bluuebunny/crossref_metadata_2025_split huggingface dataset:
Determination of a shallow velocity–depth model from seismic refraction data by coherence inversion1
Authors: Landa Evgeny , Keydar Shemer , Kravtcov Alex 
Published: April 2006

AbstractSeismic refractions have different applications in seismic prospecting. The travel‐ times of refracted waves can be observed as first breaks on shot records and used for field static calculation. A new method for constructing a near‐surface model from refraction events is described. It does not require event picking on prestack records and is not based on any approximation of arrival times. It consists of the maximization of the semblance coherence measure computed using shot gathers in a time window along refraction traveltimes. Time curves are generated by ray tracing through the model. The initial model for the inversion was constructed by the intercept‐time method. Apparent velocities and intercept times were taken from a refraction stacked section. Such a section can be obtained by appling linea moveout corrections to common‐shot records. The technique is tested successfully on synthetic and real data. An important application of the proposed method for solving the statics problem is demonstrated.
Effective filtering of artifacts for implicit finite‐difference paraxial wave equation migration1
Authors: Bunks C. 
Published: April 2006

AbstractImplicit finite‐difference implementations of the paraxial wave equation are widely used in industrial prestack and post‐stack migration programs for imaging and velocity analysis. This type of implementation gives rise to numerical artifacts which, in general, do not degrade image quality but which do impede effective velocity analysis. This paper reviews the artifacts generated by the paraxial approximation and a post‐extrapolation, spatially varying filtering scheme is described which completely eliminates these artifacts. The method is illustrated with numerous examples.
The effects of source and receiver motion on seismic data1
Authors: Hampson Gary , Jakubowicz Helmut 
Published: April 2006

AbstractThe effects of source and receiver motion on seismic data are considered using extensions of the standard convolutional model. In particular, receiver motion introduces a time‐variant spatial shift into data, while source motion converts the effect of the source signature from a single‐channel convolution in time to a multichannel convolution in time and space. These results are consistent with classical Doppler theory and suggest that Doppler shifting can introduce distortions into seismic data even at relatively slow acquisition speeds. It is shown that, while both source and receiver motion are known to be important for marine vibroseis acquisition, receiver motion alone can produce significant artifacts in marine 3D data. Fortunately, the convolutional nature of the distortions renders them amenable to correction using simple deconvolution techniques. Specifically, the effects of receiver motion can be neutralized by applying an appropriate reverse time‐variant spatial shift, while those due to source motion can be addressed by introducing time‐variant spatial shifts both before and after standard, deterministic, signature deconvolution or correlation.
Impact of processing on the amplitude versus offset response of a marine seismic data set1
Authors: Mazzotti A. , Ravagnan G. 
Published: April 2006

AbstractThe analysis of prestack reflections may have a high diagnostic potential in the evaluation of the petrophysical characteristics of subsurface targets. However, the recovery of reliable seismic responses, especially when examining amplitude versus offset (AVO) variations, is of the utmost importance and is strictly dependent on the acquisition and processing steps.In order to evaluate the impact of different processing sequences, we examine the AVO responses of three seismic events from a marine data set. Borehole data indicate that these events are related to a lignitic sand, a gas sand and a cineritic bed. The AVO analysis is focused mainly on the reflections from the gas sand. In particular, we compare the results of a standard processing sequence with results from a surface‐consistent approach and with results from a processing sequence tailored to this specific case. A decreasing AVO trend of the gas‐sand reflection results from the analyses of data that have undergone the standard and the surface‐consistent processing sequences. This contrasts with both theory and borehole information, which both predict an energy increase with offset. A detailed study shows that the receiver array attenuation, neglected in previous Studies, plays a major role in attenuating the far‐offset reflections. Other propagation factors, such as offset‐dependent geometrical spreading and Q absorption, produce only minor effects.Taking into account the above factors, we apply a third processing sequence whose impact on the AVO trend is evaluated step by step and whose results are compared with the previously applied sequences. This new sequence leads to better agreement between the AVOs predicted from borehole data and those measured on surface seismic data.
Shallow karst exploration using MT‐VLF and DC resistivity methods1
Authors: Guerin Roger , Benderitter Yves 
Published: April 2006

AbstractA geophysical test was carried out over a well‐located and fairly embedded karstic conduit. The MT‐VLF method was selected because of its high resolution and its ability to provide a resistivity parameter sensitive to water and Clay. This method was used together with DC resistivity methods which allow vertical adjustment of the VLF data and show consistency between the investigation and target depths. After correcting the deformations due to the polarization of the primary field, the MT‐VLF data show clearly, in the Central part of the site, that the conduit does not coincide with an anomaly axis but coincides with the boundary between a conductive area and a resistive area. 2D modelling confirms that direct detection of the conduit is not feasible and that the conduit is located close to a conductive zone corresponding to a completely Clay‐filled fractured zone. This situation was observed on the whole site and the conduit seemed systematically to thread its way between the conductive zones to join the outlet. The distribution of the conductive fractured zones and the direction of the hydraulic gradient were two important elements in predicting the location of the conduits. A 3D approach would increase the probability of finding the conduits in such a case.
Phase‐shift migration with a variable‐length spatial transform–an algorithm for moderately varying lateral velocities1
Authors: Benson Alvin K. 
Published: April 2006

AbstractA migration algorithm appropriate for moderately varying lateral velocity changes is developed as an extension of phase‐shift migration by using a variable‐length spatial transform. This process significantly reduces the number of lateral wave‐numbers necessary to downward continue the data, and it replaces the spatial FFT with a simple recursion relationship. For a given frequency and position x, ten lateral wavenumbers are typically sufficient, and the migration algorithm produces accurate images when the velocity structure V (X, z) changes over a few depth intervals of thickness Δz, with lateral velocity gradients up to 1.4 to 1.0.
Reflection points and surface points1
Authors: Levin Franklyn K. 
Published: April 2006

AbstractA simple expression ties the midpoint of a surface spread to reflection points on a dipping plane. If we use two coordinate systems, an unprimed one with a z‐axis perpendicular to the surface and a primed one with a z‐axis perpendicular to the reflector, we have
image

where θ is the dip angle, φ is the profile angle, X is the source‐to‐receiver separation, and D is the depth of the reflector. The reflection point is (x, yp, D) and the surface midpoint is (xc, yc, 0).Using the expression, I show that if complete azimuthal coverage is required at a CMP position, the reflection points lie on an ellipse. Similarly, a fixed reflection point generates a circle of surface midpoints. A circle of CMP positions for fixed θ and φ becomes an ellipse of reflection points and a circle of reflection points becomes an ellipse of midpoints. A user can easily find the shape and location of the reflection area generated by a surface aperture.
Fourier finite‐difference migration for steeply dipping reflectors with complex overburden1
Authors: Rühl T. , Kopp C. , Ristow D. 
Published: April 2006

AbstractThe improvement in accuracy and efficiency of wave‐equation migration techniques is an ongoing topic of research. The main problem is the correct imaging of steeply dipping reflectors in media with strong lateral velocity variations. We propose an improved migration method which is based on cascading phase‐shift and finite‐difference operators for downward continuation. Due to these cascaded operators we call this method‘Fourier finite‐difference migration’(FFD migration).In our approach we try to generalize and improve the split‐step Fourier migration method for strong lateral velocity variations using an additional finite‐difference correction term. Like most of the current migration methods in use today, our method is based on the one‐way wave equation. It is solved by first applying the square‐root operator but using a constant velocity at each depth step which has to be the minimum velocity. In a second step, the approximate difference between the correct square‐root operator and this constant‐velocity squareroot operator (the error made in the first step) is implemented as an implicit FD migration scheme, part of which is the split‐step Fourier correction term.Some practical aspects of the new FFD method are discussed. Its performance is compared with that of split‐step and standard FD migration schemes. First applications to synthetic and real data sets are presented. They show that the superiority of FFD migration becomes evident by migrating steeply dipping reflectors with complex overburden having strong lateral velocity variations. If velocity is laterally constant, FFD migration has the accuracy of the phase‐shift method. The maximum migration angle is velocity adaptive, in contrast to conventional FD migration schemes. It varies laterally depending on the local level of velocity variation. FFD migration is more efficient than higher‐order implicit FD schemes. These schemes use two cascaded downward‐continuation steps in order to attain comparable migration performance.
Detection of underground cavities with monofrequency electromagnetic tomography between boreholes in the frequency range 100 MHz to 1 GHz1
Authors: Cĉte Philippe , Degauque Pierre , Lagabrielle Richard , Levent Nathalie 
Published: April 2006

AbstractIn order to detect underground cavities, we have designed a geophysical method based on the interaction of a monochromatic electromagnetic wave in the frequency band 100 MHz to 1 GHz with the ground situated between two boreholes.Three different approaches are involved in the design of this EM tomographic method.

The application of an ‘exact’ theory is used to calculate artificial data, based on an integral equation method. These data are inverted using a tomographic algorithm whose basic assumptions are rather coarse. The results show that, however, cavities can very well be detected and their position recovered.
Data are obtained with a physical reduced‐scale model in a water tank, in which all parameters are known in advance. The inversion process confirms the validity of the method.
Underground cavities are actually detected.

The above approaches are described and discussed and results are shown. The equipment involved and its operation is quite simple: the surface devices are commercially available and only the transmitter and receiver antennae must be specially built. The method is shown to be quite efficient and its cost should be reasonably low.
An integral equation for the homogeneous Neumann condition1
Authors: Furness Peter 
Published: April 2006

AbstractModelling the theoretical response of several important geophysical systems involves the solution of Poisson's equation with homogeneous Neumann boundary conditions (i.e. a zero normal gradient) imposed over either open or closed surfaces.A simple integral equation solution to this problem is derived from first principles. It is applicable to both types of surface and in this respect represents an improvement on existing integral equation techniques. However, the present surface integral equation displays a strong singularity of order 1/R3 which requires an appropriate interpretation for its implementation.A comparison of some numerical results with analytical data taken from the literature demonstrates that the proposed integral equation technique is suitably robust, accurate and efficient for practical application in geophysical interpretation.
A note on the computation of the offset distance XY in the generalized reciprocal method of seismic refraction interpretation1
Authors: Wright C. 
Published: April 2006

AbstractA simple numerical procedure is described for measuring the distance XY in the generalized reciprocal method when there are significant measurement errors in the refracted arrival times. It is applicable when the XY value is of similar magnitude to or less than the receiver spacing. Such conditions frequently occur in using the reciprocal or generalized reciprocal methods to estimate static corrections from first‐break times measured in multifold seismic reflection profiling. The use of the method is illustrated with data from both deep and high‐resolution seismic reflection profiles.
Rapid least‐squares inversion of apparent resistivity pseudosections by a quasi‐Newton method1
Authors: Loke M.H. , Barker R.D. 
Published: April 2006

AbstractA fast inversion technique for the interpretation of data from resistivity tomography surveys has been developed for operation on a microcomputer. This technique is based on the smoothness‐constrained least‐squares method and it produces a two‐dimensional subsurface model from the apparent resistivity pseudosection. In the first iteration, a homogeneous earth model is used as the starting model for which the apparent resistivity partial derivative values can be calculated analytically. For subsequent iterations, a quasi‐Newton method is used to estimate the partial derivatives which reduces the computer time and memory space required by about eight and twelve times, respectively, compared to the conventional least‐squares method. Tests with a variety of computer models and data from field surveys show that this technique is insensitive to random noise and converges rapidly. This technique takes about one minute to invert a single data set on an 80486DX microcomputer.
Evaluation of transport and storage properties in the soil and groundwater zone from induced polarization measurements1
Authors: Börner F.D. , Schopper J.R. , Weller A. 
Published: April 2006

AbstractSpectral induced polarization as well as complex electrical measurements are used to estimate, on a non‐invasive basis, hydraulic permeability in aquifers. Basic laboratory measurements on a variety of shaly sands, silts and clays showed that the main feature of their conductivity spectra in the frequency range from 10‐3 to 103 Hertz is a nearly constant phase angle. Thus, a constant‐phase‐angle model of electrical conductivity is applied to interpret quantitatively surface and borehole spectral induced polarization measurements. The model allows for the calculation of two independent electrical parameters from only one frequency scan and a simple separation of electrical volume and interface effects. The proposed interpretation algorithm yields the true formation factor, the cation exchange capacity and the surface‐area‐to‐porosity ratio, which corresponds to the inverse hydraulic radius. Using a Kozeny–Carman‐like equation, the estimation of hydraulic permeability is possible.
A physical model for shear‐wave velocity prediction1
Authors: Xu Shiyu , White Roy E. 
Published: April 2006

AbstractThe clay‐sand mixture model of Xu and White is shown to simulate observed relationships between S‐wave velocity (or transit time), porosity and clay content. In general, neither S‐wave velocity nor S‐wave transit time is a linear function of porosity and clay content. For practical purposes, clay content is approximated by shale volume in well‐log applications. In principle, the model can predict S‐wave velocity from lithology and any pair of P‐wave velocity, porosity and shale volume. Although the predictions should be the same if all measurements are error free, comparison of predictions with laboratory and logging measurements show that predictions using P‐wave velocity are the most reliable. The robust relationship between S‐ and P‐wave velocities is due to the fact that both are similarly affected by porosity, clay content and lithology. Moreover, errors in the measured P‐wave velocity are normally smaller than those in porosity and shale volume, both of which are subject to errors introduced by imperfect models and imperfect parameters when estimated from logs.Because the model evaluates the bulk and shear moduli of the dry rock frame by a combination of Kuster and Toksöz’ theory and differential effective medium theory, using pore aspect ratios to characterize the compliances of the sand and clay components, the relationship between P‐ and S‐wave velocities is explicit and consistent. Consequently the model sidesteps problems and assumptions that arise from the lack of knowledge of these moduli when applying Gassmann's theory to this relationship, making it a very flexible tool for investigating how the vP‐vs relationship is affected by lithology, porosity, clay content and water saturation. Numerical results from the model are confirmed by laboratory and logging data and demonstrate, for example, how the presence of gas has a more pronounced effect on P‐wave velocity in shaly sands than in less compliant cleaner sandstones.
Variability in temperature regulation of CO2 fluxes and N mineralization from five Hawaiian soils: implications for a changing climate
Authors: HOLLAND ELISABETH A. , TOWNSEND ALAN R. , VITOUSEK PETER M. 
Published: April 2006

AbstractWe examined the possibility that microbial adaptation to temperature could affect rates of CO2, N2O and CH4 release from soils. Laboratory incubations were used to determine the functional relationship between temperature and CO2, N2O and CH4 fluxes for five soils collected across an elevational range in Hawaii. Initial rates of CO2 production and net N mineralization increased exponentially from 15 °C to 55 °C; initial rates of CH4 and N2O release were more complex. No optimum temperature (in which rates decline at higher and lower temperatures) was apparent for any of the gases, but respiration declined with time at higher temperatures, suggesting rapid depletion of readily available substrate. Mean Q10S for respiration varied from 1.4 to 2.0, a typical range for tropical soils. The functional relationship between CO2 production and temperature was consistent among all five soils, despite the substantial differences in mean annual temperature, soils, and land‐use among the sites. Temperature responses of N2O and CH4 fluxes did not follow simple Q10 relationships suggesting that temperature functions developed for CO2 release from heterotrophic respiration cannot be simply extrapolated. Expanding this study to tropical heterotrophic respiration, the flux is more sensitive to changes in Q10 than to changes in temperature on a per unit basis: the partial derivative with respect to temperature is 2.4 Gt C ·° C−1 with respect to Q10, it is 3.5 Gt C · Q10 unit−1. Therefore, what appears to be minor variability might still produce substantial uncertainty in regional estimates of gas exchange.
Elevated concentrations of CO2 may double methane emissions from mires
Authors: HUTCHIN PAUL R. , PRESS MALCOLM C. , LEE JOHN A. , ASHENDEN TREVOR W. 
Published: April 2006

AbstractThe potential impact of an increase in methane emissions from natural wetlands on climate change models could be very large. We report a profound increase in methane emissions from cores of mire peat and vegetation as a direct result of increasing the CO2 concentration from 355 to 550 μol mol−1 (a 60% increase). Increased CH4 fluxes were observed throughout the four month period of study. Seasonal variation in CH4 flux, consistent with that seen in the field, was observed under both ambient and elevated CO2. Under ambient CO2, methane fluxes rose from 0.02 μol m‐2 s−1 in May to 0.11 μol m−2 s−3 in July before declining again in August. Under elevated CO2 methane fluxes were at least 100% greater throughout the experiment, rising from 0.05 μol m‐2 s−1 in May to a peak of 0.27 μol m−2 s−1 in July. The stimulation of CO4 emissions was accompanied by a 100% increase in rates of photosynthesis from 4.6 (± 0.3) under ambient CO2 to 9.3 (± 0.7) μol m−2 s−1. Root and shoot biomass were unaffected.
Effects of elevated CO2 and/or O3 on growth, development and physiology of wheat (Triticum aestivum L.)
Authors: BARNES JEREMY D. , OLLERENSHAW JOHN H. , WHITFIELD CLARE P. 
Published: April 2006

AbstractTwo cultivars of spring wheat (Triticum aestivum L. cvs. Alexandria and Hanno) and three cultivars of winter wheat (cvs. Riband, Mercia and Haven) were grown at two concentrations of CO2 [ambient (355 pmol mol−1) and elevated (708 μmol mol−1)] under two O3 regimes [clean air (< 5 nmol mol−1 O3) and polluted air (15 nmol mol−1 O3 at night rising to a midday maximum of 75 nmol mol−1)] in a phytotron at the University of Newcastle‐upon‐Tyne. Between the two‐leaf stage and anthesis, measurements of leaf gas‐exchange, non‐structural carbohydrate content, visible O3 damage, growth, dry matter partitioning, yield components and root development were made in order to examine responses to elevated CO2 and/or O3.Growth at elevated CO2 resulted in a sustained increase in the rate of CO2 assimilation, but after roughly 6 weeks' exposure there was evidence of a slight decline in the photosynthetic rate (c.‐15%) measured under growth conditions which was most pronounced in the winter cultivars. Enhanced rates of CO2 assimilation were accompanied by a decrease in stomatal conductance which improved the instantaneous water use efficiency of individual leaves. CO2 enrichment stimulated shoot and root growth to an equivalent extent, and increased tillering and yield components, however, non‐structural carbohydrates still accumulated in source leaves. In contrast, long‐term exposure to O3 resulted in a decreased CO2 assimilation rate (c. ‐13%), partial stomatal closure, and the accumulation of fructan and starch in leaves in the light. These effects were manifested in decreased rates of shoot and root growth, with root growth more severely affected than shoot growth.In the combined treatment growth of O3‐treated plants was enhanced by elevated CO2, but there was little evidence that CO2 enrichment afforded additional protection against O3 damage. The reduction in growth induced by O3 at elevated CO2 was similar to that induced by O3 at ambient CO2 despite additive effects of the individual gases on stomatal conductance that would be expected to reduce the O3 flux by 20%, and also CO2‐induced increases in the provision of substrates for detoxification and repair processes. These observations suggest that CO2 enrichment may render plants more susceptible to O3 damage at the cellular level. Possible mechanisms are discussed.