bluuebunny/crossref_metadata_2025 huggingface dataset:
THE INTERPRETATION OF MAGNETIC ANOMALIES DUE TO DYKES*
Authors: BRUCKSHAW J. M. , KUNARATNAM K.
Published: April 2006

Abstract

Based on simple scaling laws, a method is developed for the routine interpretation of magnetic anomalies arising from uniformly magnetised dykes. The method can be applied to any observed component of the field, i.e. the vertical, horizontal or the total field anomaly and it is applicable for any resultant direction of magnetisation of the dyke itself and for any strike direction. The necessary data for the interpretation are given in the form of curves covering a range from o up to 15 for the ratio of the thickness of the dyke to the depth to its top. It is also shown that, by integrating in a suitable manner the observed anomaly due to inclined bodies having a horizontal upper surface and enclosed by two pairs of parallel and inclined planes, a curve is obtained which corresponds to a dyke anomaly and which can be interpreted by the same technique.
Hagedoorn's plus‐minus method: the beauty of simplicity
Authors: Van Overmeeren R.A.
Published: April 2006

Hagedoorn developed the plus‐minus method with the objective of providing the exploration world with a simple and rapid approximation of Thornburgh's wavefront reconstruction method. Straightforward adding and subtracting produces depths and velocities at all geophones where the refracted waves from a set of reciprocal shots are first arrivals. Even though seismic refraction has not kept up with the revolutionary advance of reflection technology during the past decades, the method still has a lot to offer, especially in shallow engineering, and environmental, groundwater and sea‐bed surveying. Its strong points are the ease of application, the low costs, the effectiveness in the shallow zone, the unique ability to provide detailed velocity information on the deepest refractor and the capability for producing parameters for layer and rock characterization.Because of its simplicity, the plus‐minus method is ideally suited for real‐time processing of refraction data in the field, thus monitoring the data quality and the optimal shot configuration. Today, such field processing is easily performed with a laptop and dedicated software. matlab® is a software package that enables tailor‐made processing, offering a combination of a programming language, visualization tools and a large library of ready‐made functions. This paper presents a plus‐minus program developed in matlab and illustrates its application with a case study in Yemen, where seismic refraction was used in a regional groundwater study. Here, refraction provided not only a detailed section across the recharge area of a coastal plain but also the additional information needed to reduce ambiguity in the interpretation of the vertical electrical soundings made in the area.
EINIGE ERGEBNISSE DER VIBRATORTECHNIK IN DER INGENIEURGEOPHYSIK*
Authors: MILITZER H.
Published: April 2006

Nach einer kurzen Einführung über das Prinzip der Vibratortechnik werden Ergebnisse vibratorseismischer Arbeiten auf Platten (Eis, Betonplatten, Straßendecken) mitgeteilt. Der Auswertung wurden Biegewellen zugrunde gelegt und die ermittelten experimentellen Ergebnisse mit theoretischen Dispersionskurven verglichen.
A RAPID METHOD OF DETERMINING DIP OR MAGNETIZATION INCLINATION FROM MAGNETIC ANOMALIES DUE TO DYKE‐LIKE BODIES*
Authors: POWELL D.W.
Published: April 2006

Anomalies due to igneous dykes or layering within metamorphic rocks often approximate to those calculated for geometric dyke models. In aeromagnetic surveys over outcrops of these rocks, where the ‘depth’ is generally the flying height, the ratio in which two geometric functions combine to produce such anomalies can be determined from profiles. This ratio is influenced by geomagnetic latitude, the strike and dip of the ‘dyke’ and its direction of magnetization. It is practicable to solve for either unknown, i.e. dip or magnetization inclination if the other is assumed. Since igneous dykes tend to be vertical and metamorphic rocks are generally inductively magnetized it is geologically valid to make these assumptions.
SOLUTION OF SOME INVERSE PROBLEMS OF APPLIED GEOPHYSICS*
Authors: ZIDAROV D.
Published: April 2006

An iterative process is proposed that computes a distribution of masses giving rise to a certain gravitational field. The possibility of applying a similar procedure to magnetic and electric fields is also discussed.
A GENERALIZED FORM OF NETTLETON'S DENSITY DETERMINATION*
Authors: LINSSER HELMUT
Published: April 2006

The formula for the Bouguer anomaly contains the density of the masses above the datum plane. If we use a wrong value for the density we obtain on the Bouguer map additional anomalies which coincide with the topographic features.

If we do not know the Bouguer density we can compute several Bouguer maps based on different arbitrary densities. Then we select that map which shows the least correlation with the topography and obtain in this way a reasonable value for the density. This method has been found by Nettleton.

Several authors have formulated this process mathematically and have elaborated numerical methods. This paper will present such a method. It differs from the former methods by the following:

A general formula for the Bouguer density is presented, using a non‐specified linear operator. By choosing special definition for this operator, we can obtain the formulae of several of the known methods.

It is shown that the determination of the density by “Nettleton Profiles” gives less reliable results than the statistical investigation of the whole area covered by the gravity survey.

The known methods do not consider the variations of the vertical gradient. It is shown that in areas with a regional gravity anomaly or an extended topographic feature the known methods yield poor results.

It is shown that the vertical gradient of gravity used as operator gives a most exact and simple formula.

It is shown how the electronic computer—after reading the Bouguer values based on an arbitrary initial density—can compute the residuals, derivatives etc., which are based on the corrected Bouguer density.
A METHOD OF COMPUTING TRUE INTERVAL VELOCITIES FROM EXPANDING SPREAD DATA IN THE CASE OF ARBITRARY LONG SPREADS AND ARBITRARILY DIPPING PLANE INTERFACES*
Authors: SATTLEGGER JOHANN
Published: April 2006

The method presented takes the influence of refraction rigidly into account. All travel‐times available are used in an application of the method of least squares. The solution is illustrated by a computer flow diagram.
FIELD CONTINUATION AND THE STEP MODEL IN AEROMAGNETIC INTERPRETATION*
Authors: HENDERSON ROLAND G.
Published: April 2006

Downward continuation of the field in the neighborhood of a singularity of a magnetic anomaly is used to render the anomaly more two‐dimensional, to make the bottom of the causal body more remote, and to obtain an auxiliary function, φ (O, z), by means of which the anomaly may be interpreted in terms of an equivalent vertical contact or step model. The concept of “apparent depth” is introduced and used in studying depth extent and susceptibility. The methods are illustrated with theoretical and practical examples.
THE REDUCTION OF LATERAL EFFECTS IN RESISTIVITY PROBING*
Authors: HABBERJAM G. M. , WATKINS G. E.
Published: April 2006

To be able to make use of all existing probe interpretation techniques, it is desirable that field resistivity observations be conducted in such a way that it is possible to construct an equivalent curve for the surface variation of potential about a point source. Further, the usual parallel layer interpretation will only be justified in as much as the observed potential curve is compatible with such a subsurface geometry. Thus, whilst a potential curve may be constructed from suitable finite potential differences obtained using a Wenner configuration of electrodes, it can be shown, using the tri‐potential technique of measurements, that these potential differences may arise partly from lateral resistivity variations.

In this paper, a ladder network technique is employed to display these lateral effects and an adjustment method proposed to reduce them. The adjusted potential differences obtained form a consecutive series suitable for summation to give the potential function. These values are subject to further minimum adjustments required for them to comply with slope and curvature conditions for a layered medium. After forming the potential curve by summation, a final numerical smoothing process is carried out. From this smoothed potential curve, corresponding Wenner and Schlumberger curves can readily be derived for interpretation.

The method proposed thus attempts to extract from the observed data the maximum part which can be reconciled with a purely depth variation of resistivity. A method is also proposed for compounding the ‘errors of closure’ of the network to provide a Lateral Inhomogeneity Index which gives a measure of the departure of the observed data from the basic interpretation requirements.

The method involves no great labour, but can readily be programmed for a computer if desired. Examples are given of the application of the method to field observations. In the interest of objectivity, the final smoothing has been confined to a single stage but in certain very extreme examples a further stage may prove desirable.
THREE‐DIMENSIONAL ELECTRIC MISE‐A‐LA‐MASSE SURVEY OF AN IRREGULAR LEAD‐ZINC‐COPPER DEPOSIT IN CENTRAL SWEDEN *
Authors: PARASNIS D. S.
Published: April 2006

The principle of the mise‐á‐la‐masse method is to earth one current electrode of a pair in a conducting mineral show (in a borehole, in an outcrop etc.) and measure the resulting distribution of electric potentials. The distribution will, to some extent, reflect the geometry of the ore mass of which the mineral show forms a part.

In a mise‐á‐la‐masse survey of a lead‐zinc deposit in Central Sweden electric potentials were measured on the surface of the ground as well as in some 25 drillholes, in either case with earthings (successively) in three different parts of the irregular ore deposit. Besides this, measurements were made in drillholes with earthings in two further drillholes.

Geologic correlation between the drillholes is difficult in the present case on account of the irregular geometry of the ore deposit. However, the mise‐á‐la‐masse measurements clearly showed the dip and the pitch of the ore body, established connections between the different ore widths encountered in the various holes, and yielded information about the shape of the ore mass.

In particular, the survey showed that the ore lenses must be crescent‐shaped rather than tabular, and the dip was indicated to be westerly, instead of easterly as originally presumed.

Three‐dimensional models of equipotential surfaces were constructed from the observed drillhole and surface potentials (using transparent plastic sheets and thin copper wire) and these helped to elucidate the mass geometry further.

The surface and underground potential data collected in the present case should be of nterest to geophysicists working on analytic continuation problems.
CONTINUOUS ANALYSIS OF THE VELOCITY FUNCTION AND OF THE MOVE OUT CORRECTIONS*
Authors: GAROTTA ROBERT , MICHON DOMINIQUE
Published: April 2006

Several papers presented at the last SEG Convention in Houston by Schneider, Backus et al have shown how important and fruitful it was to obtain a continuous knowledge of the velocity functions and they have solved their problem by a Dynamic Correlation Analysis. Our purpose is to introduce here a method based on the best summation of a set of traces instead of the best correlation.

Practically, this approach has several advantages:

1) Two traces only can be correlated at each step whereas the summation can bear on any number of them;

2) Optimizing the summation is actually what we are looking for since, at the long end, the success of the improvement is evaluated from the compositing of several traces either weighted or not.

On the other hand, an advantage of correlation is the possibility of adding correlations obtained at several places in a same neighbourhood in order to improve the results. With the summation method this is feasible only when dips are inexistent: we shall see that the difficulty due to the dip effect can be turned around.

The basic principle of the method can be summed up as follows: traces relating to a same reflection point are considered; several composites are made, each after applying different move out corrections ranging widely around an estimated adequate velocity function. At each time coordinate, the best adapted velocity function, i.e. the one that yields the best phase relation between reflected events, corresponds to the composite trace the average amplitude of which is the largest.

This way, the velocity function corresponding to primary reflections as well as those corresponding to multiple reflections can be established accurately.

Some examples are shown.
THE ELECTRICAL CURRENT PATTERN INDUCED BY AN OSCILLATING MAGNETIC DIPOLE IN A SEMI‐INFINITE THIN PLATE OF INFINITESIMAL RESISTIVITY*
Authors: KOEFOED O. , KEGGE G.
Published: April 2006

Several papers have been published in which the electromagnetic anomalies are described that are produced by conductive ore bodies of different shapes. No publications are available, however, in which the electrical current pattern is described that is induced in these ore bodies. Yet an insight in this electrical current pattern would be valuable in order to assess the possibilities of different electromagnetic techniques, for instance with regard to the determination of the dip and of the depth extent of plate shaped ore bodies.

In the present paper computations are given of the electrical current pattern induced by an oscillating magnetic dipole in a semi infinite plate shaped orebody of infinitesimal thickness, in which the penetration depth of the current is infinitesimal to a higher order than the thickness of the plate. The computations are based upon an equation derived by Wesley for the magnetic field produced in these conditions, combined with the relation between the electrical current density in a laminar sheet and the magnetic field produced by this current at the surface of the sheet.

The results of the computations show that, if the horizontal distance between the dipole source and the sheet is sufficiently small, the maximum current density of the return current may occur at a depth below the upper edge of the sheet which is appreciably smaller than the depth of the upper edge of the sheet below the surface. The depth of the return current becomes large when the horizontal distance between the source and the sheet is large.
COMPUTATIONS OF NETWORK MODELS OF POROUS MEDIA*
Authors: RINK MANFRED , SCHOPPER JÜRGEN R.
Published: April 2006

In a paper presented at last year's Amsterdam meeting (viz. Geophysical Prospecting 14 (1966), 3, 301–341), J. R. Schopper derived formulas relating formation factor, permeability and porosity, by means of a statistical‐network approach and by treating the electric and hydraulic resistance analogously.

The model of the porous medium consists of a network of branch resistors, their values being statistically distributed about a mean Ro with a relative standard deviation (variation coefficient) s. A properly defined total resistance R of the network can be expressed by the relationship:

image

Here α is a geometrical factor dependent only on the shape of the network (i.e. the number of meshes in the longitudinal and transversal direction), ε is a characteristical constant dependent only on the individual mesh shape (i.e. the number of nodes and branches within a mesh).

This network constant ε enters the equations relating formation factor, permeability and porosity, ε had been found to be in the range zero to one by calculating algebraically two special limiting network cases. However, for a better understanding of which value exactly this constant will have in actual porous media, networks with various mesh shapes have to be treated generally.

Because of the basically statistical approach, the networks have to be large so that a general algebraic treatment is precluded. Hence numerical methods using digital computers must be applied.

The determination of the total resistance R of any resistance network leads to the problem of solving a system of linear, inhomogeneous equations; i.e. Ohm's law written in matrix form:

image

(R) is the matrix of the coefficients, composed of the individual branch resistances.

(I) is the column vector, its components being fictitious circular mesh currents.

(U) is the inhomogeneity column, its components being source voltages within the individual meshes.

The matrix (R) has characteristic properties that depend on the mesh shape on the one hand and on the number and arrangement of the meshes on the other hand. With the regular arrangement of identical meshes investigated here, the matrix always has a banded structure and is symmetrical with respect to the main diagonal, positive definite, and non‐singular.

For the numerical determination of the wanted constant ε the coefficients matrix is provided with values having a known distribution. Here, in particular, a computer‐generated pseudorandom homogeneous distribution is used. The system, of equations is solved for R by a modified Cholesky method. Equation (1′) can then be solved fore. The main features of an ALGOL program written for this purpose and optimized with respect to storage space requirement and computer time are discussed.

Networks of triangular, square and hexagonal meshes have been investigated. The results are discussed.
THE INFLUENCE OF LATERAL INHOMOGENEITIES ON THE DIPOLE METHODS*
Authors: FRÖHLICH R. K.
Published: April 2006

The theory of electrical dipole soundings proved that this method can produce resistivity measurements, which are comparable with those obtained by electrical soundings of the Wenner or Schlumberger type. Their main advantage is the use of short cable lengths, which is important if the depth of penetration should be large.

A considerable disadvantage of the dipole method is the great sensitivity to lateral discontinuities. Though these have an influence on the Schlumberger arrangement as well, they can disturb a dipole sounding to such an extent than an interpretation based on a horizontal layer case is no more possible.

There are six different dipole arrays, which differ from each other with respect to the angle enclosed by the two dipole orientations‐the current dipole AB and the measuring dipole MN. The theoretical comparison of the dipole arrays with the Schlumberger array concerning their sensitivity to lateral discontinuities is a useful basis for the choice of the most suitable configuration.

Considering geological subsurface conditions the right choice of a dipole array can give an optimal result, i.e. a dipole sounding for which the sensitivity to lateral discontinuities is as small as possible under the given circumstances.
RESISTIVITY PROSPECTING IN A UNITED NATIONS GROUNDWATER PROJECT OF WESTERN ARGENTINA*
Authors: SERRES Y. F.
Published: April 2006

Resistivity surveys including surface measurements and electric well logging are at present carried out as routine techniques by a Groundwater Project of the United Nations Special Fund in arid valleys of the Argentine eastern Andes.

The aims of the Project are the evaluation of ground water supplies in 5 pilot areas and the training of Argentine professionals in modern techniques of groundwater investigation.

In Zonda and Tulüm Valleys, one of the five pilot areas, the groundwater body is essentially represented by alluvium of San Juan River and upper Tertiary gravels lying upon impervious clayey Tertiary sediments.

Profiles of electrical soundings—spread in the valleys normal to the structural trends—were completed as a broad reconnaissance type investigation. In the outwash plain, a thick clayey overburden sometimes covered by saline material produces a masking effect on the deep sands and gravels and makes the evaluation of true resistivities difficult. Under certain conditions, appropriate use of electric well logging results helps solving the ambiguities found in the interpretation of the electrical soundings. Two examples are discussed.

Also a typical development of surface resistivity interpretation as a whole is presented. Emphasis is placed upon the significance of the transverse resistance values to better interpret the hydrogeological picture. The last stage in data processing, presented on a map of the resistive fill and the conductive basement, provides evidence of the strong structural control of the valleys and shows the main variations in the ground water body constitution.
DIE SEISMISCHEN GESCHWINDIGKEITEN DER JUNGEREN MOLASSE IM OSTSCHWEIZERISCHEN UND DEUTSCHEN ALPENVORLAND*
Authors: LOHR J.
Published: April 2006

Abstract

According to a study of seismic velocities in the Alpine Foreland of Eastern Switzerland, the initial velocity is rather high in comparison with other Tertiary basins and shows an exceptionally high increase rate. When analysing the average and the interval velocities, especially of Tertiary strata, and when comparing them with velocities of strata of the same age and a very similar facies of the Northern Rheintalgraben, it has been found that the increase of velocity is closely related to the distance to the Subalpine Molasse. The conclusion is that the velocity of the Tertiary strata is strongly influenced by the folding pressure of the Subalpine Molasse or of the Alps. The same method has been applied to a relatively large number of wells in the area of the “German Molasse”. Not only the results in Eastern Switzerland have been confirmed, but also it has been proved that the diagenesis of the Tertiary strata and, hence, their velocities are influenced only partially by the specific depth of the basin. Velocities increase towards the Folded Molasse or the Alps. Consequently they depend on lateral folding pressure, which decreases from west to east with the increasing width of the basin. The tertiary strata of the basin have been affected by lateral folding pressurefrom south to north. However, structures with lateral compression have not been discovered yet in the German Alpine Foreland.
AN ANALYSIS OF EQUIVALENCE IN RESISTIVITY SOUNDING*
Authors: KOEFOED O.
Published: April 2006

Abstract

A mathematical analysis is given of the phenomenon of equivalence in resistivity sounding, which is based upon the properties of the raised kernel function. Analysis of this function instead of the apparent resistivity function is justified because, as has been shown in a previous publication, variations in the apparent resistivity function lead to variations in the raised kernel function with relative values of the same order of magnitude The expression for the raised kernel function is expanded into a Mac Laurin series. Equivalence can occur only if the second order term of this series is negligible. The coefficient of the first order term depends on the resistivity and the thickness of the layer under consideration. There is an infinite set of combinations of values for these two quantities, for which the coefficient of the first order term has the same value. All these combinations represent equivalent layer distributions.
COUPLAGE SOL‐GEOPHONE*
Authors: LAMER A.
Published: April 2006

Abstract

The seismometer‐ground system is represented by a damped oscillatory system. Relatively simple approximation formulas are derived to express the coupling between ground and seismometer. These expressions are applicable in seismic exploration. The coupling is that of a mass, suspended by a spring, to the surface that the ground would have in absence of the seismometer. It results in a low‐pass filtering of the ground motion, which is due to the presence of the seismometer. This effect is expressed in a unit‐impulse response. It appears that, over a sufficiently homogeneous ground and for low frequencies, one has a true coupling between ground and seismometer. To obtain a sufficiently large pass‐band, a low seismometer housing‐mass together with a not too small housing radius are necessary.
A RESPONSE OF A RANDOMLY LAYERED EARTH TO AN ELECTRIC POINT OR DIPOLE SOURCE*
Authors: NAIDU PRABHAKAR S.
Published: April 2006

Abstract

A point source or a dipole source of electric current, placed on a randomly layered semi‐infinite medium, produces an inhomogeneous random potential field on the surface. The variance of the random potential, normalized with reference to the normal field (that is, a field due to a point source or a dipole source on a homogeneous medium), falls off inversely as the distance from the source. The conductivity of the layers is assumed to vary randomly about a mean value (unity) such that the variations can be represented by a zero mean homogeneous random function. Further the variations are assumed to be small compared to the mean so that the first order perturbation is adequate. The analysis shows that the dipole field is more sensitive than the single pole field to the conductivity variations.
CAN TRANSITION IN THE ELECTRICAL PROPERTIES OF THE EARTH BE DETECTED?*
Authors: MALLICK K.
Published: April 2006

Abstract

The interface phenomena for low periods, i.e. the initial oscillations, and the cut‐off periods are two characteristic features in Magnetotelluric Sounding curves. A method is proposed to distinguish between transitional and homogeneous layered earth by means of these two features.
ANALYSIS OF SOME MULTI‐FREQUENCY, MULTI‐SEPARATION ELECTROMAGNETIC SURVEYS*
Authors: PARASNIS D. S.
Published: April 2006

Abstract

In the interpretation of anomalies obtained with moving source‐receiver dipole electromagnetic surveys, use is frequently made of the so‐called phasor or vector diagrams. Most such diagrams are based on model experiments or theoretical calculations on bodies of simple geometry. If the anomaly at some given coil‐separation and frequency is known it is possible to predict the anomaly at some other separation and frequency. While the results of such predictions often agree admirably in the laboratory experiments, wide, systematic discrepancies are observed in full‐scale field work. Some of the discrepancies can be explained by various phenomenological arguments and in this respect the effect of overburden conductivity appears to be more serious than is generally supposed. The effect of phase rotation due to the overburden is discussed in detail.

The paper is mainly a cautionary note based on some full‐scale data of multi‐frequency, multi‐separation surveys against the blind use of vector diagrams, but it also discusses the extent of the extra information and interpretation aid provided by such surveys.
TWO‐DIMENSIONAL MASS DISTRIBUTIONS FROM GRAVITY ANOMALIES: A COMPUTER METHOD*
Authors: QURESHI I. R. , MULA H. G.
Published: April 2006

Abstract

A gravity anomaly profile, considered to be associated with a two‐dimensional mass distribution of constant density contrast, may be interpreted with the help of a digital computer. Using the infinite slab formula and anomaly values at selected points of the profile, an initial model of the anomalous mass is built. The gravity effect of the model is computed with a simplified version of the formula derived by Talwani, Worzel and Landisman (1959) for line‐integration. After the effect is computed at a point, the model is adjusted depending upon the residual anomaly. The effect of the adjusted model is computed at the following point. Within one cycle of computation the model is adjusted as many times as the number of the selected points. Five such cycles are sufficient to minimize residuals in most cases. A final cycle is computed without adjustment.

Local anomaly profiles have been interpreted to obtain the shape of a sedimentary basin and of a faulted block. A regional anomaly profile has been interpreted to obtain crustal structure.
DISCOVERY OF DIRECT‐SHIPPING IRON ORE BY GEOPHYSICAL METHODS IN THE CENTRAL PART OF THE LABRADOR TROUGH*
Authors: SEGUIN MAURICE K.
Published: April 2006

Abstract

The deposits of the central sector of the Labrador Trough are magnetite‐rich iron ores subjected to enrichment by the process of leaching. In spite of the fact that some ore types are porous and low in density, most of the ore zones have undergone a net increase in density brought about by enrichment. In the course of their formation, the magnetic characteristics of the rocks have changed and a great deal of the magnetite was altered to relatively non‐magnetic iron oxides and hydrous iron oxides. Consequently, induced as well as remanent magnetization decreased, but the ratio of the latter with respect to the former increased substantially. All drift‐covered areas underlain with iron‐formation are potential ore producing areas. From measurements of the first two physical properties and determination of the thickness of overburden by means of a combined gravity‐magnetic‐seismic or resistivity survey, it is possible to detect and distinguish the iron formations and potential ore zones from the country rocks.

In spite of the fact that the iron formations are a complex assemblage of rock types, each having different physical and chemical properties, a qualitative and semi‐quantitative correlation between the results of the surveys and the geology as well as the structure of the area could be established. Consequently, in order to increase the chances of success, first a mathematical correlation of the amplitude of the gravity anomalies with the specific gravity, the iron content, the silica content, the porosity factors and the dimensions of the underlying orebodies, and secondly, a quantitative correlation of the ground magnetic intensity relative to the reference lithologie unit with the percentage of Fe3 O4 recovery and the volume magnetic susceptibility were done for many deposits.

A few discoveries were made with this method. Finally, a fully quantitative interpretation of the geophysical data using a multimodeling analysis led to the discovery of additional direct‐shipping iron ore deposits in the Schefferville mining district of Canada.
THE UTILITY OF CARBORNE RADIOMETRIC SURVEYS IN PETROLEUM EXPLORATION OF GLACIATED REGIONS*
Authors: SHIDELER GERALD L. , HINZE WILLIAM J.
Published: April 2006

Abstract

Carborne gamma radiation surveys over petroleum reservoirs and faults in the glaciated Michigan Basin suggest that this mobile technique may be useful in reconnaissance subsurface exploration, especially in fault detection. This conclusion assumes favorable surface conditions, as well as proper instrumentation, field procedures, and interpretive techniques. Glaciated regions are relatively undesirable terrains for gamma radiation surveys. Thick deposits of glacial drift appear to repress the detection of subsurface features. Furthermore, extremely heterogeneous soil conditions, which are characteristic of many glaciated regions, result in excessive radiation interference. The operational conditions which produced optimum results consisted of continuous profiling on unconsolidated roads, employing two simultaneously operated scintillation counters. Optimum detection was obtained during relatively cool nighttime traverses.
FINITE‐ELEMENT ANALYSIS OF SHALLOW TEMPERATURE ANOMALIES*
Authors: GEERTSMA J.
Published: April 2006

Abstract

Finite‐element analysis has been used in calculating shallow temperature anomalies due to both the presence of salt domes and near‐surface faults. The results of the numerical analysis are in good agreement with field observations reported earlier by Poley and van Steveninck. The calculations show how surface emissivity must be responsible for the pronounced temperature anomalies above salt domes.

Faulting is a mechanism that disorders the layering sequence of sedimentary deposits. If the disordered sedimentary layers show notable differences in heat conductivity, the layers are thick enough and the displacements along the fault plane large enough, temperature anomalies close to the free surface may be apparent because of the surface‐emissivity mechanism.

In order to explain certain characteristic temperature anomalies where a fault reaches or approaches the free surface, an additional mechanism has to be assumed; namely a distortion of texture, and consequently a change in the heat conductivity of the rock material surrounding the fault plane. The consequences of both a reduced and an increased heat conductivity of the surroundings of a near‐surface fault have been calculated, and they show remarkable agreement with temperature anomalies observed in various parts of the world.
INTERPRETATION OF GRAVITY ANOMALIES BY NON‐LINEAR OPTIMISATION*
Authors: AL‐CHALABI MAHBOUB
Published: April 2006

Abstract

The interpretation of a gravity anomaly in terms of the shape of the anomalous body is a non‐linear problem and may, therefore, be carried out using non‐linear optimisation techniques. The formulation is extended to include cases where the density contrast and the regional background are also unknown. For a given model the objective function is provided by the discrepancy between the observed anomaly and the calculated anomaly due to the model. Given an initial model, the optimisation procedure searches for a minimum of the objective function by an iterative adjustment of the parameters. A number of suitable objective functions is given. The behaviour of these functions in the parameter hyperspace is quite complex. Accordingly, direct search methods should be employed at the early stages of the search, changing to gradient methods at later stages. The use of constraints is also necessary to ensure the geological feasibility of the model. The required computer time may be largely reduced by careful programming. Two examples of interpretation by optimisation methods are given.
THE STUDY OF STREAMING POTENTIALS ON FISSURED MEDIA MODELS*
Authors: BOGOSLOVSKY V. A. , OGILVY A. A.
Published: April 2006

Abstract

In massive rocks ground waters mainly flow in fracture zones whose permeability greatly changes depending on their filling. When studying ground‐water flow in fissures, the results of observations of the electric fields of filtrational origin—which, in this case, considerably differ from those in porous media—can be used. Therefore the authors have made experiments on fissured media models. The measurements have been made in a special filtration tube with the fissured media simulated by a system of quartz glass plates. The spacings between plates were regarded as fissure widths. The observations have been made in fully open “fissures” and in those partially filled with sand or sandy‐clay material.

These experiments have resulted in establishing a dependence between the values of streaming potentials and pressure drops. The SP values have particularly been found to decrease with the opening of “fissures”. The most intensive electro‐filtrational fields were observed at 40 per cent filling with medium grained sand. Additions of argillaceous material to the sand filler brought about sharp reductions in the intensity of electro‐filtrational potentials.
A GENERALIZED METHOD OF COMPUTING SECOND DERIVATIVE OF GRAVITY FIELD*
Authors: AGARWAL B. N. P. , LAL T.
Published: April 2006

Abstract

The unsuitability of a particular method of computation of second derivative to varying types of field problems evinces the need of a method by which it may be possible to obtain a wide range of filters. This requirement is achieved by incorporating a mathematical smoothing operator, e‐λρ2, λ and ρ being the smoothing parameter and radial frequency respectively, in the theoretical frequency response of second derivative operation. A few sets of weights for various smoothing parameters, are presented. The analysis of Bouguer gravity anomaly map of Los Angeles Basin, California, U.S.A., is carried out by Elkins's (1951) and Rosenbach's (1953) methods and the sets of weights given in this paper which clearly establishes their filtering equivalence.
A NOTE ON SIGNAL DISTORTION DUE TO DYNAMIC (NMO) CORRECTIONS*
Authors: BUCHHOLTZ H.
Published: April 2006

Abstract

By applying dynamic corrections a seismic trace recorded at a distance x from the energy source should be varied in such a way as to obtain a trace which would be recorded at zero‐distance, i.e. at the source itself. Only such a zero‐offset‐trace contains the correct sequence of reflection coefficients (reflectivity function), whilst all other traces contain a distorted reflectivity function. In the simplest case, the reflectivity function is compressed over a shorter time whereas in more complicated cases a partial inversion of the reflectivity function results. This happens when some of the reflection hyperbolae intersect one another.

The reconstruction of the true zero‐offset reflectivity function by the application of dynamic corrections can only be an approximative process. In the first case mentioned we must expect a decrease in accuracy of the corrected trace in comparison with a zero‐offset‐trace. In the second case, where intersections of the hyperbolae occur, accurate reconstruction is clearly impossible.

The problems are discussed with the help of theoretical and practical examples.
THE AUTOMATIC FITTING OF A RESISTIVITY SOUNDING BY A GEOMETRICAL PROGRESSION OF DEPTHS *
Authors: MARSDEN D.
Published: April 2006

Abstract

The problem of interpretation of geoelectrical resistivity soundings has been studied and a compromise method of interpretation developed. A simple depth sequence of increasing bed thickness is taken and resistivities assigned to the various layers so that a good fit is obtained to the observed apparent resistivity curve. An initial estimate of the resistivities is made by a direct method but the model is then altered to give an improved fit by the indirect method of curve matching. The whole method is computerized and input data to the computer program consists of spacings and apparent resistivities. The method is extremely rapid, about one third of the computation time is devoted to obtaining the resistivity transform by the method of convolution. A normal type of solution, incorporating additional information if necessary, can also be obtained.
SEISMIC WAVES IN ANELASTIC NON‐LINEAR MEDIA—A THEORETICAL CONTRIBUTION *
Authors: ALFANO L.
Published: April 2006

Abstract

Numerical solutions of the wave equation for a particular type of non‐linear “constant Q” medium were carried out. These solutions were obtained after the transformation of the space derivatives in finite differences; power series of the time are used to express the solutions.

The medium is characterized by a not single valued stress‐strain relation; the stresses are greater when the absolute values of strain are increasing (passive work), and are less when they are decreasing (active work). A loss of energy follows which is constant for every cycle and independent of frequency. This model represents the simplest type of medium in agreement with the laboratory data on rock samples. For a similar medium the stress’values do not depend only on the instantaneous value of the strain, but also on the previous strain values, i.e. the history of the medium. All these characteristics are not compatible with linearity and require particular types of computing procedures similar to the one shown in this paper.

The results of calculations here shown refer both to the propagation of an isolated wave and to the generation of a wave in a spherical hole by change of pressure. They refer particularly to the shape, the amplitude and the width of the isolated wave along the propagation path. The most important results for this type of medium are the following:

a) for a plane single isolated wave, the displacement amplitude wave does not change along the propagated distance. The width increases linearly as function of the distance;

b) the corresponding particle velocity decreases in amplitude;

c) for single isolated spherical waves the displacement amplitude decreases with propagated distance only due to the geometric factor, i.e. inversely proportional to the propagated distance; its width increases in the same way as for plane waves.

The comparison between these theoretical results with the field and seismological data show a sufficiently good agreement as far as the value interval of wave width and frequencies is concerned. Less satisfactory is the comparison regarding a linear dependence of the wave width on the distance. This fact happens probably because in the field often long trains of waves and not isolated waves occur. In effect, for trains of waves the behaviour is different than that of an isolated wave; particularly, for the former the frequency variations along the travelled path is less and the displacement variations greater.

However, it seems likely that a further similar theoretical research for trains of waves propagating in this type of non‐linear medium might be carried out to complete the present research.
ANALYSIS OF THE FOURIER SPECTRUM OF THE GRAVITY EFFECT DUE TO TWO‐DIMENSIONAL TRIANGULAR PRISM *
Authors: RAO K. G. C. , AVASTHI D. N.
Published: April 2006

Abstract

Rao, K. G. C. and D. N. Avasthi, 1973, Analysis of the Fourier Spectrum of the Gravity Effect Due to Two‐Dimensional Triangular Prism, Geophysical Prospecting 21, 526‐542.

Fourier Transform of the theoretical gravity effect due to a two‐dimensional isosceles triangular prism has been derived. Evaluation of the model parameters from the analysis of the Fourier Spectrum has been outlined. Application of this method to synthetic gravity anomalies is shown.
SHAPED CHARGE—A POWERFUL EXPLOSIVE SURFACE SEISMIC SOURCE *
Authors: BAHIA R. R. , SILVA W. F.
Published: April 2006

Abstract

The directional effect of shaped charge is a well‐known feature used for a long time in military weapons, oil well guns, and steel industry. This charge was successfully applied as a seismic energy source by Petrobrás during the past three years under different surface and geological conditions.

Preliminary amplitude measurements taken with fixed gain shallow refraction instruments showed a consistent difference between conventional and shaped charges. Lately, a similar difference has been noted in deep reflection energy recorded with digital binary gain instruments as well as in deep oil well velocity surveys.

Direct comparisons along more than 50 km of multiple coverage field reflection shooting are in agreement with these results and have proved the practical advantage of this source as compared to conventional dynamite.

This source has been used since 1971 in routine seismic operation in the Amazon jungle with 300 gram unit charges distributed in small and large shot arrays increasing substantially the coverage and halving the cost at a higher record quality. A large amount of production seismic field work has been carried out in several other areas attesting the successful application of the shaped charges.
SERIES APPROXIMATION IN VELOCITY AND TRAVELTIME COMPUTATIONS *
Authors: AL‐CHALABI M.
Published: April 2006

Abstract

A rigorous proof is presented to show that over a horizontally layered ground the rms velocity cannot exceed the stacking velocity. The proof helps to illustrate the difference between stacking and rms velocities in a quantitative manner. The series of Taner and Koehler (1969) is used for the purpose. Convergence of this series is tested. Including more terms will not necessarily improve the convergence. Although the series is rapidly convergent when the spread length/depth ratio is small, strong oscillations are observed when this ratio is high.
HOW TO WRITE FOR “GEOPHYSICAL PROSPECTING”*
Authors: HELBIG K.
Published: April 2006

Abstract

The acceptance of articles in Geophysical Prospecting is strictly based on the originality and relevance of the contents. In order to be published a paper has to satisfy additional requirements on its external form and on its style and language: it should be written preferably in English though articles in French and German are permitted. A paper should consist of (i) a title page, (ii) an abstract, (iii) the text, (iv) list of references, (v) captions to illustrations, if any, (vi) tables, if any, and (vii) illustrations, if any. These seven parts should be mailed together but be physically separable and should be prepared according to rules which are either based on printing necessities or on the editorial style of the Journal.

Failure to comply with these rules may result in delay in further processing of the article. Since Geophysical Prospecting is edited with a minimal budget correspondence with authors is kept to a minimum.
ESTIMATION OF THE PRIMARY SEISMIC PULSE *
Authors: WHITE R. E. , O'BRIEN P. N. S.
Published: April 2006

Abstract

A seismic trace after application of suitable amplitude recovery may be treated as a stationary time‐series. Such a trace, or a portion of it, is modelled by the expression

image

where j represents trace number on the record, t is time, αj is a time delay, α (t) is the seismic wavelet, s(t) is the reflection impulse response of the ground and nj is uncorrelated noise. With the common assumption that s(t) is white, random, and stationary, estimates of the energy spectrum (or auto‐correlation function) of the pulse α(t) are obtained by statistical analysis of the multitrace record. The time‐domain pulse itself is then reconstituted under the assumption of minimum‐phase. Three techniques for obtaining the phase spectrum have been evaluated: (A) use of the Hilbert transform, (B) Use of the z‐transform, (C) a fast method based on inverting the least‐squares inverse of the wavelets, i.e. inverting the normal time‐domain deconvolution operator. Problems associated with these three methods are most acute when the z‐transform of α(t) has zeroes on or near the unit circle. Such zeroes result from oversampling or from highly resonant wavelets. The behaviour of the three methods when the energy spectra are perturbed by measurement errors is studied. It is concluded that method (A) is the best of the three. Examples of reconstituted pulses are given which illustrate the variability from trace‐to‐trace, from shot‐to‐shot, and from one shot‐point medium to another. There is reasonable agreement between the minimum‐phase pulses obtained by this statistical analysis of operational records and those estimated from measurements close to the source. However, this comparison incorporates a “fudge‐factor” since an allowance for absorption has to be made in order to attenuate the high frequencies present in the pulse measured close to the shot.
STRESS WAVE VELOCITY AND CRACK SYSTEM OF A MEDIUM *
Authors: SCHENK V. , SCHENKOVÁ Z.
Published: April 2006

Abstract

Two in situ measurements of stress wave velocity in a granodiorite massif were made. Firstly, the velocities close to the source were investigated (for elimination of the influence of cracks), and secondly, the velocities in different horizontal directions over a large area were studied from the viewpoint of their dependence on the crack system of the medium. A correlation between the mutual orientation of the cracks and the maximum or minimum values of the stress wave velocities was obtained.
INTER‐BOREHOLE ACOUSTIC MEASUREMENTS AND THEIR USE IN ENGINEERING GEOLOGY*
Authors: McCANN D. M. , GRAINGER P. , McCANN C.
Published: April 2006

Abstract

Well–to‐well seismic measurements are adapted to civil engineering problems by use of a sparker as a source of seismic signals and an 80 kHz hydrophone as a receiver. For display a Tektronix 549 oscilloscope is used.

Field application shows that the delineation of interfaces between homogeneous strata and the detection and delineation of localized and irregular features is possible from inter‐well travel times. In‐situ measurement of the compressional wave velocity in a medium is often complicated by refraction and wave guide effects. The degree of fracturing cannot be estimated from travel time measurements alone in a tightly jointed, saturated, rock mass, but it may be possible to correlate variations in pulse shape and length with this parameter.