bluuebunny/crossref_metadata_2025_split huggingface dataset:
AN APPLICATION OF FAST ALGORITHMS TO NUMERICAL ELECTROMAGNETIC MODELING*
Authors: BEZVODA V. , SEGETH K. 
Published: April 2006

ABSTRACTNumerical electromagnetic modeling by the finite‐difference or finite‐element methods leads to a large sparse system of linear algebraic equations. Fast direct methods, requiring an order of at most q log q arithmetic operations to solve a system of q equations, cannot easily be applied to such a system. This paper describes the iterative application of a fast method, namely cyclic reduction, to the numerical solution of the Helmholtz equation with a piecewise constant imaginary coefficient of the absolute term in a plane domain. By means of numerical tests the advantages and limitations of the method compared with classical direct methods are discussed. The iterative application of the cyclic reduction method is very efficient if one can exploit a known solution of a similar (e.g., simpler) problem as the initial approximation. This makes cyclic reduction a powerful tool in solving the inverse problem by trial‐and‐error.
LOVE SEAM‐WAVES IN A HORIZONTALLY INHOMOGENEOUS THREE‐LAYERED MEDIUM*
Authors: DOBRÓKA M. 
Published: April 2006

ABSTRACTUsing the WKBJ‐method the absorption‐dispersion relation and the amplitude functions are derived for Love seam‐waves that propagate in a horizontally inhomogeneous three‐layered medium. To describe the anelastic friction the constant Q‐model is applied. The inhomogeneity that appears in either the elastic moduli or quality factors is assumed to remain weak in the coal as well as in the adjacent layers, which are assumed to have different material properties (asymmetric channel). Using numerical solutions of the dispersion relation, it is shown that the weak horizontal inhomogeneities can be optimally detected using channel‐wave constituents of a frequency near to the Airy frequency while inhomogeneities of the adjacent rock can only be detected at frequencies close to, but higher than, the cut‐off frequency.
IDEAL PERFORMANCE CRITERIA FOR DECONVOLUTION OPERATORS*
Authors: DIMRI VIJAY P. , SRIVASTAVA KIRTI 
Published: April 2006

ABSTRACTA technique to evaluate an ideal performance of a deconvolution operator has been obtained by dividing the input trace into a number of sections. The error energy is seen to decrease with an increase in the number of sections. Numerical examples show that the error energy becomes zero following a relation between the number of sections, the length of input and the length of the filter.
THE PERFECTLY REFLECTING WEDGE USED AS A CONTROL MODEL IN SEISMIC DIFFRACTION MODELLING*
Authors: HUTTON G.D. 
Published: April 2006

ABSTRACTThe numerical modelling of seismic diffraction, e.g., at faults and other discontinuities, generally requires the use of fast approximate methods. The geophysicist responsible for the development of such numerical methods has a real need of exact solutions to certain ideal geometries to check the accuracy of his calculations.One such exact solution, which is available, is the acoustic wave solution to the perfectly reflecting wedge. The solution is three‐dimensional and the source is an explosive point source. This model is ideal for seismic diffraction; the solution has the advantage of being exact, truly three‐dimensional and of being in the convenient form of the temporal and spatial impulse response. More complicated sources which are extended in either space or time can, therefore, be modelled exactly by numerical integration.This paper presents some examples of the use of the perfectly reflecting wedge as a control model for an asymptotic high frequency diffraction modelling method. This control model has revealed that certain survey and wedge configurations can yield significant disagreement with, e.g., the Kirchhoff approximation. Such configurations could occur during VSP modelling when the survey lies in the near field or in the shadow zone of a high contrast fault. This control model has also been instructive in demonstrating why the high frequency, asymptotic, approximation is generally very good and has indicated a possible improvement to the Kirchhoff approximation for edge diffraction.
COMPUTING FIELD STATICS WITH THE HELP OF SEISMIC TOMOGRAPHY*
Authors: DE AMORIM W.N. , HUBRAL P. , TYGEL M. 
Published: April 2006

ABSTRACTField static corrections in general need be applied to all onshore seismic reflection data to eliminate the disturbing effects a weathering layer or near‐surface low velocity zone has on the continuity of deep seismic reflections. The traveltimes of waves refracted at the bottom of the low velocity zone (or intermediate refracting interfaces) can often be observed as first breaks on shot records and used to develop a laterally inhomogeneous velocity model for this layer, from which the field static corrections can then be obtained. A simple method is described for computing accurate field statics from first breaks. It is based on a linearization principal for traveltimes and leads to the algorithms that are widely and successfully applied within the framework of seismic tomography. We refine an initial model for the low velocity layer (estimated by a standard traveltime inversion technique) by minimizing the errors between the observed first arrivals on field records and those computed by ray theory through an initial model of the low velocity layer. Thus, one can include more lateral velocity variations within the low velocity layers, which are important to obtain good field static corrections. Traditional first break traveltime inversion methods cannot, in general, provide such refined velocity values. The technique is successfully applied to seismic data from the Amazon Basin. It is based on a simple model for the low velocity layer that consists of an undulating earth surface and one planar horizontal refractor overlain by a laterally changing velocity field.
MATHEMATICAL ANALYSIS OF D.C. RESISTIVITY SOUNDING OVER A PARABOLOID*
Authors: DASGUPTA S.P. 
Published: April 2006

ABSTRACTA complete mathematical analysis is proposed for direct current resistivity prospecting over the surface of a layered paraboloid. The analysis evaluates the Green's function in parabolic coordinates for a current source at the vertex. The general solution is obtained as a Fourier‐Bessel integral involving those curvilinear coordinates that have a kernel function which is similar to that of a half‐plane containing inhomogeneous layers. This similarity permits the computation of a class of sounding curves over such an oval surface providing a way to analyse field data over hilly terrain.
PARAMETER ESTIMATION AND FAULT DETECTION BY THREE‐COMPONENT SEISMIC AND GEOELECTRICAL SURVEYS IN A COAL MINE*
Authors: BREITZKE M. , DRESEN L. , CSOKAS J. , GYULAI A. , ORMOS T. 
Published: April 2006

AbstractThree‐component seismic and geoelectrical in‐mine surveys were carried out in Lyukobanya colliery near Miskolc, Hungary to determine the in situ petrophysical parameter distributions and to detect inhomogeneities in the coal seam. The seismic measurements comprise an underground vertical seismic profile, using body waves, and an in‐seam seismic amplitude‐depth distribution and transmission survey, using channel waves. The geoelectrical measurements are based on the drift‐ and seam‐sounding method.Interval traveltime‐, amplitude‐, multiple‐filter‐ and polarization analysis methods are applied to the seismic data. They lead to a five‐layer model for the strata including the coal seam. The coal seam and two underlying beds act as a seismic waveguide. The layer sequence supports the propagation of both normal and leaky mode channel waves of the Love‐ and Rayleigh type. A calculation of the total reflected energy for each interface using Knott's energy coefficients shows that the velocity ranges of high reflection energy and of normal and leaky mode wavegroups coincide. The excitation of wavegroups strongly depends on the seismic source. A simultaneous inversion of a geoelectrical drift‐ and seam‐sounding survey prevents misinterpretations of the seismic data by clearly identifying the low‐velocity coal seam as a high‐resistivity bed. Calculations of dispersion and sounding curves improve the resolution of the slowness and resistivity in each layer.Both diminished amplitudes and distortions in the polarization of transmission seismo‐grams and decreasing resistivities in a geoelectrical pseudosection of the coal seam are related to an inhomogeneity.A calculation of synthetic seismograms for Love and Rayleigh channel waves with the finite‐difference and the Alekseev‐Mikhailenko method agrees well with the field data for the main features, i.e., particular arrivals in the wave train, wavegroups, velocities and symmetries or asymmetries.This in‐mine experiment demonstrates that the simultaneous acquisition, processing and interpretation of seismic and geoelectrical data improve the lithological interpretation of petrophysical parameter distributions. Coal seam inhomogeneities can also be detected more reliably by the two independent surveys than by one alone.
CONVOLUTIONAL BACK‐PROJECTION IMAGING OF PHYSICAL MODELS WITH CROSSHOLE SEISMIC DATA1
Authors: EAST R. J. R. , WORTHINGTON M. H. , GOULTY N. R. 
Published: April 2006

ABSTRACTThe development of crosshole seismic tomography as an imaging method for the subsurface has been hampered by the scarcity of real data. For boreholes in excess of a few hundred metres depth, crosshole seismic data acquisition is still a poorly developed and expensive technology. A partial solution to this relative lack of data has been achieved by the use of an ultrasonic seismic modelling system. Such ultrasonic data, obtained in the laboratory from physical models, provide a useful test of crosshole imaging software.In particular, ultrasonic data have been used to test the efficacy of a convolutional back‐projection algorithm, designed for crosshole imaging. The algorithm is described and shown to be less susceptible to noise contamination than a Simultaneous Iterative Reconstruction Technique (SIRT) algorithm, and much more computationally efficient.
SEISMIC ATTENUATION IN THE VICINITY OF THE GEOTHERMAL ANOMALY AT URACH OBTAINED FROM NEAR‐VERTICAL REFLECTION PROFILES1
Authors: TRAPPE H. 
Published: April 2006

ABSTRACTThe attenuation in the vicinity of the geothermal anomaly at Urach was determined by means of two near‐vertical reflection profiles. The attenuation in the sediments and in the upper crust (3‐4 km depth) was estimated by interpretation of the first (refracted) arrivals. For calculating the attenuation, the amplitude decay with respect to distance was used. Corrections for the spread factor, i.e. the geometric amplitude divergence was deduced from the traveltime curves. Below the anomaly, higher attenuation values (Q−1∼ 0.008) were observed compared with those in the undisturbed crust (Q−1∼ 0.002). This effect is probably due to the cracks and fissures in the upper part of the crystalline basement.The attenuation in the middle and lower crust was determined using near‐vertical reflections from this depth interval. The use of the spectral ratio method leads to higher values of the effective attenuation Q−1eff below the heat flow anomaly compared to those of the‘ normal’crust. This zone of high Q−1eff coincides with the low velocity body below the heat flow anomaly. Both effects, the higher attenuation and the lower velocities, could be caused by high temperatures, cracks and fissures in the crust.
A NOVEL FIXED‐SOURCE ELECTROMAGNETIC SYSTEM1
Authors: JOHNSON I. , DOBORZYNSKI Z. 
Published: April 2006

ABSTRACTA novel fixed‐source electromagnetic system has been developed. The transmitter is a large rectangular loop or grounded dipole. The transmitted waveform consists of up to five superimposed pure sinusoids at well‐separated frequencies. The receiver measures the amplitude and phase at two frequencies from a single receiver coil. The amplitude ratio is routinely calculated. Field trials with both surface and downhole configurations show that the method has advantages over the more traditional frequency‐domain Turam type and downhole electromagnetic (DHEM) systems. Among such advantages are the use of a single receiver coil and the removal of noise due to free‐space variations in transmitter‐receiver geometry. The latter is important for DHEM surveys. In terms of field procedures and quantities measured, the system is similar to time‐domain methods.
SOME SOURCES OF DISTORTION IN TOMOGRAPHIC VELOCITY IMAGES1
Authors: DYER B. , WORTHINGTON M. H. 
Published: April 2006

ABSTRACTTwo particular sources of distortion, which may be encountered when applying tomographic imaging techniques to crosshole seismic data, have been investigated.Errors in survey locations of the shots and receivers can produce significant distortions in the images obtained. A simple method for solving simultaneously for the velocity field and shot and receiver location errors is presented and applied to synthetic and real data.Reflection and refraction of rays at velocity interfaces may produce poor density and angular coverage of the rays within the region of interest. It is shown that the effect of the velocity field on the ray coverage can significantly affect the resolution in the velocity image, even if ray bending is taken into account. One consequence of this effect is that, in some cases, little improvement in image quality is achieved by using curvi‐ray rather than straight‐ray inversion techniques, despite the occurrence of pronounced ray bending.
METHOD FOR DETERMINATION OF VELOCITY AND DEPTH FROM SEISMIC REFLECTION DATA1
Authors: LANDA E. , KOSLOFF D. , KEYDAR S. , KOREN Z. , RESHEF M. 
Published: April 2006

ABSTRACTThe estimation of velocity and depth is an important stage in seismic data processing and interpretation. We present a method for velocity‐depth model estimation from unstacked data. This method is formulated as an iterative algorithm producing a model which maximizes some measure of coherency computed along traveltimes generated by tracing rays through the model. In the model the interfaces are represented as cubic splines and it is assumed that the velocity in each layer is constant. The inversion includes the determination of the velocities in all the layers and the location of the spline knots.The process input consists of unstacked seismic data and an initial velocity‐depth model. This model is often based on nearby well information and an interpretation of the stacked section.Inversion is performed iteratively layer after layer; during each iteration synthetic travel‐time curves are calculated for the interface under consideration. A functional characterizing the main correlation properties of the wavefield is then formed along the synthetic arrival times. It is assumed that the functional reaches a maximum value when the synthetic arrival time curves match the arrival times of the events on the field gathers. The maximum value of the functional is obtained by an effective algorithm of non‐linear programming.The present inversion algorithm has the advantages that event picking on the unstacked data is not required and is not based on curve fitting of hyperbolic approximations of the arrival times. The method has been successfully applied to both synthetic and field data.
A NON‐GEOMETRICAL SH‐ARRIVAL1
Authors: DALEY P. F. , HRON F. 
Published: April 2006

ABSTRACTThe existence of‘*‐waves’has, in recent years, prompted a renewed interest in these non‐geometrical arrivals, which are generated by point sources located adajcent to plane interfaces. It has led to the re‐evaluation of seismic data aquisition techniques and to the question of how to use this real phenomena in enhancing existing seismic interpretation methods.This paper considers a non‐geometrical SH‐arrival which is generated by a point torque source unrealistically buried within a half‐space. The method of solution is essentially the same as presented in an earlier paper, with the modification that the limitation placed on the distance of the source from the interface has been removed in the saddle point method used to obtain a high‐frequency approximate solution. In the earlier paper, a preliminary assumption forced the saddle point, which corresponded to the *‐wave arrival, to be real when it is generally complex. However, for offsets removed from the distinct ray, the imaginary part of this complex quantity is negligible.A problem which arose when comparing exact synthetic traces with those obtained using zero‐order saddle point methods, was the inability to match either the amplitude or phase of the geometrical arrival in the range of offsets when the *‐wave and this corresponding geometrical ray were well separated. For this range of offsets the geometrical arrival was approaching grazing incidence and another term in the saddle point expansion of the integral was necessary to rectify this error. This method is also being used to validate the results for higher order terms obtained using asymptotic ray theory.Analytical formulae are given for both the *‐wave and the higher order expansion of the geometrical event, together with a comparison of synthetic seismograms using the method developed here and a numerical integration algorithm.
POST‐CRITICAL WAVELET ESTIMATION AND DECONVOLUTION1
Authors: KELAMIS P. G. , CHIBURIS E. F. 
Published: April 2006

ABSTRACTUsing synthetic data, it is demonstrated that the amplitude spectra of post‐critical plane‐wave components are stable and equal to the amplitude spectrum of the input wavelet (critical reflection theorem). Our analysis and physical explanation of the theorem are based only on amplitude versus offset arguments. The stability of the spectra in the post‐critical region is directly related to a high amplitude post‐critical reflection that dominates the trace in the slant‐stack domain. The validity of the theorem for both the acoustic and elastic cases, its assumptions and limitations, are also examined with emphasis on applications for processing seismic reflection data. Based on the theorem, a deterministic procedure is developed (assuming minimum‐phase properties) for wavelet estimation and subsequent deconvolution. We call this method Post‐critical Deconvolution, which emphasizes reliance on post‐critical reflection data. The performance of the method is shown with real data and the results are compared to those obtained with conventional deconvolution techniques.
A PULSED PLASMA JET ACOUSTIC SOURCE FOR PROFILING THE OCEAN FLOOR1
Authors: PITT P. L. , SMITH R. D. , SHEFFLER G. C. , WARREN R. D. , CLEMENTS R. M. , HAMILTON T. S. 
Published: April 2006

ABSTRACTA plasma gun source has been successfully used to obtain sub‐bottom profiles. The profiles show better penetration than with a 3‐5 kHz source and more resolution than with an air gun. The plasma gun source is compact, self‐contained, and requires no complex auxiliary equipment. The device was deployed from large research vessels and a small boat. The plasma gun produces sufficient acoustic energy and with its characteristically short pulse length and broad bandwidth it is an attractive sub‐bottom profiler, especially in shallow fresh or salt water.
ELASTIC WAVE PROPAGATION IN MEDIA WITH PARALLEL FRACTURES AND ALIGNED CRACKS1
Authors: SCHOENBERG M. , DOUMA J. 
Published: April 2006

ABSTRACTA model of parallel slip interfaces simulates the behaviour of a fracture system composed of large, closely spaced, aligned joints. The model admits any fracture system anisotropy: triclinic (the most general), monoclinic, orthorhombic or transversely isotropic, and this is specified by the form of the 3 × 3 fracture system compliance matrix. The fracture system may be embedded in an anisotropic elastic background with no restrictions on the type of anisotropy. To compute the long wavelength equivalent moduli of the fractured medium requires at most the inversion of two 3 × 3 matrices. When the fractures are assumed on average to have rotational symmetry (transversely isotropic fracture system behaviour) and the background is assumed isotropic, the resulting equivalent medium is transversely isotropic and the effect of the additional compliance of the fracture system may be specified by two parameters (in addition to the two isotropic parameters of the isotropic background). Dilute systems of flat aligned microcracks in an isotropic background yield an equivalent medium of the same form as that of the isotropic medium with large joints, i.e. there are two additional parameters due to the presence of the microcracks which play roles in the stress‐strain relations of the equivalent medium identical to those played by the parameters due to the presence of large joints. Thus, knowledge of the total of four parameters describing the anisotropy of such a fractured medium tells nothing of the size or concentration of the aligned fractures but does contain information as to the overall excess compliance due to the fracture system and its orientation. As the aligned microcracks, which were assumed to be ellipsoidal, with very small aspect ratio are allowed to become non‐fiat, i.e. have a growing aspect ratio, the moduli of the equivalent medium begin to diverge from the standard form of the moduli for flat cracks. The divergence is faster for higher crack densities but only becomes significant for microcracks of aspect ratios approaching 0.3.
ARTEFACTS IN ZERO‐OFFSET MIGRATION1
Authors: MÆLAND E. 
Published: April 2006

ABSTRACTMigration of zero‐offset data from a point diffractor gives an elliptic or hyperbolic ‘smile’ when the velocity is too high or too low, respectively (over‐ and undermigration).The phase‐shift within the smile is positive (approximately +π/4) when the velocity is too high, and negative (approximately –π/4) when the velocity is too low. The phase‐shift is calculated from the stationary phase approximation of the Kirchhoff integral.