bluuebunny/crossref_metadata_2025_split huggingface dataset:
REFLECTION OF ELASTIC WAVES FROM PERIODICALLY STRATIFIED MEDIA WITH INTERFACIAL SLIP*
Authors: SCHOENBERG M. 
Published: April 2006

AbstractA periodically stratified elastic medium can be replaced by an equivalent homogeneous transverse isotropic medium in the long wavelength limit. The case of a homogeneous medium with equally spaced parallel interfaces along which there is imperfect bonding is a special instance of such a medium. Slowness surfaces are derived for all plane wave modes through the equivalent medium and reflection coefficients for a half‐space of such a medium are found. The slowness surface for the SH mode is an ellipsoid. The exact solution for the reflection of SH‐waves from a half‐space with parallel slip interfaces is found following the matrix method of K. Gilbert applied to elastic waves. Explicit results are derived and in the long wavelength limit, shown to approach the results for waves in the equivalent homogeneous medium. Under certain conditions, a half‐space of a medium with parallel slip interfaces has a reflection coefficient independent of the angle of incidence and thus acts like an acoustic reducing mirror. The method for the reflection of P‐ and SV‐waves is fully outlined, and reflection coefficients are shown for a particular example. The solution requires finding the eigenvalues of a 4 × 4 transfer matrix, each eigenvalue being associated with a particular wave. At higher frequencies, unexpected eigenvalues are found corresponding to refracted waves for which shear and compressional parameters are completely coupled. The two eigenvalues corresponding to the transmitted wavefield give amplitude decay perpendicular to the stratification along with up‐ and downgoing phase propagation in some other direction.Much of this work was performed while the author was at the Department of Geophysics and Planetary Sciences, Tel‐Aviv University, Ramat‐Aviv, Israel. The author is grateful for illuminating discussions with K. Helbig and K. Gilbert.
PROBLEMS IN THE USE OF GEOPHYSICAL INFORMATION WHEN INTERPRETING THE DATA OF SPACECRAFT OBSERVATIONS*
Authors: BOGOSLOVSKY V. A. , IL'INA E. B. , KUZMINA E. N. , OGILVY A. A. 
Published: April 2006

AbstractObservations made from space have become almost a standard method of geologic investigation. However, interpretation of space photographs is not always unambiguous. Particularly, the nature and spatial location of various objects photographed from space remain obscure. It is especially difficult to investigate territories where basement rocks are overlain by sedimentary cover.From the example of investigations carried out in the central part of the Russian Platform it is shown how separate lineaments and ovoids reflect the peculiar features of geologic, hydrogeologic and geophysical structure of the region. It has been established that—within the sedimentary cover—they correspond usually to zones of increased fissuredness and are associated with low‐amplitude tectonic dislocations.The authors concentrate on the elucidation of the transfer of deep geologic information to the surface. The sedimentary cover is regarded as a communication channel which makes it possible to employ the concepts of information theory for assessing its “transparency”. The contrast due to heterogeneities of the sedimentary cover in space photographs and, consequently, the degree of their manifestation in geologic and physical fields, depends on the water content of the rocks composing the section. Along with hydrogeologic characteristics, geo‐electric parameters, such as specific electric resistivity, longitudinal conductivity and transverse resistivity, can serve as criteria of the degree of rock moistening.When the location of lineaments does not coincide with the plane axes of geophysical anomalies, an inclination of the fault plane can be supposed. The angle of inclination is characterized by the deviation of the lineament from the axis of the anomaly.The data resulting from the investigation of selected standard structures were used to develop a classification based on the degree of their expression in hydrodynamic, geoelectric, gravitational, and magnetic fields and depending on the spatial position and peculiarities of geologic arrangement. For geophysical investigations of unknown structures detected by space observations, this classification can be used as the basis for solving inverse problems by the methods of mathematical statistics.
REGIONAL GEOTHERMAL EXPLORATION IN EGYPT*
Authors: MORGAN P. , BOULOS FOUAD K. , SWANBERG C. A. 
Published: April 2006

AbstractAlthough Egypt is not characterized by abundant Cenozoic igneous activity, its location in the northeastern corner of the African plate suggests that it may possess geothermal resources, especially along its eastern margin. Regional geothermal exploration has been carried out in Egypt using the thermal gradient/heat flow technique and groundwater temperature/chemistry technique. In the thermal gradient/heat flow study, existing oil‐well bottom‐hole temperature data as well as subsurface temperature measurements in existing boreholes were utilized before special thermal gradient holes were drilled. Groundwater temperature and chemistry data were used to extend the geographic range of the direct subsurface thermal measurements. On a very modest budget, a regional thermal high has been discovered along the eastern margin of Egypt, and a local thermal anomaly has been discovered in this zone. Published geological information suggests that the sandstones of the Nubian Formation may be a suitable reservoir for geothermal fluids. The new data indicate that temperatures of 150°C or higher may be found in this reservoir in the Gulf of Suez and Red Sea coastal zone where it lies at a depth of 4 km and deeper.
MIGRATION BY EXTRAPOLATION OF TIME‐DEPENDENT BOUNDARY VALUES*
Authors: McMECHAN G. A. 
Published: April 2006

AbstractMigration of an observed zero‐offset wavefield can be performed as the solution of a boundary value problem in which the data are extrapolated backward in time. This concept is implemented through a finite‐difference solution of the two‐dimensional acoustic wave equation. All depths are imaged simultaneously at time 0 (the imaging condition), and all dips (right up to vertical) are correctly migrated. Numerical examples illustrate this technique in both constant and variable velocity media.
THE INTEGRAL‐FINITE‐DIFFERENCE METHOD FOR CALCULATING TRANSIENT ELECTROMAGNETIC FIELDS IN A HORIZONTALLY STRATIFIED MEDIUM*
Authors: GOLDMAN M. M. 
Published: April 2006

AbstractA finite‐difference solution and an integral algorithm are developed for computing time‐domain electromagnetic fields generated by an arbitrary source located in horizontally stratified earth.The finite‐difference problem is first solved for the kernel of an integral Bessel transform of the desired field and then an inverse transformation is performed either by numerical integration or by linear filtering. A description of the appropriate algorithm as well as numerical results are presented for two cases: an axially symmetric source (a vertical magnetic dipole or a circular loop) placed on the surface of a horizontally layered section overlying either an insulating or an ideally conducting basement.In comparison with the conventional methods used for computing transient electromagnetic fields, the integral‐finite‐difference method is more universally applicable.
GEOELECTRICAL MODELS INVOLVING LAYERS WITH A LINEAR CHANGE IN RESISTIVITY AND THEIR USE IN THE INVESTIGATION OF CLAY DEPOSITS*
Authors: MUNDRY E. , ZSCHAU H.‐J. 
Published: April 2006

AbstractA model has been set up for the interpretation of geoelectrical sounding data for certain kinds of clay deposits containing gradually varying amounts of sand. The model assumes that in the so‐called gradient layer, resistivity varies linearly with depth. Model calculations show how such a layer can be replaced by two homogeneous layers.An inversion procedure using the Marquardt algorithm has been developed for the interpretation of sounding data obtained with the Schlumberger array; it assumes a gradient layer beneath several covering layers. The procedure is demonstrated on two clay deposits. A comparison is made between the newly developed interpretation, the traditional approach using model curves, and computerized inversion for a homogeneous layer model.
EXPLORATION OF HIGH RESISTIVITY BASEMENT USING ELECTRICAL AND MAGNETIC FIELDS OF QUASI‐STATIC POINT SOURCES*
Authors: SZARKA L. 
Published: April 2006

AbstractDetermination of thickness of sediments (usually of high conductivity) overlying a high‐resistivity basement is one of the basic problems of electrical exploration methods. This paper proposes to determine horizontal electrical conductance on the basis of impedance calculated from electrical and magnetic fields of distant quasi‐static (low‐frequency) point sources. Using the proposed method, horizontal conductance of the sediments can be determined also from artificial quasi‐static noise‐impulses coming from sources of unknown position and intensity. The results of analogue modeling and field examples prove the potential of the proposed technique.
SHEAR WAVES BY AN EXPLOSIVE POINT‐SOURCE: THE EARTH SURFACE AS A GENERATOR OF CONVERTED P‐S WAVES*
Authors: FERTIG J. 
Published: April 2006

AbstractThe most common source of seismic energy is an explosion at some depth in a borehole. The radiated waves are reflected not only at the subsurface layers but also at the free surface. The earth's surface acts as a generator of both P‐ and S‐waves.If the source depth is much less than the dominant wavelength the reflected waves resemble closely the waves generated by a single force. Theoretical seismograms were computed with different methods to look for the relevance of the surface‐reflected waves. The numerical experiments show reflected shear waves even for small shotpoint—receiver distances. Due to their polarization these waves can be detected most easily on in‐line horizontal geophones. The existence of these waves was examined during a conventional survey in Northern Germany. Conventional data analysis shows a large variability in the νp/νs ratio. The method used here produced a shear‐wave section with a rather good signal‐to‐noise ratio down to 4 s S‐wave reflection time.
A THREE‐DIMENSIONAL NUMERICAL BOTTOM‐HOLE TEMPERATURE STABILIZATION MODEL*
Authors: JONES F. W. , RAHMAN M. , LEBLANC Y. 
Published: April 2006

AbstractThe heat flow equation in cylindrical coordinates is solved numerically for any general distribution of thermal diffusivity. The temperature stabilization of a borehole is considered, and solutions for the case where thermal diffusivity is a function of radial distance from the borehole are obtained and compared to solutions for uniform diffusivity. The results are discussed in terms of thermal diffusivities that are different for the well contents and for the surrounding material. It is found that the approach to formation temperature is affected by differences between well contents and the surrounding region.
A DIGITAL LINEAR FILTER FOR RESISTIVITY SOUNDING WITH A GENERALIZED ELECTRODE ARRAY*
Authors: O'NEILL D. J. , MERRICK N. P. 
Published: April 2006

AbstractThis paper presents a digital linear filter which maps composite resistivity transforms to apparent resistivities for any four—electrode array over a horizontally layered earth. A filter is provided for each of three sampling rates; the choice of filter will depend on resistivity contrasts and computational facilities.Two methods of filter design are compared. The Wiener‐Hopf least‐squares method is preferable for low sampling rate filters. The Fourier transform method is more successful in producing a filter with a high sampling rate which can handle resistivity contrasts of 100 000: 1.
GENERALISATION ET OPTIMALISATION DE LA METHODE DES DIFFERENCES FINIES POUR LA MODELISATION EN MAGNETO‐TELLURIQUE (MT)*
Authors: DOUCET D. , VAN NGOC PHAM 
Published: April 2006

AbstractAfter a brief discussion of MT modeling methods, we expose a particular application of finite elements that may be assimilated to a finite‐difference method. The resulting linear equations obtained are similar to the transmission network equations for two‐dimensional media. The introduction of resistivity discontinuities is more rigorous than in the usual finite‐difference development. The consequence is an improved accuracy.We also present some conclusions about various problems encountered in modeling, such as the choice of network boundary conditions, linear system resolution and the final derivation of apparent resistivity. Application of successive over‐relaxation is discussed and we detail rules for mesh design that control result accuracy and iterative convergence.
COMPUTER MODELING OF VERTICAL SEISMIC PROFILING*
Authors: YOUNG T. K. , MONASH C. B. , TURPENING R. M. 
Published: April 2006

AbstractThere are numerous modeling techniques commonly employed for the computer simulation of seismic wave propagation. The capabilities of these techniques vary according to the theoretical foundations and subsequent approximations upon which the algorithms are based. This paper constitutes a comparative review of seven modeling methods: geometric ray theory, asymptotic ray theory, generalized ray theory, Kirchhoff wave theory, Fourier synthesis, finite differences, and finite elements.These methods can be categorized as ray or wave, acoustic or elastic, and can be contrasted according to their relative abilities to simulate such behavior as wave interference effects, diffractions, and mode conversions. As is implied by their names, geometric ray theory and asymptotic ray theory are both ray methods. The other five methods provide wave theory simulations. Geometric ray theory and Kirchhoff wave theory are normally implemented in acoustic form, while the other methods are readily adapted for computing elastic theory solutions. Generalized ray theory and Fourier synthesis are more limited in the complexity of geological model they can accommodate than are the other techniques. The methods which typically demand the greatest computer resources are the finite‐difference and the finite‐element techniques. All methods can incorporate at least some multiple events. Diffractions, however, are only inherent in the solutions computed by Kirchhoff wave theory, finite differences and finite elements. Attenuation is readily incorporated in both the Fourier synthesis and the finite‐element methods.As an example of the application of seismic modeling, a geological model representative of a typical petroleum exploration target is used to compare vertical seismic profiles calculated by different modeling methods.
SIMULATIONS OF REFLECTIONS AND DIFFRACTIONS FROM TARGETS BURIED IN A MEDIUM WITH LINEAR VARIATION OF VELOCITY WITH DISTANCE*
Authors: CHANDER RAMESH , RADHAKRISHNA BHAT C 
Published: April 2006

AbstractA combination of ray theory and diffraction theory has been used to simulate reflections of seismic pulses from a full plane and reflections and diffractions from a half plane buried horizontally in earth models with linear increase of velocity with depth. Wave forms of signals reflected from a full plane in this case are modifications of the source pulse, even if the medium is assumed to be perfectly elastic. The extent of pulse modification increases with increase of the velocity gradient. The duration of the reflected pulse is always longer than that of the source pulse. The pulses diffracted from the edge of the half plane in such cases do not lie along hyperbolic curves in the time‐offset plane. Diffracted signals can be observed only up to a limited distance from the edge of the half plane equalling half the horizontal range of the ray which is tangential to the half plane and has its end points at the level of the source and the receiver.
SEISMIC REFRACTION METHOD TO STUDY THE FOUNDATION ROCK OF A DAM*
Authors: DUTTA N. P. 
Published: April 2006

AbstractDynamic elastic moduli like E, μ, K and μ of the foundation rock of a dam have been determined by finding Vp‐ and Vs‐velocities by seismic refraction with a hammer as source. Some parameters such as “fracture frequency” and “rock quality designation” (RQD) of the foundation rock have been derived using “average regression curves” and Vp‐velocities. By comparing K/μ with Vp/Vs, a few locations showing weathered conditions have been demarcated. This compares well with RQD values of those locations.
INVERSION METHOD IN THE SLANT STACK DOMAIN USING AMPLITUDES OF REFLECTION ARRIVALS*
Authors: CARRION Ph.M. , KUO J. T. , STOFFA P. L. 
Published: April 2006

AbstractAlmost all ray‐tracing methods ignore the analysis of the amplitudes of seismic arrivals and therefore utilize only half of the available information. We propose a method which is a combination of ray‐tracing imaging and transformation of the amplitudes of wide‐aperture data.Seismic data in the conventional X‐T domain are first transformed to the domain of intercept time τ and ray parameter p to recover the plane wave response. The next step is the derivation of a series of plane wave reflection coefficients, which are mapped as a function of τ and p. The reflection coefficients R(τ, p) for two arbitrarily chosen traces can then be used in our inversion method to derive a slowness‐depth and a density‐depth profile. It is shown that the inclusion of amplitudes of seismic arrivals (in this method, we consider the acoustic case) makes the inverse method highly stable and accurate. In a horizontally stratified medium one can recover separate profiles of velocity and density. Since this method utilizes large‐offset data, it can be used for separate recovery of velocity and density to a greater depth.
A LABORATORY INVESTIGATION OF THE INDUCED POLARIZATION OF THE TRIASSIC SHERWOOD SANDSTONE OF LANCASHIRE AND ITS HYDROGEOLOGICAL APPLICATIONS*
Authors: OLORUNFEMI M. O. , GRIFFITHS D. H. 
Published: April 2006

ABSTRACTThe time‐domain induced polarization (IP) of saturated Sherwood Sandstone correlates significantly with the intergranular permeability and the matrix conductivity but only at low electrolyte concentrations (< 500 p.p.m. NaCl). An increase in the magnitude of sandstone IP with increase in the valence of the electrolyte cation is pronounced but occurs only at intermediate concentrations, i.e., between 100 and 2500 p.p.m. Surface IP and resistivity depth sounding measurements, supplemented by data from laboratory measurements, can be used to estimate the groundwater conductivity and hence the salinity in a moderate to strongly saline sandstone aquifer.
THE GRAVITY AND MAGNETIC FIELDS FROM ELLIPSOIDAL BODIES IN THE WAVENUMBER DOMAIN*
Authors: PEDERSEN L. B. 
Published: April 2006

ABSTRACTWavenumber domain expressions for bodies with elliptical cross‐section and of ellipsoidal shape have been developed both for homogeneous bodies and for certain bodies of density/magnetization varying linearly with depth or, more generally, according to a polynomial with depth. The simple expressions thus obtained lend themselves to an easy analysis, especially for long and short wavelengths. At the long‐wavelength end of the spectra their decay is governed by an exponential with a decay “depth” equal to the depth to the center of mass. At the short‐wavelength end this depth is replaced by the depth to the upper focus of the ellipsoid (or the elliptic cross‐section). For vertically inhomogeneous ellipsoids the decay rate is also dependent on the product of the vertical gradient of density/magnetization and their semi‐axes.
SEISMIC MODELING IN THE IMPLICIT MODE*
Authors: MUFTI I. R. 
Published: April 2006

AbstractFinite‐difference seismic models are often saddled with huge memory requirements for data manipulation, a prohibitive amount of CPU time and even approximate results. At least part of these costs may be due to the fact that most of the work reported on this subject is devoted to the development of explicit models which suffer from severe limitations of stability and necessitate extremely fine time sampling of the wavefield.A new method of seismic modeling which works in the implicit mode and is unconditionally stable is put forward. It is based on the self‐adjoint version of the acoustic wave equation. The evaluation of the wavefield is done by using a highly efficient splitting algorithm which does not require transposing the field data at the various time steps. Moreover, it can accommodate anisotropic media as well as three‐dimensional structures. Computational efficiency is achieved by introducing an unconventional procedure which yields the sum of the values of the wavefield corresponding to a new time step and a previous time step. The new value can be obtained from this sum by a simple subtraction.A number of numerical examples, some obtained by using time steps about five times larger than the largest permissible time step in the corresponding explicit model, are presented. The effect of topographic changes on the growth of the wavefield due to a source located near the surface of the ground is investigated.
DATA QUALITY ASSESSMENT OF TIME‐DOMAIN IP DECAY CURVES*
Authors: BUSBY J. P. 
Published: April 2006

AbstractData quality assessments have been made on time‐domain induced polarization decay curves taken over sulphide ore bodies of Cyprus. The data were taken with the dipole‐dipole, pole‐dipole and symmetrical quadripole arrays. The results indicate that at larger array separations decay curves taken with the dipole‐dipole and pole‐dipole arrays are often seriously affected by noise, but that curves taken with the symmetrical quadripole array are not so affected. Optimum pseudo‐sections designed to minimize the noise show that the noise results in spurious highs and lows.This work was carried out as part of a NERC research studentship at the University of Leicester.
A NUMERICAL STUDY OF LAMB'S PROBLEM*
Authors: KUHN M. J. 
Published: April 2006

AbstractThis paper considers propagation of elastodynamic waves in an imperfectly elastic half‐space. Two different excitation modes are investigated: a buried source of compressional waves and a vertically directed areal load applied to the surface. Numerical integration of the analytical solution of the wave equation allows study of the vertical and horizontal components of displacement and/or particle velocity anywhere in the half‐space. One case of particular interest concerns the examination of particle displacement and velocity at the surface in a circular area above the source. In another application seismograms generated by an explosive buried source are contrasted with seismograms generated by the transient application of a vertically directed load to the free surface. Still another application of considerable practical interest concerns the study of the nongeometrical pS—wave, in particular its characteristics as functions of range and depth. Finally, in the last application the behavior of a rarely observed wave (denoted here by the letter U) is studied in both elastic and visco‐elastic half‐spaces.
THE APPLICATION OF VHF MEASUREMENTS TO EARTHQUAKE PREDICTION*
Authors: SINGH R. P. , RANKIN D. 
Published: April 2006

AbstractThe use of Very High Frequency measurements applied to the detection of the variation in dielectric constant as a precursor to earthquakes is examined. It is concluded that such a technique is feasible and that the installation of source and sensor in separate deep boreholes provides a suitable environment for this type of study. The boreholes could also be used for a considerable range of other types of instrumentation.