bluuebunny/crossref_metadata_2025_split huggingface dataset:
Compound events decomposition and the interaction between AVO and velocity information1
Authors: Spagnolini Umberto 
Published: April 2006

AbstractMinimization of seismic residuals does not guarantee uniqueness of the model, and this implies ambiguities in the inversion. Amplitude vs. offset (AVO) inversion does not lead to a unique solution of single elastic interface parameters unless converted and S‐wave or critical angle reflections are available. Given the ambiguity of AVO inversion, this paper discusses the interaction between AVO and velocity estimation. The number of independent parameters necessary to describe an isolated reflection with AVO behaviour and residual velocity error is determined. Statistical analysis allows the establishment of an approximate equivalence of the effects of AVO and slight velocity variations; this equivalence cannot be solved without geologicala prioriinformation (kinematic equivalence). The data are then decomposed into compound events (i.e. sequences ofNinterfaces that follow each other at a fixed time lag). The decomposition is obtained by extrapolating the results of the analysis from narrowband to wideband data. Compound events decomposition demonstrates that AVO inversion is ambiguous, not only in the physical parameter space (P‐ and S‐wave velocities, and density) but also kinematically. As an example of compound event decomposition, a medium is derived. This medium is geologically implausible but is kinematically equivalent.
Experimental and finite difference modelling of borehole Mach waves1
Authors: Cheng Ningya , Zhu Zhenya , Cheng C.H. , Toksöz M.N. 
Published: April 2006

AbstractA series of model experiments was performed in an ultrasonic laboratory to study the radiation of downhole sources in a variety of formations. Three models were used in the experiments. They were a Lucite model, a Lucite model with a free glass pipe in the centre, and a glass‐cased soil model. In addition, a finite‐difference modelling technique was used to simulate the wave propagation in these models and the results of the laboratory and numerical experiments are compared. In the Lucite borehole model the waveforms recorded in the experiment agree very well with the finite‐difference synthetics. The snapshots of the wavefield from the finite‐difference simulation show the radiation pattern of the P‐ and S‐waves in the Lucite formation. These patterns are consistent with the theoretical calculations. In the Lucite model with the free glass pipe, the finite‐difference synthetics are also in good agreement with the experimental observations, especially for the conical P‐wave arrival. The angle between the wavefront of the conical P‐wave and the borehole axis, observed from the snapshot, agrees with the theory. In the cased soil model, the arrival time of the finite‐difference synthetics is in good agreement with the laboratory measurements. The relative amplitudes of the P‐wave and the Mach wave are not correctly modelled because intrinsic attenuation is not included in the finite‐difference calculation. The Mach cone angle from the snapshot agrees with the theoretical prediction. Finally, a finite‐difference method was used to simulate Mach‐wave propagation in a formation with two horizontal layers. In the case of two slow layers, the Mach‐wave generated in the first layer is reflected back from and transmitted through the boundary and another Mach wave is generated at the second layer when the Stoneley wave travels into the second layer. In the case of a formation having one slow and one fast layer, the Mach wave generated in the slow layer is reflected back at the boundary and leaked into the fast layer. There is no Mach wave in the fast layer.
Upward continuation of potential fields from a polyhedral surface1
Authors: Ivan M. 
Published: April 2006

AbstractAn equivalent source procedure is derived for upward continuation of unevenly spaced gravity and magnetic data. The dipole layer is placed on a topographic relief approximated by a polyhedral surface, the stations being the vertices of the triangular faces. The dipoles have linear magnitudes, being directed along the normal vector over each triangle. The unknown values of the dipole magnitudes at each station are obtained by a suitable modification of the usual integral equation considering the discontinuity of the normal vector at each vertex of the dipole surface. Profile data processing is also studied. A numerical test outlines the accuracy and the limitations of the model for the case of a magnetic field significantly perturbed by a rough topographic relief.
Inversion of reflection seismograms by differential semblance analysis: algorithm structure and synthetic examples1
Authors: Symes William W. , Kern Michel 
Published: April 2006

AbstractSeismograms predicted from acoustic or elastic earth models depend very non‐linearly on the long wavelength components of velocity. This sensitive dependence demands the use of special variational principles in waveform‐based inversion algorithms. The differential semblance variational principle is well‐suited to velocity inversion by gradient methods, since its objective function is smooth and convex over a large range of velocity models. An extension of the adjoint state technique yields an accurate estimate of the differential semblance gradient. Non‐linear conjugate gradient iteration is quite successful in locating the global differential semblance minimum, which is near the ordinary least‐squares global minimum when coherent data noise is small. Several examples, based on the 2D primaries‐only acoustic model, illustrate features of the method and its performance.
Radar experiments in isotropic and anisotropic geological formations (granite and schists)1
Authors: Tillard Sylvie 
Published: April 2006

AbstractIn order to understand various aspects of radar wave propagation, a survey of electromagnetic wave behaviour relative to the geological characteristics of the formations prospected was undertaken. The sites chosen for the tests were a granite quarry and an underground schist working. By investigating an electrically resistive isotropic site and a conductive anisotropic site, it was demonstrated that non‐conventional use of a radar system (antennae raised, various orientations of the transmitter/receiver, etc.) could improve data quality, and could allow information other than reflector depth to be collected (volume scattering intensity, isotropy, etc.). By studying wave propagation velocities, we underlined the difficulties encountered in establishing a velocity versus depth law, despite recourse to seismic data processing, such as NMO corrections. The results of field experiments, complemented by laboratory measurements of dielectric permittivities, clearly showed anisotropy effects: in the case of a path that is perpendicular to the schistosity plane, an electromagnetic wave propagates more slowly and is more attenuated than a wave parallel to the schistosity plane.
Line integrals of potential field data1
Authors: Ivan M. 
Published: April 2006

AbstractInformation on the mass and the spatial location of an arbitrary source body can be obtained by performing suitable integrations of 3D gravity and magnetic data along an infinite straight line. No assumptions on the density/magnetization distribution or the shape and location of the source are required.For an oblique borehole, a relationship between the lower limit of the source mass and the distance to the body is obtained. The mass contrast and the magnetic moment of the source can also be estimated.For a vertical borehole, both gravity and vertical magnetic component anomalies have equal areas to the left and right of the depth axis.The particular case of a horizontal gallery not intersecting the body is also studied. If the source is intersected, a lower limit is estimated for the maximum thickness of the body along the gallery. Information on the vertical coordinate of the centre of mass of the source can also be obtained.Numerical tests with synthetic gravity data support the theoretical results.
Location of secondary faults from cross‐correlation of the second vertical derivative of gravity anomalies1
Authors: Zeng Hualin , Zhang Qinghe , Liu Jun 
Published: April 2006

AbstractA method is described to locate secondary faults, which can be difficult to identify on the Bouguer gravity map. The method is based on cross‐correlation between the theoretical anomaly due to a vertical step and the second vertical derivative of the Bouguer anomaly. Faults are located from the closed maxima and minima on the cross‐correlation contour map calculated for two perpendicular directions. One‐dimensional model computations show that the magnitude of the extremum of the cross‐correlation is related to the depth to the top of the hanging wall and the throw of the fault. Application of the method to the Bouguer gravity map of the former mouth of the Yellow River in the Shengli Oilfield area near the Bo Hai Sea shows the effectiveness of the method.
Synthetic seismograms from generalized ray tracing1
Authors: Hanyga Andrzej , Helle Hans B. 
Published: April 2006

AbstractA new method of numerical computation of elastic wavefields in regions containing caustics is tested. The method is an extension of the asymptotic ray theory (ART). The essential features of the method consist of the application of expressions which are well defined at caustics and expressed in terms of ray tracing combined with complex ray tracing in caustic shadows. The method and an outline of the underlying theory are briefly presented, followed by a comparison with finite differences on a test model involving a caustic cusp. The comparison reveals the unexpectedly high degree of accuracy of the new method.
Bootstrap statistics for velocity tomography: application of a new information criterion1
Authors: Nishizawa Osamu , Nero Harufumi 
Published: April 2006

AbstractA new information criterion, the extended information criterion (EIC) was applied in order to determine an optimum solution in simultaneous iterative reconstruction technique (SIRT) P‐wave velocity tomography. The EIC is derived from information theory and statistics, and it measures the goodness‐of‐fit between the true (unknown) data distribution and the observed data distribution: the former gives the probability of data realization from the true (unknown) model, whereas the latter gives a probability of data realization calculated from a particular model of which parameters are estimated. The EIC is calculated using bootstrap statistics, a numerical technique for calculating statistical estimators. Bootstrap statistics enables us to obtain the bias between the log likelihood and the expected log likelihood, and then to obtain the expected log likelihood from the log likelihood. Since the EIC is obtained numerically, we can use it for most problems of model parameter estimation without employing the maximum likelihood method. Taking weak anisotropy into account, we reconstructed the P‐wave velocity structure of a rock sample during water infiltration under differential stress loading conditions. The results indicate that we can remove unrealistic solutions sometimes encountered when too many iterations are made. In spite of much computation time, the EIC is a promising technique for the near future, prompted by the rapid progress in current computer technology.
Restoration of missing offsets by parabolic Radon transform1
Authors: Kabir M.M. Nurul , Verschuur D.J. 
Published: April 2006

AbstractRestoration of missing offsets and trace interpolation is an interesting and important problem in seismic data processing. Based on the parabolic Radon transform, a method is presented for missing offset restoration, resampling and regularization of prestack individual CMP gathers. The method is also valid for resampling spatially aliased seismic data.The method is based on the parabolic assumption of the seismic events which is generally verified after a partial NMO correction in the CMP organization of the data. The essence of the method consists of a band‐limited forward parabolic Radon transform of the data containing zero traces at the missing offset locations. The curvature range is chosen to map properly the coherent energy while the zero traces map beyond that range. After inverse transform the originally zero traces are partly filled with information. Several iterations of forward and inverse transform, every time replacing the zero traces in the original gather with the partially reconstructed ones, almost fully restore the zero traces.Efficient and fast algorithms can be built up to process data having a uniform geometry. Examples on synthetic as well as on field data demonstrate clearly the robustness of the method.
Anisotropic velocity analysis1
Authors: Sayers C.M. 
Published: April 2006

AbstractResults from walkaway VSP and shale laboratory experiments show that shale anisotropy can be significantly anelliptic. Heterogeneity and anellipticity both lead to non‐hyperbolic moveout curves and the resulting ambiguity in velocity analysis is investigated for the case of a factorizable anisotropic medium with a linear dependence of velocity on depth. More information can be obtained if there are several reflectors. The method of Dellinger et al. for anisotropic velocity analysis in layered transversely isotropic media is examined and is shown to be restricted to media having relatively small anellipticity. A new scheme, based on an expansion of the inverse‐squared group velocity in spherical harmonics, is presented. This scheme can be used for larger anellipticity, and is applicable for horizontal layers having monoclinic symmetry with the symmetry plane parallel to the layers. The method is applied to invert the results of anisotropic ray tracing on a model Sand/shale sequence. For transversely isotropic media with small anisotropy, the scheme reduces to the method of Byun et al. and Byun and Corrigan. The expansion in spherical harmonics allows the P‐phase slowness surface of each layer to be determined in analytic form from the layer parameters obtained by inversion without the need to assume that the anisotropy is weak.
A new method of quantitative interpretation of SP anomalies produced by a polarized inclined sheet1
Authors: Skianis George , Papadopoulos Taxiarchis , Vaiopoulos Dimitrios , Nikolaou Sotiris 
Published: April 2006

AbstractA new method is proposed for the quantitative interpretation of SP field data produced by a polarized ore body and simulated by an inclined sheet. The theoretical concept is based on the study of the amplitude Spectrum. It is shown that the SP amplitude Spectrum is not continuous at zero frequency; this leads to the dip angle determination of the inclined sheet. It is also shown that the SP amplitude spectrum is practically nullified at a characteristic amortization frequency that depends on the depth of the polarized body. The maximum amplitude Spectrum value of the SP gradient is used to estimate the depth to the bottom of the polarized body. Thus, the geometrical parameters h and H, the depths to the top and bottom, respectively, as well as the dip angle of the inclined sheet, can be satisfactorily determined. Some problems may arise in the determination of these parameters, affecting their accuracy, whenever unwanted frequency noise is present.
On the application of geophysics in the exploration for copper and chrome ores in Albania1
Authors: Frasheri Alfred , Lubonja Ligor , Alikaj Perparim 
Published: April 2006

AbstractSome generalized results of geophysical exploration for copper sulphide and chromite ores in Albania are presented. The most important geophysical methods used are electrical prospecting, gravity, magnetics and electromagnetics. Physical properties of the ores, genesis and geological problems to be solved have determined the proper choice of any of these methods in the complex exploration.
Conventional and modern seismic investigations for rock quality determination at a dam site—a case history1
Authors: Louis J. , Papadopoulos T. , Drakatos G. , Pantzartzis P. 
Published: April 2006

AbstractRefraction seismics and modern tomographic surveys were employed at the Platanovrissi dam site on the Nestos river in northern Greece in an effort to investigate the dynamic elastic properties and rockmass condition of gneiss‐schist outcropping in the area under investigation. Six seismic refraction traverses were run on the ground surface, inside the exploratory tunnels and between tunnels and the ground surface. The results provided information about the low‐velocity surface layer parameters and the stress relief zones on the walls of all the tunnels in which seismic tomography tests were performed. Ultrasonic tests, conducted both in situ and in the laboratory, enabled the determination of the ratio Vp/Vs and the Poisson's ratio calculation. From the tomography data analysis and the geological interpretation of the results, three main velocity zones, ranging between 3500 and 4000, 3000 and 3500 and less than 3000m/s, were detected in each tunnel. High‐velocity zones were found to be compatible with the hard rockmass of gneiss‐schist material, while moderate velocities are closely related to jointed and fractured zones. Finally, low‐velocity zones were well correlated with weathered gneiss‐schist material.
Finite‐difference elastic wave propagation in 2D heterogeneous transversely isotropic media1
Authors: Juhlin Christopher 
Published: April 2006

AbstractThe velocity‐stress formulation for propagation of elastic seismic waves through 2D heterogeneous transversely isotropic media of arbitrary orientation is presented. The equations are recast into a finite‐difference scheme and solved numerically using fourth‐order spatial operators and a second‐order temporal operator on a staggered grid. Absorbing, free‐surface and symmetry boundary conditions have been implemented. Test cases compare well with other published solutions. Synthetic seismograms are calculated over two idealized models: (i) vertical fractures in granite with a dolerite sill reflector and (ii) a dipping anisotropic shale. Comparisons with the isotropic counterparts show significant differences which may have to be accounted for in seismic processing in the future.
Computation of apparent resistivity profiles from VLF‐EM data using linear filtering1
Authors: Chouteau Michel , Zhang Ping , Chapellier Dominique 
Published: April 2006

AbstractA simple filter is developed which transforms VLF‐EM real magnetic field transfer functions into apparent resistivities. It is based on the relationship between the horizontal derivative of the surface electric field and the vertical magnetic field at the surface of a two‐dimensional earth model. The performance of this simple autoregressive filter is tested for modelled and real survey data. The technique yields profiles of apparent resistivity very similar, both in magnitude and in wavelength, to those which would have been obtained using VLF‐EM resistivity measurements or d.c. resistivity profiling. This low‐pass filter has the advantage of reducing high‐wavenumber noise in the data; therefore only the major features of the VLF‐EM profile are displayed.
Extreme climate and variation in tree growth: individualistic response in subalpine fir (Abies lasiocarpa)
Authors: ETTL GREGORY J. , PETERSON DAVID L. 
Published: April 2006

AbstractDendroecological techniques were used to describe the variation in growth response of subalpine fir (Abies lasiocarpa)to climate across a range of elevations (1350–1850 m) and annual precipitation (125–350 cm y−1) in the Olympic Mountains, Washington. Correlation analysis is used to describe individual growth‐climate relationships. Growth response is quantified in years with unusually warmer, colder, wetter, and drier climates during the period 1895–1990. Combinations of climatic variables that result in unusually fast or slow growth years are also described. Differences in growth‐climate relationships among sites, and among individuals from the same site, emphasize within‐species variability in response to climate. Growth was not significantly faster or slower on the majority of sites for extreme climate years examined. Few climate variables are correlated with growth of the majority of individuals on most sites, suggesting that some individuals are relatively unresponsive to climate. Individual growth‐climate correlations also indicate an increase in the percentage of individuals whose growth is significantly correlated with a climate variable, as the value of the mean site growth correlation increases for that climate variable. Individual differences in growth‐climate relationships probably result from microsite variation (soil depth, soil moisture, wind, insolation) and from individual genetic differences. Descriptions of tree species response to climate change need to incorporate both individual and site variation in growth response to climate in order to accurately represent existing environmental heterogeneity.
Simple carbon assimilation response functions from atmospheric CO2, and daily temperature and shortwave radiation
Authors: WILKS D.S. , WOLFE D.W. , RIHA S.J. 
Published: April 2006

AbstractA global ‘CO2 fertilizer effect’ multiplier is often used in crop or ecosystem models because of its simplicity. However, this approach does not take into account the interaction between CO2, temperature and light on assimilation. This omission can lead to significant under‐ or overestimation of the magnitude of beneficial effects from elevated CO2, depending on environmental conditions. We use a mechanistic model of the biochemistry of photosynthesis to represent the response of net assimilation to different levels of CO2, temperature and radiation, on the daily time scale. Instantaneous assimilation rates for an idealized canopy model are integrated through diurnal cycles of environmental variables derived from historical climate data at three locations in North America. The calculated CO2 fertilizer effect is greatest at high light and warm temperatures. The results are summarized by assimilation response surfaces specified by the CO2 concentration, the canopy leaf area index, and by daily values of temperature and radiation available from climatic records. These summary functions are suitable for incorporation into crop or ecosystem models for predicting carbon assimilation or biomass production on a daily time step. An example application of the function reveals that for a relatively cool, high latitude location, the beneficial effects from a CO2 doubling would be negligible during the early spring, even assuming a + 4°C global warming scenario. In contrast, the beneficial effects from increasing CO2 at a relatively warm, lower latitude location are greatest in the spring, but decline in late summer because of excessively warm temperatures with a + 4°C global warming.