bluuebunny/crossref_metadata_2025_split huggingface dataset:
The problem of velocity inversion in refraction seismics: some observations from modelling results
Authors: Krishna  , Rao  , Sarkar  
Published: March 2003

The applicability of seismic refraction profiling for the detection of velocity inversion, which is also known as a low‐velocity layer (LVL), is investigated with the aid of synthetic seismogram computations for a range of models. Our computational models focus on the inherent ambiguities in the interpretation of first‐arrival time delays or ‘skips’ in terms of LVL model parameters. The present modelling results reveal that neither the measure nor even the existence of a shadow zone and/or a time shift (skip) in first arrivals is necessarily indicative of an LVL. Besides attenuation effects, the cap‐layer velocity gradient is a critical parameter, determining the termination point of the cap‐layer diving wave and thus the time skip.We suggest that shallow LVLs can be delineated more reliably by traveltime and amplitude modelling of coherent phases reflected from their top and bottom boundaries, often clearly observed in the pre‐ and near‐critical ranges in seismogram sections of refraction profiling experiments with a close receiver spacing. We demonstrate the applicability of this approach for a field data set of a refraction profile in the West Bengal Basin, India. The inferred LVL corresponds to the Gondwana sediments underlying the higher‐velocity layer of the Rajmahal Traps. This interpretation is consistent with the data from a nearby well in the region.
Mise‐à‐la‐masse interpretation using a perfect conductor in a piecewise uniform earth
Authors: Furness Peter 
Published: March 2003

A useful analysis of the mise‐à‐la‐masse problem can be made by considering a perfectly conducting orebody in a piecewise uniform conducting earth. While the use of a perfect conductor is clearly an idealization of the true geological conditions it provides several advantages for the present purpose.The electric field associated with the above model can be expressed in terms of a surface integral of the normal potential gradient over the boundary of the conductor, where the normal gradient satisfies a well‐posed Fredholm integral equation of the first kind. This integral equation formulation remains unchanged when the conductor is arbitrarily located in the conducting earth, including the important case when it crosses surfaces of conductivity discontinuity. Moreover, it is readily specialized to the important case of a thin, perfectly conductive lamina.Consideration of the boundary value problem relevant to a conductive body fed by a stationary current source suggests that under certain circumstances, equivalent mise‐à‐la‐masse responses will result from any perfect conductor confined by the equipotential surfaces of the original problem. This type of equivalence can only be reduced by extending the potential measurements into or on to the conductor itself.This ambiguity in the interpretation of mise‐à‐la‐masse surveys suggests a simple if approximate integral solution to the mise‐à‐la‐masse problem. The solution is suitable for modelling the responses of perfect conductors and could possibly be used as the basis of a direct inversion scheme for mise‐à‐la‐masse data.
Source/receiver array directivity effects on marine seismic attenuation measurements
Authors: Hustedt Bernhard , Clark Roger A. 
Published: March 2003

Attenuation of seismic waves, quantified by the seismic quality factor Q, holds important information for seismic interpretation, due to its sensitivity to rock and fluid properties. A recently published study of Q, based on surface seismic reflection data, used a modified spectral ratio approach (QVO), but both source and receiver responses were treated as isotropic, based on simple raypath arguments. Here, this assumption has been tested by computing apparent attenuation generated by frequency‐dependent directivity of typical marine source and receiver arrays and acquisition geometries. Synthetic wavelet spectra were computed for reflected rays, summed over the first Fresnel zone, from the base of a single interval, 50–3000 m thick and velocity 2000 m/s, overlying a 2200 m/s half‐space, and for offsets of 71–2071 m. The source and receiver geometry were those of an actual survey.The modelled spectra are clearly affected by directivity, most strongly because of surface ghosts. In general, the strong high‐frequency component, produced by the array design, leads to apparently negative attenuation in individual reflection events, though this is dependent on offset and target depth. For shallow targets (less than 400–500 ms two‐way traveltime (TWT) depth), apparent Q‐values as extreme as −50 to −100 were obtained. For deeper target depths, the directivity effect is far smaller. The implications of the model study were tested on real data. QVO was applied to 20 true‐spectrum‐processed CMPs, in a shallow (405–730 ms TWT) and a deeper (1000–1300 ms TWT) interval, firstly using a measured far‐field source signature (effectively isotropic), and secondly using computed directivity effects instead. Mean interval Q−1‐values for the deeper interval, 0.029 ± 0.011 and 0.027 ± 0.018 for conventional and directional processing, respectively, suggested no directivity influence on attenuation estimation. For the shallow interval (despite poor spectral signal‐to‐noise ratios and hence scattered attenuation estimates), directional processing removed directivity‐generated irregularities from the spectral ratios, resulting in an improvement from Q−1int = −0.036 ± 0.130 to a realistic Q−1int = 0.012 ± 0.030: different at 94% confidence level. Equivalent Q‐values are: for the deeper interval, 35 and 37 for conventional and directional processing, respectively, and −28 and 86 for the shallow interval.These results support the conclusions of the model studies, i.e. that source/receiver directivity has a negligible effect except for shallow targets (e.g. TWT depth ≤ 500 ms) imaged with conventional acquisition geometry. In such cases directivity corrections to spectra are strongly recommended.
Green's function interpolations for prestack imaging
Authors: Mendes Manuela 
Published: March 2003

A new interpolation method is presented to estimate the Green's function values, taking into account the migration/inversion accuracy requirements and the trade‐off between resolution and computing costs. The fundamental tool used for this technique is the Dix hyperbolic equation (DHE). The procedure, when applied to evaluate the Green's function for a real source position, uses the DHE to derive the root‐mean‐square velocity, vRMS, from the precomputed traveltimes for the nearest virtual sources, and by linear interpolation generates vRMS for the real source. Then, by applying the DHE again, the required traveltimes and geometrical spreading can be estimated.The inversion of synthetic data demonstrates that the new interpolation yields excellent results which give a better qualitative and quantitative resolution of the imaging sections, compared with those carried out by conventional linear interpolation. Furthermore, the application to synthetic and real data demonstrates the ability of the technique to interpolate Green's functions from widely spaced virtual sources. Thus the proposed interpolation, besides improving the imaging results, also reduces the overall CPU time and the hard disk space required, hence decreasing the computational effort of the imaging algorithms.
A generalized Biot–Gassmann model for the acoustic properties of shaley sandstones1
Authors: Carcione José M. , Gurevich Boris , Cavallini Fabio 
Published: March 2003

We obtain the wave velocities of clay‐bearing sandstones as a function of clay content, porosity and frequency. Unlike previous theories, based simply on slowness and/or moduli averaging or two‐phase models, we use a Biot‐type three‐phase theory that considers the existence of two solids (sand grains and clay particles) and a fluid. The theory, which is consistent with the critical porosity concept, uses three free parameters that determine the dependence of the dry‐rock moduli of the sand and clay matrices as a function of porosity and clay content.Testing of the model with laboratory data shows good agreement between predictions and measurements. In addition to a rock physics model that can be useful for petrophysical interpretation of wave velocities obtained from well logs and surface seismic data, the model provides the differential equation for computing synthetic seismograms in inhomogeneous media, from the seismic to the ultrasonic frequency bands.
Automatic cross‐well tomography: an application of the differential semblance optimization to two real examples
Authors: Plessix René‐Edouard 
Published: March 2003

An automatic tomography algorithm, based on differential semblance optimization (DSO), has been used to invert real cross‐well seismic data for the background velocity. The method relies on the first‐arrival transmitted waves. Given a background velocity model, the traveltimes between the sources and the receivers are computed, then semblance panels are created by back‐propagating the data traces. If the velocity model is correct all the first‐arrival transmitted waves will be aligned in the semblance panels. The DSO method consists of finding the background velocity by minimizing the L2‐norm of the difference between adjacent back‐propagated traces. Thanks to the good behaviour of this DSO cost function about the solution, a local (gradient) optimization can be performed. This provides a relatively fast algorithm when ray tracing and analytic computation of the gradient are used.Unfortunately the method fails in the presence of caustics in the data. However, this difficulty can be circumvented by applying suitable masks to the data. This approach is first applied to a synthetic example then to two real data sets: the McElroy data set recorded in West Texas and the NIMR data set recorded in Oman. The results are quite encouraging and similar to those obtained with classical tomography.
The Buffon's needle problem and the design of a geophysical survey
Authors: Sambuelli Luigi , Strobbia Claudio 
Published: March 2003

A logical procedure for designing a geophysical survey when sampling an area with a regular grid can be summarized as follows: model the expected anomaly, estimate the expected noise level, estimate the area of the anomaly above the noise level, choose the spacing, in both the x‐ and y‐directions, of the measurement grid. This last step can be approached according to two main strategies: either when applying the sampling theorem to the shortest dimension of the anomaly or when using a coarser grid, leaving a more complete definition of the anomaly to a later fitting. When following this second option, it can be constructive to estimate the probability of intercepting a given anomaly with a specific segment of profile and a given profile spacing. This latter procedure is analysed by considering a rectangle approximating the plane projection of the anomaly shape and taking into account various ratios between the grid spacing and the rectangle sides. The formulae for estimating the probability of intersecting a given anomaly with a given segment of a given profile spacing are calculated. To demonstrate the accuracy of the results, a Monte‐Carlo simulation on a synthetic magnetic map was performed, obtaining, for different ratios between the sides, segment length and profile interval, an agreement better than 0.1% with the analytical formulae.
Effects of an anisotropic overburden on azimuthal amplitude analysis in horizontal transverse isotropic media
Authors: Maultzsch Sonja , Horne Steve , Archer Stewart , Burkhardt Hans 
Published: March 2003

Azimuthal amplitude variation in fractured media, commonly used to characterize fracture systems, is a function not only of reflection at the target but also of transmission through the overburden. This study investigates the sensitivity of amplitudes to various anisotropic overburden effects in horizontal transverse isotropic (HTI) media. Issues considered here are geometric spreading, transmission coefficients and attenuation due to fluid flow. Their influence on the azimuthal amplitude variation is evaluated quantitatively over a wide model space.Only the variation of transmission coefficients with azimuth proves to be negligible. Geometric spreading alters the amplitude signature significantly over a relatively narrow range of models, and its influence increases with layer thickness. The most severe effect of an anisotropic fractured overburden is attenuation due to fluid flow between the cracks or the cracks and pores in the surrounding matrix. The relative changes in amplitudes between the symmetry directions due to anisotropic absorption are of the same order of magnitude as the changes in the reflection coefficient. The effect is significant over a very wide range of petrophysical parameters. Thus it leads to considerable problems in the amplitude analysis for almost any case of an overburden that contains cracks and pores. A correct amplitude analysis at the target will not be possible unless the effect of attenuation is removed.
Interannual and interdecadal variation in California Current zooplankton: Calanus in the late 1950s and early 1990s
Authors: Mullin Michael M. 
Published: March 2003

AbstractSamples from the southern California sector of the California Current System were examined to test for interannual changes in winter–spring abundance of the planktonic copepod, Calanus pacificus, coincident with the 1992–93 and 1958–59 El Niños, each evaluated relative to immediately preceding years, and for interdecadal change (the early 1990s relative to the late 1950s). Calanus was anomalously rare in both of the El Niño periods, as was macrozooplankton, but (unlike macrozooplanktonic biomass) was not rarer in the early 1990s than in the late 1950s. The El Niño anomalies in Calanus’s abundance and macrozooplanktonic biomass were not spatially correlated on the mesoscale, implying that different proximate ecological causes may dominate at this scale.
Organic matter, heterotrophic activity, and NO· consumption in soils
Authors: Dunfield Peter F. , Knowles Roger 
Published: March 2003

AbstractWhen the effect of water content was minimized, soil CO2 evolution and soil organic matter content were good predictors of aerobic NO. uptake rate constants across a wide range of soil types. Field manure application to a Gleysol stimulated NO. uptake rate constants and lowered NO. compensation points compared to unfertilized or NH4NO3‐fertilized soil. This effect lasted for months after manure application. In a laboratory experiment, addition of manure reduced the NO. efflux associated with nitrification of NH4 Cl fertilizer, and manured soils had a greater capacity to remove NO. from polluted air. Evidence is presented that these observations result from NO. oxidation during heterotrophic microbial activity in soil.
Influence of O2availability on NO and N2O release by nitrification and denitrification in soils
Authors: Bollmann Annette , Conrad Ralf 
Published: March 2003

AbstractThe availability of O2is believed to be one of the main factors regulating nitrification and denitrification and the release of NO and N2O. The availability of O2in soil is controlled by the O2partial pressure in the gas phase and by the moisture content in the soil. Therefore, we investigated the influence of O2partial pressures and soil moisture contents on the NO and N2O release in a sandy and a loamy silt and differentiated between nitrification and denitrification by selective inhibition of nitrification with 10 Pa acetylene. At 60% whc (maximum water holding capacity) NO and N2O release by denitrification increased with decreasing O2partial pressure and reached a maximum under anoxic conditions. Under anoxic conditions NO and N2O were only released by denitrification. NO and N2O release by nitrification also increased with decreasing O2partial pressure, but reached a maximum at 0.1–0.5% O2and then decreased again. Nitrification was the main source of NO and N2O at O2partial pressures higher than 0.1–0.5% O2. At lower O2partial pressures denitrification was the main source of NO and N2O. With decreasing O2partial pressure N2O release increased more than NO release, indicating that the N2O release was more sensitive against O2than the NO release. At ambient O2partial pressure (20.5% O2) NO and N2O release by denitrification increased with increasing soil moisture content. The maximum NO and N2O release was observed at soil moisture contents of 65–80% whc and 100% whc, respectively. NO and N2O release by nitrification also increased with increasing soil moisture content with a maximum at 45–55% whc and 90% whc, respectively. Nitrification was the main source of NO and N2O at soil moisture contents lower than 90% whc and 80% whc, respectively. Higher soil moisture contents favoured NO and N2O release by denitrification. Soil texture had also an effect on the release of NO and N2O. The coarse‐textured sandy silt released more NO than N2O compared with the fine‐textured loamy silt. At high soil moisture contents (80–100% whc) the fine‐textured soil showed a higher N2O release by denitrification than the coarse‐textured soil. We assume that the fine‐textured soil became anoxic at a lower soil moisture content than the coarse‐textured soil. In conclusion, the effects of O2partial pressure, soil moisture and soil texture were consistent with the theory that denitrification increasingly contributes to the release of NO and in particular N2O when conditions for soil microorganisms become increasingly anoxic.
Soil biota and global change at the ecosystem level: describing soil biota in mathematical models
Authors: SMITH PETE. , ANDRÉN OLOF. , BRUSSAARD LIJBERT , DANGERFIELD MARK. , EKSCHMITT KLEMENS , LAVELLE PATRICK , TATE KEVIN 
Published: March 2003

AbstractAll current mathematical models of the soil system are underpinned by a wealth of research into soil biology and new research continues to improve the description of the real world by mathematical models. In this review we examine the various approaches for describing soil biology in mathematical models and discuss the use of each type of model in global change research. The approaches represented among models participating in the Global Change and Terrestrial Ecosystems (GCTE) Soil Organic Matter Network (SOMNET) are described. We examine the relative advantages and constraints of each modelling approach and, using these, suggest appropriate uses of each. We show that for predictive purposes at ecosystem scale and higher, process‐orientated models (which have only an implicit description of soil organisms) are most commonly used. As a research tool at the ecosystem level, both process‐orientated and organism‐orientated models (in which functional or taxonomic groups of soil organisms are explicitly described) are commonly used. Because of uncertainties introduced in internal model parameter estimation and system feedbacks, the predictive use of organism‐orientated models at the ecosystem scale and larger is currently less feasible than is the use of process‐orientated models. In some specific circumstances, however, an explicit description of some functional groups of soil organisms within models may be required to adequately describe the effects of global change. No existing models can adequately predict the feedback between global change, a change in soil community function, and the response of the changed system to future global change. To find out if these feedbacks exist and to what extent they affect future global change, more research is urgently required into the response of soil community function to global change and its potential ecosystem‐level effects.
Elevated CO2 and temperature impacts on different components of soil CO2 efflux in Douglas‐fir terracosms
Authors: Lin Guanghui , Ehleringer James R. , Rygiewicz PauL. T. , Johnson Mark G. , Tingey David T. 
Published: March 2003

AbstractAlthough numerous studies indicate that increasing atmospheric CO2 or temperature stimulate soil CO2 efflux, few data are available on the responses of three major components of soil respiration [i.e. rhizosphere respiration (root and root exudates), litter decomposition, and oxidation of soil organic matter] to different CO2 and temperature conditions. In this study, we applied a dual stable isotope approach to investigate the impact of elevated CO2 and elevated temperature on these components of soil CO2 efflux in Douglas‐fir terracosms. We measured both soil CO2 efflux rates and the 13C and 18O isotopic compositions of soil CO2 efflux in 12 sun‐lit and environmentally controlled terracosms with 4‐year‐old Douglas fir seedlings and reconstructed forest soils under two CO2 concentrations (ambient and 200 ppmv above ambient) and two air temperature regimes (ambient and 4 °C above ambient). The stable isotope data were used to estimate the relative contributions of different components to the overall soil CO2 efflux. In most cases, litter decomposition was the dominant component of soil CO2 efflux in this system, followed by rhizosphere respiration and soil organic matter oxidation. Both elevated atmospheric CO2 concentration and elevated temperature stimulated rhizosphere respiration and litter decomposition. The oxidation of soil organic matter was stimulated only by increasing temperature. Release of newly fixed carbon as root respiration was the most responsive to elevated CO2, while soil organic matter decomposition was most responsive to increasing temperature. Although some assumptions associated with this new method need to be further validated, application of this dual‐isotope approach can provide new insights into the responses of soil carbon dynamics in forest ecosystems to future climate changes.
The effects of chamber pressurization on soil‐surface CO2 flux and the implications for NEE measurements under elevated CO2
Authors: Lund C. P. , Riley W. J. , Pierce L. L. , Field C. B. 
Published: March 2003

AbstractSoil and ecosystem trace gas fluxes are commonly measured using the dynamic chamber technique. Although the chamber pressure anomalies associated with this method are known to be a source of error, their effects have not been fully characterized. In this study, we use results from soil gas‐exchange experiments and a soil CO2 transport model to characterize the effects of chamber pressure on soil CO2 efflux in an annual California grassland. For greater than ambient chamber pressures, experimental data show that soil‐surface CO2 flux decreases as a nonlinear function of increasing chamber pressure; this decrease is larger for drier soils. In dry soil, a gauge pressure of 0.5 Pa reduced the measured soil CO2 efflux by roughly 70% relative to the control measurement at ambient pressure. Results from the soil CO2 transport model show that pressurizing the flux chamber above ambient pressure effectively flushes CO2 from the soil by generating a downward flow of air through the soil air‐filled pore space. This advective flow of air reduces the CO2 concentration gradient across the soil–atmosphere interface, resulting in a smaller diffusive flux into the chamber head space. Simulations also show that the reduction in diffusive flux is a function of chamber pressure, soil moisture, soil texture, the depth distribution of soil CO2 generation, and chamber diameter. These results highlight the need for caution in the interpretation of dynamic chamber trace gas flux measurements. A portion of the frequently observed increase in net ecosystem carbon uptake under elevated CO2 may be an artifact resulting from the impact of chamber pressurization on soil CO2 efflux.