bluuebunny/crossref_metadata_2025 huggingface dataset:
A THEOREM FOR DIRECT CURRENT REGIMES AND SOME OF ITS CONSEQUENCES *
Authors: ROY A.
Published: April 2006

Abstract

For any direct current regime, the theorem

image

holds, where φp is the total measured or calculated potential at any point P, φ is the potential distribution known a priori, r is the distance between P and any volume element dV, the gradients are evaluated at the element, and the current sources and sinks have finite dimensions. Thus, each space element behaves as a dipole of moment (1/4π) ∇φdV and contributes its share of signal or potential accordingly. By suitable summation or integration, the contribution from any assigned portion of space to the total measured signal can be determined. Except for the chargeability factor m, the formula also establishes Seigel's initial postulate for the time domain induced polarisation theory.

The contribution depends on the potential gradient, not the current density, and the integration extends over the entire space. Although an insulating target carries no current, it contributes a signal that is in general larger than normal by virtue of its higher potential gradient, and thus helps in creating an overall positive anomaly or resistivity high. On the other hand, an infinitely conducting target—even though it supports a larger amount of current than normal—contributes nothing to the measured signal as the potential gradient is zero everywhere inside. Thus, by contributing less than normal, a conducting target promotes the creation of what is usually a resistivity low. In all cases, the contributions from the space elements add up exactly to the measured or total calculated value.

Some other consequences of the theorem are also discussed, especially in relation to a simple two‐layer earth. For instance, the contribution from the upper half‐space (air) turns out to be equal to that from the lower (real ground), for all observation points on the ground surface and for any ground configuration.
INDUCED POLARIZATION RESPONSE OF A POLARIZABLE INHOMOGENEOUS SPHERE *
Authors: RATHOR B. S.
Published: April 2006

Abstract

Expressions for the I. P. activity coefficient for an inhomogeneous sphere with the conductivity increasing/decreasing from the core towards the periphery according to a power law have been derived.

In certain geometrical situations of the electrode configurations a negative I. P. signal is obtained. The negative I. P. may be understood as due to modifications in the discharge current from the polarized medium during the discharge process.

The results of the investigation may be useful for more accurate interpretation of I. P. anomalies due to isometric inhomogeneous bodies.
MAGNETO‐TELLURIC EXPERIMENTS IN NORTHERN FINLAND *
Authors: BENDERITTER Y. , HÉRISSON C. , KORHONEN H. , PERNU T.
Published: April 2006

Abstract

Magneto‐telluric experiments were performed in August 1974 in the Muhos Jotnian sedimentary formation and the Koillismaa Precambrian basement complex by the Centre de Recherches Géophysiques, Garchy, and the Department of Geophysics, University of Oulu, employing the “Résistivimètre magnéto‐tellurique”, an instrument which permits apparent resistivity values to be observed at eight pre‐selected frequencies between 8–1700 Hz.

Preliminary soundings were made on the Muhos formation and on its surroundings, after which a magneto‐telluric profile was measured involving eleven sites on a line 13 km long extending across the formation. Some further soundings were made at certain sites of known stratigraphy. The resistivity values in the sedimentary formation were found to vary between 10 ωm and 200 ωm, while those in the basement were markedly higher. This resistivity contrast suggests that the method seems to be suitable for delimiting the sedimentary formation.

Soundings made on the basement at Koillismaa show apparent resistivity values in the range 1000–20000 ωm. The interpretation of these soundings was in agreement with other, independently obtained geophysical results.
ON GETTING REFLECTION COEFFICIENTS FROM WAVES *
Authors: HUBRAL P.
Published: April 2006

Abstract

A horizontally layered non‐absorptive system of homogeneous layers may be specified by giving the reflection coefficients at each interface. Provided the layers have equal vertical travel time and a perfect reflector as a free surface, the reflection coefficients are generally reconstructed from the reflected pulses by way of solving simultaneous equations of the Toeplitz matrix form with the Levinson recursion method. There exists an alternative approach to solving this problem which by simple reasoning immediately turns out the (Levinson) recursion scheme. The method is based on formulas that relate to solving the forward problem. It resembles Kunetz's (1962) original inverse solution in as much as the computation of the reflection coefficients is based on the idea of separating the contribution of a primary from the sum of all multiples.
FINITE DIFFERENCE AND WAVE NUMBER MIGRATION *
Authors: HOOD P.
Published: April 2006

Abstract

Finite difference migration has been developed and popularized by J. F. Claerbout of Stanford University and is now widely used in seismic processing. For most sections finite difference migration gives results comparable to those obtained by conventional Kirchhoff migration and, where events are not dipping too much, a cleaner appearance is often apparent. However, there are two practical limitations to the method, and these occur in regions of very steep dip and where there is a large variation of the velocity in the lateral direction.

It is possible to develop successively more accurate equations to deal with the steep dip problem, but above third order these schemes become prohibitively expensive to implement. The finite difference method itself introduces errors and so imposes further limitations on the angle of dip. For the effective treatment of steeply dipping beds there appears to be no method available in the time domain which does not suffer from dispersion inaccuracies. However, by developing wavenumber migration, an exact one‐way wave equation can be used, and this eliminates any error except that caused by finite sampling.

The other difficulty with wave migration is the correct migration in regions with lateral velocity variation. A number of approaches are possible of which three are discussed here. The first uses an exact theory, the second is based on the deviation from a depth stratified model, and the third uses a transformation to a depth co‐ordinate system. All methods are discussed with their advantages and limitations. Finally, some examples are shown of wave migration applied to synthetic and real data.
A SPATIAL ANALYSIS OF UPWARD CONTINUATION OF POTENTIAL FIELD DATA *
Authors: TSAY L. J.
Published: April 2006

Abstract

A spatial analysis of both continuous and discrete operators for upward continuation help us realize the problems and limitations which have been encountered before (Henderson 1960, Kontis 1971) but remained unsolved in practical application of upward continuation computation due to the finite length of data and operator in spatial domain. Various numerical examples show that an improvement of accuracy of continuation computations can be achieved through proper sampling and sufficient length of data.
ACOUSTICAL IMAGING OF SOURCE RECEIVER COINCIDENT PROFILES*
Authors: KUHN M. J.
Published: April 2006

Abstract

The 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

Abstract

Nonlinear 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

Abstract

Computations 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

Abstract

The 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

Abstract

Theoretical 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

Abstract

The 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

Abstract

For 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

Abstract

The 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

Abstract

A 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

Abstract

Mallick 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

Abstract

Geophysical 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

Abstract

The 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

Abstract

For 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

Abstract

The 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

Abstract

A 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

Abstract

The 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

Abstract

In 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

Abstract

Air 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

Abstract

The 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.
SOURCE ARRAY SCALING FOR WAVELET DECONVOLUTION*
Authors: ZIOLKOWSKI A.M.
Published: April 2006

Abstract

A seismic source array is normally composed of elements spaced at distances less than a wavelength while the overall dimensions of the array are normally of the order of a wavelength. Consequently, unpredictable interaction effects occur between element and the shape of the far field wavelet, which is azimuth‐dependent, can only be determined by measurements in the far field. Since such measurements are very often impossible to make, the shape of the wavelet—particularly its phase spectrum—is unknown.

A theoretical design method for overcoming this problem is presented using two scaled arrays. The far field source wavelets from the source arrays have the same azimuth dependence at scaled frequencies, and the far field wavelets along any azimuth are related by a simple scaling law. Two independent seismograms are generated by the two scaled arrays for each pair of source‐receiver locations, the source wavelets being related by the scaling law.

The technique thus permits the far field waveform of an array to be determined in situations where it is impossible to measure it. Furthermore it permits the array design criteria to be changed: instead of sacrificing useful signal energy for the sake of the phase spectrum, the array may be designed to produce a wavelet with desired amplitude characteristics, without much regard for phase.
INMOD—TWO DIMENSIONAL INVERSE MODELING ALGORITHM BASED ON RAY THEORY*
Authors: SATTLEGGER J.W. , ROHDE J. , EGBERS H. , DOHR G.P. , STILLER P.K. , ECHTERHOFF J.A.
Published: April 2006

Abstract

Two dimensional inverse modeling, a process to be applied after standard processing and interpretation, uses interfaces picked by the user. These interfaces are transformed into an approximate subsurface model.

The subsurface model is represented by curved interfaces and interval velocities. The interfaces have to be unique functions of the line coordinate. Otherwise they may be arbitrarily curved and may begin or terminate anywhere along the section, e.g., at faults, pinchouts, salt domes and the like. Interval velocities may vary laterally along the section. The inverse modeling algorithm then modifies the model until traveltimes calculated from this model match the traveltimes observed as closely as possible in a least squares sense.

The traveltimes corresponding to the model are obtained through ray tracing taking exact account of refraction. The traveltimes observed are the arrival times of single impulses before stacking contributing to the interfaces. These traveltimes are provided by ANAKON, a continuous interface analysis system.

The comparison of INMOD results with those of well measurements and those of classical interval velocity computation from seismic data shows the accuracy of the method. Deviations of INMOD derived interface depths are within 2% of well data.
THE SOLUTION OF THE STATIONARY ELECTRIC FIELD STRENGTH AND POTENTIAL OF A POINT CURRENT SOURCE IN A 2 1/2‐ DIMENSIONAL ENVIRONMENT*
Authors: ESKOLA L. , HONGISTO H.
Published: April 2006

Abstract

A method is given for solving the dc electric field problem of a point current source in an anisotropic 2 1/2‐dimensional structural model. The surface integral equation of the field strength is given. Parallel to the strike the field strength is represented by a Fourier series. On the plane perpendicular to the strike each term of the field strength series is solved by means of the method of sub‐sections, where linear behaviour of field strength over the sub‐sections is assumed. Some numerical examples for different galvanic effects are given.
FLUID‐FLOW MONITORING USING ELECTROMAGNETIC PROBING*
Authors: LYTLE R.J. , LAGER D.L. , LAINE E.F. , SALISBURY J.D. , OKADA J.T.
Published: April 2006

Abstract

High‐frequency electromagnetic probing is used to monitor the rate and direction of flow of fluids injected into the ground. This method shows the potential for providing more detailed information than procedures presently used. The experimental technique and the test‐of‐concept experimental results are discussed. This technique has applications in oil‐reservoir engineering and in hydrology studies concerning storage of chemical wastes.
RECURSIVE MIGRATION IN THREE DIMENSIONS*
Authors: BERKHOUT A.J. , DE JONG B.A.
Published: April 2006

Abstract

A simple theory is presented to derive explicit recursive migration schemes in three dimensions which are suitable for situations with lateral velocity variations. The proposed schemes can be realized by one‐dimensional convolutions along the x‐axis and y‐axis. An investigation is made on the errors of each scheme. For each frequency component the amplitude error and the phase error are computed as a function of the immergence angle (dip). Based on the error analysis a critical discussion is given on the stability of explicit schemes.
VELOCITY ANALYSIS USING ITERATIVE STACKING*
Authors: NAESS O.E. , BRULAND L.
Published: April 2006

Abstract

An iterative stacking algorithm (superstack) has been applied in velocity analysis as a velocity filter prior to the coherency measurements. The effect of this filtering is demonstrated on both synthetic and real data using the following three methods of velocity analysis: Constant velocity stack, constant velocity gather, and velocity spectrum. The experimental results indicate that the velocity analysis can be improved by this approach.
BOUNDARY ELEMENT METHOD FOR THE ARBITRARY INHOMOGENEITIES PROBLEM IN ELECTRICAL PROSPECTING*
Authors: OKABE M.
Published: April 2006

Abstract

The generalized integral equation for the electric potential governed by a quasi‐harmonic equation can be derived via a variational formulation. For surface current distributions it is not always a Fredholm integral equation of the second kind. Numerical solutions of the general heterogeneous problem can be obtained with the “reciprocal averaging technique”, where the solution is obtained a second time after exchange of source and field points.
THE AIRBORNE ELECTROMAGNETIC METHOD AS A TOOL OF GEOLOGICAL MAPPING*
Authors: PALACKY G.J.
Published: April 2006

Abstract

Since its development some thirty years ago, the airborne electromagnetic (AEM) method has been primarily used as a tool for mineral prospecting. However, advanced AEM systems are capable of other tasks, such as geological mapping and groundwater exploration. Excellent correlation between maps of apparent conductivity and geological maps was observed in several regions of Brazil where AEM surveys were performed. The degree of correlation seems to depend on the local climate. In humid and subhumid tropical regions, a weathered layer develops whose thickness and conductivity depend upon bedrock lithology. Therefore certain lithological types can be recognized from their conductivity signature; e.g., granites and Precambrian coarse clastic rocks are resistive, metavolcanic (particularly mafic) and volcanic rocks are conductive, Phanerozoic sediments are generally highly conductive.

Two geophysical surveys are analyzed in the paper. The first was conducted with the time‐domain, towed‐bird AEM system in the Itapicuru greenstone belt in the state of Bahia. The apparent conductivity map correlated better with the local lithology than the magnetic map. Results of the AEM survey were successfully used to improve the regional geological map. A helicopter EM system was used in the second survey, which covered a portion of the Precambrian shield of Rio Grande do Sul. Also in this region ground checks confirmed the usefulness of conductivity surveys in geological mapping.

The technique outlined in the paper holds great promise for countries of humid tropical climate, where few outcrops exist and access is often difficult. The tests performed in Brazil indicate that by executing AEM/aeromagnetic surveys during initial stages of mapping and exploration programs, time and expenditure required for geological field work can be considerably reduced. The resulting geological maps are more accurate and the inventory of mineral occurrences becomes more complete.
A NUMERICAL METHOD OF CALCULATING THE KERNEL FUNCTION FROM SCHLUMBERGER APPARENT RESISTIVITY DATA*
Authors: SANTINI R. , ZAMBRANO R.
Published: April 2006

Abstract

A method to calculate the resistivity transform of Schlumberger VES curves has been developed. It consists in approximating the field apparent resistivity data by utilizing a linear combination of simple functions, which must satisfy the following requirements: (i) they must be suitable for fitting the resistivity data; (ii) once the fitting function has been obtained they allow the kernel to be determined in an analytic way.

The fitting operation is carried out by the least mean squares method, which also accomplishes a useful smoothing of the field curve (and therefore a partial noise filtering). It gives the possibility of assigning different weights to the apparent resistivity values to be approximated according to their different reliability.

For several examples (theoretical resistivity curves in order to estimate the precision of the method and with field data to verify the practicality) yield good results with short execution time independent of shape the apparent resistivity curve.
ON THE SIGNIFICANCE OF AMPLITUDE STUDIES IN SHALLOW REFRACTION SEISMICS*
Authors: KUMAR G. NANDA , RAGHAVA M.S. VIJAYA
Published: April 2006

Abstract

An analysis of amplitudes of refraction records of some shallow refraction profiles shot primarily for detailing the near‐surface structure in a granitic terrain has yielded information on refractor properties: reduced amplitudes are plotted on amplitude‐distance graphs. The negative power n to which distance should be raised to represent (elastic) amplitude decay with respect to distance due to spreading of the critically refracted wave involved is examined. Computed values of this “spreading index”n are close to n = 2 as predicted by the theory.

With this value of n, amplitude data are processed to determine residual attenuation attributable to elastic absorption in the bedrock. A graphical approach for this purpose from comparison of amplitude‐distance graphs with the plots of amplitude decay due to spreading which is applicable to flat and horizontal refractor situations is suggested. Assuming residual attenuation to represent absorption in the granite bedrock, the computed coefficients of absorption, which vary from 0.5 to 3.90 km−1 for a frequency of 50 Hz, are obtained.

From amplitude graphs of reversed profiles it is shown that the amplitude differences plot bears a relation to lateral velocity changes in the refractor. From comparison of practical amplitude decay graphs with those computed for different subsurface models, it appears possible to detect fractured rock occurrences in the refractor.
UNDERGROUND GRAVITY SURVEY IN ALPINE REGIONS*
Authors: HUSSAIN A. , WALACH G. , WEBER F.
Published: April 2006

Abstract

An underground gravity study was carried out under extreme conditions of the Alpine regions. The lead–zinc mine Bleiberg, Carinthia, was selected as an example to show the possibilities and limitations of the subsurface gravity method. For in situ density determinations, gravity measurements were made in two vertical mine shafts passing through Triassic sedimentary rocks of the Bleiberg Unit. The main prblem in gravity data reduction in extremely rugged topography is the accurate calculation of the terrain effect on underground stations. A general discussion of the various corrections required for the gravity measurements in the mine is presented. The mean interval densities in the two shafts, in limestone, dolomite, and schists formations, were determined as 2.76 and 2.77 g/cm3, respectively, with an accuracy of better than 0.01 g/cm3 for a depth interval of 50 m. The interval densities provide valuable information about the lithological and structural changes in the shaft surroundings and also agree well with the representative hand sample densities.

In the second part, the applications of subsurface horizontal gravity surveys in exploration are discussed. Since the influence of topography is less underground because of the greater distance to the surface, subsurface surveys have definite advantages over surface surveys and can be very helpful in locating anomalous density zones in the mines. An example of gravity survey with a station spacing of 10 m at a depth of about 540 m is presented.
ELECTROMAGNETIC RESPONSE OF FOLD MODELS IN A UNIFORM FIELD*
Authors: VERMA O.P.
Published: April 2006

Abstract

The electromagnetic response of anticline fold models in a uniform field has been studied using scale‐modeling. With the increasing conductance the in‐phase anomaly increases initially but then starts to decrease and finally attains a negative saturations, whilst the quadrature anomaly reaches a positive saturation. E.M. response of folds is similar to that of an interaction between two weakly coupled circuits in a dipolar field. The base of the fold is one of the characteristic parameters to determine the measured response.
A TUNNEL RESOLUTION INVESTIGATION USING AN AUTOMATED RESISTIVITY TANK ANALOG*
Authors: KARWATOWSKI J. , HABBERJAM G.M.
Published: April 2006

Abstract

The examination of apparent resistivity space sections over two‐dimensional targets requires the accumulation of large amounts of data, if the resistivity field is to be adequately described.

Whilst such data may be obtained from an intensive sounding traverse in the field, it is desirable for interpretation purposes to be able to generate such space sections for a range of model variations.

Within their limitations, tank analogs can be used to provide such interpretation material, but the collection of data can itself be time‐consuming. In this paper, an account is given of the development of an automated tank analog, where an entire space section can be scanned and the output obtained in paper tape format for later processing on a digital computer. The resistance measurement accuracy attained is comparable with normal field measurement accuracy.

A practical example of the use of this analog in investigating the problem of resolving and discriminating between two horizontal cylindrical tunnels is given. This problem is basic to an understanding of the minimum size and disposition of targets which can be observed on a space section. The square array has been used in this example because of its good resolution properties in general prospecting rather than its particular suitability for the targets investigated. The directional response of the array is also illustrated and discussed.
SEISMIC REFLECTION APPLIED TO SEDIMENTOLOGY AND GAS DISCOVERY IN THE GULF OF CADIZ*
Authors: DELAPLANCHE J. , LAFET Y. , SINERIZ B. G. , GIL M. A. REMON
Published: April 2006

Abstract

In the Gulf of Cadiz, a Tertiary basin became filled by clastic series during Miocene and Pliocene times. This terrigenous influx, derived from the Iberic Meseta in the north, is characterized by a sandy episode during the Tortonian and Messinian. The sand deposits were probably connected with uplift and major erosion of the Meseta during the sliding of the olistostrome, which occupied the south of the basin from late Helvetian to middle Tortonian.

High resolution seismic techniques produced a good picture of the stratigraphy and of the depositional environment of the sands.

A further study, using the amplitude of the reflections, inversion of seismic traces into acoustic impedance traces, and modeling, provides a remarkable example of the possibilities of seismic stratigraphy for depicting the lateral evolution of facies and localizing hydrocarbon occurrences. Out of seven exploratory wells based upon seismic information, six encountered gas‐bearing sands with economic potential.