bluuebunny/crossref_metadata_2025_split huggingface dataset:
THE SCATTERING OF SH‐CHANNEL WAVES BY A FAULT IN A COAL SEAM*
Authors: BUCHANAN D.J. 
Published: April 2006

ABSTRACTThe imaging of faults in coal seams by the in‐seam seismic method has now become standard practice. In the UK over 300 surveys have been undertaken and the technique is now part of the exploration arsenal of colliery planners. From these users comes the pressure for two major improvements, namely an increase in range and target identification. This paper is directed towards the latter problem.It has long been recognized that the reflected channel waves must contain information on the fault structure that caused the reflection, and model experiments have been undertaken to investigate the reflection process. Only recently, however, have attempts been made to quantify the reflection process. Calculations using both the finite‐difference and finite‐element techniques have been carried out, and estimates of the reflection coefficient as a function of frequency have been obtained. The object of this paper is to extend these considerations by calculating analytically the scattering matrix of an SH‐channel wave after interaction at a fault plane. The scattering matrix is calculated as a function of frequency, hade angle, and fault throw.The method employed is based on the decomposition of the incident SH‐channel wave into Fourier components, the calculation of plane wave reflection and transmission coefficients within the constraints of geometrical acoustics, and finally the synthesis of the scattering matrix by application of the Helmholtz‐Kirchhoff integral. The calculation throughout is restricted to normal modes.
TUNING EFFECTS, LITHOLOGICAL EFFECTS AND DEPOSITIONAL EFFECTS IN THE SEISMIC RESPONSE OF GAS RESERVOIRS*
Authors: BROWN A.R. , WRIGHT R.M. , BURKART K.D. , ABRIEL W.L. , McBEATH R.G. 
Published: April 2006

ABSTRACTSynergism of 3‐D seismic data, wavelet processing, colour display and interactive interpretation has been exploited in the study of a Gulf of Mexico gas reservoir. Seismic amplitude has been used as a measure of the proportion of a sand/shale reservoir capable of producing gas. This has led to the mapping of net producible thickness of gas sand. The tuning phenomena resulting from geometric effects alone were studied in detail, and tuning curves of various levels of sophistication were used as the basis for amplitude editing. Statistical tuning curves were derived by interactive cross‐plotting and deterministic curves by wavelet extraction. Multiple wavelet side lobes cause multiple maxima in the tuning curve. Depositional effects and intrareservoir communication have also been studied by interactive cross‐plotting.
COMPUTER TECHNIQUES FOR THE ZONING AND CORRELATION OF WELL‐LOGS*
Authors: HOYLE I. B. 
Published: April 2006

ABSTRACTThis paper reviews computer techniques used in the automatic zoning and correlation of well‐logs. Prior to correlating, well‐logs are to be segmented–or ‘zoned’–so as to delineate sections that have similar properties. Techniques discussed include statistical methods such as variance tests and Student's t‐test, linguistic analysis, the use of Walsh functions and spectral analysis.Well‐log correlation, which may be between traces from different wells or between traces from the same hole (as in dip logs), is used in basin studies and the determination of structural dip. A variety of methods are reviewed including conventional time and frequency correlation, sequence slotting, pattern recognition and frequency analysis. Future directions for investigation are proposed.
SHEAR‐WAVE REFLECTION PROFILING FOR NEAR‐SURFACE LIGNITE EXPLORATION*
Authors: MILKEREIT B. , STÜMPEL H. , RABBEL W. 
Published: April 2006

ABSTRACTWe present the results of a shear‐wave reflection experiment and in situ measurements in opencast lignite exploration. Near‐surface coal seams have lower shear‐wave velocities (∼ 200 m/s) and lower densities than sand and clay layers. Due to strong reflection coefficients, a shear‐wave reflection survey provides a powerful tool in lignite prospecting. Due to shorter seismic wavelengths shear waves will yield a higher resolution of shallow subsurface structure than compressional waves. Low shear‐wave velocities and strong lateral velocity variations, however, require a dense data acquisition in the field. The variation of stacking velocities can exceed ± 15% within a profile length of 300 m. The different steps in processing and interpretation of results are described with actual records. The final CMP‐stack shows steep‐angle fault zones with maximum dislocations of 20 m within a coal seam.
SYNTHESIS OF ACOUSTIC WELL‐LOGGING WAVEFORMS ON AN IRREGULAR GRID*
Authors: SALERNO M. , ORLANDI G. , MARTINELLI G. , BURRASCANO P. 
Published: April 2006

ABSTRACTThe need for synthetic seismograms arises in several acoustic well‐logging contexts. We propose a numerical algorithm for computing the seismograms, characterized by a very reduced computational cost. Unlike previous contributions appearing in the technical literature, the proposed algorithm is based on a non‐uniform sampling step and on linear interpolation for computing the integrals involved in the synthesis.
EXPERIMENTAL STUDIES OF ELASTIC‐WAVE PROPAGATION IN A COLUMNAR‐JOINTED ROCK MASS*
Authors: KING M.S. , MYER L.R. , REZOWALLI J.J. 
Published: April 2006

ABSTRACTResults are presented of a series of cross‐hole acoustic measurements made between four horizontal boreholes drilled from a near‐surface underground opening situated in a basaltic rock mass. The objectives of the program were to assess the extent of blast damage around the opening, and to evaluate the rock mass characteristics and their spatial variation around the opening. The acoustic velocity and attenuation data are indicative of an anisotropic, jointed rock mass, with a greater intensity of jointing along travel paths in the horizontal than the vertical direction. Low acoustic P‐ and S‐velocities are indicative of blast damage and of zones of intense jointing or fracturing. In this case blast damage extends to approximately 1.5 m from the face. Attenuation data appear to be less sensitive in distinguishing between the blast‐damaged zone and intense vertical jointing and fracturing in the virgin rock mass. Taken together with field data, laboratory measurements of P‐ and S‐wave velocities on intact core samples suggest that the rock mass is probably water saturated.
SOME NEW MAGNETIC TRANSFORMATIONS*
Authors: ROY A. , AINA A.O. 
Published: April 2006

AbstractAll magnetic transformations are governed by one simple differential relation between the observed and the transformed quantities. A magnetic map for any component, at any location, and for any given direction of magnetization can be converted into one for which any one, two, or all three parameters differ.Three new magnetic transformations are introduced: (i) reduction to equator, (ii) orthogonal reduction, and (iii) elimination of remanence. The first eliminates (or minimizes) the asymmetry and the lateral shift of the measured total field anomalies, exactly as in Baranov's reduction to pole. The second produces perfect asymmetry so that a symmetrical target lies vertically below the zero anomaly point, midway between the maximum and minimum. When remanence is a contributing factor, the direction of resultant magnetization must be known a priori in all cases, except for transformation of one component into another in the same area.Explicit working formulae are presented for reduction to equator and pole, and orthogonal reduction.
DISPERSION CALCULATIONS FOR SH AND P‐SV WAVES IN MULTILAYERED COAL SEAMS*
Authors: BUCHANAN D.J. 
Published: April 2006

ABSTRACTRecent work by Räder, Schott, Dresen and Rüter has provided a means of calculating dispersion and amplitude distribution curves for SH channel waves in multilayered media. An alternative calculation procedure is presented and the P‐SV wave type is also included. Numerical results are given for a coal seam containing a band of dirt of either higher or lower seismic velocities than the coal itself.
DATA ACQUISITION AND PROCESSING OF CONVERTED pS‐WAVES*
Authors: FERTIG J. , HENTSCHKE M.K. 
Published: April 2006

ABSTRACTThe earth's surface can be an effective means of generating converted pS‐waves. Due to their nearly symmetrical ray path, conventional processing techniques can be used. As the wave is generated by reflection at the surface or at the base of surface layers one can expect a general filtering effect in the data for individual ray paths of a single shot gather.To balance the spectra of the traces a multiple‐trace filter was used. This filter can be fully determined in the time domain using the prediction‐error operators of the individual traces. The preferred mean spectrum to colour the traces was the geometric mean.As the process of spectral balancing requires a minimum‐delay wavelet, the recording instrument was replaced by its corresponding minimum‐phase equivalent. This process can also be carried out effectively in the time domain.Results of the application of minimum‐delay transform and spectral balancing are discussed for single shot gathers and for the general improvement of the final stack.
DETECTION OF THIN BEDS WITH THE PSEUDO‐ACOUSTIC IMPEDANCE SECTION*
Authors: SZULYOVSZKY I. 
Published: April 2006

AbstractThe conventional seismic response of a thin bed approximates the time derivative of the incident wavelet, whereas the pseudo‐impedance response approximates the incident wavelet. Consequently the pseudo‐impedance response of a geological sequence composed of thin beds is simpler and easier to interpret than the conventional response.By calibrating the sonic log data with check‐shot data and performing zero‐phase seismic processing, the fit of the sonic log and pseudo‐velocity section is improved. Discrepancies in amplitude and phase, however, generally remain.A five‐step processing and interpretation procedure, which benefits from multichannel interpretation along the model seismic section generated from the sonic logs, is described.The method has been tested with field data. In the test the detection of thin beds and the estimation of the natural gas content was more reliable with the proposed procedure than with the conventional method.
MODELING OF SH‐WAVE PROPAGATION IN AN IRREGULARLY LAYERED MEDIUM‐APPLICATION TO SEISMIC PROFILES NEAR A DOME*
Authors: CAMPILLO M. 
Published: April 2006

ABSTRACTAn approach that relies on a discrete representation of seismic wavefields allows the computation of synthetic SH‐seismograms in a laterally varying medium with plane and curved interfaces in the two dimensional (2‐D) case. The diffracting interface is represented by an array of body forces located along the interface at equal spacing. The numerical treatment is limited to the irregular boundary while the propagation in flat layered zones is obtained by the reflection‐transmission matrix method. As an example we have studied the case of a dome in a stratified medium. The solutions obtained verify the reciprocity theorem with good accuracy. The computation of vertical profiles and of surface reflection profiles illustrates the effects of diffraction and the importance of lateral propagation in such a structure.
A METHOD FOR DETECTION OF DIFFRACTED WAVES ON COMMON‐OFFSET SECTIONS*
Authors: LANDA E. , SHTIVELMAN V. , GELCHINSKY B. 
Published: April 2006

ABSTRACTA method of detection of diffracted waves on common‐offset sections is proposed. The method utilizes the main kinematic and dynamic properties of the diffracted waves. The detection algorithm is defined by an automatic procedure including phase correlation of the diffracted waves and the application of certain statistical criteria. This procedure enables us to make decisions with regard to the presence of the diffracted waves and also to estimate parameters of the scattering objects. The method is applied to synthetic and field data and, even for a relatively low signal‐to‐noise ratio, it gives reliable results.
TEMPERATURE ANOMALIES OVER UNDERGROUND CAVITIES*
Authors: MOŚCICKI W.J. 
Published: April 2006

ABSTRACTTheoretical and practical aspects of a new method for underground cavity localization are presented. The method is based on shallow temperature measurements. It is shown that such measurements can complement the geophysical methods most commonly used for cavity location.The results from finite‐difference numerical calculations indicate possible origins of temperature anomalies–the existence of which is confirmed experimentally, primarily by field measurements. Temperature anomalies up to 1°C have been recorded over underground cavities. A satisfactory agreement between measurements and theory is found.
COMPUTATIONAL ASPECTS OF THE CHOICE OF OPERATOR AND SAMPLING INTERVAL FOR NUMERICAL DIFFERENTIATION IN LARGE‐SCALE SIMULATION OF WAVE PHENOMENA*
Authors: HOLBERG O. 
Published: April 2006

ABSTRACTConventional finite‐difference operators for numerical differentiation become progressively inaccurate at higher frequencies and therefore require very fine computational grids. This problem is avoided when the derivatives are computed by multiplication in the Fourier domain. However, because matrix transpositions are involved, efficient application of this method is restricted to computational environments where the complete data volume required by each computational step can be kept in random access memory.To circumvent these problems a generalized numerical dispersion analysis for wave equation computations is developed. Operators for spatial differentiation can then be designed by minimizing the corresponding peak relative error in group velocity within a spatial frequency band. For specified levels of maximum relative error in group velocity ranging from 0.03% to 3%, differentiators have been designed that have the largest possible bandwidth for a given operator length.The relation between operator length and the required number of grid points per shortest wavelength, for a required accuracy, provides a useful starting point for the design of cost‐effective numerical schemes. To illustrate this, different alternatives for numerical simulation of the time evolution of acoustic waves in three‐dimensional inhomogeneous media are investigated. It is demonstrated that algorithms can be implemented that require fewer arithmetic and I/O operations by orders of magnitude compared to conventional second‐order finite‐difference schemes to yield results with a specified minimum accuracy.
EXPERIMENTAL COMPARISON BETWEEN SPECTRAL RATIO AND RISE TIME TECHNIQUES FOR ATTENUATION MEASUREMENT*
Authors: TARIF P. , BOURBIE T. 
Published: April 2006

ABSTRACTTwo techniques for the measurement of attenuation–spectral ratio and rise time techniques–were tested and compared in the laboratory. The spectral ratio technique proved to be reliable and easy to implement for intermediate values (5 < Q < 50) of attenuation. For low (Q > 50) and high attenuations, the spectral ratio technique is inaccurate. Calculating the rise time on simulated signals, we found a relation between rise time τ and the ratio travel‐time to quality factor T/Q which could be approximated in intervals by the linear relation τ=τ+C*T/Q. The constants τ and C depend on the absolute value of T/Q and on the initial source signal. The rise time technique, performed on the first quarter period of the signal, enables high attenuations (Q < 5) to be measured. The determination of the relation between τ and T/Q is possible if one knows the initial source. We theoretically approximate this relation through a simulation using a realistic propagation model. With laboratory measurements made on Fontainebleau sandstone, we show that the rise time technique using the theoretical relation τ=τ(T/Q) gives comparable values of Q to those obtained from the spectral ratio technique. In borehole seismics, where it is often difficult to remove undesired signals, the rise time technique applied with the right (τ, T/Q) relation is the best method to use.
THREE‐DIMENSIONAL INTERPRETATION OF GRAVITY DATA FROM SEDIMENTARY BASINS USING AN EXPONENTIAL DENSITY‐DEPTH FUNCTION*
Authors: GRANSER H. 
Published: April 2006

ABSTRACTIn mapping the topography of the basement of deep sedimentary basins by gravity modelling, the accuracy can be improved by incorporating an exponential increase in density with depth. For calculating the gravity effect of a three‐dimensional (3D) structure with such an exponential density‐depth relation a frequency‐domain forward algorithm based on series expansion is presented, the numerical evaluation of which can be performed efficiently by fast Fourier transform. The algorithm can be applied in a recursive procedure to give the inverse solution in terms of basement relief.The inversion procedure is satisfactorily tested on a 2D synthetic example and a 3D field example of gravity data from the western margin of the Pannonian Basin in eastern Austria, where up to 2.2 km of Tertiary sediments overlie an igneous or metamorphic basement. The results are confirmed by basement intersections in several wells.
CORRECTING FOR COLOURED PRIMARY REFLECTIVITY IN DECONVOLUTION1
Authors: WALDEN A. T. , NUNN K. R. 
Published: April 2006

ABSTRACTStatistical deconvolution, as it is usually applied on a routine basis, designs an operator from the trace autocorrelation to compress the wavelet which is convolved with the reflectivity sequence. Under the assumption of a white reflectivity sequence (and a minimum‐delay wavelet) this simple approach is valid. However, if the reflectivity is distinctly non‐white, then the deconvolution will confuse the contributions to the trace spectral shape of the wavelet and reflectivity.Given logs from a nearby well, a simple two‐parameter model may be used to describe the power spectral shape of the reflection coefficients derived from the broadband synthetic. This modelling is attractive in that structure in the smoothed spectrum which is consistent with random effects is not built into the model. The two parameters are used to compute simple inverse‐ and forward‐correcting filters, which can be applied before and after the design and implementation of the standard predictive deconvolution operators. For whitening deconvolution, application of the inverse filter prior to deconvolution is unnecessary, provided the minimum‐delay version of the forward filter is used.Application of the technique to seismic data shows the correction procedure to be fast and cheap and case histories display subtle, but important, differences between the conventionally deconvolved sections and those produced by incorporating the correction procedure into the processing sequence. It is concluded that, even with a moderate amount of non‐whiteness, the corrected section can show appreciably better resolution than the conventionally processed section.
LARGE‐OFFSET APPROXIMATIONS IN THE MODIFIED CAGNIARD METHOD FOR COMPUTING SYNTHETIC SEISMOGRAMS: A SURVEY1
Authors: DE HOOP A. T. 
Published: April 2006

ABSTRACTLarge horizontal offset and large vertical offset approximations in the modified Cagniard method for computing synthetic seismograms in a horizontally layered model of the earth are derived. They apply to each of the generalized‐ray constituents into which the seismic wave motion is decomposed. For the results applying to large horizontal offset, which are known in the seismological literature, a simplified derivation is given. The results for large vertical offset, which are of particular interest to vertical seismic profiling, are new. The use of the large‐offset approximations, both horizontal and vertical, leads to a considerable reduction in computation time for synthetic seismograms compared with the full three‐dimensional version of the modified Cagniard method.
MIGRATION OF MIXED‐MODE VSP WAVEFIELDS1
Authors: DILLON P. B. , AHMED H. , ROBERTS T. 
Published: April 2006

ABSTRACTTwo‐dimensional VSP surveys are often conducted to provide structural illumination of the subsurface away from the borehole. The illumination is achieved through offsetting the source with respect to the downhole geophone. This inevitably gives rise to mode‐conversions in both downgoing and upgoing wavefields.Migration of mixed‐mode wavefields is complex because the velocity profile used for wavefield extrapolation is valid only for a particular propagation mode; the other mode always propagates at a different velocity. It is therefore advisable to separate the wave‐types (P‐wave and SV‐wave) prior to migration. This may be achieved through wavemode filtering, a multichannel process which exploits the relation between propagation velocity, slowness of events at the recording array and particle motion. The necessary information about particle motion is available only if the VSP data are acquired with a three‐component downhole geophone assembly.The wavemode filter partitions wave‐types at the recording array; it provides no information about the various changes of propagation mode experienced by the energy as it travels from source to geophone. For the purpose of migration, the intermediate modes of propagation must be deduced.Much of the energy arriving at the receivers is P‐wave which has followed the P‐wave velocity profile from the source. It can therefore be imaged by conventional (Kirchhoff) migration. As an example of SV‐wave imaging, a common mode‐code is P‐wave from source to reflector and SV‐wave from reflector to geophone. Migration of such data calls for back‐propagation of the geophone array wavefield, at SV‐wave velocity, to the point in the subsurface where it is time‐coincident with the forward propagated downwave, at P‐wave velocity.
BAYESIAN ESTIMATION IN SEISMIC INVERSION. PART II: UNCERTAINTY ANALYSIS1
Authors: DUIJNDAM A. J. W. 
Published: April 2006

ABSTRACTA parameter estimation or inversion procedure is incomplete without an analysis of uncertainties in the results. In the fundamental approach of Bayesian parameter estimation, discussed in Part I of this paper, the a posteriori probability density function (pdf) is the solution to the inverse problem. It is the product of the a priori pdf, containing a priori information on the parameters, and the likelihood function, which represents the information from the data. The maximum of the a posteriori pdf is usually taken as a point estimate of the parameters. The shape of this pdf, however, gives the full picture of uncertainty in the parameters. Uncertainty analysis is strictly a problem of information reduction. This can be achieved in several stages. Standard deviations can be computed as overall uncertainty measures of the parameters, when the shape of the a posteriori pdf is not too far from Gaussian. Covariance and related matrices give more detailed information. An eigenvalue or principle component analysis allows the inspection of essential linear combinations of the parameters.The relative contributions of a priori information and data to the solution can be elegantly studied. Results in this paper are especially worked out for the non‐linear Gaussian case. Comparisons with other approaches are given. The procedures are illustrated with a simple two‐parameter inverse problem.