bluuebunny/crossref_metadata_2025_split huggingface dataset:
Error estimation and optimization of gravity surveys1
Authors: Keating Pierre 
Published: April 2006

AbstractGravity data are often acquired over long periods of time using different instruments and various survey techniques, resulting in data sets of non‐uniform accuracy. As station locations are inhomogeneously distributed, gravity values are interpolated on to a regular grid to allow further processing, such as computing horizontal or vertical gradients. Some interpolation techniques can estimate the interpolation error. Although estimation of the error due to interpolation is of importance, it is more useful to estimate the maximum gravity anomaly that may have gone undetected by a survey. This is equivalent to the determination of the maximum mass whose gravity anomaly will be undetected at any station location, given the data accuracy at each station. Assuming that the maximum density contrast present in the survey area is known or can be reasonably assumed from a knowledge of the geology, the proposed procedure is as follows: at every grid node, the maximum mass whose gravity anomaly does not disturb any of the surrounding observed gravity values by more than their accuracies is determined. A finite vertical cylinder is used as the mass model in the computations. The resulting map gives the maximum detection error and, as such, it is a worst‐case scenario. Moreover, the map can be used to optimize future gravity surveys: new stations should be located at, or near, map maxima. The technique is applied to a set of gravity observations obtained from different surveys made over a period of more than 40 years in the Abitibi Greenstone Belt in eastern Canada.
Effect of clayey media parameters on the negative response of a coincident loop1
Authors: El‐Kaliouby Hesham M. , Hussain Safwat A. , Bayoumi Abd El‐Rahim , A.El‐diwany Essam , Hashish Essam A. 
Published: April 2006

AbstractThe aim of this work is to consider the phenomenon of negative response and its detectability in a polarizable half‐space formed of Clay‐water mixture which commonly exists in water aquifers and which can be used as a simple indicator of the presence of Underground water.We make use of an analytical treatment for the transient voltage induced in a coincident loop lying on a half‐space as a basis for our computations. A Cole‐Cole model is used to represent the Clay with appropriate parameters.As saturated Clay is characterized by high conductivity and moderate chargeability, it is hardly detectable relative to the practical noise level. However, there are optimum Clay parameters that offer a maximum negative response. It is also observed that for a very short time constant τ in the Cole‐Cole model, the amplitude of the negative response decreases with the decrease of the time constant due to the fast decay of the polarization current.Finally, it is found that there is an optimum value for the loop radius which gives the largest value of the negative response and this loop radius depends on the model parameters.
P‐ and S‐wave anisotropy of a synthetic sandstone with controlled crack geometryl1
Authors: Rathore J.S. , Fjaer E. , Holt R.M. , Renlie L. 
Published: April 2006

AbstractA method is reported for the production of synthetic porous sandstones containing cracks of known dimensions and geometry with respect to the matrix. A synthetic sandstone was manufactured from Sand cemented with an epoxy glue. The cracks of known geometry were introduced into the material in the manufacturing stage, by emplacing thin metallic discs in the Sand‐epoxy matrix. These discs were chemically leached out of the consolidated porous sandstone. Acoustic anisotropy. and shear‐wave splitting were observed in the synthetic sandstones. For the dry sample the observed angular dependence of the P‐ and S‐ wave velocities (at 100 kHz) compares well, qualitatively, with the theoretical models of Hudson and of Thomsen. Quantitatively, however, the experimental data fits Hudson's model better. For the case of a saturated sample the experimental results are in excellent agreement with Thomsen's model. Hudson's model, on the other hand, predicts a different angular dependence for P‐waves. This demonstrates that the concept of fluid transfer between cracks and the ambient porosity can be a significant process. The results reported here are from the first successful experiment in which the theoretical models were tested on a porous material containing a known crack geometry.
Use of traveltime skips in refraction analysis to delineate velocity inversion1
Authors: Tewari H.C. , Dixit M.M. , Murty P.R.K. 
Published: April 2006

AbstractFirst arrival refraction data does not normally provide any indication of the velocity inversion problem. However, under certain favourable circumstances, when the low‐velocity layer (LVL) is considerably thicker than the overlying higher‐velocity layer (HVL), the velocity inversion can be seen in the form of a traveltime skip. Model Studies show that in such cases the length of the HVL traveltime branch can be used to determine the thickness of the HVL and the magnitude of the traveltime skip in order to determine the thickness of the LVL. This is also applicable in the case of field data.
Frequency‐domain modelling of airborne electromagnetic responses using staggered finite differences1
Authors: Newman Gregory A. , Alumbaugh David L. 
Published: April 2006

AbstractA 3D frequency‐domain EM modelling code has been implemented for helicopter electromagnetic (HEM) simulations. A vector Helmholtz equation for the electric fields is employed to avoid convergence problems associated with the first‐order Maxwell's equations when air is present. Additional stability is introduced by formulating the problem in terms of the scattered electric fields. With this formulation the impressed dipole source is replaced with an equivalent source, which for the airborne configuration possesses a smoother spatial dependence and is easier to model. In order to compute this equivalent source, a primary field arising from dipole sources of either a whole space or a layered half‐space must be calculated at locations where the conductivity is different from that of the background.The Helmholtz equation is approximated using finite differences on a staggered grid. After finite‐differencing, a complex‐symmetric matrix system of equations is assembled and preconditioned using Jacobi scaling before it is solved using the quasi‐minimum residual (QMR) method. The modelling code has been compared with other 1D and 3D numerical models and is found to produce results in good agreement.We have used the solution to simulate novel HEM responses that are computationally intractable using integral equation (IE) solutions. These simulations include a 2D conductor residing at a fault contact with and without topography. Our simulations show that the quadrature response is a very good indicator of the faulted background, while the in‐phase response indicates the presence of the conductor. However when interpreting the in‐phase response, it is possible erroneously to infer a dipping conductor due to the contribution of the faulted background.
Full frequency‐range transient solution for compressional waves in a fluid‐saturated viscoacoustic porous medium1
Authors: Carcione José M. , Ouiroga‐Goode Gerardo 
Published: April 2006

AbstractAn analytical transient solution is obtained for propagation of compressional waves in a homogeneous porous dissipative medium. The solution, based on a generalization of Biot's poroelastic equations, holds for the low‐ and high‐frequency ranges, and includes viscoelastic phenomena of a very general nature, besides the Biot relaxation mechanism. The viscodynamic operator is used to model the dynamic behaviour associated with the relative motion of the fluid in the pores at all frequency ranges. Viscoelasticity is introduced through the standard linear solid which allows the modelling of a general relaxation spectrum. The solution is used to study the influence of the material properties, such as bulk moduli, porosity, viscosity, permeability and intrinsic attenuation, on the kinematic and dynamic characteristics of the two compressional waves supported by the medium. We also obtain snapshots of the static mode arising from the diffusive behaviour of the slow wave at low frequencies.
Near‐surface layer traveltime inversion: a synthetic example1
Authors: Bloor Robert 
Published: April 2006

AbstractThe near‐surface layer is modelled as a constant‐velocity layer with varying thickness. The base of the layer is described by a B‐spline curve. The optimum model is calculated by minimizing, with respect to the model parameters, the difference between traveltimes predicted by the model and those observed in the data. Once a model has been produced, corrections that are dependent on the raypath geometry through the near‐surface layer can be calculated.The effect of the near‐surface layer is normally considered to be consistent at each shot or geophone station for all traveltimes arriving at that location (the surface‐consistent approximation). This assumption linearizes the problem, allowing timeshifts to be calculated and the traveltimes corrected to a chosen datum, representing static corrections. The single correction at each point is an averaged correction, based on an assumption that is particularly inaccurate in the presence of lateral variations of velocity or thickness of the surface layer, in the presence of large surface layer velocities or in the presence of a thick surface layer. The method presented considers the non‐linear relationship between data and model explicitly, hence the correction that is dependent on the raypath. Linearization removes this dependence and reduces the problem to a surface‐consistent approximation.The method is applied to synthetic data calculated from a model with surface layer variations. Comparisons are made between the corrected data resulting from the method described here and the conventional surface‐consistent approach. From these results it becomes apparent that the near‐surface layer inversion method presented here can reproduce accurate models and correct for near‐surface layer effects in cases where conventional methods encounter difficulties. Additionally the method can be readily extended to 3D.
Two field applications of the rotating current EM method1
Authors: Hall S.H. 
Published: April 2006

AbstractThe rotating current EM method has been applied to the delineation of two conductive orebodies, Elura near Cobar, NSW, and Thalanga near Charter's Towers, Queensland. The field data were collected in the form of observations of the vertical magnetic field strength ratio and phase difference using a Turam‐style receiver with twin vertical coils. By reconstituting this data back to the ring source field and phase, i.e. the observed Hz, phasor, it is possible to present contoured maps of the EM field. Anomaly phasors are obtained by subtracting theoretical phasors from the observed phasors in the complex plane of the Hz phasor. The theoretical phasors for the finite source are based on horizontally layered, half‐space earth models, computed at each point of the survey grids, then normalized to selected points of the observed fields. Use is made of the intrinsic circular symmetry of the method in X–Y plots of field versus source‐receiver distance to ascertain geoelectric parameters for the earth models. A steel picket fence at Thalanga is modelled by a line source grounded at each end and its Hz, phasor is removed by the same process.A considerable improvement in anomaly delineation is gained over previous Turam‐style anomalies and the two survey examples illustrate the limitations of the method in the presence of a conductive overburden (Elura) and its abilities in the absence of a conductive overburden (Thalanga).
Seismic inversion through Tabu Search1
Authors: Vinther Rikke , Mosegaard Klaus 
Published: April 2006

AbstractHighly non‐linear seismic trace inversion problems can be solved efficiently by an implementation of Tabu Search, a meta‐heuristic method related to artificial intelligence. The implementation under consideration is a deterministic, global search that combines the advantages ofa local search, giving a quick descent to local misfit minima, with an ability to cross misfit barriers in the model space. Once Tabu Search has found an area of low misfit, it performs an extensive exploration of its deepest points. This property makes it possible to use Tabu Search for a semiquantitative resolution and uncertainty analysis of the inverse problem.
Event‐oriented velocity estimation based on prestack data in time or depth domain1
Authors: Iversen Einar , Gjøystdal Håvar 
Published: April 2006

AbstractA 2D reflection tomographic method is described, for the purpose of estimating an improved macrovelocity field for prestack depth migration. An event‐oriented local approach of the ‘layer‐stripping’ type has been developed, where each input event is defined by its traveltime and a traveltime derivative, taken with respect to one of four coordinates in the source/receiver and midpoint half‐offset systems.Recent work has shown that the results of reflection tomography may be improved by performing event picking in a prestack depth domain. We adopt this approach and allow events to be picked either before or after prestack depth migration. Hence, if events have been picked in a depth domain, such as the common‐shot depth domain or the common‐offset depth domain, then a depth‐time transformation is required before velocity estimation. The event transformation may, for example, be done by conventional kinematic ray tracingr and with respect to the original depth‐migration velocity field. By this means, we expect the input events for velocity updating to become less sensitive to migration velocity errors.For the purpose of velocity estimation, events are subdivided into two categories; reference horizon events and individual events. The reference horizon events correspond to a fixed offset in order to provide basic information about reflector geometry, whereas individual events, corresponding to any offset, are supposed to provide the additional information needed for velocity estimation. An iterative updating approach is used, based on calculation of derivatives of event reflection points with respect to velocity. The event reflection points are obtained by ray‐theoretical depth conversion, and reflection‐point derivatives are calculated accurately and efficiently from information pertaining to single rays. A number of reference horizon events and a single individual event constitute the minimum information required to update the velocity locally, and the iterations proceed until the individual event reflection point is consistent with those of the reference horizon events. Normally, three to four iterations are sufficient to attain convergence. As a by‐product of the process, we obtain so‐called uncertainty amplification factors, which relate a picking error to the corresponding error in the estimated velocity or depth horizon position.The vector formulation of the updating relationship makes it applicable to smooth horizons having arbitrary dips and by applying velocity updating in combination with a flexible model‐builder, very general macro‐model structures can be obtained. As a first step in the evaluation of the new method, error‐free traveltime events were generated by applying forward ray tracing within given macrovelocity models. When using such ‘perfect’ observations, the velocity estimation algorithm gave consistent reconstructions of macro‐models containing interfaces with differential dip and curvature, a low‐velocity layer and a layer with a laterally varying velocity function.
Impact of climate change on grassland production and soil carbon worldwide
Authors: PARTON W.J. , SCURLOCK J.M.O. , OJIMA D.S. , SCHIMEL D.S. , HALL D.O. ,   SCOPEGRAM GROUP MEMBERS
Published: April 2006

AbstractThe impact of climate change and increasing atmospheric CO2 was modelled for 31 temperate and tropical grassland sites, using the CENTURY model. Climate change increased net primary production, except in cold desert steppe regions, and CO2 increased production everywhere. Climate change caused soil carbon to decrease overall, with a loss of 4 Pg from global grasslands after 50 years. Combined climate change and elevated CO2 increased production and reduced global grassland C losses to 2 Pg, with tropical savannas becoming small sinks for soil C. Detection of statistically significant change in plant production would require a 16% change in measured plant production because of high year to year variability in plant production. Most of the predicted changes in plant production are less than 10%.
Historical records of the occurrence of butterflies in Britain: examples showing associations between annual number of records and weather
Authors: POLLARD E. , MOSS D. 
Published: April 2006

AbstractHistorical records (1900–66) from the Lepidoptera Recording Scheme were used to examine associations with weather for three butterfly species Ladoga Camilla, Celastrina argiolus and Nymphalis polychloros. For each species, a series of equations was examined in which the number of records per year was considered to be a function of (1) the number of records in the previous year and (2) temperature or rainfall in the months leading up to the current flight‐period. Significant associations with weather were found for all species. Further multiple regression equations were obtained, using a stepwise procedure to select weather variables contributing separately to annual variation in the number of records. For L. Camilla and C. argiolus, predictions from these equations were significantly correlated with data on changes in abundance for the later period 1976–91. No recent data exist for JV. polychloros, which is now very rare, or possibly extinct, in Britain. Similar historic data are available for other butterfly species and for other taxa. It is suggested that these types of data have potential for the study, and perhaps prediction, of some effects of climate change.
Contrasting effects of tropospheric ozone on five native herbs which coexist in calcareous grassland
Authors: WARWICK KEVIN ROBERT , TAYLOR GAIL 
Published: April 2006

AbstractThe aim of this study was to examine the effects of increased tropospheric ozone concentrations on the growth and morphology of five native herbs commonly found to coexist in calcareous grassland in areas of Britain and continental Europe: Anthyllis vulneraria L., Cirsium acaule (L.) Scop., Festuca ovina L., Pilosella offtcinarum F. Shultz & Shultz‐Bip and Lotus comiculatus L.In a chronic fumigation (mean O3 concentration of 71 ppb (71 nl 1−1) for 7 h d−1 AOT40 4585 ppb‐h) which lasted for 21 d, the effects of ozone were assessed using classical growth analysis. Large reductions in mean relative growth rates for shoot and root weight and root length were observed for the two legumes (Fabaceae) Lotus corniculatus and Anthyllis vulneraria, although these were only statistically significant for Lotus corniculatus. Significant reductions in specific root length (length per unit dry weight) were found for Cirsium acaule and Pilosella officinarum (Asteraceae), while for Festuca ovina (Poaceae) the allometric coefficient was reduced significantly following exposure to ozone.An acute fumigation (mean O3 concentration of 196 ppb, 7 h) resulted in a range of visible injury, from no injury (Festuca ovina and Pilosella officinarum) through moderate levels of injury (Cirsium acaule and Lotus corniculatus) to extensive and widespread injury (Anthyllis vulneraria). Scoring of visible damage showed that this was only statistically significant for the two legumes, Lotus corniculatus and Anthyllis vulneraria.These results suggest that native herbs may differ in their sensitivity to tropospheric ozone. Both chronic and acute exposures revealed that members of the Fabaceae may be most sensitive to ozone pollution, but the study also suggests that subtle changes in root morphology occurred for members of the Asteraceae. These findings are discussed in relation to the critical levels of ozone set recently for plants and the implications of increasing tropospheric ozone for the conservation of native plant communities.
Estimating canopy conductance to ozone uptake from observations of evapotranspiration at the canopy scale and at the leaf scale*
Authors: MASSMAN W.J. , GRANTZ D.A. 
Published: April 2006

AbstractStomatal uptake by vegetation is often the major sink for the destruction of tropospheric ozone. Using data obtained during the summer of 1991 at a grape vineyard and a cotton field in the San Joaquin Valley of California, we compare canopy (stomatal) conductances to ozone estimated (1) from eddy covariance ozone flux data (2) from eddy covariance evapotranspiration data and (3) by scaling leaf trans pi rational conductance to the canopy level using a canopy radiative transfer model. These simultaneous data, obtained at two levels of biological organization and for two trace gases, allow us to contrast the pathways for canopy‐atmosphere exchange of water vapour and ozone, to evaluate limitations to scaling from leaf to canopy, and to predict ozone uptake parameters from those governing transpiration. At the vineyard site the eddy covariance ozone results underestimate the ET‐based (eddy covariance and leaf scaling) approaches between 25% and 36%. At the cotton site the ozone‐based results overestimate the ET‐based approaches between 9% and 62%. A number of modelling and measurement uncertainties are of appropriate magnitude to reconcile these estimates. Some of the possible causes for these discrepancies that are discussed include NO effects, mesophyll resistances to ozone uptake and flaws in the K‐theory (first‐order closure) approach on which the canopy‐scale analysis is based. Nevertheless, both canopy and single leaf measurements of conductance for water vapour provide acceptable estimates of conductance for ozone, but further experiments in which all are measured simultaneously are suggested.