bluuebunny/crossref_metadata_2025_split huggingface dataset:
DECOMPOSITION OF MULTICOMPONENT SEISMIC DATA INTO PRIMARY P‐ AND S‐WAVE RESPONSES1
Authors: WAPENAAR C. P. A. , HERRMANN P. , VERSCHUUR D. J. , BERKHOUT A. J. 
Published: April 2006

AbstractInversion of multicomponent seismic data can be subdivided in three main processes: (1) Surface‐related preprocessing (decomposition of the multicomponent data into ‘primary’ P‐and S‐wave responses). (2) Prestack migration of the primary P‐ and S‐wave responses, yielding the (angle‐dependent) P‐P, P‐S, S‐P and S‐S reflectivity of the subsurface. (3) Target‐related post‐processing (transformation of the reflectivity into the rock and pore parameters in the target). This paper deals with the theoretical aspects of surface‐related preprocessing.In a multicomponent seismic data set the P‐ and S‐wave responses of the subsurface are distorted by two main causes: (1) The seismic vibrators always radiate a mixture of P‐ and S‐waves into the subsurface. Similarly, the geophones always measure a mixture of P‐ and S‐waves. (2) The free surface reflects any upgoing wave fully back into the subsurface. This gives rise to strong multiple reflections, including conversions.Therefore, surface‐related preprocessing consists of two steps: (1)Decomposition of the multicomponent data (pseudo P‐ and S‐wave responses) into true P‐ and S‐wave responses. In practice this procedure involves (a) decomposition per common shot record of the particle velocity vector into scalar upgoing P‐ and S‐waves, followed by (b) decomposition per common receiver record of the traction vector into scalar downgoing P‐ and S‐waves. (2) Elimination of the surface‐related multiple reflections and conversions. In this procedure the free surface is replaced by a reflection‐free surface. The effect is that we obtain ‘primary’ P‐and S‐wave responses, that contain internal multiples only.An interesting aspect of the procedure is that no knowledge of the subsurface is required. In fact, the subsurface may have any degree of complexity. Both the decomposition step and the multiple elimination step are fully determined by the medium parameters at the free surface only. After surface‐related preprocessing, the scalar P‐ and S‐wave responses can be further processed independently by existing scalar algorithms.
ENHANCED MIGRATION OF SEISMIC DATA1
Authors: CARRION PHILIP M. 
Published: April 2006

AbstractThe so‐called ‘enhanced migration’ which uses diffraction tomography as the ‘repair tool’ for correction of amplitudes (reflection coefficients) of migrated sections is discussed. As with any linearized procedure, diffraction tomography requires knowledge of the initial model. It is suggested that the initial model is taken as the migrated image. It will be demonstrated that diffraction tomography applied to the data residuals improves the amplitudes of the migrated images. Migration is redefined as the reconstruction of the wavefront sets of distributions (reflection interfaces), and the inversion process as tomographic correction of migrated images.
NUMERICAL MODELLING OF STANDARD AND CONTINUOUS VERTICAL ELECTRICAL SOUNDINGS1
Authors: MOLANO C. E. , SALAMANCA M. , VAN OVERMEEREN R. A. 
Published: April 2006

AbstractAnalytical solutions of vertical electrical soundings (VES) have mostly been applied to groundwater exploration and monitoring groundwater quality on terrains of fairly simple geology and geomorphology on which the electrode arrays are symmetrical (e.g. Schlumberger or Wenner configurations). The sounding interpretation assumes flat topography and horizontally stratified layers. Any deviations from these simple situations may be impossible to interpret analytically. The recently developed GEA‐58 geoelectrical instrument can make continuous soundings along a profile with any colinear electrode configuration.This paper describes the use of finite‐difference and finite‐element methods to model complex earth resistivity distributions in 2D, in order to calculate apparent resistivity responses to any colinear current electrode distribution in terrains in which the earth resistivities do not vary along the strike. The numerical model results for simple situations are compared with the analytical solutions. In addition, a pseudo‐depth section of apparent resistivities measured in the field with the GEA‐58 is compared with the numerical solution of a real complex resistivity distribution along a cross‐section. The model results show excellent agreement with the corresponding analytical and experimental data.
MEASURED UNDERWATER NEAR‐FIELD E‐PATTERNS OF A PULSED, HORIZONTAL DIPOLE ANTENNA IN AIR: COMPARISON WITH THE THEORY OF THE CONTINUOUS WAVE, INFINITESIMAL ELECTRIC DIPOLE1
Authors: WENSINK W. A. , GREEUW G. , HOFMAN J. , VAN DEEN J. K. 
Published: April 2006

AbstractAt Delft Geotechnics the technique of ground‐penetrating radar is in use for the detection of buried objects such as pipes. To enable us to give our ‘measurements in the field’ a more quantitative interpretation than can be deduced from these alone, a series of experiments has been started under well‐defined conditions. A cylindrical vessel containing water simulates wet soil. Mounted horizontally above the water surface is a pulsed triangular half‐wave dipole which is used as a transmitting antenna (TA). It has a carrier‐frequency of about 160 MHz and a pulse repetition‐frequency of about 50 kHz.A movable receiving dipole (‘probe’) in the water measures the transverse, mutually orthogonal Eφ,‐ and Eθ‐components of the pulses as a function of probe‐position (r, θ, φ) and of the height h of the TA above the water surface. When these pulses are Fourier‐transformed, the transverse electric fields Eφ and Eθ at 200 MHz are obtained. The resulting field patterns are compared with computational results on the basis of the theory of the continuous wave, infinitesimal electric dipole (‘point dipole’). It can be concluded that:

Far‐field conditions have not fully developed at a depth of about 2.50 m, the largest value of the radius r at which field patterns were measured, although it represents a distance of about 15 wavelengths.
The attenuation constant of the tapwater used, as deduced from E‐field measurements for θ= 0, 2.50 m < r < 2.75 m, is slightly less than the value measured using a network analyser and air line combination, in agreement with (1).
E
φ field patterns calculated using the value of the conductivity σ corresponding to the former value of the attenuation constant agree reasonably well with the measured patterns for r≤ 2.50 m and for θ < 20° at all antenna heights considered. Calculated Eφ patterns do not agree so well with the measured patterns when h is close to zero. With increasing height the agreement inproves.
In accordance with the theory of the point‐dipole, the angular distribution of the radiation patterns of the TA becomes wider as the frequency decreases.
The normalized underwater pulse‐spectra shift to lower frequencies with increasing r. This can be explained since the attenuation constant of the water rises with rising frequency.
THE USE OF MARINE SEISMIC PROFILING FOR ENVIRONMENTAL ASSESSMENT1
Authors: McGEE T. M. 
Published: April 2006

AbstractOne application of marine seismic profiling that has an especially auspicious future is the delineation, and possible identification, of anomalously reflective material accumulated in the sediments of rivers, bays and estuaries. Analogue field data are presented which illustrate that such sediments are associated with strong sequences of multiple reflections. The rate of decay of these sequences indicates the magnitude of the reflection coefficients and the relative polarity of wavelets indicates whether they are positive or negative. If zero‐offset field geometry and digital recording are used, it is possible to compare wavelets within a sequence and thereby determine acoustic impedance contrasts from which sediment density can be inferred. If the source is sufficiently broadband, it becomes possible to determine absorption characteristics as well. Two principal problems arise when analysing multiple sequences; adequate data acquisition and suitable geometric corrections. Having solved these, this type of analysis could lead to the seismic identification of sea‐floor materials. Since anomalous material is often associated with effluent, the method can become an important tool in the growing effort to monitor and improve environmental quality.
SEISMIC MODELLING WITH CHANNEL WAVES IN SEAM STRUCTURES INFLUENCED BY MYLONITE ZONES1
Authors: GELDMACHER I. M. , DRESEN L. , STÜRZNICKEL T. 
Published: April 2006

AbstractChannel waves generated in coal‐seams and their reflections from discontinuities are widely used to indicate the tectonic and stratigraphic features of coal deposits, resulting in greater efficiency and safety in coal‐mining. In the mining area of Ibbenbüren (F.R.G.) seam structures sometimes contain so‐called mylonite zones, which are crushed coal deposits capable of binding gas. If mining hits a mylonite zone this would probably not only reduce output of the mine, but could even cause gas explosions. To investigate the influence of a mylonite zone on the propagation of channel waves, Rayleigh channel wave measurements for 2D analogue models were performed and synthetic seismograms of Love channel waves were calculated.Analogue modelling of the mylonite zone using Rayleigh seam waves within the seam was carried out using a perforation technique. Calculations were made to obtain an estimate of velocity reduction due to perforation. The results agree well with velocity values measured up to a perforation of 25% in a 2D epoxy resin model. Reflected channel wave energy was found by applying dispersion analysis in the case where the impedance reduction between the mylonite seam structure and the undisturbed seam was approximately 5%. The horizontal width of the mylonite structure was detectable from the time lag between reflected channel wave signals from both in‐seam borders of the mylonite zone. Resolution of two discrete borders was possible for a width of 1.5 λ's. The influence of a vertical fault, positioned within the mylonite zone, could only poorly be resolved.Numerical model investigations of Love seam waves were concerned mainly with the variation of the horizontal width of the mylonite zone. Mylonite zones with dimensions of the order of 0.4 λ's allow definite statements about their widths from dispersion and spectral analyses. For zones with smaller widths down to 0.2 λ's, it was found that reflectivity and transmissivity analyses give a qualitative criterion for distinguishing a mylonite structure surrounding a fault from a pure fault.
AN ANALYTICAL RAYPATH APPROACH TO THE REFRACTION WAVEFRONT METHOD1
Authors: AK M. ALI 
Published: April 2006

AbstractSeismic refraction surveying is still an important tool for determining the geometries and elastic wave propagation velocities of near‐surface layers. Many analytical and graphical methods have been developed over the years for refraction interpretation, and these can be classified into two basic groups. The first group visualizes critically refracted rays converging on a common surface position, while the second group, which includes the wavefront methods, makes use of the critical rays emerging from a common point on the refractor.The method described in this paper is an analytical approach to the wavefront methods. The reverse refracted ray received by a geophone is intersected by the forward refracted rays received by subsequent geophones and a common critical refraction point on the refractor is estimated after a series of comparisons. This process is repeated for each geophone to yield the geometry and the velocity of the refractor. Several interpolations are performed to achieve a better accuracy.Palmer's models are used to test the efficiency of the algorithm. The results are presented together with those of other methods applied to the same models.
THE DETERMINATION OF THE SEISMIC QUALITY FACTOR Q FROM VSP DATA: A COMPARISON OF DIFFERENT COMPUTATIONAL METHODS1
Authors: TONN RAINER 
Published: April 2006

AbstractTen methods for the computation of attenuation have been investigated, namely: amplitude decay, analytical signal, wavelet modelling, phase modelling, frequency modelling, rise‐time, pulse amplitude, matching technique, spectral modelling and spectral ratio. In particular, we have studied the reliability of each of these methods in estimating correct values ofQusing three synthetic VSP seismograms for plane P‐waves with different noise contents.The investigations proved that no single method is generally superior. Rather, some methods are more suitable than others in specific situations depending on recording, noise or geology. The analytical signal method has been demonstrated to be superior if true amplitude recordings are available. Otherwise spectral modelling or, in the ‘ noise‐free’ case the spectral ratio method, is optimal.Finally, two field VSPs in sediments are investigated. Only in the case of the highest quality VSP can significant information be deduced from the computed attenuation.
SEISMO‐ELECTRIC EXPLORATION: EXPECTED SIGNAL AMPLITUDES1
Authors: RUSSELL R. D. , BARKER A. S. 
Published: April 2006

AbstractFor more than 20 years, Soviet scientists have published papers and registered patents describing the conversion of seismic to electromagnetic energy in geological environments and the detection of the electric or magnetic signals as a method of geophysical exploration. Because of the potential importance of a reliable geophysical technique for locating quartz veins, we have been conducting extensive laboratory and field tests of the phenomena.For the purposes of designing appropriate field tests we need to know approximate signal magnitudes, but little has been published on them. The present paper describes a simplified model from which order‐of‐magnitude estimates of expected electric and magnetic signal strengths can be made with sufficient accuracy for such purposes. For mathematical convenience we model the target as a homogeneous sphere in which the seismic input induces uniform, time‐varying electric polarization. More realistic configurations can be described by linear superposition of the potentials of appropriate sub‐elements.True piezo‐electricity is, by definition, linear. Therefore, responses should have the same frequency content as the seismic input. Combining the low‐frequency form of our results with the assumption that the entire thickness of a target vein responds in phase to a seismic excitation, we obtain the following estimates of the maximum electric and magnetic fields at a distance r from the target:
image

where p is the resistivity and μ0 the dielectric constant of the ground in which E is measured, V is the effective volume of the target, P is the polarization of the vein, s is the seismic stress at the target, and a is the effective piezo‐electric coefficient. Signals observed in experiments at the Erickson Mine, British Columbia, gave electric fields in acceptable agreement with our theoretical predictions.We conclude by considering plausible relationships for the high‐frequency signals observed from sulfide minerals, assuming that they represent the release of stored stress triggered by the seismic arrival.
τ‐p SEISMIC DATA FOR VISCOELASTIC MEDIA – PART 1: MODELLING1
Authors: MARTINEZ RUBEN D. , McMECHAN GEORGE A. 
Published: April 2006

AbstractViscoelastic modelling reveals that the interaction of compressional‐wave velocity Cp, compressional‐wave quality factor Qp, shear‐wave velocity Cs, shear‐wave quality factor Qs and Poisson's ratio as a function of time intercept τ and ray parameter p, is complicated; however, distinct, potentially diagnostic behaviours are seen for different combinations of viscoelastic parameters.Synthetic seismograms for three viscoelastic reservoir models show that variations in the Poisson's ratio produce visible differences when compared to the corresponding elastic synthetic seismograms; these differences are attributable to interaction of the elastic parameters with Qp and Qs.When the P‐wave acoustic impedance contrast is small, viscoelastic effects become more apparent and more useful for interpretation purposes. The corresponding amplitude and net phase spectra reveal significant differences between the elastic and the viscoelastic responses. When P‐wave reflectivities are large, they tend to dominate the total response and to mask the Q reflectivity effects. The attenuation effects are manifested as an amplitude decay that increases with both time and ray parameter.The sensitivity of the computed seismic responses for various combinations of viscoelastic parameters suggests the opportunity for diagnostic interpretation of τ‐p seismic data. The interpretation of the viscoelastic parameters can permit a better understanding of the rock types and pore fluid distribution existing in the subsurface.
EXTENSIVE‐DILATANCY ANISOTROPY: RELATIVE INFORMATION IN VSPs AND REFLECTION SURVEYS1
Authors: YARDLEY GARETH S. , CRAMPIN STUART 
Published: April 2006

AbstractShear‐waves have complicated interactions with the free surface, particularly in the presence of low‐velocity surface layers, topographic irregularities, and the expected near‐surface crack and stress anomalies. Consequently, it has been suggested that shear‐waves should be recorded subsurface in vertical seismic profiles (VSPs), in order to extract accurate information about the in situ crack and stress geometry contained in shear‐wave splitting. This paper compares the information in synthetic shear‐waves in reflection gathers and VSPs, in order to assess the relative merits of the two techniques for investigating shear‐wave splitting. Synthetic seismograms demonstrate that in the presence of even very simple surface layers, shear‐waves recorded in reflection surveys at the surface have polarizations which may not indicate crack and stress geometry at depth. In contrast, shear‐waves recorded in VSPs are relatively unaffected by surface layers and near‐surface stress and crack anomalies, and the behaviour of shear‐wave splitting is dominated by the structure of the rock mass in the vicinity of subsurface geophones. Matrix rotations of multicomponent‐multisource shear‐wave reflection data to extract the information contained in the split shear‐waves, are found to be directly meaningful only in situations where crack orientations do not change with depth.
TUNNELLING AND THE GENERALIZED RAY METHOD IN PIECEWISE HOMOGENEOUS MEDIA1
Authors: DRIJKONINGEN GUY G. 
Published: April 2006

AbstractThe contributions of the tunnelled constituents to a seismic wave are analysed in two different configurations pertaining to homogeneous acoustic media: a thin high‐velocity layer, present in a plane‐layered configuration, and a thin layer in media separated by dipping interfaces. The generalized ray method in the far‐field is used to determine them. We expand around the relevant ray parameters in order to determine the characteristics of the tunnelling ray and find that the most important feature of this type of ray is a phase (in terms of asymptotic ray theory) which has a real and an imaginary part. Numerical results illustrate this.
TOMOGRAPHIC INVERSION OF NORMALIZED DATA: DOUBLE‐TRACE TOMOGRAPHY ALGORITHMS1
Authors: DOBROKA MICHAEL , DRESEN LOTHAR , GELBKE CHRISTOPH , RÜTER HORST 
Published: April 2006

AbstractTomography is widely used in geophysics as a technique for imaging geological structures by means of data that are line integrals of physical characteristics. In some transmission measurements, due to various kinds of normalization, the measured data are related to two (the current and the reference) raypaths and can be expressed as a function of differences between line integrals. This is the case, for example, in seismo‐acoustic emission measurements, when (since the exact start time is unknown) only the differences between traveltimes (differences between line integrals of the slowness) can be determined. Similarly the use of normalized Fourier amplitudes results in data dependent upon the difference between line integrals of the absorption coefficient (computed along the actual and the reference raypaths).In order to invert these data the ordinary tomography algorithms should be modified. Some generalizations are presented for series expansion tomography methods in order to make them applicable to reconstruction problems in which the input data are differences between two line integrals. The conjugate gradient and the simultaneous iterative reconstruction technique (SIRT) methods were adapted and tested. It is shown that the modified tomography algorithms are stable and sufficiently accurate for practical use. In the reconstruction of noise‐free difference data, the conjugate gradient algorithm is found to be faster and more accurate while, in the case of noisy difference data, the modified SIRT algorithm is more stable and insensitive to noise.
NATURAL SMOOTHNESS CONSTRAINTS IN CROSS‐HOLE SEISMIC TOMOGRAPHY1
Authors: PILKINGTON M. , TODOESCHUCK J. P. 
Published: April 2006

AbstractTomographic inversion problems are ill‐posed and therefore solutions must be either damped or regularized to produce results that are geologically reasonable. We introduce a priori information in terms of parameter covariances to constrain the solution. We use slowness logs to determine the appropriate parameter covariances for the inversion of traveltime data collected for a cross‐hole geometry. We find that the logs exhibit power spectra proportional to a power α of the frequency. The value of α controls the smoothness of the inversion solution. For α < 0, the solution smoothness is maximized. Thus, knowing the correct value of α, which can be found from well logs, we can specify the appropriate amount of smoothing, rather than using some arbitrary level.Inversions were carried out on synthetic data for the case of α= 0 and α=−1. The use of α= 0 implies uncorrelated model parameters and is equivalent to standard damped least‐squares methods. We find that solutions for α= 0 show greater complexity than for α=−1 but this level of resolution can be illusory. An example from the Midale field, Saskatchewan, Canada, is inverted using both α= 0 and α=−2, given by the observed parameter covariances. The solution for α= 0 exhibits spurious detail which must be smoothed away. For α=−2, the solution smoothness is maximized and we recover only that structure which is required to fit the data.
NON‐LINEAR AVO INVERSION FOR A STACK OF ANELASTIC LAYERS1
Authors: DAHL TERJE , URSIN BJØRN 
Published: April 2006

AbstractParameters in a stack of homogeneous anelastic layers are estimated from seismic data, using the amplitude versus offset (AVO) variations and the travel‐times. The unknown parameters in each layer are the layer thickness, the P‐wave velocity, the S‐wave velocity, the density and the quality factor.Dynamic ray tracing is used to solve the forward problem. Multiple reflections are included, but wave‐mode conversions are not considered. The S‐wave velocities are estimated from the PP reflection and transmission coefficients. The inverse problem is solved using a stabilized least‐squares procedure. The Gauss‐Newton approximation to the Hessian matrix is used, and the derivatives of the dynamic ray‐tracing equation are calculated analytically for each iteration.A conventional velocity analysis, the common mid‐point (CMP) stack and a set of CMP gathers are used to identify the number of layers and to establish initial estimates for the P‐wave velocities and the layer thicknesses. The inversion is carried out globally for all parameters simultaneously or by a stepwise approach where a smaller number of parameters is considered in each step.We discuss several practical problems related to inversion of real data. The performance of the algorithm is tested on one synthetic and two real data sets. For the real data inversion, we explained up to 90% of the energy in the data. However, the reliability of the parameter estimates must at this stage be considered as uncertain.