bluuebunny/crossref_metadata_2025 huggingface dataset:
CALCULATION OF POTENTIAL FIELDS FOR STOCHASTIC MODELS OF HETEROGENEOUS GEOLOGICAL OBJECTS*
Authors: VAKHROMEYEV G.S. , DAVYDENKO A.Y.
Published: April 2006

The solution of the direct problem of magnetic and gravity prospecting for heterogeneous objects is discussed from the view‐point of the probability description of object models and the anomalous effects caused by them. Three stochastic models for the determination of the basic characteristics of non‐stationary geophysical fields above the finite objects are suggested: linear, with a random distribution of the physical properties and fixed source geometry; non‐linear, of a single object with random parameters, and an ensemble of random sources. With the methods suggested for the formalized description of the structure of heterogeneous objects for those models, expressions are derived enabling the calculation of mathematical expectations, dispersions and correlation functions of anomalous effects.

Model examples illustrate the application of stochastic modeling for the study of the influence of the character and dimensions of heterogeneities of the internal structure of the disturbing bodies on the probability characteristics of expected anomalous effects. It is shown that the assumption of the random character of the anomalous effect can be used for direct calculation of the optimum Wiener filter enabling the recovery of the shape of anomaly contaminated by random noise, as well as for calculating an observation step that is sufficient for recording the expected anomalous effects with prescribed accuracy.
WAVE SCATTERING DECONVOLUTION BY SEISMIC INVERSION*
Authors: SARWAR A.K.M. , SMITH D.L.
Published: April 2006

We propose a wave scattering approach to the problem of deconvolution by the inversion of the reflection seismogram. Rather than using the least‐squares approach, we study the full wave solution of the one‐dimensional wave equation for deconvolution. Randomness of the reflectivity is not a necessary assumption in this method. Both the reflectivity and the section multiple train can be predicted from the boundary data (the reflection seismogram). This is in contrast to the usual statistical approach in which reflectivity is unpredictable and random, and the section multiple train is the only predictable component of the seismogram. The proposed scattering approach also differs from Claerbout's method based on the Kunetz equation.

The coupled first‐order hyperbolic wave equations have been obtained from the equation of motion and the law of elasticity. These equations have been transformed in terms of characteristics. A finite‐difference numerical scheme for the downward continuation of the free‐surface reflection seismogram has been developed. The discrete causal solutions for forward and inverse problems have been obtained. The computer algorithm recursively solves for the pressure and particle velocity response and the impedance log. The method accomplishes deconvolution and impedance log reconstruction. We have tested the method by computer model experiments and obtained satisfactory results using noise‐free synthetic data. Further study is recommended for the method's application to real data.
INVERSION OF AN UNMIGRATED STACKED SECTION TO DETERMINE AN INTERVAL VELOCITY MODEL*
Authors: SUTTON G.R. , MOORE B.J.
Published: April 2006

A seismic inversion procedure is developed that inverts data available from an unmigrated stacked section to produce an interval velocity model. It attempts to overcome some of the limitations of existing methods by using a generalized linear inversion technique. The inversion process incorporates several features: (i) Lateral interval velocity variations are permitted, (ii) A fast accurate forward model was developed, (iii) Input data is weighted according to the accuracy with which it has been acquired. The procedure is applied to seismic data from the Gippsland Basin, an area offshore South‐East Australia.
SHOULD THE ELECTRIC LINE BE STRAIGHT IN MAGNETOTELLURIC SURVEYS?*
Authors: GOMEZ‐TREVINO E.
Published: April 2006

The usual description of electric field measurements in terms of potential differences is not entirely adequate at high frequencies. In general, the telluric electric field is non‐conservative and voltage measurements depend on the contour described by the cable. A simple error analysis helps to recognize those situations where systematic errors may be significant for present day standards.
EXAMINATION OF SOUNDING CURVE EXTRAPOLATION USED BY THE OFFSET WENNER SYSTEM1
Authors: WHITE P. A. , SCOTT D. M.
Published: April 2006

The Offset Wenner resistivity sounding system provides for the extrapolation of the Wenner resistivity curve. The extrapolation technique was applied to data measured in the Solomon Islands and it is shown to be unreliable. An accurate method of predicting the reliability of extrapolation using measured resistances could not be found.
APPROXIMATE INVERSION OF AIRBORNE EM DATA FROM A MULTILAYERED GROUND1
Authors: SENGPIEL K.‐P.
Published: April 2006

A complex transfer function c (or generalized skin depth) can be derived from data for the secondary magnetic field measured by a dipole system with small coil spacing at height h above the ground. This function has a useful property: For a uniform or layered ground, the real part of c yields the‘ centroid depth’z* of the in‐phase current system as a function of frequency.

This parameter can be combined with the apparent resistivity ρa derived by conventional methods. The function ρa(z*), if known over a broad frequency range, yields a smoothed approximation of the true distribution ρ(z) without an initial model. The relations between ρa(z*) and ρ(z) are studied for a number of multilayer models. An example of the application of the ρa*) algorithm to data from a groundwater survey is given.
STATISTICAL TECHNIQUES APPLIED TO BOREHOLE GEOPHYSICAL DATA IN GOLD EXPLORATION1
Authors: URBANCIC T. I. , BAILEY R. C.
Published: April 2006

Gold, as a trace element, is not directly detectable by geophysical borehole logging techniques. Geophysical logging methods have therefore been used to infer the presence of structural features and alteration processes associated with gold. Since these features and processes differ with the style of gold mineralization, a unique relation between geophysical anomalies and gold cannot be established. However, in a particular environment, such an association can be established. We examined geophysical log data (self potential, induced polarization, resistivity, gamma, temperature, and temperature gradient) and drill core from the Barber‐Larder property in NE Ontario. The geophysically detectable alteration processes associated with gold were identified as sericitization and pyritization, by means of factor analysis. A linear discriminant function was constructed which allowed the zones of economic mineralization (nominally 0.05 oz/ton or 1.5 ppm) to be identified with a 75% success rate based on geophysical log data alone. This rate was achieved without sophisticated non‐linear data transformations.
TRANSIENT EM ANALOGUE MODELLING FOR KOREAN TREASURE HUNTING WITH THE SIROTEM SYSTEM1
Authors: LEE SANG KYU , BUSELLI G.
Published: April 2006

The inductive transient electromagnetic method (TEM) shows good potential for the detection of metallic relics of historical interest, such as buried antique bells, Buddhist idols, or precious metal nuggets. The effectiveness of the method was investigated with analogue models, using transmitter‐receiver loop configurations with sizes or receiver spacings slightly different from those usually applied in earth resources exploration with the Sirotem system.

The analogue modelling results show that the location and depth of the buried treasure may be obtained from the Sirotem data. A solid metallic object such as an antique bell could be detected to a depth about 12 times greater than its size, and treasure consisting of separate metallic objects, such as gold nuggets, could be detected to a depth 5 times its linear dimensions with a transmitter current of ˜ 20 A.

In scaling down the dimensions of a target, its conductivity should be increased in order to preserve the same TEM conditions found in the field. However, since the buried treasure consists of gold or copper objects, it is not possible to properly scale the conductivity. Hence, in the field, the depth detection limit is expected to be greater than that derived from analogue modelling.
A STUDY OF DIFFERENT METHODS OF WAVEFIELD SEPARATION FOR APPLICATION TO VSP DATA1
Authors: KOMMEDAL J. H. , TJØSTHEIM B. A.
Published: April 2006

Abstract

During the last couple of years there has been much research in the area of wavefield separation of borehole seismic data, and several articles have been published on various separation techniques. Methods involving the application of two‐dimensional Fourier transformation, the Radon transformation, multi‐level median filters or optimal filters, are all suggested as possible approaches to the wavefield separation problem.

This paper compares some of these methods commonly used in the industry.

The theories of the chosen methods are described to see how they are related. Using the different methods on synthetic and real data, we show how this theoretical relation is reflected in the relatively similar results obtained. We also show how the different filters treat coherent and random noise.
MAGNETIC WAVETILT MEASUREMENTS FOR GEOLOGICAL FRACTURE MAPPING1
Authors: SINHA AJIT K.
Published: April 2006

Abstract

The measurement of wavetilt is diagnostic for determining the electrical characteristics of the upper layers of the ground at VLF and LF frequency ranges. Theoretical and field studies have indicated that electric wavetilt using the transverse magnetic (TM) waves detects lateral inhomogeneities virtually instantly as abrupt changes in electrical properties are encountered. Theoretical studies have also indicated that magnetic wavetilt measurements using transverse electric (TE) waves are superior to electric wavetilts for such purposes.

An experimental survey was conducted at two locations near Atikokan, Ontario, to verify the theoretical predictions. The survey area, forming a part of a large granitic pluton, was mapped earlier by various geophysical techniques, including the ground VLF‐EM method, to detect weak conductors formed either by the presence of fractures in the bedrock filled with water and/or clay, or overburden filling bedrock depressions.

A small, multi‐turn, horizontal loop was used during the survey as the transmitter to generate TE waves at eleven frequencies from 10.7 to 58.5 kHz. The magnetic wavetilt measurements detected all previously known conductors at the two locations. In addition, the survey detected several weak conductors that were missed by the VLF survey. Thus, the survey indicated the usefulness of magnetic wavetilt results for detection of weak conductors at shallow depths, which may have application in engineering geophysical surveys. The multi‐frequency wavetilt data also provided some indications of the depth and depth extent of such conductors.
STATIC CORRECTIONS ON CRYSTALLINE ROCKS1
Authors: DAHL‐JENSEN TRINE
Published: April 2006

Static corrections for reflection seismic data shot on high velocity crystalline rock can be calculated directly from the data using a global, iterative stackpower optimization. The reflections consist of several peaks, creating the possibility of aligning the wrong peak in the signals. Due to misalignments a large number of local maxima in the stackpower exist in a cyclic manner, only slightly smaller than thes stackpower of the‘best’stack. Therefore the search must be of a global nature. A Monte Carlo search requires long run times. A global search method is presented using a varying sequence of parameters within each iteration. The ability for a set of static corrections and the associated stackpower to move from a local maximum to a larger maximum is enhanced. The performance of the iterative process is improved, so a relatively small number (about 20) of iterations is needed to obtain the optimum set of corrections. The risk of misalignment of traces by using the wrong peak in a signal consisting of several peaks is diminished, and it is less important that the initial set of corrections is close to the final set. The method is illustrated by a synthetic example and on a data set shot on the granites of Dalarna, Sweden.
RESOLUTION AND NMO‐STRETCH: IMAGING BY STACKING1
Authors: HALDORSEN JAKOB B. U. , FARMER PAUL A.
Published: April 2006

Abstract

We analyse the stacking process within the framework of imaging techniques. Our results show how the NMO stretch, traditionally looked upon as giving a negative contribution, can be utilized to improve the vertical resolution of the stacked data from a source deficient in low frequencies. The added bandwidth is provided by the spatial coherency of the energy emitted by a point source.
OPTIMIZED RECEIVER ARRAY SIMULATION BASED UPON RESOLUTION CONSTRAINTS1
Authors: CARLINI A. , MAZZOTTI A.
Published: April 2006

The use of cables with a large number of closely spaced receivers has become increasingly common in marine seismic acquisition; this gives geophysicists the opportunity to improve data quality in processing centres. In particular, arrays need not be fixed in the field, but instead can be flexibly simulated in the computer.

The paper defines a strategy to optimize the application of receiver‐array simulation, based on the requirement that signal resolution should not be compromised. The approach consists in the design of k‐domain filters through the use of Chebyshev polynomials that define the spatial response of the desired array, with array length that varies with time according to spatial and vertical resolution constraints set by the interpreter.

An example, taken from a marine 240‐channel seismic line, ties the underlying ideas to practical application and shows that appropriate array processes can contribute to improving the quality of seismic data, especially in areas with complex geology and strong coherent noise.
RESIDUAL DEPTH‐MIGRATION: ENHANCING DIPS1
Authors: BENSON ALVIN K.
Published: April 2006

Residual migration can correct for migration done with the wrong velocity. The initial migration can be done with a less accurate, fast time‐migration algorithm followed by a residual depth‐migration operator. This combines the speed and accuracy of different migration schemes.

The residual depth‐migration equation can be obtained by an appropriate transformation of a typical finite‐difference, depth‐migration equation. Since the cheaper, initial time‐migration has already partially migrated the data, the residual depth‐migration has less work to do than if the depth‐migration were done in a single step. As a result, the overall process has increased the speed and reduced the dip limitations associated with typical depth‐migration operators. Results on a steeply dipping model demonstrate the validity and usefulness of residual depth‐migration.
IMPROVED LOW‐FREQUENCY DECAY ESTIMATION USING THE MULTITAPER SPECTRAL ANALYSIS METHOD1
Authors: WALDEN A. T.
Published: April 2006

Abstract

Spectral analysis is one of the most ubiquitous signal processing tools used in exploration geophysics. Among many applications, it is used simply to look at the frequency content of seismic traces, to find notches, to estimate wavelets under the minimum‐phase assumption, and to match broadband synthetic seismograms to seismic data.

Seismic spectra exhibit very large dynamic ranges, particularly at low frequencies. Estimation of low‐frequency decay is very important for accurate modelling. However, when using traditional spectral estimates incorporating smoothing windows, too much sidelobe energy leaks from high power into low power areas, spoiling our ability to estimate low‐frequency spectral decay. The multitaper method of spectral analysis due to D. Thomson does not employ just a single window, but rather a set of orthogonal data tapers. It is possible to have much less sidelobe contamination, while maintaining a stable estimate.

The trace is tapered by each of a subset of the orthogonal tapers, and a raw spectral estimate produced in each case. These are combined to produce a final spectral estimate. The technique can be made adaptive by applying different weights to the different raw spectra at different frequencies.

A comparison of seismic spectral estimation using this multitaper technique with a traditional approach having the same analysis bandwidth and stability demonstrates the very different estimates of spectral decay in the areas of high dynamic range. The multitaper approach provides estimates with much reduced sidelobe leakage, and hence is a very appealing method for reflection seismology.
SHEAR‐WAVE VELOCITY ESTIMATION FROM FULL SONIC WAVEFORM BY THE EXTENDED‐LATTICE PREDICTOR1
Authors: ANGELERI G. P. , BURRASCANO P. , MARTINELLI G. , ORLANDI G.
Published: April 2006

Abstract

One of the basic parameters of the rock formation surrounding a fluid‐filled borehole to be estimated is the shear‐wave velocity. In the present contribution a novel method for carrying out this estimate, based on the use of linear prediction techniques, is proposed. It is assumed that the shape and energy content of each wave can be accurately modelled by an ARMA (Auto Regressive Moving Average) impulsive process and by an appropriate delay. The overall seismogram is then considered to be a multiple impulsive ARMA process and the estimation is carried out by using the residual at the output of an extended version of the Burg lattice predictor. The resulting algorithm is very effective as illustrated by several examples performed on synthetic and real seismograms.
RESIDUAL STATICS BY CDP‐LOCALIZED STACK OPTIMIZATION1
Authors: KIRCHHEIMER F.
Published: April 2006

Abstract

Many conventional schemes of automated residual statics estimate time lags between prestack traces (sorted as CDP gathers) and a model section, and transform the collected lags into surface‐consistent residuals. The method discussed in this paper aims at improving the lag estimator. ‘Externally generated’ reference traces are avoided and a principle of localized stack optimization is introduced whereby application of a multichannel filter to the stack and evaluation of the normalized power of every filtered trace gives a measure of the stack quality. One may consider the power as a function of all variable (inconsistent) shifts applied to the prestack traces. To obtain a set of optimal lag estimates the power function must be maximized. This power function is complex and the number of its variables prohibits a straightforward search for the maximum. Thus an iterative method must be employed, and steepest descent schemes have proved the most satisfactory. In the actual calculation, the repeated evaluation of the objective can be replaced by the computation of certain cross‐correlations. At the last iteration (after five to ten coordinate sweeps), the global behaviour of this correlation gives some indications of how well a prestack trace is adapted to the filtered stack. This information is used to compute a weighting factor to be stored with the lag estimate. At this stage simple statistical procedures are run to eliminate the most unlikely estimates. The remaining ones are transformed to surface‐consistent residuals by means of a weighted least‐squares inversion according to a model which takes into account the fact that the lags have no fixed reference datum.

The efficiency of the method is demonstrated by a field data example into which synthetic anomalies were introduced, and the effect of the new process is compared with that of a ‘classical’ production program using field data with genuine static problems.
INTERPRETATION OF VLF‐EM IN‐PHASE DATA USING CURRENT DENSITY PSEUDOSECTIONS1
Authors: OGILVY R. D. , LEE A. C.
Published: April 2006

Abstract

The interpretation of VLF‐EM surveys in terms of buried conductors can be assisted by the application of a linear filter to the observed in‐phase component of the vertical magnetic field. One such scheme is examined critically by using the calculated response from a variety of synthetic models to compute theoretical current density pseudosections.

The results confirm that this filter technique provides a useful complementary tool for studying the third (i.e. depth) dimension. For single, steeply dipping plates (> 45°) diagnostic information may be derived concerning the depth, size, lateral location, and direction of dip, since the current density maxima seems always to occur within the conductor or at least within one data interval.

However, there are some limitations which do not appear to be widely recognized. For single plates the angle of dip cannot be resolved by the current density transformation. Pattern distortions can occur where targets are in close proximity, where the cross‐sectional form of the conductor is complex or where the dip is shallow (< 45°). In these latter cases the current density maxima may not occur within the conductive structure and therefore cannot be used to infer depth of burial and/or conductor shape with the same degree of confidence.
AN ALTERNATIVE STRATEGY FOR NON‐LINEAR INVERSION OF SEISMIC WAVEFORMS1
Authors: SAMBRIDGE M. S. , TARANTOLA A. , KENNETT B. L. N.
Published: April 2006

Abstract

A common example of a large‐scale non‐linear inverse problem is the inversion of seismic waveforms. Techniques used to solve this type of problem usually involve finding the minimum of some misfit function between observations and theoretical predictions. As the size of the problem increases, techniques requiring the inversion of large matrices become very cumbersome. Considerable storage and computational effort are required to perform the inversion and to avoid stability problems. Consequently methods which do not require any large‐scale matrix inversion have proved to be very popular. Currently, descent type algorithms are in widespread use. Usually at each iteration a descent direction is derived from the gradient of the misfit function and an improvement is made to an existing model based on this, and perhaps previous descent directions.

A common feature in nearly all geophysically relevant problems is the existence of separate parameter types in the inversion, i.e. unknowns of different dimension and character. However, this fundamental difference in parameter types is not reflected in the inversion algorithms used. Usually gradient methods either mix parameter types together and take little notice of the individual character or assume some knowledge of their relative importance within the inversion process.

We propose a new strategy for the non‐linear inversion of multi‐offset reflection data. The paper is entirely theoretical and its aim is to show how a technique which has been applied in reflection tomography and to the inversion of arrival times for 3D structure, may be used in the waveform case. Specifically we show how to extend the algorithm presented by Tarantola to incorporate the subspace scheme. The proposed strategy involves no large‐scale matrix inversion but pays particular attention to different parameter types in the inversion.

We use the formulae of Tarantola to state the problem as one of optimization and derive the same descent vectors. The new technique splits the descent vector so that each part depends on a different parameter type, and proceeds to minimize the misfit function within the sub‐space defined by these individual descent vectors. In this way, optimal use is made of the descent vector components, i.e. one finds the combination which produces the greatest reduction in the misfit function based on a local linearization of the problem within the subspace. This is not the case with other gradient methods. By solving a linearized problem in the chosen subspace, at each iteration one need only invert a small well‐conditioned matrix (the projection of the full Hessian on to the subspace). The method is a hybrid between gradient and matrix inversion methods. The proposed algorithm requires the same gradient vectors to be determined as in the algorithm of Tarantola, although its primary aim is to make better use of those calculations in minimizing the objective function.
TWO‐POINT RAY TRACING IN GENERAL 3D MEDIA1
Authors: PEREYRA V.
Published: April 2006

Abstract

The modelling of realistic 3D geology and wave‐propagation phenomena is an important aspect of exploration geophysics. Ongoing research on a new model representation in terms of strongly reflecting interfaces described by surface patches, and on two‐point (i.e. source‐receiver) non‐zero‐offset ray tracing in such models, is presented.

Examples are used to demonstrate how this model representation facilitates the description of geological unconformities such as pinched‐out layers, overhangs and reverse faults. Neither pseudo‐layers, nor ‘transparent’ or zero‐impedance extensions are needed for surfaces that do not naturally cover the geological window under study. These are therefore no longer layered models and consequently they require new ray‐tracing techniques which are described and exemplified.
RADON TRANSFORM APPLICATION TO THE IMPROVED GRIDDING OF AIRBORNE GEOPHYSICAL SURVEY DATA1
Authors: YUNXUAN ZHOU
Published: April 2006

Abstract

The 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

Abstract

Traditionally, 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

Abstract

Migration 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

Abstract

Based 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

Abstract

An 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

The 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

Fluid 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

We 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

Twelve 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

All 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

High‐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

We 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

Air 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.
A STUDY OF THE DEPTH OF GERMINATION OF TROPICAL GRASSES: A NEW APPROACH
Authors: Bogdan A. V.
Published: April 2006

A suggested new method for studying the depth of grass seed germination and emergence is described.
GRASS GROWTH IN MID‐SUMMER: A RE‐INTERPRETATION OF PUBLISHED DATA*
Authors: Brougham R W. , Glenda A. C.
Published: April 2006

In a recent paper by Anslow it was concluded that there was no significant relationship between Leaf‐area Index, light interception and growth rate in a perennial ryegrass sward. The author further concluded that higher growth rates would not be obtained by attention to this relationship. The data presented by Anslow are re‐examined in this paper and the results of the analysis showed that: (a) recovery growth of the swards followed a sigmoid pattern of growth, (b) at low values of LAI growth rate was low and maximum growth rate was attained when most of the visible light was intercepted by leaves, and (c) the relationship between yield and LAI was linear, indicating that leaf‐area increase after defoliation also followed a sigmoid pattern. These results are discussed in relation to Anslow's interpretation.
MICROWAVE DRYING OF HERBAGE
Authors: Jones D. I. H. , ap Griffith G.
Published: April 2006

Microwave heating has recently been suggested as a method of drying herbage. A slightly modified commercial microwave cooking‐oven has been tested for this purpose and was found to dry 400 g of fresh herbage in 15‐20 min. The water‐soluble‐carbohydrate contents of herbage samples dried (a) by microwaves and (b) by freezing were similar.