bluuebunny/crossref_metadata_2025 huggingface dataset:
Geyser watching
Authors: Waltham Tony
Published: March 2003

Yellowstone is justifiably famed for its geysers, but Iceland, New Zealand and Kamchatka all have their own special geothermal features.
Blind date: The importance of chronology in reconstructing the past
Authors: Plater Andrew J.
Published: March 2003

The dating of Quaternary sedimentary records provides detailed information for understanding the response of Earth surface environments to climate change and human activity. Although its roots lie in the most fundamental of geological principles, palaeoenvironmental research is an essential component of present‐day global environmental change research.
Parsimonious Bayesian Markov chain Monte Carlo inversion in a nonlinear geophysical problem
Authors: Malinverno Alberto
Published: March 2003

A key element in the solution of a geophysical inverse problem is the quantification of non-uniqueness, that is, how much parameters of an inferred earth model can vary while fitting a set of measurements. A widely used approach is that of Bayesian inference, where Bayes' rule is used to determine the uncertainty of the earth model parameters a posteriori given the data. I describe here, a natural extension of Bayesian parameter estimation that accounts for the posterior probability of how complex an earth model is (specifically, how many layers it contains). This approach has a built-in parsimony criterion: among all earth models that fit the data, those with fewer parameters (fewer layers) have higher posterior probabilities.

To implement this approach in practice, I use a Markov chain Monte Carlo (MCMC) algorithm applied to the nonlinear problem of inverting DC resistivity sounding data to infer characteristics of a 1-D earth model. The earth model is parametrized as a layered medium, where the number of layers and their resistivities and thicknesses are poorly known a priori. The algorithm obtains a sample of layered media from the posterior distribution; this sample measures non-uniqueness in terms of how many layers are effectively resolved by the data and of the range of layer thicknesses and resistivities consistent with the data.

Because the complexity of the model is effectively determined by the data, the solution does not need to be regularized. This is a desirable feature, because requiring the solution to be smooth beyond what is implied by prior information can lead to underestimating posterior uncertainty. Letting the number of layers be a free parameter, as done here, broadens the space of earth models possible a priori and makes the determination of posterior uncertainty less dependent on the parametrization.
Automated 1D interpretation of resistivity soundings by simultaneous use of the direct and iterative methods*
Authors: Başokur Ahmet T.
Published: March 2003

Noise contamination of measured data greatly affects the final results of inversion. Three types of noise source — random and systematic errors and the uncertainties due to the inadequacy of the mathematical model in representing the actual physical conditions — are discussed in the framework of resistivity sounding data. Two methods are proposed for describing these uncertainties. The first possibility is to smooth the measured data by a combination of simple fitting functions that satisfies the ‘1D smoothness’ criteria and consequently simulates the behaviour of a 1D Schlumberger apparent resistivity curve. The second method is to derive weight coefficients from the differences between the measured and the smoothed data sets. Both methods are carried out under the control of the interpreter.

The relative merits and drawbacks of the direct and iterative interpretation methods used for the estimation of the parameters of the layered earth model are summarized. Two variants of the combination of these methods are presented to obtain more powerful and automatic interpretation schemes. In the sequential interpretation, an initial guess supplied by the direct method is improved by the iterative method to obtain a reasonable fit between the measured data and the model response. In the simultaneous interpretation, the successive application of the direct and iterative methods is carried out, starting from the first branch of the apparent resistivity curve. The operation is then shifted to subsequent branches that represent the deeper parts of the geoelectric section. This is similar to the data acquisition applied in direct current sounding in which the depth penetration is increased by expanding the current electrode spacings. The proposed sequential and simultaneous interpretation algorithms require minimum aids and efforts of the interpreter.
Weighted sum method for calculating ground force: an evaluation by using a portable vibrator system
Authors: Van Der Veen Michiel , Brouwer Jan , Helbig Klaus
Published: March 2003

Using a lightweight portable vibrator, we have evaluated the accuracy of the ‘weighted sum’ method for calculating ground force. Experiments in which the vibrator was suspended elastically have shown that, contrary to expectations based on standard theory, the amplitude of the weighted sum ground force was significantly above zero at high frequencies (> 500 Hz). Complementary investigations with load cells confirmed these results. If not accounted for, these deviations may introduce significant ‘vibroseis‐correlation noise’ in processed records. Furthermore, we have demonstrated that ground force and base‐plate velocity can be used to estimate the radiation impedance, which describes the interaction of (vibratory) sources with the ground. Using the mechanical characteristics of the system (i.e. maximum displacement, maximum velocity and maximum acceleration of the base‐plate) and the radiation impedance, the behaviour of the portable vibrator on typical Dutch soil types was evaluated. We found that for the same sweep, more high‐frequency energy could be generated on hard grounds (e.g. concrete) characterized by a higher radiation impedance than on softer grounds (e.g. clay or sand). Knowledge of this behaviour may provide important information for use in data interpretation.
Anti‐aliased Kirchhoff–Helmholtz transformations
Authors: Druzhinin Alexander
Published: March 2003

Non‐aliased integral (Kirchhoff‐type) transformations for forward and downward wavefield extrapolations in inhomogeneous media with interfaces are described. Special weights are computed to compensate for operator aliasing and finite‐aperture effects, even when the data are spatially aliased and irregularly sampled. Basic components of the algorithm, such as Green's function computation, can be replaced by alternative solutions in conjunction with ray‐tracing methods. Applications of this algorithm to model and real data in both two and three dimensions are discussed in terms of its impact on seismic modelling, multiple prediction and prestack imaging.
Stacked
Authors: Hendrickson
Published: March 2003

Analysis of prestack P‐wave seismic data yields information about both the P‐ and S wave properties of the earth. An anticipated advantage of having two measurements (P and S) is that they can be combined into a new measurement that is less sensitive to lithology variations and more sensitive to fluid effects. The amplitude‐variation‐with‐offset (AVO) gradient is one such measure that is often used qualitatively as a fluid indicator. The gradient always becomes softer (more negative) when hydrocarbon replaces brine in the pore spaces but the overall AVO response is dominated by the lithology. Fluid effects are expressed primarily by the normal‐incidence P‐wave response and only secondarily by the offset dependence. The gradient often does not function as an effective fluid indicator. This is partially due to the fact that the gradient is roughly twice as sensitive to S‐ than to P‐wave properties. More importantly, effective random noise in the CMP gathers introduces a strong correlation between the AVO intercept and gradient and, hence, between the measured P‐ and S‐wave properties. This correlation in the AVO attributes corresponds to a significant error in the estimation of the S‐wave properties and can dominate the measurements from many of the popular AVO techniques. A simple method to minimize the effect of this noise‐induced correlation is to stack the data. The stack corresponds to a coordinate rotation in elastic space with the stack amplitudes measured along one of the new axes and the other (unmeasured) axis naturally tending to line up with the noise and thus suppressing it. Fluid effects cause the data to move roughly perpendicular to this noise trend. The stack axis is then in the direction of the fluid effect. The stack thus combines both the P‐ and S‐wave (normal and oblique incidence) information into a single measurement which can be made to optimally suppress background noise and highlight fluid effects. A major consequence of this interpretation is the simplicity of both prospect identification and quantitative amplitude analysis.
Surface NMR within a geophysical study of an aquifer at Haldensleben (Germany)
Authors: Yaramanci Ugur , Lange Gerhard , Knödel Klaus
Published: March 2003

The surface nuclear magnetic resonance (SNMR) method has been tested at a site in Haldensleben, northern Germany, to assess the suitability of this new method for groundwater exploration and environmental investigations. More information is obtained by SNMR, particularly with respect to aquifer parameters, than with other geophysical techniques. SNMR measurements were carried out at three borehole locations, together with 2D and 1D direct current geoelectrics, as well as ground‐penetrating radar, and well logging (induction log, gamma‐ray log and pulsed neutron‐gamma log). Permeabilities were calculated from the grain‐size distributions of core material determined in the laboratory. It is demonstrated that the SNMR method is able to detect groundwater and the results are in good agreement with other geophysical and hydrogeological data. Using the SNMR method, the water content of the unsaturated and saturated zones (i.e. porosity of an aquifer) can be reliably determined. This information and resistivity data permit in situ determination of other aquifer parameters. Comparison of the SNMR results with borehole data clearly shows that the water content determined by SNMR is the free or mobile water in the pores. The permeabilities estimated from the SNMR decay times are similar to those derived from sieve analysis of core material. Thus, the combination of SNMR with geoelectric methods promises to be a powerful tool for studying aquifer properties.
Diffraction problems of 3D seismic imaging
Authors: Zavalishin B.R.
Published: March 2003

Migration is essential to seismic imaging. It is carried out by backward extrapolation of the wavefield registered on the observation surface. The quality of images depends on the accuracy of the wavefield reconstruction at interior subsurface points. From the theory based on the exact solution of the scalar wave equation it is known that, for accurate wave extrapolation, data must be obtained from an infinite observation surface. Limiting of migration apertures, which is inevitable in practice, leads to artefacts in extrapolated fields. The distortion they cause in 2D and 3D imaging is different. In 2D migration, the artefacts known as truncation effects are much weaker than the signals being extrapolated and for this reason attract no special attention. In 3D migration, diffractions caused by an aperture edge are stronger and may create serious problems. For a circular aperture, their amplitudes are comparable to the amplitudes of the signals themselves. The study of aperture diffractions is intended to help in the search for ways of either suppressing them efficiently or deliberately utilizing them in order to improve imaging.

In optics, diffractions by an aperture play a constructive role in image making. This research shows that the same may take place in seismic imaging.
Use of marine controlled‐source electromagnetic sounding for sub‐basalt exploration
Authors: MacGregor Lucy , Sinha Martin
Published: March 2003

The problem of imaging sedimentary structure obscured by high‐velocity layers, such as carbonate, basalt or salt, using conventional seismic techniques is well known. When this problem is encountered in offshore areas, marine electromagnetic data can provide valuable, alternative and complementary constraints on the structure. We concentrate on the use of marine controlled‐source electromagnetic (CSEM) sounding in the frequency domain. The feasibility of mapping sub‐basalt sedimentary structure using this technique is explored by means of modelling studies which mimic a type of survey which could readily be employed in practice. As a starting point the problem is addressed in terms of 1D resistivity structure. We show that sub‐basalt sediments can be detected and their depth of burial quantified to within 200 m in the examples shown. The presence of small‐scale inhomogeneities in the basalt (which cause much of the scattering in seismic data) is shown to have no appreciable effect on the ability of the CSEM data to detect the sediments. The modelling is then extended to 2.5 dimensions. Again the presence of sub‐basalt sediments can be readily detected and their properties and geometry quantified, especially if the electromagnetic data are combined with constraints from complementary geophysical techniques.
Self‐potential anomalies in Cerro de Pasco and Hualgayoc areas (Peru) revisited
Authors: Vagshal D.S. , Belyaev S.D.
Published: March 2003

Self‐potential anomalies with unexplained high values have been observed in various places. Such anomalies were reported in Peru more than 30 years ago. An explanation is proposed based on the electrokinetic effect obtained when a topographic high occurs above a rock layer through which water filtrates and which itself lies over a highly resistive layer. In such a case a fairly high self‐potential may occur. Conversely, when one of these conditions is not met, the self‐potential anomaly at the earth's surface resulting from the electrokinetic effect is strongly attenuated.
Klauder wavelet removal before vibroseis deconvolution
Authors: Robinson Enders A. , Saggaf Muhammad
Published: March 2003

The spiking deconvolution of a field seismic trace requires that the seismic wavelet on the trace be minimum phase. On a dynamite trace, the component wavelets due to the effects of recording instruments, coupling, attenuation, ghosts, reverberations and other types of multiple reflection are minimum phase. The seismic wavelet is the convolution of the component wavelets. As a result, the seismic wavelet on a dynamite trace is minimum phase and thus can be removed by spiking deconvolution. However, on a correlated vibroseis trace, the seismic wavelet is the convolution of the zero‐phase Klauder wavelet with the component minimum‐phase wavelets. Thus the seismic wavelet occurring on a correlated vibroseis trace does not meet the minimum‐phase requirement necessary for spiking deconvolution, and the final result of deconvolution is less than optimal. Over the years, this problem has been investigated and various methods of correction have been introduced. In essence, the existing methods of vibroseis deconvolution make use of a correction that converts (on the correlated trace) the Klauder wavelet into its minimum‐phase counterpart. The seismic wavelet, which is the convolution of the minimum‐phase counterpart with the component minimum‐phase wavelets, is then removed by spiking deconvolution. This means that spiking deconvolution removes both the constructed minimum‐phase Klauder counterpart and the component minimum‐phase wavelets. Here, a new method is proposed: instead of being converted to minimum phase, the Klauder wavelet is removed directly. The spiking deconvolution can then proceed unimpeded as in the case of a dynamite record. These results also hold for gap predictive deconvolution because gap deconvolution is a special case of spiking deconvolution in which the deconvolved trace is smoothed by the front part of the minimum‐phase wavelet that was removed.
2D surface topography boundary conditions in seismic wave modelling
Authors: Ruud Bent , Hestholm Stig
Published: March 2003

New formulations of boundary conditions at an arbitrary two‐dimensional (2D) free‐surface topography are derived. The top of a curved grid represents the free‐surface topography while the grid's interior represents the physical medium. The velocity–stress version of the viscoelastic wave equations is assumed to be valid in this grid. However, the rectangular grid version attained by grid transformation is used to model wave propagation in this work in order to achieve the numerical discretization. We show the detailed solution of the particle velocities at the free surface resulting from discretizing the boundary conditions by second‐order finite‐differences (FDs). The resulting system of equations is spatially unconditionally stable. The FD order is gradually increased with depth up to eighth order inside the medium. Staggered grids are used in both space and time, and the second‐order leap‐frog and Crank–Nicholson methods are used for time‐stepping. We simulate point sources at the surface of a homogeneous medium with a plane free surface containing a hill and a trench. Applying parameters representing exploration surveys, we present examples with a randomly realized surface topography generated by a 1D von Kármán function of order 1. Viscoelastic simulations are presented using this surface with a homogeneous medium and with a layered, randomized medium realization, all generating significant scattering.
Porosity and permeability prediction from wireline logs using artificial neural networks: a North Sea case study
Authors: Helle Hans B. , Bhatt Alpana , Ursin Bjørn
Published: March 2003

Estimations of porosity and permeability from well logs are important yet difficult tasks encountered in geophysical formation evaluation and reservoir engineering. Motivated by recent results of artificial neural network (ANN) modelling offshore eastern Canada, we have developed neural nets for converting well logs in the North Sea to porosity and permeability. We use two separate back‐propagation ANNs (BP‐ANNs) to model porosity and permeability. The porosity ANN is a simple three‐layer network using sonic, density and resistivity logs for input. The permeability ANN is slightly more complex with four inputs (density, gamma ray, neutron porosity and sonic) and more neurons in the hidden layer to account for the increased complexity in the relationships. The networks, initially developed for basin‐scale problems, perform sufficiently accurately to meet normal requirements in reservoir engineering when applied to Jurassic reservoirs in the Viking Graben area. The mean difference between the predicted porosity and helium porosity from core plugs is less than 0.01 fractional units. For the permeability network a mean difference of approximately 400 mD is mainly due to minor core‐log depth mismatch in the heterogeneous parts of the reservoir and lack of adequate overburden corrections to the core permeability. A major advantage is that no a priori knowledge of the rock material and pore fluids is required. Real‐time conversion based on measurements while drilling (MWD) is thus an obvious application.
Some SH‐wave seismic reflections from depths of less than three metres
Authors: Deidda Gian Piero , Ranieri Gaetano
Published: March 2003

Shallow SH‐wave reflections are far from routine, although their study can provide insights into important properties of near‐surface materials that cannot be inferred from P‐wave data alone. Difficulties in separating SH‐wave reflections from Love waves are generally considered the major obstacle to progress in shallow SH‐wave seismic reflection. This may be the case in surveys undertaken at great depths, but it is not necessarily true for reflection data gathered at shallow and ultra‐shallow depths. This paper shows that when SH‐wave data possess wavelengths greater than the thickness of the superficial layer, Love waves are not greatly dispersed. In this case, misinterpretation between parts of reflection hyperbolae and waveguide arrivals is sufficiently limited. In a one‐layer model earth, which well approximates typical situations of the near‐surface underground, the most energetic modes (the lowermost modes) of the dispersed surface waves have a dominant frequency band that falls below the wavelet spectrum of the shallow reflections; therefore, they can be filtered out in the frequency domain. Higher modes, although their spectral content overlaps that of the reflections, exhibit small amplitudes on seismograms and leave strong reflections unaffected.We present field examples from three different sites where we were able to obtain ultra‐shallow reflections (< 3 m) in unconsolidated sediments. The high level of resolution (vertical resolution up to 15 cm) suggests that SH‐wave reflection imaging has the potential to complement other high‐resolution techniques, such as P‐wave reflection and ground‐penetrating radar (GPR) imaging, allowing a better and more complete characterization of the near‐surface environments.
3D refraction statics in the wavenumber domain
Authors: Zanzi Luigi , Bellotti Sabrina , Stucchi Eusebio
Published: March 2003

The wavenumber iterative modelling (WIM) method was first introduced to estimate the static corrections for 2D land profiles by performing first‐break inversion in the wavenumber domain. The WIM algorithm presents some useful advantages, of robustness, stability and flexibility. Robustness is obtained by intensive exploitation of all the available data and by application of an automatic function for mispick removal. Stability is the result of an iterative procedure that ensures convergence towards a stable and plausible solution even at the end of the profile where the problem is normally ill‐posed. Finally, flexibility is due to the possibility of solving for multilayered structures and of estimating vertical gradients of the velocity.This work extends the WIM method to three dimensions. The extension is feasible because the three‐dimensional (3D) problem can be decomposed into a number of small independent problems, one for any pair of wavenumbers kx, ky. The extension preserves the above‐mentioned advantages. The parameters of the estimated model are affected differently by noise: the analysis of the input/output noise transfer function demonstrates that the high spatial frequencies of the velocity distributions are the components that are most affected by noise; thus, the algorithm includes a gradual damping of the higher wavenumbers of the velocity parameter. Although the WIM 3D algorithm requires a larger amount of RAM compared with other standard approaches, considerable reduction in CPU run time can be achieved as every wavenumber pair can be treated as an independent linear problem.
Second‐order interpolation of traveltimes
Authors: Vanelle Claudia , Gajewski Dirk
Published: March 2003

To carry out a 3D prestack migration of the Kirchhoff type is still a task of enormous computational effort. Its efficiency can be significantly enhanced by employing a fast traveltime interpolation algorithm. High accuracy can be achieved if second‐order spatial derivatives of traveltimes are included in order to account for the curvature of the wavefront. We suggest a hyperbolic traveltime interpolation scheme that permits the determination of the hyperbolic coefficients directly from traveltimes sampled on a coarse grid, thus reducing the requirements in data storage. This approach is closely related to the paraxial ray approximation and corresponds to an extension of the well‐known method to arbitrary heterogeneous and complex media in 3D. Application to various velocity models, including a 3D version of the Marmousi model, confirms the superiority of our method over the popular trilinear interpolation. This is especially true for regions with strong curvature of the local wavefront. In contrast to trilinear interpolation, our method also provides the possibility of interpolating source positions, and it is 5–6 times faster than the calculation of traveltime tables using a fast finite‐difference eikonal solver.
Anisotropic inversion of refracted waves in vertical cable data in the presence of dip
Authors: Wang Hejie , Li Xiang‐Yang
Published: March 2003

Refracted arrivals are analysed to estimate the near‐surface anisotropy of marine sediments using a vertical‐cable (VC) configuration. In the presence of dip, the horizontal and vertical ray‐slownesses are obtained from the observed apparent slownesses in the up‐ and downdip directions using a sum or difference at each azimuth. The multiple azimuths generated by a VC geometry permit the ray‐slowness distribution of the marine sediments to be determined.

An inversion procedure is developed to provide dip and anisotropy parameters for refractive layers from the measured refraction traveltimes in multilayered azimuthally isotropic and anisotropic media. Two sets of transversely isotropic models are used to analyse the azimuthal variations of apparent and ray slownesses. In the first set, we fix the anisotropic parameters of the models but vary the dip (0°, 5° and 10°) to test the effects of the presence of dip. In the second set, we vary the P‐wave anisotropy strength (5.2%, 10.3%, 15.8% and 22.0%) to examine the sensitivity and accuracy of ray‐slowness approximations which are independent of dip. We test this inversion procedure on synthetic P‐wave VC data calculated for six different models by a finite‐difference method. The results of applications to real VC data acquired from the North Sea are also presented.
Out‐of‐plane geometrical spreading in anisotropic media
Authors: Ettrich Norman , Sollid Anders , Ursin Bjørn
Published: March 2003

Two‐dimensional seismic processing is successful in media with little structural and velocity variation in the direction perpendicular to the plane defined by the acquisition direction and the vertical axis. If the subsurface is anisotropic, an additional limitation is that this plane is a plane of symmetry. Kinematic ray propagation can be considered as a two‐dimensional process in this type of medium. However, two‐dimensional processing in a true‐amplitude sense requires out‐of‐plane amplitude corrections in addition to compensation for in‐plane amplitude variation. We provide formulae for the out‐of‐plane geometrical spreading for P‐ and S‐waves in transversely isotropic and orthorhombic media. These are extensions of well‐known isotropic formulae.

For isotropic and transversely isotropic media, the ray propagation is independent of the azimuthal angle. The azimuthal direction is defined with respect to a possibly tilted axis of symmetry. The out‐of‐plane spreading correction can then be calculated by integrating quantities which describe in‐plane kinematics along in‐plane rays. If, in addition, the medium varies only along the vertical direction and has a vertical axis of symmetry, no ray tracing need be carried out. All quantities affecting the out‐of‐plane geometrical spreading can be derived from traveltime information available at the observation surface.

Orthorhombic media possess no rotational symmetry and the out‐of‐plane geometrical spreading includes parameters which, even in principle, are not invertible from in‐plane experiments. The exact and approximate formulae derived for P‐ and S‐waves are nevertheless useful for modelling purposes.
3D description and inversion of reflection moveout of PS‐waves in anisotropic media
Authors: Tsvankin Ilya , Grechka Vladimir
Published: March 2003

Common‐midpoint moveout of converted waves is generally asymmetric with respect to zero offset and cannot be described by the traveltime series t2(x2) conventionally used for pure modes. Here, we present concise parametric expressions for both common‐midpoint (CMP) and common‐conversion‐point (CCP) gathers of PS‐waves for arbitrary anisotropic, horizontally layered media above a plane dipping reflector. This analytic representation can be used to model 3D (multi‐azimuth) CMP gathers without time‐consuming two‐point ray tracing and to compute attributes of PS moveout such as the slope of the traveltime surface at zero offset and the coordinates of the moveout minimum.

In addition to providing an efficient tool for forward modelling, our formalism helps to carry out joint inversion of P and PS data for transverse isotropy with a vertical symmetry axis (VTI media). If the medium above the reflector is laterally homogeneous, P‐wave reflection moveout cannot constrain the depth scale of the model needed for depth migration. Extending our previous results for a single VTI layer, we show that the interval vertical velocities of the P‐ and S‐waves (VP0 and VS0) and the Thomsen parameters ε and δ can be found from surface data alone by combining P‐wave moveout with the traveltimes of the converted PS(PSV)‐wave.

If the data are acquired only on the dip line (i.e. in 2D), stable parameter estimation requires including the moveout of P‐ and PS‐waves from both a horizontal and a dipping interface. At the first stage of the velocity‐analysis procedure, we build an initial anisotropic model by applying a layer‐stripping algorithm to CMP moveout of P‐ and PS‐waves. To overcome the distorting influence of conversion‐point dispersal on CMP gathers, the interval VTI parameters are refined by collecting the PS data into CCP gathers and repeating the inversion.

For 3D surveys with a sufficiently wide range of source–receiver azimuths, it is possible to estimate all four relevant parameters (VP0, VS0, ε and δ) using reflections from a single mildly dipping interface. In this case, the P‐wave NMO ellipse determined by 3D (azimuthal) velocity analysis is combined with azimuthally dependent traveltimes of the PS‐wave. On the whole, the joint inversion of P and PS data yields a VTI model suitable for depth migration of P‐waves, as well as processing (e.g. transformation to zero offset) of converted waves.
Fast modelling of marine surface multiples
Authors: Taylor David B.
Published: March 2003

In the development and testing of water‐surface multiple‐removal algorithms, it is valuable to have accurate synthetic seismograms which exhibit multiples, for which the multiple‐free solution is known. A method is presented for constructing 2D and 3D solutions of the acoustic wave equation in water, by combining the solution from a primary source with other scaled solutions of secondary sources, which simulate diffractors. The computation involves function evaluation rather than numerical solution of differential equations and is consequently accurate and comparatively fast. The analytic formulae on which the method is based give insights into methods for multiple removal. Generalized reflection coefficients, defined on a horizontal plane above the diffractors, are derived and used to construct the integral equations which are the basis for many multiple‐removal schemes.
Wavelet correlation filter for wide‐angle seismic data
Authors: Carrozzo M.T. , De Franco R. , De Luca L. , Luzio D. , Primiceri R. , Quarta T. , Vitale M.
Published: March 2003

A new filtering technique for single‐fold wide‐angle reflection/refraction seismic data is presented. The technique is based on the wavelet decomposition of a set of adjacent traces followed by coherence analysis. The filtering procedure consists of three steps. In the first, a wavelet decomposition of traces into different detail levels is performed. In the second, the coherence attributes for each level are evaluated by calculating cross‐correlation functions of detail portions contained in a space–time moving window. Finally, the filtered traces are obtained as a weighted reconstruction of the trace details. Each weight is obtained from the coherence‐attributes distribution estimated in a proper interval. A sequence of tests is then conducted in order to select possible optimum or unsuitable wavelet bases. The efficiency of the filter proposed was assessed by calculating some properly designed parameters in order to compare it with other standard de‐noising techniques. The proposed method produced a clear signal enhancement in high‐density wide‐angle seismic data, thus proving that it is a useful processing tool for a reliable correlation of seismic phases.
Velocities of compressional and shear waves in limestones
Authors: Assefa Solomon , McCann Clive , Sothcott Jeremy
Published: March 2003

Carbonate rocks are important hydrocarbon reservoir rocks with complex textures and petrophysical properties (porosity and permeability) mainly resulting from various diagenetic processes (compaction, dissolution, precipitation, cementation, etc.). These complexities make prediction of reservoir characteristics (e.g. porosity and permeability) from their seismic properties very difficult. To explore the relationship between the seismic, petrophysical and geological properties, ultrasonic compressional‐ and shear‐wave velocity measurements were made under a simulated in situ condition of pressure (50 MPa hydrostatic effective pressure) at frequencies of approximately 0.85 MHz and 0.7 MHz, respectively, using a pulse‐echo method. The measurements were made both in vacuum‐dry and fully saturated conditions in oolitic limestones of the Great Oolite Formation of southern England. Some of the rocks were fully saturated with oil. The acoustic measurements were supplemented by porosity and permeability measurements, petrological and pore geometry studies of resin‐impregnated polished thin sections, X‐ray diffraction analyses and scanning electron microscope studies to investigate submicroscopic textures and micropores. It is shown that the compressional‐ and shear‐wave velocities (Vp and Vs, respectively) decrease with increasing porosity and that Vp decreases approximately twice as fast as Vs. The systematic differences in pore structures (e.g. the aspect ratio) of the limestones produce large residuals in the velocity versus porosity relationship. It is demonstrated that the velocity versus porosity relationship can be improved by removing the pore‐structure‐dependent variations from the residuals. The introduction of water into the pore space decreases the shear moduli of the rocks by about 2 GPa, suggesting that there exists a fluid/matrix interaction at grain contacts, which reduces the rigidity. The predicted Biot–Gassmann velocity values are greater than the measured velocity values due to the rock–fluid interaction. This is not accounted for in the Biot–Gassmann velocity models and velocity dispersion due to a local flow mechanism. The velocities predicted by the Raymer and time‐average relationships overestimated the measured velocities even more than the Biot model.
Elevated CO2reduces the nitrogen concentration of plant tissues
Authors: Cotrufo M. Francesca , Ineson Phil , Scott AndY.
Published: March 2003

We summarize the impacts of elevated CO2on the N concentration of plant tissues and present data to support the hypothesis that reductions in the quality of plant tissue commonly occur when plants are grown under elevated CO2. Synthesis of existing data showed an average 14% reduction of N concentrations in plant tissue generated under elevated CO2regimes. However, elevated CO2appeared to have different effects on the N concentrations of different plant types, as the reported reductions in N have been larger in C3 plants than in C4 plants and N2‐fixers. Under elevated CO2plants changed their allocation of N between above‐ and below‐ground components: root N concentrations were reduced by an average of 9% compared to a 14% average reduction for above‐ground tissues. Although the concentration of CO2treatments represented a significant source of variance for plant N concentration, no consistent trends were observed between them.
Soil water content and temperature as independent or confounded factors controlling soil respiration in a temperate mixed hardwood forest
Authors: Davidson EriC. A. , Belk Elizabeth , Boone Richard D.
Published: March 2003

Variation in soil temperature can account for most of the seasonal and diel variation in soil CO2 efflux, but the temperature effect is not always consistent, and other factors such as soil water content are known to influence soil respiration. The objectives of this research were to study the spatial and temporal variation in soil respiration in a temperate forested landscape and to evaluate temperature and soil water functions as predictors of soil respiration. Soil CO2 fluxes were measured with chambers throughout an annual cycle in six study areas at the Harvard Forest in central Massachusetts that include soil drainage classes from well drained to very poorly drained. The mean annual estimate of soil CO2 efflux was 7.2 Mg ha–1, but ranged from 5.3 in the swamp site to 8.5 in a well‐drained site, indicating that landscape heterogeneity is related to soil drainage class. An exponential function relating CO2 fluxes to soil temperature accounted for 80% of the seasonal variation in fluxes across all sites (Q10 = 3.9), but the Q10 ranged from 3.4 to 5.6 for the individual study sites. A significant drought in 1995 caused rapid declines in soil respiration rates in August and September in five of the six sites (a swamp site was the exception). This decline in CO2 fluxes correlated exponentially with decreasing soil matric potential, indicating a mechanistic effect of drought stress. At moderate to high water contents, however, soil water content was negatively correlated with soil temperature, which precluded distinguishing between the effects of these two confounded factors on CO2 flux. Occurrence of high Q10 values and variation in Q10 values among sites may be related to: (i) confounding effects of high soil water content; (ii) seasonal and diel patterns in root respiration and turnover of fine roots that are linked to above ground phenology and metabolism; and (iii) variation in the depth where CO2 is produced. The Q10 function can yield reasonably good predictions of annual fluxes of CO2, but it is a simplification that masks responses of root and microbial processes to variation in temperature and water content throughout the soil.
Effects of elevated CO2 on the chrysanthemum leaf‐miner, Chromatomyia syngenesiae: a greenhouse study
Authors: Smith P. H. D. , Jones T. H.
Published: March 2003

Although feeding behaviour of Chromatomyia syngenesiae on plants grown in elevated CO2 (ambient + 200ppm) was unaffected, leaf‐miner development was slower in elevated compared to ambient CO2 atmospheres. Pupal weight was lower at high CO2 and correlated with the area of leaf mined; no such correlation existed in ambient CO2. There appears to be no compensatory feeding by the leaf‐miner for the reduced food quality of plants growing in elevated CO2. The implications of these findings are discussed.
Impact of global warming on the distribution and survival of the gelada baboon: a modelling approach
Authors: Dunbar R. I. M.
Published: March 2003

The gelada baboon is a graminivorous primate whose ecology is unusually sensitive to ambient temperature. A systems model of the socio‐ecology of the gelada is used to predict the impact of global warming on the species’ altitudinal distribution. The species’ lower altitudinal limit will rise by ≈ 500 m for every 2 °C increase in global mean temperature. A 7 °C rise in temperature would be sufficient to result in the species being confined to a small number of isolated mountain peaks, where its chances of survival will be greatly reduced. Changes in local climate are also likely to have significant effects on agricultural practice on the Ethiopian highlands, and this in turn is likely to have repercussions for the distribution patterns of the gelada by further constraining the habitat available to them.
Long‐term measurements of boreal forest carbon balance reveal large temperature sensitivity
Authors: Lindroth Anders , Grelle Achim , Morén Ann‐Sofie
Published: March 2003

We present results from two years’ net ecosystem flux measurements above a boreal forest in central Sweden. Fluxes were measured with an eddy correlation system based on a sonic anemometer and a closed path CO2 and H2O gas analyser. The measurements show that the forest acted as a source during this period, and that the annual balance is highly sensitive to changes in temperature. The accumulated flux of carbon dioxide during the full two‐year period was in the range 480–1600 g CO2 m–2. The broad range is caused by uncertainty regarding assessment of the night‐time fluxes. Although annual mean temperature remained close to normal, the results are partly explained by higher than normal respiration, due to abnormal temperature distribution and reduced soil moisture during one growing season. The finding that a closed forest can be a source of carbon over such a long period as two years contrasts sharply with the common belief that forests are always carbon sinks.
Effects of elevated atmospheric CO2 on soil microbiota in calcareous grassland
Authors: Niklaus Pascal A.
Published: March 2003

Microbial responses to three years of CO2 enrichment (600 μL L–1) in the field were investigated in calcareous grassland. Microbial biomass carbon (C) and soil organic C and nitrogen (N) were not significantly influenced by elevated CO2. Microbial C:N ratios significantly decreased under elevated CO2 (– 15%, P = 0.01) and microbial N increased by + 18% (P = 0.04). Soil basal respiration was significantly increased on one out of 7 sampling dates (+ 14%, P = 0.03; December of the third year of treatment), whereas the metabolic quotient for CO2 (qCO2 = basal respiration/microbial C) did not exhibit any significant differences between CO2 treatments. Also no responses of microbial activity and biomass were found in a complementary greenhouse study where intact grassland turfs taken from the field site were factorially treated with elevated CO2 and phosphorus (P) fertilizer (1 g P m–2 y–1). Previously reported C balance calculations showed that in the ecosystem investigated growing season soil C inputs were strongly enhanced under elevated CO2. It is hypothesized that the absence of microbial responses to these enhanced soil C fluxes originated from mineral nutrient limitations of microbial processes. Laboratory incubations showed that short‐term microbial growth (one week) was strongly limited by N availability, whereas P was not limiting in this soil. The absence of large effects of elevated CO2 on microbial activity or biomass in such nutrient‐poor natural ecosystems is in marked contrast to previously published large and short‐term microbial responses to CO2 enrichment which were found in fertilized or disturbed systems. It is speculated that the absence of such responses in undisturbed natural ecosystems in which mineral nutrient cycles have equilibrated over longer periods of time is caused by mineral nutrient limitations which are ineffective in disturbed or fertilized systems and that therefore microbial responses to elevated CO2 must be studied in natural, undisturbed systems.
N‐poor ecosystems may respond more to elevated [CO2] than N‐rich ones in the long term. A model analysis of grassland.
Authors: Cannell M. G. R. , Thornley J. H. M.
Published: March 2003

The Hurley Pasture Model was used to examine the short and long‐term responses of grazed grasslands in the British uplands to a step increase from 350 to 700 μmol mol–1 CO2 concentration ([CO2]) with inputs of 5 or 100 kg N ha–1 y–1. In N‐rich grassland, [CO2] doubling quickly increased net primary productivity (NPP), total carbon (Csys) and plant biomass by about 30%. By contrast, the N‐poor grassland underwent a prolonged ‘transient’, when there was little response, but eventually NPP, Csys and plant biomass more than doubled. The ‘transient’ was due to N immobilization and severe depletion of the soil mineral N pool. The large long‐term response was due to slow N accumulation, as a result of decreased leaching, decreased gaseous N losses and increased N2‐fixation, which amplified the CO2 response much more in the N‐poor than in the N‐rich grassland. It was concluded that (i) ecosystems use extra carbon fixed at high [CO2] to acquire and retain nutrients, supporting the contention of Gifford et al. (1996), (ii) in the long term, and perhaps on the real timescale of increasing [CO2], the response (in NPP, Csys and plant biomass) of nutrient‐poor ecosystems may be proportionately greater than that of nutrient‐rich ones, (iii) short‐term experiments on nutrient‐poor ecosystems may observe only the transient responses, (iv) the speed of ecosystem responses may be limited by the rate of nutrient accumulation rather than by internal rate constants, and (v) ecosystem models must represent processes affecting nutrient acquisition and retention to be able to simulate likely real‐world CO2 responses.
Do slow‐growing species and nutrient‐stressed plants respond relatively strongly to elevated CO2?
Authors: POORTER HENDRIK
Published: March 2003

Mainly based on a simulation model,Lloyd & Farquhar (1996;Functional Ecology, 10, 4–32) predict that inherently slow‐growing species and nutrient‐stressed plants show a relatively strong growth response to an increased atmospheric CO2concentration. Compiling published experiments, I conclude that these predictions are not supported by the available data. On average, inherently fast‐growing species are stimulated proportionately more in biomass than slow‐growing species and plants grown at a high nutrient supply respond more strongly than nutrient‐stressed plants.
Solar UV‐B and visible radiation in tropical forest gaps: measurements partitioning direct and diffuse radiation
Authors: FLINT STEPHAN D. , CALDWELL MARTYN M.
Published: March 2003

Solar ultraviolet‐B (UV‐B) radiation penetrates plant canopies to a different degree than photosynthetically active radiation (PAR) because UV‐B is diffused to a greater degree by the atmosphere. We measured both global (total) and diffuse solar radiation in canopy gaps of a semideciduous tropical forest in Panama. Measurements were simultaneously made in the UV‐B and PAR wavebands. Compared to unobstructed measurements taken outside the forest, the sunlit portions of gaps were depleted in the proportion of UV‐B relative to PAR, especially at midday. Shaded areas, in contrast, were always richer in UV‐B relative to PAR, but the magnitude of the change varied greatly. Presumably this variation was due to the differences in the directional nature of diffuse solar UV‐B radiation as compared to diffuse PAR. Measurements in the gaps showed substantial reductions in the proportion of radiation in the diffuse components of both the UV‐B and PAR wavebands. However, because of the greater proportion of UV‐B which is diffuse, it tended to predominate in shaded areas. Similar patterns were seen in measurements taken at temperate latitudes. Response of shade‐ and gap‐dwelling plants to these high UV‐B:PAR ratios has received little attention.