bluuebunny/crossref_metadata_2025_split huggingface dataset:
ACOUSTICAL IMAGING OF SOURCE RECEIVER COINCIDENT PROFILES*
Authors: KUHN M. J. 
Published: April 2006

AbstractThe first part of this paper examines a special case of acoustical imaging in which the source and the receiver coincide. The benefits of weighting and muting are studied in detail by means of computer modeling. The test model consists of a single planar interface z=z1, abruptly terminated at x= o. The amplitude and phase responses are computed in the plane z=z0= o for two separations of neighboring stations, Δx=λ/10 and Δx=λ/2. Six different weighting factors are used in the test. However, in this source‐receiver coincident case, three of the weighting factors produce identical responses, so that all six test factors may be represented by only four curves. It is found that when the spatial sampling at the aperture approaches the condition of critical sampling, i.e. Δx=λ/2, only the weighting factor which implicitly takes into account beam steering along the specular reflection path is acceptable. This factor alone keeps the amplitude and the phase curves undistorted until the difference 2 ·ΔR between two neighboring paths reaches approximately λ/2. If we set 2 ·ΔR=λ/2, we may construct a set of curves which we may call quite appropriately muting curves. These curves are physically interpretable only for station separation Δx > λ/4. The muting curves are symmetrical about the line x= 0 and their angular opening depends on spatial separation Δx, depth z, and wavelength λ (which may vary with depth).The second part of this paper suggests how the weighting factor with implicit beam steering can be applied to reconstruction of two and three‐dimensional wavefields. Seismic migration of common depth point (CDP) stacked line data is also discussed. This is a hybrid case which presents certain theoretical difficulties. We shall also mention the velocity problem which is inherent to migration of CDP stacked data.The third and final part concerns implementation of the migration of CDP stacked data. When the spatial sampling is between λ/4 and λ/2, the migration process will benefit from beam steering and from muting. The benefits are more subtle when the separation of the traces is less than λ/4. However, in that case the cost of data collection is considerable and often prohibitive. In either case the migration of seismic data can be expedited by use of precalculated tables of migration velocities, ray path distances, and weights (including muting).
COMBISWEEP—A CONTRIBUTION TO SWEEP TECHNIQUES*
Authors: WERNER H. , KREY TH. 
Published: April 2006

AbstractNonlinear sweeps have often successfully been employed in the 1960s. However, this area of sweep technology has been neglected since the introduction of digital recording techniques in the Vibroseis system. Now the advent of computerized recording instruments yields a new economical possibility of forming approximately nonlinear sweeps by combining several linear sweeps with or without time gaps to a “Combisweep”. The total duration of a Combisweep may be as long as the maximum available recording time, for example 32 s.Beside the attenuation of correlation noise, the new method has further merits, such as the weighting of predetermined frequency ranges, in order to effect a certain kind of optimum filtering on the emitter side, or in order to compensate to some degree for frequency dependent absorption.In all these applications the Combisweep is considered as one signal in the correlation process. But by correlating with the individual sweeps or a partial combination of them and by applying automatic switching at predetermined times within the gaps between the individual sweeps additional possibilities arise, such as obtaining in one run with a twenty‐four channel recording unit twenty‐four traces with small distances between vibrators and geophones for shallow reflections and another twenty‐four traces with larger distances for deeper reflections. Various Combisweeps and their applications are presented.
TIME‐DOMAIN ELECTROMAGNETIC SOUNDING—COMPUTATION OF MULTI‐LAYER RESPONSE AND THE PROBLEM OF EQUIVALENCE IN INTERPRETATION*
Authors: MALLICK K. , VERMA R. K. 
Published: April 2006

AbstractComputations of the time‐domain electromagnetic response of a multi‐layered earth have been carried out for different source‐receiver coil systems. The primary excitation is a train of half‐sinusoidal waveforms of alternating polarity. The conversion into the time‐domain involves Fourier series summation of the matched complex mutual coupling ratios of the layered earth models computed by a digital linear filter method. As an example, the response of a perpendicular coil system on the ground surface for two source‐receiver separations has been presented for a five‐layer earth model. This has been compared with the responses of homogeneous, two‐layer, three‐layer, and four‐layer models.Next, the investigations have been extended to study the problems of equivalence of three‐layer models, the intermediate layer of which is either conductive or resistive. For an intermediate conductive layer (H‐type), the studies show that in the early portion of the signal the interpretation of a true three‐layer earth is possible to some extent, whereas the ambiguity due to equivalence persists in the late samples. On the other hand, for an intermediate resistive layer (K‐type), the three‐layer earth and its equivalent model cannot be distinguished from each other over the entire sampling period.On the basis of a computational approach, equivalence has been empirically established as √h/ρ=constant for H‐type earth‐sections, and as h2ρ=constant for K‐type earth sections, where h and ρ are respectively the thickness and resistivity of the intermediate layer.
COMPARISON BETWEEN SCHLUMBERGER ELECTRODE ARRANGEMENT AND SOME FOCUSED ELECTRODE ARRANGEMENTS IN RESISTIVITY PROFILES*
Authors: BRIZZOLARI E. , BERNABINI M. 
Published: April 2006

AbstractThe Mining Geophysics Institute of the University of Rome has tested focused electrode arrangements. These have been used both in the field for resistivity profiles in investigations for buried cavities and in the laboratory for model tests in an electrolytic tank with cylindrical and parallelepipedal bodies.Anomalous potentials due to superficial noise, assumed to be a hemisphere having an equatorial plane coinciding with the earth surface, have been calculated for focused current electrode arrangements. Similar anomalies have also been calculated for the Schlumberger arrangement.Model tests in an electrolytic tank have also been carried out for a Schlumberger arrangement and parallelepipedal buried bodies.It is interesting to know whether the focused current arrangements give better results than the Schlumberger arrangement. Since it is not possible to compare directly the results of the two arrangements, the comparison is made by considering the ratio of the anomalies to the superficial noise (considered as hemisphere) for Schlumberger and focused arrangements. The results of the comparison show that, generally, the focused arrangements give better results even in the presence of superficial noise.
THEORETICAL REFLECTION SEISMOGRAMS FOR ELASTIC MEDIA*
Authors: KENNETT B. L. N. 
Published: April 2006

AbstractTheoretical seismograms for an explosive source in a multilayered elastic medium are constructed by Fourier synthesis and plane wave superposition. The calculation scheme which builds up a reflection matrix layer by layer in the frequency and wave number domain allows the inclusion of attenuation and a choice of the level of internal multiples in each layer.Comparative calculations of theoretical seismograms for an elastic model and in the acoustic approximation, neglecting shear, show that the main differences arise at large offsets. The inclusion of shear waves leads to lower reflected P wave amplitudes at the end of the spread but only small amounts of converted phases.
PATTERN SPACE OF SEISMIC ANOMALIES ASSOCIATED WITH HYDROCARBON DEPOSITS*
Authors: KHATTRI K. , MITHAL R. , GAUR V. 
Published: April 2006

AbstractThe quantitative estimate of the anomalies in the seismic signal associated with a realistic subsurface hydrocarbon bearing anticline is discussed. This estimate is derived by the calculation of synthetic seismograms for a dissipative media. The reflection amplitude anomaly associated with gas deposit are as much as three times the background level. However, oil deposits give rise to much weaker anomalies. A decrease of the reflection amplitude by about fifty percent occurs for horizons lying below the gas bearing zone, causing a shadow zone. Time sag anomaly is dependent chiefly on the thickness of the gas bearing zone. The appearance of the anomaly on the time section is modified by the structure, and flat reflectors may be seen over the structure owing to the mutual cancellation of the structural rise and the time sag. Flat spots corresponding to the horizontal fluid interfaces may be expected at the edges of the pool only. The presence of hydrocarbon bearing layers causes a diagnostic character in the composite reflection signal which is very clearly seen in the spectra, the cross correlation functions, and the cross‐spectra in profiles crossing the hydrocarbon bearing structures. These are potential diagnostic tools for locating hydrocarbon deposits, particularly when entrapped in thin layers.
REFRACTOR VELOCITY DETERMINATION—CAUSE AND NATURE OF SOME ERRORS*
Authors: SJÖGREN B. 
Published: April 2006

AbstractFor detailed determination of refractor velocities of in‐line reversed profiles, two principally different systems have been employed, the ABC method and Hales's method. The two systems differ in the travel‐path arrangements used. The more conventional approach to the problem, the ABC method, makes use of critically refracted rays converging on a common surface position, while Hales's method deals with the common position on the refractor surface from which critically refracted rays diverge towards the ground surface.Because of the travel‐path system used, Hales's method has proved to be particularly applicable to high‐relief structures and to cases where the refractor lies at considerable depth. Some of the ambiguities in more commonly used interpretation techniques can be solved by Hales's method.Some attention has also been paid to errors caused by non‐critical refractions and to the diffraction problem.
NUMERICAL MODELLING OF A NEW MULTI‐FREQUENCY ELECTROMAGNETIC SYSTEM*
Authors: LEE T. , RICHARDSON R. L. , VOZOFF K. 
Published: April 2006

AbstractThe amplitude of the horizontal magnetic field in the ground between two parallel wires, both carrying an alternating current in the same direction, is likely to have a saddle point if the separation between the wires is small and the frequency is low. The amplitude has a maximum in the vertical direction and a minimum in the horizontal.Rectangular geological structures in the ground which are centered between the wires have a varying effect on the magnetic fields at the surface. In general, the vertical magnetic field “crosses over” at the center of the structure.A shallow and flat lying conductor displays a broad flat type of profile when the horizontal magnetic field between the wires is measured. Changing the structure to a narrower but more conducting one at depth will provide a more pointed but still broad profile. The phase of the horizontal field is also increased.When the structure is a thin vertical dyke, the amplitude of the horizontal magnetic field anomaly due to the dyke rapidly decreases as the depth of the dyke is increased. The phase of the horizontal field is less sensitive to changes in depth of the dyke but is more sensitive to the conductivity ratio of the dyke and the half‐space. The amplitude of the vertical magnetic field anomaly due to the dyke is only slightly influenced by conductivity contrast or the depth of the dyke. The phase of the vertical magnetic field, however, is strongly influenced by the conductivity contrast, particularly if the conductivity frequency product is greater than hundred.In essence, the field behaves like that of the conventional vertical loop source, but the fields are uniform over much larger areas. This suggests the possibility of using dip angle measurements for rapid reconnaissance.
CALCULATION OF GALVANIC EFFECTS BY MEANS OF THE METHOD OF SUB‐AREAS*
Authors: ESKOLA L. 
Published: April 2006

AbstractA numerical method is proposed for solving the problem of steady current flow. The electrodynamic model is replaced by the equivalent stationary charge distribution obtained by Poisson's analysis, in which the surface integral equation for field intensity is reduced to a set of simultaneous linear algebraic equations by means of the method of sub‐areas. The solution of the set allows the calculation of an approximation for the charge density distribution on the discontinuity surfaces of conductivity.The method is valid for complex conductivities, whereby the apparent phase shift of IP can be calculated from the complex potential or field intensity. The phase shift anomaly calculated as an application is very similar to the corresponding frequency effect anomaly.The method allows the calculation of the mise‐à‐la‐masse effect as a solution to a potential problem, in which the primary current electrode is located within the body to be surveyed.
RESISTIVITY SOUNDING ON AN EARTH MODEL CONTAINING TRANSITION LAYERS WITH LINEAR CHANGE OF RESISTIVITY WITH DEPTH*
Authors: KOEFOED O. 
Published: April 2006

AbstractMallick and Roy solved the problem of determining the apparent resistivity function for a three‐layer stratification in which the central layer is a transition layer with linear change of the conductivity with depth. In the present paper the problem is solved for a transition layer with linear change of the resistivity with depth, a type of change that seems to be more common in nature than the type considered by Mallick and Roy. The solution is extended to layer stratifications involving an arbitrary number of transition layers and of homogeneous layers. The solution is given in the form of a modification of the recurrence relation that was derived by Pekeris for homogeneous layers.
SIGNIFICANCE OF INFORMATION UNCERTAINTY IN SETTING UP THE INTERPRETATIVE PATTERNS OF GEOPHYSICAL DATA*
Authors: MARCAK H. 
Published: April 2006

AbstractGeophysical interpretation consists of the identification of geological parameters carried out in the state of uncertainty. From this follows that the identificative model must correspond to the degree of uncertainty. As uncertainty measure of the surveyed material the informative distance between different interpretive models is introduced. The influence of the measurement noise and the accuracy of measurement devices as well as the number of measurement sets worked out simultaneously for this survey is analyzed. Numerical examples indicate that the material considered to be not interpretable from the point of view of a certain accurate model may be interpreted by the use of a less accurate model.
RESIDUAL STATIC CORRECTIONS—ITERATIVE SOLUTION ANALYSIS*
Authors: ZELEI A. , SÁGHY G. 
Published: April 2006

AbstractThe iterative estimation process of residual static corrections published earlier is further analyzed.The convergence and convergence rate of the iterative solution are analysed for components of different relative wave length with results of both theoretical and practical value.Relative wave length components practically determinable and indeterminable are defined.One model example is presented for illustrative purposes.
COMPUTATION OF THE NORMAL MOVEOUT VELOCITY IN 3D LATERALLY INHOMOGENEOUS MEDIA WITH CURVED INTERFACES*
Authors: HUBRAL P. 
Published: April 2006

AbstractFor a 3D velocity model of curved first order interfaces and layer velocities which are arbitrary smooth functions of the space coordinates, the normal moveout (NMO)‐velocity can be computed by numerically integrating a system of first order ordinary differential equations for a hypothetical wavefront that originates at the normal incidence point of the normal ray and moves up along the ray to the common mid‐point of the common datum point (CDP) profile.
DIRECT RESISTIVITY INTERPRETATION BY ACCUMULATION OF LAYERS*
Authors: SZARANIEC E. 
Published: April 2006

AbstractThe asymptotic approximation of Pekeris is replaced by two new procedures referred to as the two‐point method and the multilayer method, other steps in the direct interpretation remaining unmodified. The new methods are based on the assumption that there are at least one or two consecutive sample points of the kernel curve containing the information on a particular layer and containing no information on the deeper layers. In any step, the identified covering layers are accumulated and the interpretation progresses to the successive deeper layer.The multilayer method is oriented towards interpretation of data severly contaminated by noise. The elimination of noise with simultaneous averaging of layer parameters is performed in the domain of Dar Zarrouk parameters.
THEORY FOR THE BIPOLE‐DIPOLE METHOD OF RESISTIVITY SOUNDING*
Authors: DAS U.C. , VERMA SAURABH K. 
Published: April 2006

AbstractA theory for the bipole‐dipole method of resistivity sounding is developed. Bipole‐dipole apparent resistivities are related to Schlumberger apparent resistivities at two spacings. The theory can also be used to compute exact dipole‐dipole apparent resistivity curves providing an improvement over the existing techniques which involve far field approximations. A comparison of bipole‐dipole and dipole‐dipole systems reveals the similarity between the two. However, the resolution of the bipole‐dipole system depends on the azimuth angle. The flexibility of the theoretical expressions lead to a generalized field scheme independent of the bipolar or dipolar nature of the current source.
ON SOME RESISTIVITY LOG INTERPRETATION CHARTS*
Authors: ROY A. 
Published: April 2006

AbstractThe paper relates primarily to the borehole and the side bed correction charts for Latero‐logs 3 and 7. Versions of some of these charts published by different companies—or even by the same company in different years—exhibit significant variations. Usually, such publications do not contain adequate information on how the charts were constructed and do not explain why discrepancies occur.Because of these reasons, an attempt has been made in this paper to reconstruct the borehole correction chart for Laterolog 7 and the shoulder bed correction charts for Latero‐logs 3 and 7. For the latter two, the results found differ substantially from those published earlier.The paper demonstrates how departure (response) curves and correction charts for the lateral and the Laterolog 7 sondes can be computed from those for the normal sonde. An apparent resistivity formula is suggested for Laterolog 7 in which all currents that exist in the ground at the time of measurement and that produce the signal are monitored and used. Response curves and correction charts for Laterolog 7, based on such a formula, are presented as illustrations.
DIGITAL LINEAR FILTER FOR COMPUTING TYPE CURVES FOR THE TWO‐ELECTRODE SYSTEM OF RESISTIVITY SOUNDING*
Authors: DAS U.C. , VERMA S.K. 
Published: April 2006

AbstractIn this paper a technique for computing type curves for the two‐electrode resistivity soundings is presented. It is shown that the apparent resistivity due to the system can be represented by a convolution integral. Thus, it is possible to apply the principle of digital linear filtering and compute the desired type‐curves. The filter function required for the purpose is found to be identical with that used to compute the EM sounding curves for the two coplanar horizontal loop system.It is further shown that from the two‐electrode apparent‐resistivity expression one can easily derive the apparent resistivities for other configurations. A comparison of depths of investigation for various systems reveals that the two‐electrode system has greater depth of investigation than other conventional systems. This is also supported by the field example presented in the end, which illustrates the relative performance of the two‐electrode system vis‐a‐vis the Wenner system.
PERFORMANCE OF 2000 AND 6000 PSI AIR GUNS: THEORY AND EXPERIMENT*
Authors: JOHNSTON R.C. 
Published: April 2006

AbstractAir guns have been used in various applications for a number of years. They were first used in coal‐mining operations and were operated at up to 16000 psi charge pressures. Later, single air guns, operated at 2000 psi, found application as an oceanographic survey tool. Air gun arrays were first used in offshore seismic exploration in the mid‐1960's. These early arrays were several hundred cubic inches in total volume and were operated at 2000 psi; they were either tuned arrays or several large guns of the same size with wave‐shape kits. Today's arrays have total volumes greater than 5000 cu in. and are typically operated at 2000 psi. Recently, higher‐pressure, lower‐volume arrays operated at 4000–5000 psi have been introduced; guns used in these arrays are descendants of the coal‐mining gun.On first thought one would equate increased gun pressure linearly with the amplitude of the initial pulse. This is approximately true for the signature radiated by a “free‐bubble” (no confining vessel) and recorded broadband. The exact relation depends on the depth at which the gun is operated; from solution of the free‐bubble oscillation equation, the relation is
image
If Pc,1= 6014.7 psia, Pc,2= 2014.7 psia and PO, 1=PO, 2= 25.8 psia (corresponding to absolute pressure at 25 ft water depth), then
image
Experiments were conducted offshore California in deep water to determine the performance of several models of air guns at pressures ranging from 2000 to 6000 psi and gun volumes ranging from 5 to 300 cu in. At a given gun pressure, the initial acoustic pulse Pa correlated with gun volume Vc according to the classical relation
image
For 1 ms sampled data the ratio
image
varied between 4.5 and 5.5 dB depending on gun model. Pulse width of the 2000 psi signatures indicated they are compatible with 2 ms sample‐rate recording while pulse width of the 6000 psi signatures was greater, indicating they are less compatible with 2 ms sample‐rate recording.Conclusions reached were that 2000 psi air guns are more efficient than higher pressure guns and are more compatible with 2 ms sample‐rate requirements.
COMMON OFFSET PLANE MIGRATION (COPMIG)*
Authors: SATTLEGGER J.W. , STILLER P.K. , ECHTERHOFF J.A. , HENTSCHKE M.K. 
Published: April 2006

AbstractThe quality of results of migration before stack is sensitive to inaccuracies in the velocity field applied. This does not hold if only traces of similar sources‐receiver distances (common offset traces) enter the migration process. In this case, velocity deviations generate minor shifts in travel times of migrated interfaces but no deterioration in quality. These time shifts are proportional to both the velocity error and the square of the source‐receiver distance.The above observations suggest the following migration scheme: migrate separately the traces of the various common offset planes or groups of neighbouring common offset planes; for every common midpoint plane and as a function of travel‐time perform a residual NMO search to find trajectories t) =t)o+px)2 of maximum coherency along which migrated events are aligned; correct for residual NMO and stack the migration results obtained in the various common offset planes to obtain the final migration result.This process not only takes care of inaccurate migration velocities but also corrects partly for effects of refraction.It is shown by means of an example that good migration results are generated even with a considerably deviating velocity field.