bluuebunny/crossref_metadata_2025_split huggingface dataset:
ON THE EFFECTS OF THICKNESS OF THE HALF‐PLANE MODEL IN HLEM INDUCTION PROSPECTING OVER SULPHIDE DYKES IN A HIGHLY RESISTIVE MEDIUM1
Authors: JOSHI M. S. , GUPTA O. P. , NEGI J. G. 
Published: April 2006

ABSTRACTIn the quantitative data interpretation for HLEM induction prospecting, a vertical half‐plane model in an insulating medium is widely employed. For this assumption to be valid, the steeply dipping massive sulphide dykes must have large strike lengths and depth extents, but small thickness.We report investigations, using the laboratory scale‐modelling method, on the response variation of large vertical conductors as the thickness is varied. We conclude that a steeply dipping large dyke can be approximated by a half‐plane model only if its thickness is less than half the skin depth. An inductively thick conductor produces larger amplitudes and relatively higher quadrature compared to a thin conductor, even if both have the same induction number.
STUDY AND APPLICATIONS OF SPATIAL DIRECTIONAL FILTERING IN THREE‐COMPONENT RECORDINGS1
Authors: BENHAMA A. , CLIET C. , DUBESSET M. 
Published: April 2006

AbstractThree‐component recordings permit the construction of particle trajectories. These three‐dimensional pictures of particle motion show successive predominant directions of polarization and allow wave modes with distinct polarization directions to be recognized. A polarization selection, called ‘spatial directional filtering’, can be accomplished by several methods; four techniques are described. The application of these polarization filters to a noise shot, offset VSPs and CDP stack are also presented. This type of filtering is shown to cancel waves with undue polarization and to enhance the signal‐to‐noise ratio.
EM MODELLING USING SURFACE INTEGRAL EQUATIONS1
Authors: DOHERTY J. 
Published: April 2006

ABSTRACTThe theory by which the Surface Integral Equation method may be applied to the solution of electromagnetic transmission boundary value problems is presented. For a 3D target of arbitrary electrical property contrast with its host medium excited by an arbitrary time‐harmonic source, two integral equations are derived which need to be simultaneously solved for tangential electric and magnetic source density on the target's surface. If the target is 2D, though still excited by an arbitrary source (the 2½ D case), the problem is best solved in the transform domain for a number of different wavenumbers in the target's strike direction. Then a set of four simultaneous scalar integral equations needs to be solved for the components of the surface source density transforms in the target's strike direction and in the direction of the tangent vector to the target's cross‐sectional contour.Examples are presented in which the 2½D problem is solved numerically using the method of moments with piecewise linear basis functions. Although the results generally compare well with analytical solutions, or solutions obtained numerically by other means, errors appear in the calculation of the real response of these targets to excitation by a magnetic dipole source at low frequencies. This is attributed to ill‐conditioning of the system resulting from a non‐unique solution at zero frequency.
TIME‐DOMAIN COMPUTATION OF NON‐NORMAL INCIDENCE WAVEFIELDS IN PLANE LAYERED MEDIA1
Authors: FERBER R.‐G. 
Published: April 2006

ABSTRACTA new time‐domain method is introduced for the calculation of theoretical seismograms which include frequency dependent effects like absorption. To incorporate these effects the reflection and transmission coefficients become convolutionary operators. The method is based on the communication theory approach and is applicable to non‐normal incidence plane waves in flat layered elastic media. Wave propagation is simulated by tracking the wave amplitudes through a storage vector inside the computer memory representing a Goupillaud earth model discretized by equal vertical transit times. Arbitrary numbers of sources and receivers can be placed at arbitrary depth positions, while the computational effort is independent of that number. Therefore, the computation of a whole plane‐wave vertical seismic profile is possible with no extra effort compared to the computation of the surface seismogram. The new method can be used as an aid to the interpretation of plane‐wave decomposed reflection data where the whole synthetic vertical seismic profile readily gives the interpreter the correct depth position of reflection events.
BAYESIAN ESTIMATION IN SEISMIC INVERSION. PART I: PRINCIPLES1
Authors: DUIJNDAM A. J. W. 
Published: April 2006

ABSTRACTThis paper gives a review of Bayesian parameter estimation. The Bayesian approach is fundamental and applicable to all kinds of inverse problems. Its basic formulation is probabilistic. Information from data is combined with a priori information on model parameters. The result is called the a posteriori probability density function and it is the solution to the inverse problem. In practice an estimate of the parameters is obtained by taking its maximum. Well‐known estimation procedures like least‐squares inversion or l1 norm inversion result, depending on the type of noise and a priori information given. Due to the a priori information the maximum will be unique and the estimation procedures will be stable except (in theory) for the most pathological problems which are very unlikely to occur in practice. The approach of Tarantola and Valette can be derived within classical probability theory.The Bayesian approach allows a full resolution and uncertainty analysis which is discussed in Part II of the paper.
INTERPRETATION OF RESISTIVITY MEASUREMENTS OVER 2D STRUCTURES1
Authors: SCHULZ R. , TEZKAN B. 
Published: April 2006

ABSTRACTResistivity measurements were carried out in a survey area in the south of Germany. This area is characterized by complicated subsurface geology. Schlumberger full‐arrays and their respective half‐arrays were recorded simultaneously. The results obtained by the one‐dimensional (1D) interpretation of the full‐array measurements were incorrect because of a resistivity discontinuity. This discontinuity, under a relatively thick overburden, could only be located by the half‐array soundings. Its exact location and the resistivity distribution in the subsurface were ascertained by comparing the sounding curves with 2D model curves, which are calculated by a finite‐difference method.
FURTHER DEVELOPMENT OF A HIGH‐FREQUENCY SEISMIC SOURCE FOR USE IN BOREHOLES1
Authors: BARIA R. , JACKSON P. D. , McCANN D. M. 
Published: April 2006

AbstractA borehole sparker seismic source enclosed in a semi‐flexible tube has been developed to produce a short compressional seismic pulse with a frequency content in the range 250 Hz to 3.5 kHz with a peak power at 570 Hz. The pulse shape and frequency content are shown to be a function of the input power, the diameter of the spark chamber, the salinity of the electrolyte, the material of the spark chamber and the electrode configuration. When in a borehole, the source produces a vertically polarized shear wave but, being similar to a small explosive charge does not allow phase reversal as a means of identifying the shear wave in the received pulse train. The source is shown to be ideal for tomographic imaging surveys because of its repetitive nature, high frequency content and reliability. Very high resolution seismic reflection surveys are also shown to be possible under favourable circumstances.
CONTRIBUTION DES MESURES DE L'ANISOTROPIE ÉLECTRIQUE À LA RECHERCHE DES AQUIFÈRES DE FRACTURE EN MILIEU CRISTALLIN AU BÉNIN1
Authors: DARBOUX‐AFOUDA R. , LOUIS P. 
Published: April 2006

AbstractA particular methodology adapted to crystalline formations with a thin weathered zone was developed for a village hydrological project, in Benin. A combination of electrical profiles, Schlumberger and square arrays, was able to locate the most fractured zones in the basement. We present some results obtained from theoretical models as well as from field data.The suggested methodology uses both measurements of resistivity and anisotropy. Strong anisotropy and low resistivity indicate the most productive hydrogeological areas.
DECOMPOSED VIBRATOR PATTERNS TO IMPROVE SEISMIC SURVEY RESULTS1
Authors: EDELMANN H. A. K. , KIRCHHEIMER F. , SCHULTE L. 
Published: April 2006

AbstractEffective noise reduction of single‐sweep recorded data is achieved by application of a velocity filter process on a decomposed vibrator pattern. This technique promises high resolution results with a minimum effect on signal characteristic. A comparison of the stacked section of records vertically stacked in the field with the stacked section of velocity‐filtered receiver gathers shows a significant increase in resolution and signal‐to‐noise ratio.
MATRIX METHOD FOR THE TRANSFORMATION OF RESISTIVITY SOUNDING DATA OF ONE ELECTRODE CONFIGURATION TO THAT OF ANOTHER CONFIGURATION1
Authors: NIWAS SRI , ISRAIL M. 
Published: April 2006

AbstractMatrix equations are derived to transform the resistivity sounding data obtained in one type of a four‐electrode array to the corresponding resistivity sounding data that would be obtained using a different four‐electrode array. These expressions are based primarily on recent work in which we have established a linear relation between the apparent resistivity and the kernel function by using a powerful exponential approximation for the kernel function. It is shown that the resistivity sounding data of two different four‐electrode arrays have a linear relation through an essentially non‐singular matrix operator and, as such, one is derivable from the other for a one‐dimensional model and it can also be extended to two‐dimensions.Some numerical examples considering synthetic data are presented which demonstrates the efficiency of the method in such transformations. Two published field examples are also considered for transformation giving a reliable interpretation.
APPARENT DENSITY MAPPING AND 3D GRAVITY INVERSION IN THE EASTERN ALPS1
Authors: GRANSER H. , MEURERS B. , STEINHAUSER P. 
Published: April 2006

ABSTRACTPower spectrum analysis of the Bouger gravity values in the Eastern Alps suggests that the gravity field may be separated into long and short wavelength components. The long wavelength component is assumed to be caused by Alpine crustal thickening. This long wavelength component was subjected to a gravimetric single density‐interface inversion procedure, giving a gravimetric Mohorovičić model which is generally in good agreement with Moho‐depths derived by refraction and reflection seismology.The residual high‐frequency gravity component correlates well with the main surface geological units in the Eastern Alps.Apparent density mapping by applying an inverse density deconvolution filter to the short wavelength gravity component gives density values for the upper crust which correspond well with averaged density values from rock samples.
EFFICIENT 2D AND 3D SHOT RECORD REDATUMING1
Authors: KINNEGING N. A. , BUDEJICKY V. , WAPENAAR C. P. A. , BERKHOUT A. J. 
Published: April 2006

ABSTRACTIn order to make 3D prestack depth migration feasible on modern computers it is necessary to use a target‐oriented migration scheme. By limiting the output of the migration to a specific depth interval (target zone), the efficiency of the scheme is improved considerably. The first step in such a target‐oriented approach is redatuming of the shot records at the surface to the upper boundary of the target zone. For this purpose, efficient non‐recursive wavefield extrapolation operators should be generated. We propose a ray tracing method or the Gaussian beam method. With both methods operators can be efficiently generated for any irregular shooting geometry at the surface. As expected, the amplitude behaviour of the Gaussian beam method is better than that of the ray tracing based operators.The redatuming algorithm is performed per shot record, which makes the data handling very efficient. From the shot records at the surface‘genuine zero‐offset data’are generated at the upper boundary of the target zone. Particularly in situations with a complicated overburden, the quality of target‐oriented zero‐offset data is much better than can be reached with a CMP stacking method at the surface. The target‐oriented zero‐offset data can be used as input to a full 3D zero‐offset depth migration scheme, in order to obtain a depth section of the target zone.
CURRENT CHANNELLING IN SQUARE PLATES WITH APPLICATIONS TO MAGNETOMETRIC RESISTIVITY1
Authors: CHEESMAN S. J. , EDWARDS R. N. 
Published: April 2006

ABSTRACTThe magnetometric resistivity (MMR) method uses a sensitive magnetometer to measure the low‐level, low‐frequency magnetic fields associated with the galvanic current flow between a pair of electrodes. While the MMR anomalies of simple structures such as dikes and vertical contacts have been determined analytically, there is a lack of systematic information on the expected responses from simple three‐dimensional bodies. We determine the characteristic anomalies associated with square, plate‐like conductors, which are excellent models of many base metal mineral deposits.The anomalies of plates of finite size are determined numerically using an integral equation method. A plate is subdivided into many sections and the current flow within each section is solved by equating the electrical field within each section to the tangential electrical field just outside it. When the plate size is small in relation to either the depth or the transmitter spacing, the shape and amplitude of the anomaly produced is closely approximated by a current dipole model of the same length and depth. At the other extreme, a large plate is represented by a half‐plane. The dipole and half‐plane models are used to bracket the behaviour of plates of finite size.The form of a plate anomaly is principally dependent on the shape, depth and orientation of the plate. A large, dipping plate near the surface produces a skewed anomaly highly indicative of its dip, but the amount of skew rapidly diminishes with increased depth or decreased size. Changes in plate conductivity affect the amplitude of the anomaly, but have little effect on anomaly shape. A current channelling parameter, determined from the conductivity contrast, can thus be used to scale the amplitude of an anomaly whose basic shape has been determined from geometrical considerations.The separation into geometrical and electrical factors greatly simplifies both the interpretation and modelling of MMR anomalies, particularly in situations with multiple plates. An empirical formula, using this separation, predicts the anomaly of two or more parallel plates with different conductances. In addition, the relation between the resolution of two vertical, parallel plates of equal conductance and their separation is determined.The ability of the integral equation method to model plate‐like structures is demonstrated with the interpretation of an MMR anomaly in a survey conducted at Cork Tree Well in Western Australia. The buried conductor, a mineralized graphitic zone, is modelled with a vertical, bent plate. The depth to the top of the plate, and the plate conductance, is adjusted to fit the anomaly amplitude as closely as possible. From the modelling it would appear that this zone is not solely responsible for the observed anomaly.
3D MODELLING OF DIFFRACTIONS OBSERVED ON DEEP REFLECTION LINE DEKORP 2‐S1
Authors: SADOWIAK P. , VOSS J. , MEISSNER R. 
Published: April 2006

ABSTRACTDEKORP 2‐S is the first profile carried out in the German continental reflection seismic programme. Besides numerous reflections in the lower crust, the seismic section is characterized by strongly curved events that are interpreted as diffractions. These diffractions occur as clusters, mainly in two areas of the profile: the Dinkelsbühl and the Spessart area. This paper deals with the Dinkelsbühl diffractions where three‐dimensional control is available. The control is provided by two additional profiles P‐1 and Q‐40 which run parallel and perpendicular to the main line, DEKORP 2‐S. The type and the location of the diffractors are determined by traveltime‐modelling using crustal velocity functions derived from in‐line wide‐angle observations. A model with inclined line diffractors provides the best fit to the data for all three profiles. Projections of these line diffractors to the surface show that they are aligned parallel to the strike direction of the Variscides. This suggests that the diffractions are associated with the suture zone between the Saxothuringian and Moldanubian geological provinces.
3D SEISMIC MODELLING OF EDGE DIFFRACTIONS1
Authors: PEDERSEN H. M. , GELIUS L.‐J. , STAMNES J. J. 
Published: April 2006

ABSTRACTAn accurate, fast, and simple algorithm for 3D modelling of seismic edge diffractions is presented. It is based on a generalized Kirchhoff theory that applies also to inhomogeneous (non‐uniform) media. Both the boundary values and the Green functions in the Kirchhoff diffraction integral are determined by dynamic ray tracing, and each ray event is treated separately to obtain a description with clear physical interpretation. For each event the resulting Kirchhoff diffraction integral is evaluated by means of a uniform asymptotic technique that remains valid for receiver points near shadow boundaries. Since all parameters needed in the computations are obtained from dynamic ray tracing, the algorithm can readily be incorporated in existing software packages for 3D seismic ray modelling.
TWO METHODS FOR CONTINUOUS MONITORING OF HARMONIC DISTORTION IN VIBROSEIS SIGNALS1
Authors: MARTIN J. E. , WHITE R. E. 
Published: April 2006

ABSTRACTThe quality of Vibroseis survey data can be improved by continuously monitoring the vibrator's baseplate and reaction mass accelerations. Equipment failures can be detected as they occur, rather than relying on similarity trials at the beginning and end of the day's production. Equipment faults can then be corrected as they happen and thus would not have a detrimental effect on the quality of the survey data. Source efficiency can be optimized by monitoring the amount of harmonic distortion generated by the vibrator at different drive levels on the different surfaces which may be encountered during a survey. Phase problems introduced by poor coupling of the baseplate to the ground can also be identified and addressed in the field.Rapid analysis of vibrator signals is required if continuous monitoring is to be useful. Frequency‐time (f‐t) analyses of vibrator signals are often used in processing centres, but are slow and require a large storage capacity which makes the technique unsuitable for a field analysis system.The two methods proposed to analyse vibrator signals entail the use of hodograms and time‐varying notch filters. Hodograms provide a qualitative analysis of harmonic distortion and vibrator performance. A fast, time‐varying notch filter gives quantitative and qualitative information about the harmonic distortion present in the signal and can be used to identify problems with vibrator behaviour. Both the hodogram and fast, time‐varying notch filter methods can analyse the vibrator's reaction mass and baseplate accelerations as it progresses through its sweep and can present automatically interpreted results to the operator before moving to the next vibrator point.
3D TABLE‐DRIVEN MIGRATION1
Authors: BLACQUIÈRE G. , DEBEYE H. W. J. , WAPENAAR C. P. A. , BERKHOUT A. J. 
Published: April 2006

ABSTRACTAn efficient full 3D wavefield extrapolation technique is presented. The method can be used for any type of subsurface structure and the degree of accuracy and dip‐angle performance are user‐defined. The extrapolation is performed in the space‐frequency domain as a space‐dependent spatial convolution with recursive Kirchhoff extrapolation operators.To get a high level of efficiency the operators are optimized such that they have the smallest possible size for a specified accuracy and dip‐angle performance. As both accuracy and maximum dip‐angle are input parameters for the operator calculation, the method offers the possibility of a trade‐off between these quantities and efficiency. The operators are calculated in advance and stored in a table for a range of wavenumbers. Once they have been calculated they can be used many times.At the basis of the operator design is the well‐known phase‐shift operator. Although this operator is exact for homogeneous media only, it is assumed that it may be applied locally in case of inhomogeneities. Lateral velocity variations can then be handled by choosing the extrapolation operator according to the local value of the velocity. Optionally the operators can be designed such that they act as spatially variant high‐cut filters. This means that the evanescent field can be suppressed in one pass with the extrapolation. The extrapolation method can be used both in prestack and post‐stack applications. In this paper we use it in zero‐offset migration. Tests on 2D and 3D synthetic and 2D real data show the excellent quality of the method. The full 3D result is much better then the result of two‐pass migration, which has been applied to the same data.The implementation yields a code that is fully vectorizable, which makes the method very suitable for vector computers.
ESTIMATION OF THE DEGREE OF POLYNOMIAL FITTED TO GRAVITY ANOMALIES AND ITS APPLICATION1
Authors: ZENG HUALIN 
Published: April 2006

ABSTRACTA method for estimating the degree of polynomial fitted to gravity anomalies to evaluate the regional anomaly is presented. The anomaly can be fitted by polynomials of different degrees with the least‐squares method and the optimum degree of the polynomial evaluating this anomaly can be estimated from the point of discontinuity of the gradient on a graph of variance against the polynomial degree. The Bouguer gravity is initially separated by upward continuation to a proper height and then the degree of regional polynomial to fit the Bouguer anomaly can be estimated. Theoretical and field examples show the effectiveness of the method.
SEISMIC TOMOGRAPHIC IMAGING OF A BURIED CONCRETE TARGET1
Authors: BISHOP I. , STYLES P. 
Published: April 2006

AbstractThis paper describes a field evaluation of an algebraic reconstruction technique for the tomographic imaging of sub‐surface velocity anomalies. We describe the construction of a three‐dimensional concrete model and the acquisition and processing of seismic traveltime data through the model. Image reconstructions of the data sets, using an algebraic reconstruction technique and incorporating prior knowledge are presented and these are compared with the actual model. Reconstructions show that it is essential that accurate data are obtained as we demonstrate that relatively small errors in the traveltime data can seriously degrade the reconstruction. We also show that raypath effects are very important limiting factors to the analysis.
CROSS‐BOREHOLE OBSERVATION OF MODE CONVERSION FROM BOREHOLE STONELEY WAVES TO CHANNEL WAVES AT A COAL LAYER1
Authors: ALBRIGHT JAMES N. , JOHNSON PAUL A. 
Published: April 2006

AbstractConversion of borehole Stoneley waves to channel waves was observed in data from a seismic cross‐borehole experiment conducted between wellbores penetrating a thin coal layer at 2022 m depth, near Rifle, Colorado. Traveltime moveout observations show that borehole Stoneley waves underwent partial conversion to channel waves at the coal layer. The channel waves were detected directly in an adjacent borehole 35 m away at receiver positions within the coal. Stoneley waves, subsequently produced by partial conversion of channel waves, were also detected at receiver positions located up to 50 m above and below the coal layer in the adjacent borehole. We infer the channel wave to be the first‐higher Rayleigh mode by comparing the observed group velocity with theoretically derived dispersion curves. Identifying the conversion between borehole and stratigraphically guided waves is significant because coal penetrated by multiple wells may be detected without placing a transmitter or receiver in the coal itself.