bluuebunny/crossref_metadata_2025_split huggingface dataset:
ON THE ABSORPTION‐DISPERSION CHARACTERISTICS OF CHANNEL WAVES PROPAGATING IN COAL SEAMS OF VARYING THICKNESS1
Authors: DOBRÓKA M. 
Published: April 2006

ABSTRACTThe WKB‐method is used for the derivation of both the complex dispersion relation and displacement functions for Love channel‐waves that propagate in a coal seam of varying thickness. The constant Q‐model is used to describe the anelastic friction. With numerical solutions of the absorption‐dispersion relation, the influence of thickness changes on the phase velocity and absorption coefficient of Love seam‐waves is analysed at various frequencies. It is shown that the changes in the seam thickness can be optimally detected around the average Airy‐phase frequency. An equivalence is pointed out between the wave guide structures: homogeneous with varying seam thickness and horizontally inhomogeneous with constant seam thickness.
THE NON‐LINEAR INVERSION OF SEISMIC WAVEFORMS CAN BE PERFORMED EITHER BY TIME EXTRAPOLATION OR BY DEPTH EXTRAPOLATION1
Authors: TARANTOLA A. , JOBERT G. , TREZEGUET D. , DENELLE E. 
Published: April 2006

ABSTRACTThe classical aim of non‐linear inversion of seismograms is to obtain the earth model which, for null initial conditions and given sources, best predicts the observed seismograms. This problem is currently solved by an iterative method: each iteration involves the resolution of the wave equation with the actual sources in the current medium, the resolution of the wave equation, backwards in time, with the current residuals as sources; and the correlation, at each point of space, of the two wavefields thus obtained.Our view of inversion is more general: we want to obtain a whole set of earth model, initial conditions, source functions, and predicted seismograms, which are the closest to some a priori values, and which are related through the wave equation. It allows us to justify the previous method, but it also allows us to set the same inverse problem in a different way: what is now searched for is the best fit between calculated and a priori initial conditions, for given sources and observed surface displacements. This leads to a completely different iterative method, in which each iteration involves the downward extrapolation of given surface displacements and tractions, down to a given depth (the‘bottom’), the upward extrapolation of null displacements and tractions at the bottom, using as sources the initial time conditions of the previous field, and a correlation, at each point of the space, of the two wavefields thus obtained. Besides the theoretical interest of the result, it opens the way to alternative numerical methods of resolution of the inverse problem. If the non‐linear inversion using forward‐backward time propagations now works, this non‐linear inversion using downward‐upward extrapolations will give the same results but more economically, because of some tricks which may be used in depth extrapolation (calculation frequency by frequency, inversion of the top layers before the bottom layers, etc.).
COMMON REFLECTION POINT DATA‐STACKING TECHNIQUE FOR CONVERTED WAVES1
Authors: TESSMER G. , BEHLE A. 
Published: April 2006

ABSTRACTFor converted waves stacking requires a true common reflection point gather which, in this case, is also a common conversion point (CCP) gather. We consider converted waves of the PS‐ and SP‐type in a stack of horizontal layers.The coordinates of the conversion points for waves of PS‐ or SP‐type, respectively, in a single homogeneous layer are calculated as a function of the offset, the reflector depth and the velocity ratio vp/vs. Knowledge of the conversion points enables us to gather the seismic traces in a common conversion point (CCP) record. Numerical tests show that the CCP coordinates in a multilayered medium can be approximated by the equations given for a single layer. In practical applications, an a priori estimate of vp/vs is required to obtain the CCP for a given reflector depth.A series expansion for the traveltime of converted waves as a function of the offset is presented. Numerical examples have been calculated for several truncations. For small offsets, a hyperbolic approximation can be used. For this, the rms velocity of converted waves is defined. A Dix‐type formula, relating the product of the interval velocities of compressional and shear waves to the rms velocity of the converted waves, is presented.
SEISMIC MODELLING: HIGH‐FREQUENCY RANGE1
Authors: ALMOSSAWI H. I. H. 
Published: April 2006

ABSTRACTAn increase in the grain/pore size can cause the received echo signal to be distorted due to the preferential attenuation of the high‐frequency components of the transmitted acoustic signal. Therefore, a rock acts as a low‐pass filter and the property of the filter has a linear relation to the grain/pore size. The results have shown that P‐ and S‐wave attenuations can be used to characterize selected internal characteristics of a rock. For example, an increase in the differential stress can cause an increase in the attenuation, due to the sensitivity of the S‐wave to microcracks.
THE DETECTION OF BURIED PLACER DEPOSITS BY GROUND MAGNETIC SURVEY1
Authors: SCHWARZ E. J. , WRIGHT N. 
Published: April 2006

ABSTRACTA method is proposed to detect buried magnetite concentrations within river sediments. Model studies show that relatively small (thickness 1 m, width 10 m) plate‐shaped volumes with magnetite contents of 0.5% within a non‐magnetic matrix can be detected by a detailed ground magnetic survey using a sensitive (0.1 nT) magnetometer. Tests along lines perpendicular to the river but at a different angle to the bedrock trend (or the aeromagnetic anomaly trend) reveal that essentially all total field and vertical’gradient’anomalies detected in the profiles are elongated in the river and/or valley direction. This shows that the anomaly sources are (buried) volumes of higher magnetite content within the river sediments. Further evidence for this is (1) that higher intensity total field and vertical’gradient’anomalies occur in an area where the river cuts through strongly magnetic serpentinites, (2) the absence of high frequency anomalies over unsorted glacial till, and (3) some features on power spectra. Relatively large volumes of high magnetite concentrations within the river sediments may well contain the highest concentration of heavy economic minerals such as gold. Consequently, the areas of greatest interest in the exploitation of buried placers may be selected from maps obtained by detailed magnetic surveys either on the ground or at low altitude by helicopter.
A GYRO‐ORIENTED 3‐COMPONENT BOREHOLE MAGNETOMETER FOR MINERAL PROSPECTING, WITH EXAMPLES OF ITS APPLICATION1
Authors: BOSUM W. , EBERLE D. , REHLI H.‐J. 
Published: April 2006

ABSTRACTA triple axis borehole magnetometer is described that consists of a Förster‐probe (fluxgate) triplet (sensitivity 1 n T), a Förster‐probe gradiometer (sensitivity 2 nT/40 cm), a gyro unit (mean angular drift approx. 0.5°/h) which is equipped with accelerometers (sensitivity 1/100°), and a data transmission unit (with multiplexer and 16‐bit AD converter). The sensitive fluxgate‐magnetometer can detect weakly magnetic or small source bodies. Data from the gyro and the accelerometers allow the 3‐component magnetic field values to be transformed to north, east and vertical components. Since they do not rely on magnetically‐determined directional data, the results are not disturbed by local anomalies of the magnetic declination. Furthermore, the magnetometer can also be used in vertical boreholes. 3‐component measurements are carried out at discrete points in the neighbourhood of a source body to locate its position, and within the source body to determine the direction of magnetization. The strength of magnetization and information on magnetic classification are obtained by continuous measurement of one or more components within the source body. Calculation algorithms and computer programs are available to simplify data processing and interpretation. Survey examples are discussed.
SEISMIC INVERSION BY A RMS BORN APPROXIMATION IN THE SPACE‐TIME DOMAIN1
Authors: FOKKEMA JAKOB T. , VAN DEN BERG PETER M. 
Published: April 2006

AbstractA new approximate method to calculate the space‐time acoustic wave motion generated by an impulsive point source in a horizontally layered configuration is presented. The configuration consists of a stack of fluid layers between two acoustic half‐spaces where the source and the receiver are located in the upper half‐space. A distorted‐wave Born approximation is introduced; the important feature of the method is the assumption of a background medium with vertical varying root‐mean‐square acoustic wave speed. A closed‐form expression for the scattered field in space and time as a function of the contrast parameters is deduced. The result agrees closely with rigorously calculated synthetic seismograms. In the inverse scheme the wave speed and mass density can be reconstructed within a single trace. Results of the inversion scheme applied to synthetic data are shown.
RECENT DEVELOPMENTS IN UNDERGROUND GRAVITY SURVEYS1
Authors: CASTEN U. , GRAM CHR. 
Published: April 2006

AbstractUnderground gravity observations in deep coal mines using the conventional gravity meters Worden (type Master) and LaCoste‐Romberg (model D), both of which have been adapted to the fire damp regulations, can be accurate to ± 10 and ± 3 μgal, respectively. For underground determination of the vertical gradient of gravity the LaCoste‐Romberg meter is used together with a specially designed measuring tower. Using this euipment an accuracy in tower gradient observations of ± 30 E was obtained.To apply the equipment to precision gravity observations in underground situations an additional correction, i.e. a gallery correction, is needed. High accuracy in correction is achieved by a new method of three‐dimensional modelling. The gravity effect is computed for bodies with a surface approximated by triangular elements, which are generated from corner points of the body. The combination of gallery correction with tower gradient data leads to a new method for in situ density determination. It offers the possibility of horizontal instead of vertical density profiling.To demonstrate the effectiveness of the developments in underground observations the localization of a pump room is presented. Microgravity and tower gradient observations were carried out to detect the cavity. The horizontal gradient was also calculated to give a more reliable location.
3D ACOUSTIC REVERSE‐TIME MIGRATION1
Authors: CHANG WEN‐FONG , McMECHAN GEORGE A. 
Published: April 2006

ABSTRACTAcoustic reverse‐time finite‐difference migration for zero‐offset data is extended from two‐ to three‐dimensional media. The formulation is based on the full three‐dimensional acoustic wave equation and so has no dip restrictions and it involves extrapolation in a velocity distribution variable in three dimensions. The algorithm is demonstrated by successful migration of synthetic data sets for three models: a point diffractor, an oblique pinch‐out, and a dome overlying a planar reflector.
DIRECT CURRENT ELECTRIC POTENTIAL FIELD ASSOCIATED WITH TWO SPHERICAL CONDUCTORS IN A WHOLE‐SPACE1
Authors: ALDRIDGE D. F. , OLDENBURG D. W. 
Published: April 2006

ABSTRACTBispherical coordinates are used to derive an exact mathematical solution for the potential field generated by direct current electric conduction in an earth model consisting of two spherical inclusions in a uniform whole‐space. The solution takes the form of a spherical harmonic expansion in bispherical coordinates; coefficients in the expansion are obtained by solving sets of linear equations. Rapid forward modelling of numerous interesting situations in d.c. resistivity prospecting is facilitated by the generality and computational efficiency inherent to this new solution. For example, the accuracy of image (or superposition) methods for calculating potential solutions can be quantified. Similarly, the ability of d.c. conduction methods to resolve two distinct bounded bodies in three‐dimensional space can be examined by repeatedly calculating the secondary potential or apparent resistivity response of an earth model as a selected parameter is varied. Synthetic mise à la masse, crosshole, or areal potential data sets can be generated for subsequent use in inversion studies. Improvements in solution technique derived here also apply to a simpler model consisting of a single sphere buried in a half‐space.
VELOCITY‐STACK PROCESSING1
Authors: YILMAZ ÖZDOǦAN 
Published: April 2006

AbstractA conventional velocity‐stack gather consists of constant‐velocity CMP‐stacked traces. It emphasizes the energy associated with the events that follow hyperbolic traveltime trajectories in the CMP gather. Amplitudes along a hyperbola on a CMP gather ideally map onto a point on a velocity‐stack gather. Because a CMP gather only includes a cable‐length portion of a hyperbolic traveltime trajectory, this mapping is not exact. The finite cable length, discrete sampling along the offset axis and the closeness of hyperbolic summation paths at near‐offsets cause smearing of the stacked amplitudes along the velocity axis. Unless this smearing is removed, inverse mapping from velocity space (the plane of stacking velocity versus two‐way zero‐offset time) back to offset space (the plane of offset versus two‐way traveltime) does not reproduce the amplitudes in the original CMP gather. The gather resulting from the inverse mapping can be considered as the model CMP gather that contains only the hyperbolic events from the actual CMP gather. A least‐squares minimization of the energy contained in the difference between the actual CMP gather and the model CMP gather removes smearing of amplitudes on the velocity‐stack gather and increases velocity resolution. A practical application of this procedure is in separation of multiples from primaries.A method is described to obtain proper velocity‐stack gathers with reduced amplitude smearing. The method involves a t2‐stretching in the offset space. This stretching maps reflection amplitudes along hyperbolic moveout curves to those along parabolic moveout curves. The CMP gather is Fourier transformed along the stretched axis. Each Fourier component is then used in the least‐squares minimization to compute the corresponding Fourier component of the proper velocity‐stack gather. Finally, inverse transforming and undoing the stretching yield the proper velocity‐stack gather, which can then be inverse mapped back to the offset space. During this inverse mapping, multiples, primaries or all of the hyperbolic events can be modelled. An application of velocity‐stack processing to multiple suppression is demonstrated with a field data example.
SOME EFFECTS OF MULTIPLE LATERAL IN HOMOGENEITIES IN MAGNETOTELLURICS1
Authors: GROOM R. W. , BAILEY R. C. 
Published: April 2006

ABSTRACTThe analytical solution, for the H‐polarization magnetotelluric impedance, of a series of multiple, vertical, conducting slabs (dikes) embedded in a host medium is extended to an infinite array in order to model an anisotropic layer. The solution is used to study the effects of such strongly anisotropic media on the surface impedance. At low frequencies such vertically laminated structures behave as a bulk material. It is shown that the effective bulk parameters are those expected from d.c. theory. However, practical electrode separations may not be long enough or adequately positioned to correctly obtain these bulk parameters from the measured impedance. Thus, such structures can masquerade as quite different one‐dimensional structures. A haphazard use of long electrode spacings will not necessarily produce correct results.
ITERATIVE TOMOGRAPHIC METHODS TO LOCATE SEISMIC LOW‐VELOCITY ANOMALIES: A MODEL STUDY1
Authors: KRAJEWSKI CHRISTINA , DRESEN LOTHAR , GELBKE CHRISTOPH , RÜTER HORST 
Published: April 2006

ABSTRACTThe possibilities for reconstructing seismic velocity distributions containing low‐velocity anomalies by iterative tomographic methods are examined studying numerical and analogue 2D model data. The geometrical conditions of the model series were designed to generalize the geometrical characteristics of a typical cross‐hole tomographic field case. Models with high (30%) and low (8%) velocity contrasts were realized. Traveltimes of 2D ultrasonic P‐waves, determined for a dense net of raypaths across each model, form the analogue data set. The numerical data consists of traveltimes calculated along straight raypaths. Additionally, a set of curved‐ray traveltimes was calculated for a smoothed version of the high‐contrast model.The Simultaneous Iterative Reconstruction Technique (SIRT) was chosen from the various tomographic inversion methods. The abilities of this standard procedure are studied using the low‐contrast model data. The investigations concentrate on the resolving power concerning geometry and velocity, and on the effects caused by erroneous data due to noise or a finite time precision. The grid spacing and the source and receiver patterns are modified. Smoothing and slowness constraints were tested. The inversion of high‐contrast analogue model data shows that curved raypaths have to be considered. Hence, a ray‐tracing algorithm using velocity gradients was developed, based on the grid structure of the tomographic inversion. This algorithm is included in the SIRT‐process and the improvements concerning anomaly localization, resolution and velocity reconstruction are demonstrated. Since curved‐ray tomography is time‐consuming compared with straight‐ray SIRT, it is necessary to consider the effects of grid spacing, ray density, slowness constraints and the
ON THE ROLE OF PARTIAL RAY EXPANSION IN THE COMPUTATION OF RAY SYNTHETIC SEISMOGRAMS FOR LAYERED STRUCTURES1
Authors: COVEY J. D. , HRON F. , PEACOCK K. L. 
Published: April 2006

ABSTRACTThis paper presents results of testing an efficient ray generation scheme needed whenever ray synthetic seismograms are to be computed for layered models with more than 10‘ thick’layers. Our ray generation algorithm is based on the concept of kinematically equivalent waves (the kinematic analogs) having identical traveltimes along different ray‐paths between the source and the receiver, both located on the surface of the model. These waves, existing in any medium composed of laterally homogeneous parallel layers, interfere at any location along the recording surface, thereby producing a composite wavelet whose amplitude and shape depend directly on the number of kinematic analogs (the multiplicity factor). Hence, explicit knowledge of the multiplicity factor is crucial for any analysis based on the amplitude and shape of individual wavelets, such as wavelet shaping, Q estimation, or linearized wavelet inversion.For unconverted waves, such as those discussed in this paper, the multiplicity factor can be computed analytically using formulae given in the Appendix; for converted waves, the multiplicity factor should be computed numerically, using the algorithm employed for the computation of the seismograms presented in a previous paper by one of the authors.
DOWNHOLE SYNTHETIC SEISMIC PROFILES IN ELASTIC MEDIA1
Authors: XU XIAOHONG 
Published: April 2006

AbstractA multichannel lattice filter structure is utilized to represent seismic waves propagating in adjacent layers in an elastic medium. Using this model, an explicit time‐domain solution for arbitrary source and receiver locations is obtained as an ARMA (AutoRegressive and Moving‐Average) process. The lattice and ARMA structures have given rise to an effective algorithm for the calculation of offset/downhole synthetic seismograms.A large range of recently developed offset/downhole seismic survey geometries, such as the ‘Yo‐Yo’ arrangement, can thus be simulated. In addition, the explicit solutions for upgoing and downgoing waves provide new insight into the properties of general downhole seismic signals, including wave‐mode conversion effects and multiple reflections. Furthermore, offset/downhole seismograms generated by a line source (i.e. 2D point source) can also be constructed by superposition of plane waves with different incidence angles.Synthetic seismograms generated using a different source‐receiver arrangement indicate that the properties especially associated with offset/downhole seismic signals can be predicted by this modelling method. These properties include arrival times, amplitude attenuation and wave‐mode conversion effects. Finally, utilizing this numerical modelling method to a real downhole survey with Yo‐Yo geometry may lead to a proper data acquisition and processing procedure, and improves the interpretation confidence of the field section.
INVERSION OF FIRST‐BREAK TRAVELTIME DATA OF DEEP SEISMIC REFLECTION PROFILES1
Authors: RÜHL THOMAS , LÜSCHEN EWALD 
Published: April 2006

AbstractFirst breaks of 2D deep reflection data were used to construct velocity‐depth models for improved static corrections to a deeper datum level and for geological interpretations. The highly redundant traveltime data were automatically picked and transformed directly into a velocity‐depth model by maximum depth methods such as the Giese‐ and the Slichter‐method. Comparisons with the results of synthetic calculations and a tomographic approach using iterative inversion methods (ART, SIRT) showed that maximum depth methods provide reliable velocity models as a basis for the computation of static corrections. These methods can economically be applied during data acquisition in the field. They provide particularly long‐period static anomalies, which are of the order of 20–40 ms (0.5‐1 wavelength) within CMP gathers of an example of a deep reflection profile in SW‐Germany sited on crystalline basement. Reprocessing of this profile, which was aimed at the comparison between the effects of the originally used and the new statics, did not result in dramatically improved stacking quality but showed a subtle influence on the detailed appearance of deep crustal events.
MINIMUM RELATIVE ENTROPY INVERSION OF 1D DATA WITH APPLICATIONS1
Authors: ULRYCH T. , BASSREI A. , LANE M. 
Published: April 2006

AbstractThe pioneering work of E. T. Jaynes in the field of Bayesian/Maximum Entropy methods has been successfully explored in a number of disciplines. The principle of maximum entropy (PME) is remarkably powerful and versatile and leads to results which are devoid of spurious structure. Minimum relative entropy (MRE) is a method which has all the important attributes of the maximum‐entropy (ME) approach with the advantage that prior information may be easily included. These ‘soft’ prior constraints play a fundamental role in the solution of underdetermined problems. The MRE approach, like ME, has achieved considerable success in the field of spectral analysis where the spectrum is estimated from incomplete autocorrelations. In this paper we apply the MRE philosophy to 1D inverse problems where the model is not necessarily positive, and thus we show that MRE is a general method of tackling linear, underdetermined, inverse problems. We illustrate our discussion with examples which deal with the famous die problem introduced by Jaynes, the question of aliasing, determination of interval velocities from stacking velocities and, finally, the universal problem of band‐limited extrapolation. It is found that the MRE solution for the interval velocities, when a uniform prior velocity is assumed, is exactly the Dix formulation which is generally used in the seismic industry.