bluuebunny/crossref_metadata_2025_split huggingface dataset:
RADON TRANSFORM APPLICATION TO THE IMPROVED GRIDDING OF AIRBORNE GEOPHYSICAL SURVEY DATA1
Authors: YUNXUAN ZHOU 
Published: April 2006

AbstractThe Radon transform is applied to airborne geophysical data, which consist of parallel profiles, analogous to a seismic record. The plane‐wave decomposition (PWD) thus becomes the strike‐direction decomposition (SDD) since the observed spatially distributed information is represented by its strike directions in a domain achieved by the transformation. It is important that, after the SDD, we can identify anomalies and work on them according to their strikes. In particular, for gridding purposes, we may guide the second interpolation of the bi‐directional gridding approach along the strike directions.In principle, the proposed Radon transform gridding method (RTGM) transforms the observed parallel profiles into a domain where information is mapped as its strike‐direction ‘traces’ against its wavelengths. The number of strike directions into which the data are decomposed is equal to the number of lines to be interpolated. The Fourier spectrum of the grid is reconstructed from the strike‐wavenumber domain by using the projection‐slice theorem and the final square grid is obtained by performing an inverse Fourier transformation on the spectrum.The SDD is restricted to the Nyquist wavenumber bandwidth imposed by the survey line‐spacing, so that there is no addition of ambiguous short wavelengths in the gridded data. A tapering window is employed to prevent any Gibb's oscillation in the final grid because of the sharp Nyquist cut‐off in the reconstructed spectrum due to the survey line‐spacing.The RTGM is first tested on a set of synthetic line‐based data. It is also applied to aeromagnetic profile data from northern Botswana as a practical example.
RESIDUAL STATICS ESTIMATION BY STACK‐POWER MAXIMIZATION IN THE FREQUENCY DOMAIN1
Authors: NØRMARK E. 
Published: April 2006

AbstractTraditionally, residual static corrections are based on timeshifts estimated for individual CMP sorted traces, which are later resolved into surface‐consistent statics. This is a stable and attractive procedure because the data flow is simple and the memory storage required is limited. An alternative station‐oriented method maximizing the stack‐power estimates surface‐consistent static corrections directly. The statics evaluation in this method involves several CMP gathers, which should improve the prediction of statics on noise‐contaminated data. In this paper the performance of the above methods will be compared using synthetic as well as real seismic data. Neither method is capable of estimating large statics compared to the dominating period, because local optimization might fail. Global Monte Carlo search by, for instance, simulated annealing has been used to overcome the cycle‐skipping problems when proper field statics are missing. Although this procedure is computationally very heavy, it may be the only way to deal with large residual statics. In order to enlarge the operational field for local optimization, it is suggested that the stack‐power in the frequency domain is maximized. This makes it easy to change the frequency band during the optimization. Making use of the frequency domain will also normally be faster than the traditional time‐domain optimization even for a limited number of iterations. Moreover, the main memory storage required can be significantly reduced, since it is only necessary to keep the frequency band in the memory, where the signal‐to‐noise ratio is good.
MIGRATION TO ZERO OFFSET (DMO) FOR A CONSTANT VELOCITY GRADIENT: AN ANALYTICAL FORMULATION1
Authors: DIETRICH MICHEL , COHEN JACK K. 
Published: April 2006

AbstractMigration to zero offset (MZO) is a prestack partial migration process that transforms finite‐offset seismic data into a close approximation to zero‐offset data, regardless of the reflector dips that are present in the data. MZO is an important step in the standard processing sequence of seismic data, but is usually restricted to constant velocity media. Thus, most MZO algorithms are unable to correct for the reflection point dispersal caused by ray bending in inhomogeneous media.We present an analytical formulation of the MZO operator for the simple possible variation of velocity within the earth, i.e. a constant gradient in the vertical direction. The derivation of the MZO operator is carried out in two steps. We first derive the equation of the constant traveltime surface for linear V(z) velocity functions and show that the isochron can be represented by a fourth‐degree polynomial in x, y and z. This surface reduces to the well‐known ellipsoid in the constant‐velocity case, and to the spherical wavefront obtained by Slotnick in the coincident source‐receiver case.We then derive the kinematic and dynamic zero‐offset corrections in parametric form by using the equation of the isochron. The weighting factors are obtained in the high‐frequency limit by means of a simple geometric spreading correction. Our analytical results show that the MZO operator is a multivalued, saddle‐shaped operator with marked dip moveout effects in the cross‐line direction. However, the amplitude analysis and the distribution of dips along the MZO impulse response show that the most important contributions of the MZO operator are concentrated in a narrow zone along the in‐line direction. In practice, MZO processing requires approximately the same trace spacing in the in‐line and cross‐line directions to avoid spatial aliasing effects.
ON THE CONSTRUCTION OF THE 3D BAND‐LIMITED EXTRAPOLATION OPERATOR IN THE SPACE‐FREQUENCY DOMAIN1
Authors: MAELAND EINAR 
Published: April 2006

AbstractBased on an expansion of the band‐limited 3D extrapolation operator in terms of orthogonal Chebyshev polynomials, a closed form expression of the space‐frequency response is presented. A key step is an evaluation of the (inverse) 2D Fourier transform of circularly symmetric functions, which is related to the (zero‐order) Hankel transform. Hankel transforms of individual members of the orthogonal set of polynomials are available from tables and summation of series; hence, the real and the imaginary parts of the space‐frequency response can be found in terms of cylindrical and spherical Bessel functions, respectively. The procedure permits an efficient and accurate evaluation of the space‐frequency response.
SIMULTANEOUS VIBROSEIS RECORDING1
Authors: MARTIN J. E. 
Published: April 2006

AbstractAn experiment was undertaken at BP's Fulbeck Geophysical test site to compare the viability of various simultaneous vibroseis recording techniques, which are often recommended as a means of improving data acquisition production rates for 3D seismic surveys. Of particular interest were: (a) the ability to separate the signals from each source during processing, (b) the generation and suppression of harmonics and (c) the effects of any source interaction.Two vibrators were deployed with a baseplate separation of 10 m, about a borehole containing a vertical array of geophones. Our analysis concentrated on the groundforce signals measured at each vibrator and the far‐field signatures measured using a vertical geo‐phone at a depth of 204 m.By comparing single vibrator records with similar but separated records from a simultaneous recording sequence, signal separability, harmonic suppression and vibrator interaction could be fully studied.Separated far‐field signatures from simultaneous vibroseis methods using combinations of up and downsweeps exhibited unsuppressed harmonics and substantial energy from the undesired source which leaked through the correlation process. The ‘up/down’ method was capable of separating the signal from each source by only 12.7 dB, and is therefore unsuitable as a field technique.The variphase simultaneous vibroseis methods studied afforded some harmonic suppression and gave signal separations of about 30.0 dB. Use of variphase simultaneous vibroseis methods will compromise the quality of the data recorded, when compared with single‐source acquisition methods.None of the simultaneous vibroseis methods tested provided adequate signal separation and, therefore, cannot be recommended as data acquisition techniques. The ‘alternate sweeping’ method coupled with multispread recording will give the desired improvement in data acquisition rates, while preserving the necessary quality of our seismic data.
Tectonic modelling in the Bjørnøya West Basin of the Western Barents Sea1
Authors: Liu Guojiang 
Published: April 2006

AbstractThe Bjøirnøya West Basin lies between latitudes 73° and 74°, longitudes 16°E and 18°E, contains at least 8 km of sediments deposited from the Late Jurassic, and is of considerable interest for hydrocarbon exploration. The Cenozoic extensional tectonics in the basin can be clearly seen from seismic data with normal faulting and from subsidence curves with rapid subsidence. The extension occurred during the Late Palaeocene with active extension lasting about 6 million years (m.y.) followed by thermal cooling. The tectonic subsidence within the study area shows a three‐phase development: phase 1, synrift (58–52 Ma (million years before the present day)), is characterized by rapid subsidence; phase 2, postrift (52–5 Ma), by slow subsidence with occasional uplift; and phase 3 (5–0 Ma), by rapid subsidence. An adaptive finite‐element model, with consideration of the radiogenic heat production in the lithosphere, has been used to model the subsidence and heat flow. The modelling of subsidence shows the β‐factor distribution varying from 1.9 to 3.5 with an average of 2.4 for the uniform lithospheric extension. The heat‐flow modelling predicts a rapid increase of heat flow during the Early Palaeocene. The maximum heat flow at about 52 Ma, which could be as much as 3.0 hfu (10−6 cal/cm2/s), was followed by a decrease in heat flow. A plate‐weakening model has been proposed to explain the rapid subsidence for the last 5 m.y. by flexure of the elastic lithosphere which is weakened by a decrease in elastic thickness caused by an increase of the temperature gradient in the lithosphere. The plate‐weakening model predicts a heat‐flow increase at 5 Ma of up to 2.0 hfu. Our study, using quantitative modelling of the tectonic subsidence, provides a partial (if not a full) understanding of the tectonic development and thermal evolution of the Bjønøya West Basin.
Pore pressure profiles in fractured and compliant rocks1
Authors: Yilmaz Özdoǧan , Nolen‐Hoeksema Richard C. , Nur Amos 
Published: April 2006

AbstractFluid permeability in fractured rocks is sensitive to pore‐pressure changes. This dependence can have large effects on the flow of fluids through rocks. We define the permeability compliance γ= 1/k(k/δpp)pc, which is the sensitivity of the permeability k to the pore pressure pp at a constant confining pressure pc, and solve the specific problems of constant pressure at the boundary of a half‐space, a cylindrical cavity and a spherical cavity. The results show that when the magnitude of permeability compliance is large relative to other compliances, diffusion is masked by a piston‐like pressure profile. We expect this phenomenon to occur in highly fractured and compliant rock systems where γ may be large. The pressure profile moves rapidly when fluids are pumped into the rock and very slowly when fluids are pumped out. Consequently, fluid pressure, its history and distribution around injection and production wells may be significantly different from pressures predicted by the linear diffusion equation. The propagation speed of the pressure profile, marked by the point where δpp/δx is a maximum, decreases with time approximately as and the amplitude of the profile also dissipates with time (or distance).The effect of permeability compliance can be important for fluid injection into and withdrawal from reservoirs. For example, excessive drawdown could cause near‐wellbore flow suffocation. Also, estimates of the storage capacity of reservoirs may be greatly modified when γ is large. The large near‐wellbore pressure gradients caused during withdrawal by large γ can cause sanding and wellbore collapse due to excessive production rates.
Resistivity inversion of cross‐hole and borehole‐to‐surface EM data using axially symmetric models1
Authors: Sasaki Yutaka , Matsuo Koichi , Yokoi Koichi 
Published: April 2006

AbstractWe describe a least‐squares inversion approach to estimating the subsurface resistivity structure from cross‐hole or borehole‐to‐surface electromagnetic data. It is assumed that the resistivity distribution is symmetric about the axis of a borehole and that vertical magnetic dipoles are located on the borehole axis. The receivers are placed either in another borehole or on the earth's surface. The inversion scheme uses the finite‐element and smoothness‐constrained least‐squares methods. The computational effort required to obtain partial derivatives is reduced considerably by using the reciprocity principle. Numerical simulations show that the reconstructions are generally in good agreement with the true structures when the assumption of an axisymmetric earth structure holds. An example involving the breakdown of this assumption, which can be obtained by interchanging the source and receiver boreholes, suggests that the inversion result may also be useful for locating a general 3D anomaly although artifacts are present.
Ground penetrating radar antennae frequencies and transmitter powers compared for penetration depth, resolution and reflection continuity1
Authors: Jol Harry M. 
Published: April 2006

AbstractTwelve ground penetrating radar (GPR) experiments were conducted on the modern, wave‐influenced William River delta, on the Southern shore of Lake Athabasca in northern Saskatchewan, Canada. The delta is a well‐sorted, quartzoserich, clean, sand‐dominated, water‐saturated geomorphic feature which provided an ideal site to test GPR. Penetration depths, resolution and continuity of reflections were compared for different antennae frequencies (25, 50, 100, 200 MHz) and transmitter powers (pulser voltage: 400 V, 1000 V). The data show significant variations in vertical resolution from 0.15 m to 0.76 m (200‐25 MHz), depth of penetration from 14 m‐28 m (200‐25 MHz), and continuity of reflections. Increasing the transmitter power from 400 V to 1000 V increases the depth of penetration by 5 to 14% and improves the continuity of reflections with little effect on the resolution.
Elastic anisotropy due to aligned cracks in porous rock1
Authors: Thomsen Leon 
Published: April 2006

AbstractAll theoretical expressions which relate the characteristics of saturated aligned cracks to the associated elastic anisotropy are restricted in some important way, for example to the case of stiff pore fluids, or of the absence of equant porosity, or of a moderately high frequency band. Because of these restrictions, previous theory is not suitable for application to the upper crust, where the pore fluid is brine (Kf≅ K820), the equant porosity is often substantial (φp > 0.1), and the frequency band is sonic to seismic. This work removes these particular restrictions, recognizing in the process an important mechanism of dispersion. A notable feature of these more general expressions is their insensitivity, at low frequency, to the aspect ratio of the cracks; only the crack density is critical. An important conclusion of this more general model is that many insights previously achieved, concerning the shear‐wave splitting due to vertical aligned saturated cracks, are sustained. However, conclusions on crack orientation or crack aspect ratio, which were derived from P‐wave data or from shear‐wave‘critical angles’, may need to be reconsidered. Further, the non‐linear coupling between pores and cracks, due to pressure equalization effects, means that the (linear) Schoenberg‐Muir calculus may not be applied to such systems. The theory receives strong support from recent data by Rathore et al. on artificial samples with controlled crack geometry.
High‐speed photography of the bubble generated by an airgun1
Authors: Langhammer Jan , Landrø Martin 
Published: April 2006

AbstractHigh‐speed photography has been used visually to study the shape, surface, turbulence and behaviour of an underwater oscillating bubble generated by an airgun. The source was a BOLT airgun with a chamber volume of 1.6cu.in., placed in a 0.85m3 tank at 0.5m depth. Near‐field signatures were also recorded in order to compare the instant photographs of the oscillating bubble with the pressure field recorded about 25 cm from the gun. Estimations of the bubble‐wall velocity and bubble radius estimated from high‐speed film sequences are also presented, and are compared with modelled results. The deviation between the modelled and measured bubble radii was at most 9%. In order to check the capacity for transmission of light through the bubble, a concentrated laser beam was used as illumination. We found that the air bubble is a strong scattering medium of laser light, hence the bubble is opaque.
Seedling density modifies the growth responses of yellow birch maternal families to elevated carbon dioxide
Authors: WAYNE P.M. , BAZZAZ F.A. 
Published: April 2006

AbstractWe studied seedling growth responses to ambient and elevated CO2(350 and 700 μL L−1) of three maternal families of yellow birch (Betula alleghaniensis), raised both individually and in high‐density stands. Seedlings in competitive, dense stands exhibited markedly lower average CO2‐induced growth enhancements than individually grown plants (16% vs. 49%). Maternal families differed in their growth responses to elevated CO2. However, differences among families were contingent upon density; families which exhibited the greatest CO2‐induced growth at low density exhibited the least CO2‐responsiveness at high density. These data are discussed in two separate contexts; the reliability of estimates of the CO2fertilization potential of forest species based solely on individually grown plants, and the potential evolutionary consequences of rising CO2on regenerating forest tree populations.
The expected effects of climate change on wheat development
Authors: MIGLIETTA F. , TANASESCU M. , MARICA A. 
Published: April 2006

AbstractAir temperature and the atmospheric concentrations of carbon dioxide are expected to rise. These two factor have a great potential to affect development, growth and yield of crops, including wheat. Rising air temperature may affect wheat development more than rising atmospheric CO2 as there is not yet evidence that elevated CO2 concentrations can directly induce changes in wheat development. In winter wheat, temperature has a complex effect on development due to its strong interaction with vernalization and photoperiod. In this paper, potential effects of rising temperature on the development of winter wheat from sowing to heading are considered in the light of this complex controlling mechanism. Data from a large series of field trials made in Romania is analysed at first and, subsequently, the IATA‐Wheat Phenology model is used to calculate the impact of air warming on wheat development under different climate change scenarios. Data from the field trials showed very clearly the occurrence of a complex temperature/photoperiod/vernalization interaction for field sown crops and demostrated that the photoperiodic and vernalization responses have a key role in controlling the duration of the emergence‐heading period. Temperature plays, instead, a central role in controlling seed germination and crop emergence as well as leaf inititiation and leaf appearance rate. The results of model analysis showed very well that the impact of an even or uneven distribution of warning effects may be very different. In the first case, the model predicted that the duration of the vegetative period was at least partly reduced in some years. In the second case, the model suggested that if warming will be more pronounced in winter than in spring, as predicted for some areas of the world by General Circulation Models, we may expect an increase in the duration of the vegetative phase of growth. On the contrary, in case of a spring warming but unchanged winter temperatures, we may expect a substantial decrease in the duration of the vegetative period.
THE DRY‐WEIGHT‐RANK METHOD FOR THE BOTANICAL ANALYSIS OF PASTURE
Authors: Mannetje Lt , Haydock† K. P. 
Published: April 2006

A method is presented which gives an accurate estimate of the botanical composition of grassland on a dry‐weight basis, without the necessity of cutting and hand‐separating samples. In a number of quadrats an observer estimates which species take first, second, and third place in terms of dry‐weight. The data are tabulated to give the proportion of quadrats in which each species received first, second, and third place. These proportions are multiplied by 70·2, 21·1, and 8·7, respectively, and added to give the dry‐weight percentages of each species.The dry‐weight‐rank method was tested four times by comparing the results with those of hand‐separating cut samples. No great differences between the two methods were detected when exact rankings, obtained from hand‐separating, were used, and it was found that ranks could be estimated accurately, provided that sufficient training had been given to the observers.
THE EFFECT OF METHOD OF DETERMINATION OF ACID‐DETERGENT FIBRE ON ITS RELATIONSHIP WITH THE DIGESTIBILITY OF GRASSES
Authors: McLeod M. N. , Minson D. J. 
Published: April 2006

ABSTRACTThe acid‐detergent fibre (ADF) contents of 50 grass samples of known in vivo and in vitro digestibility were determined by hydrolysing with acid for 1, 2, 3,4, 5, and 6 h. The 50 samples used consisted of 6 Lolium perenne, 9 Cenchrus ciliaris, 15 Chloris gayana, 10 Digitaria spp., and 10 Setaria spp. Regression and residual standard deviations (RSD) were calculated for each hydrolysis time for each grass and for all 50 samples.Increasing the hydrolysis time decreased the RSD for Lolium perenne and Setaria, had no effect on Chloris gayana and Digitaria and increased the RSD for Cenchrus ciliaris regressions. For regressions based on all 50 samples die RSD varied between ±4.8 and 5.0 digestibility units with only minor differences between hydrolysis times. Correcting the ADF for contaminating ash generally increased the RSD.Changing the acid strength from the 10 N recommended by van Soest (8) did not lead to any major improvement in the RSD for regressions based on Cenchrus ciliaris, but with Setaria the RSD was reduced from ±2.8 to 1.7 digestibility units when 2N acid was used and hydrolysis was extended to 2 h.It was concluded that unless the optimum conditions for determining ADF for each grass are known then the in vitro technique provides digestibility estimates with a lower error.