in terms of the quality factor Q from the Fourier spectral ratios, using the frequency spectral ratios technique. The corrections necessary for the effects of diffraction due to the finite size of the ultrasonic transducers have been carried out for the case of measurements under lower confining stress. To interpret the laboratory measured velocity and attenuation data under the physical conditions of this study and to estimate the effects of pore structure, numerical modelling of velocities and attenuation as functions of the confining stress have been performed, based on the MIT model. Theoretical models based on several hypothesized attenuation mechanisms are considered in relation to laboratory data of the effects of confining pressure, fluid saturation and pore structure on attenuation. Numerical calculations using these models with the experimental data indicate that friction on thin cracks and grain boundaries is the dominant attenuation mechanism for dry and brine‐saturated sandstones at low effective stresses for the frequencies tested. However, for brine‐saturated sandstones at moderately high effective stresses, fluid flow could play a more important role in ultrasonic S‐wave attenuation, depending on the pore structure of the sample.
Harmonic resonance structure and chaotic dynamics in the earth‐vibrator system1
Authors: Walker David 
Published: April 2006

AbstractSource‐generated energy in seismic vibrator records includes ultraharmonics, subharmonics, ultra‐subharmonics and possibly chaotic oscillatory behaviour. Nonlinear behaviours can be modelled using a ‘hard‐spring’ form of the Duffing equation. Modelling indicates that a qualitatively similar harmonic resonance structure is present for a broad range of possible mathematical descriptions. Qualitative global system behaviours may be examined without knowledge of actual earth parameters. Non‐linear resonances become stronger, relative to fundamental sweep frequencies, as the driving force increases or damping decreases. System response energy levels are highest when non‐linear resonances are strong. The presence of chaotic energy can indicate the highest energy state of a system reponse. Field data examples are consistent with behaviours predicted by modelling. Conventional correlation and stack uses a fraction of the energy produced in the earth‐vibrator system. A correlation and filtering process that uses a representation of the source dynamics based on the system response can reduce signal degradation due to non‐linear resonance.
Laboratory technique for measurement of spectral induced polarization response of soil sampies1
Authors: Vanhala Heikki , Soininen Heikki 
Published: April 2006

AbstractLaboratory measurements of soil samples are necessary to assess the effect of mineralogy, grain size distribution, moisture content, and electrolyte composition on the resistivity spectrum of soil material. Laboratory results are also required for the interpretation of field data. Induced polarization phenomena in glacial soils are poorly understood and so far no convenient laboratory techniques are available for its measurement. Coarse grain size and the need to measure unsaturated samples and to monitor the homogeneity of the sample require a sample holder–electrode construction that differs from those presented in Clay mineral Studies.This study presents a spectral induced polarization laboratory system that is suitable for measuring fine‐ and coarse‐grained and both saturated and unsaturated soil samples. The noise caused by the electrode–electrolyte interface is studied in detail. It is shown that easy‐to‐use platinum or acid‐free steel potential electrodes are convenient over a broad frequency band ranging from 0.016Hz up to more than 1000 Hz. The laboratory experiments and comparisons between laboratory and field results also indicate that sampling and sample packing procedures have only a minor influence on the phase spectrum of glacial soils.
Monitoring subsurface changes over time with cross‐well electromagnetic tomographyt1
Authors: Alumbaugh David L. , Morrison H.F. 
Published: April 2006

AbstractA fast imaging technique is developed to deduce the spatial conductivity distribution in the earth from low‐frequency (> 1 MHz) cross‐well electromagnetic measurements. A sinusoidally oscillating, vertically orientated, magnetic dipole employed as a source, and it is assumed that the scattering bodies are azimuthally symmetric about the source dipole axis. The use of this model geometry reduces the 3D vector problem to a more manageable 2D scalar form. Additional efficiency is obtained by using the Born series approximation which is derived from nonlinear integral equations that account for the scattered magnetic fields generated by inhomogeneities embedded in a layered earth. Stabilization of the inversion problem is accomplished through the use of bounding constraints and a regularization method which results in a smooth model that fits the data to the desired noise level.The applicability of cross‐well electromagnetics for imaging and monitoring changes caused by subsurface processes has been tested by simulating plumes of conductive fluid with 2D models. The images that result from inverting these synthetic data indicate that the vertical resolution of the method is better than the horizontal, increasing the noise decreases the image resolution, and incorporating a priori knowledge in the form of positivity constraints improves the results. Although higher operating frequencies are usually associated with better resolution, frequencies as low as 100 Hz can produce acceptable images in simulated oilfield environments.The imaging scheme has been applied to data collected during a salt‐water injection experiment at the Richmond Field Station test site in Richmond, California. Both the data and the resulting images clearly reveal the presence of the plume and indicate that it is migrating towards the north‐northwest rather than spreading symmetrically about the injection well. Applying the imaging code to synthetic data generated by a 3D sheet model verifies the interpretation of these results.
Accurate and efficient shot‐gather dip moveout processing in the log‐stretch domain1
Authors: Zhou B. , Mason I.M. , Greenhalgh S.A. 
Published: April 2006

AbstractAn accurate analytical expression for shot‐gather dip‐moveout (DMO) in the timespace log‐stretch domain has until now not been published. We present a simpler, alternative derivation of the exact DMO relationships of Blacket al. which correctly take account of the repositioning of the midpoint. A new computationally efficient frequency‐wavenumber(F‐K)DMO operator for shot profiles is then derived, based on these DMO relationships in the time‐space log‐stretch domain. The newly derived DMO operator is, unlike most other log‐stretch DMO operators) accurate for the full range of reflector dips. Along with other schemes which are performed in the log‐stretch domain, it offers considerable time savings over conventional DMO processing. We have compared numerically the impulse response of the new operator with those of a number of other shot‐gather DMO operators, and found it to be superior and well match to the theoretical elliptical DMO response.
Numerical modelling of airborne electromagnetic anomalies originating from low‐conductivity 3D bodies1
Authors: Peltoniemi Markku , Bärs Rainer , Newman Gregory A. 
Published: April 2006

AbstractResponses of a multifrequency, multicoil airborne electromagnetic (AEM) system were modelled numerically for 3D electrical conductors embedded in a resistive bedrock and overlain by an overburden of low to moderate conductivity. The results cover a horizontal coplanar coil configuration and two frequencies, 7837 Hz and 51 250 Hz. The models studied are single or multiple, poor conductors (conductance lower than 0.1 S) embedded in a host rock of high but finite resistivity (5000 Ωm) and overlain by a layer of overburden with finite thickness and low to moderate conductivity (conductance up to 2 S).On the basis of the modelling results, limits of detectability for poor conductors have been studied for the various model structures. The results indicate that the anomaly from a steeply dipping, plate‐like conductor will decrease significantly when the conductor is embedded in a weakly conductive host rock and is overlain by a conductive overburden. However, an anomaly is obtained, and its magnitude can even increase with increasing overburden conductivity or frequency. The plate anomaly remains practically constant when only the overburden thickness is varied. Changes in overburden conductivity will cause the plate‐anomaly values to change markedly. If the plate conductance is less than that of the overburden, a local anomaly opposite in sign to the normal type of anomaly will be recorded. Another major consequence is that conductors interpreted with free‐space models will be heavily overestimated in depth or underestimated in conductance, if in reality induction and current channelling in the host rock and overburden make even a slight contribution to the anomalous EM field.The lateral resolution for the horizontal coplanar coil system was found to be about 1.7 times the sensor altitude. Similarly, the lateral extension of a horizontal conductive ribbon, required to reach the semi‐infinite (half‐space) behaviour, is more than three times the sensor altitude. Finally, screening of a steeply dipping plate, caused by a small, conductive horizontal ribbon, is much more severe than screening of the same plate by an extensive horizontal layer.
A new method for geoelectrical investigations underwater1
Authors: Baumgartner François 
Published: April 2006

AbstractA new electrical method is proposed for determining the apparent resistivity of multi‐earth layers located underwater. The method is based on direct current geoelectric sounding principles. A layered earth model is used to simulate the stratigraphic target. The measurement array is of pole‐pole type; it is located underwater and is orientated vertically. This particular electrode configuration is very useful when conventional electrical methods cannot be used, especially if the water depth becomes very important. The calculated apparent resistivity shows a substantial quality increase in the measured signal caused by the underwater targets, from which little or no response is measured using conventional surface electrode methods. In practice, however, different factors such as water stratification, underwater streams or meteorological conditions complicate the interpretation of the field results. A case study is presented, where field surveys carried out on Lake Geneva were interpreted using the calculated apparent resistivity master‐curves.
Fluxes of carbon dioxide and water vapour over an undisturbed tropical forest in south‐west Amazonia
Authors: GRACE JOHN , LLOYD JON , MCINTYRE JOHN , MIRANDA ANTONIO , MEIR PATRICK , MIRANDA HELOISA , MONCRIEFF JOHN , MASSHEDER JON , WRIGHT IVAN , GASH JOHN 
Published: April 2006

Abstract

Carbon dioxide and water vapour fluxes were measured for 55 days by eddy covariance over an undisturbed tropical rain forest in Rondonia, Brazil. Profiles of CO2 inside the canopy were also measured.
During the night, CO2 concentration frequently built up to 500 ppm throughout the canopy as a result of low rates of exchange with the atmosphere. In the early morning hours, ventilation of the canopy occurred.
Ecosystem gas exchange was calculated from a knowledge of fluxes above the canopy and changes of CO2 stored inside the canopy. Typically, uptake by the canopy was 15 μmol m−2 s−1 in bright sunlight and dark respiration was 6‐7 μmol m−2 s−1 The quantum requirement at low irradiance was: 40 mol photons per mol of CO2.
Bulk stomatal conductance of the ecosystem was maximal in the early morning (0.4‐1.0 mol m−2 s−1) and declined over the course of the day as leaf‐to‐air vapour pressure difference increased.
Global climate models and ‘dynamic’ vegetation changes
Authors: HENDERSON‐SELLERS A. , McGUFFIE K. 
Published: April 2006

AbstractModels of global change must come to incorporate changes in terrestrial vegetation. Here we choose a 1‐ year meshing (coupling) period to link a global climate model to a well‐known biophysical representation of the continental surface by means of eleven vegetation functional types. This coupled model is used to answer two questions: Can a ‘standard’ GCM ‘cope' with sudden switches in continental characteristics?’ and Does the climate ‘care’ about the changing underlying vegetation? We find affirmative answers to both questions. Our results also suggest that those content to generate vegetation post facto from climate output have incomplete results.
Was low atmospheric CO2 during the Pleistocene a limiting factor for the origin of agriculture?
Authors: SAGE ROWAN F. 
Published: April 2006

AbstractAgriculture originated independently in many distinct regions at approximately the same time in human history. This synchrony in agricultural origins indicates that a global factor may have controlled the timing of the transition from foraging to food‐producing economies. The global factor may have been a rise in atmospheric CO2 from below 200 to near 270 μol mol−1 which occurred between 15,000 and 12,000 years ago. Atmospheric CO2 directly affects photosynthesis and plant productivity, with the largest proportional responses occurring below the current level of 350 μol mol−1. In the late Pleistocene, CO2 levels near 200 μol mol−1 may have been too low to support the level of productivity required for successful establishment of agriculture. Recent studies demonstrate that atmospheric CO2 increase from 200 to 270 μol mol−1 stimulates photosynthesis and biomass productivity of C3 plants by 25% to 50%, and greatly increases the performance of C3 plants relative to weedy C4 competitors. Rising CO2 also stimulates biological nitrogen fixation and enhances the capacity of plants to obtain limiting resources such as water and mineral nutrients. These results indicate that increases in productivity following the late Pleistocene rise in CO2 may have been substantial enough to have affected human subsistence patterns in ways that promoted the development of agriculture. Increasing CO2 may have simply removed a productivity barrier to successful domestication and cultivation of plants. Through effects on ecosystem productivity, rising CO2 may also have been a catalyst for agricultural origins by promoting population growth, sedentism, and novel social relationships that in turn led to domestication and cultivation of preferred plant resources.
Past vegetation changes in the Brazilian Pantanal arboreal–grassy savanna ecotone by using carbon isotopes in the soil organic matter
Authors: VICTORIA REYNALDO LUIZ , FERNANDES FERNANDO , MARTINELLI LUIZ ANTONIO , DE CÁSSIA PICCOLO MARISA , DE CAMARGO PLINIO BARBOSA , TRUMBORE SUSAN 
Published: April 2006

AbstractMeasurements of the organic carbon inventory, its stable isotopic composition and radiocarbon content were used to deduce vegetation history from two soil profiles in arboreal and grassy savanna ecotones in the Brazilian Pantanal. The Pantanal is a large floodplain area with grass‐dominated lowlands subject to seasonal flooding, and arboreal savanna uplands which are only rarely flooded. Organic carbon inventories were lower in the grassy savanna site than in the upland arboreal savanna site, with carbon decreasing exponentially with depth from the surface in both profiles. Changes in 13C of soil organic matter (SOM) with depth differed markedly between the two sites. Differences in surface SOM 13C values reflect the change from C3 to C4 plants between the sites, as confirmed by measurements of 13C of vegetation and the soil surface along a transect between the upland closed‐canopy forest and lowland grassy savanna. Changes of 13C in SOM with depth at both sites are larger than the 3–4 per mil increases expected from fractionation associated with organic matter decomposition. We interpret these as recording past changes in the relative abundance of C3 and C4 plants at these sites. Mass balances with 14C and 13C suggest that past vegetational changes from C3 to C4 plants in the grassy savanna, and in the deeper part of the arboreal savanna, occurred between 4600 and 11 400 BP, when major climatic changes were also observed in several places of the South American Continent. The change from C4 to C3, observed only in the upper part of the arboreal savanna, was much more recent (1400 BP), and was probably caused by a local change in the flooding regime.
Plant growth and reproduction along CO2 gradients: non‐linear responses and implications for community change
Authors: ACKERLY D.D. , BAZZAZ F.A. 
Published: April 2006

AbstractThe effects of rising atmospheric CO2 concentrations on natural plant communities will depend upon the cumulative responses of plant growth and reproduction to gradual, incremental changes in climatic conditions. We analysed published studies of plant responses to elevated CO2 to address whether reproductive and total biomass exhibit similar enhancement to elevated vs. ambient CO2 concentrations, and to assess the patterns of plant response along gradients of CO2 concentrations. In six annual plant species, mean enhancement at double ambient vs. ambient CO2 was 1.13 for total biomass and 1.30 for reproductive biomass. The two measures were significantly correlated, but there was considerable scatter in the relationship, indicating that reproductive responses cannot be consistently predicted from enhancement of total biomass. Along experimental CO2 gradients utilizing three concentrations, there was a great diversity of response patterns, including positive, negative, non‐monotonic and non‐significant (flat) responses. The distribution of response patterns differed for plants grown in stands compared to those grown individually. Positive responses were less frequent in competitive environments, and non‐monotonic responses were more frequent. These results emphasize that interpolation of plant response based on enhancement ratios measured at elevated vs. ambient CO2 concentrations is not sufficient to predict community responses to incremental changes in atmospheric conditions. The consequences of differential response patterns were assessed in a simulation of community dynamics for four species of annual plants. The model illustrates that the final community composition at a future point in time depends critically on both the magnitude and the rate of increase of atmospheric CO2.
Land‐use change and the carbon cycle
Authors: HOUGHTON R. A. 
Published: April 2006

AbstractChanges in land use between 1850 and 1980 are estimated to have increased the global areas in croplands, pastures, and shifting cultivation by 891, 1308, and 30 × 106ha, respectively, reducing the area of forests by about 600 × 106ha, releasing about 100 PgC to the atmosphere, and transferring about 23 PgC from live vegetation to dead plant material and wood products. Another 1069 × 106ha are estimated to have been logged during this period, and the net release of carbon from the combined processes of logging and regrowth contributed 23 PgC to the 100‐PgC release. Annual rates of land‐use change and associated emissions of carbon have decreased over the last several decades in temperate and boreal zones and have increased in the tropics. The average release of carbon from global changes in land use over the decade of the 1980s Is estimated to have been 1.6 ± 0.7 PgC y−1almost entirely from the tropics. This estimate of carbon flux is higher than estimates reported in recent summaries because it is limited here to studies concerned only with changes in land use. Other recent analyses, based on data from forest inventories, have reported net accumulations of carbon as high as 1.1 PgC y−1in temperate and boreal zones. Because the accumulation of carbon in forests may result from natural processes unrelated to land‐use change, estimates based on these inventories should be distinguished from estimates based on changes in land use. Both approaches identify terrestrial sinks of carbon. The argument is made here, however, that differences between the two approaches may help identify the location and magnitude of heretofore ‘missing’ sinks. Before different estimates can be used in this way, analyses must consider similar geographical regions and dates, and they must account for the accumulation and loss of carbon in forest products in a consistent fashion.
Effects of temperature on aphid phenology
Authors: ZHOU XILONG , HARRINGTON RICHARD , WOIWOD IAN P. , PERRY JOE N. , BALE JEFFREY S. , CLARK SUZANNE J. 
Published: April 2006

AbstractDaily samples between 1964 and 1991 from suction traps throughout Great Britain were used to study the migration phenologies of five aphid species: Brachycaudus helichrysi, Elatobium abietinum, Metopolophium dirhodum, Myzus persicae and Sitobion avenae, and their relationship with temperature. Regression relationships have been established between characteristics of aphid phenology and temperature, latitude and longitude for each species. There were differences between species in the period for which temperature was most strongly associated with aphid phenology. The study indicates that temperature, especially winter temperature, is the dominant factor affecting aphid phenology, for all five species. A 1 °C increase in average winter temperature advanced the migration phenology by 4–19 days depending on species. Effects of temperature on the aphid phenology are similar between holocyclic and anholocyclic species, unlike the effects of temperature on date of first flight record which have been previously shown to be important only in anholocyclic species.