bluuebunny/crossref_metadata_2025 huggingface dataset:
SOURCES OF ERROR IN A SEISMIC DIGITAL RECORDING SYSTEM*)
Authors: DONNELL W. F.
Published: April 2006

The use of digital recorders and computers in seismic exploration promises major enhancement of the quality of final documents available to interpreters. The ultimate objectives of recording and processing remain what they always have been:

1 Record the reflection wavelet as a function of time; this requirement has been met with satisfactory accuracy for a number of years.

2. Record the reflection wavelets with sufficient fidelity to permit the interpreter to recognize them.

Various factors affect our ability to achieve this second objective. Certain recording errors are associated with digital recording systems. However, an understanding of the sources of error will enable the operator to use his system properly and to estimate the noise level or inaccuracy of field recordings. Field operations do not require rigorous error analysis; in most cases a satisfactory approximation can be obtained from simple calculations.

Three types of “noise”–seismic, instrument and power line–introduce errors. Factors which contribute to over‐al recording system error include specifically input noise, power supply ripple, crosstalk, A‐D conversion error, quantizing noise, aliasing, distortion. Examination of each component of a recording system, permits the determination of its ultimate effect on the over‐all noise level–or error level–of the entire system.

Many of the error sources produce statistically independent noise which is not correlative. Where this is true, error voltages from various sources may be combined by taking the square root of the sum of the mean square noise voltages, giving a result slightly greater than the largest single voltage if one source is much greater than any other source. This simplification can be used to estimate over‐all system noise levels. Distortion and crosstalk depend on signal amplitude and should be added algebraically in each category. Each final sum should be used as a statistically independent noise source with respect to other system noise sources.

Using the foregoing examples and simplified system for estimating over‐all system noise, and assuming that much of the distortion (which limits signal/instrument noise ratio to 54 db) can be removed by filtering, we determine that the combined effect of all sources of error is to reduce the system S/N ratio to approximately 74 db.

With proper care digital field recording systems can produce very good field records, and exotic computer processes can enhance signal and reduce various forms of noise. However, one always must recall that the level of confidence which one can place in an interpretation of seismic data must be dependent on a knowledge of the accuracy of the basic data.
USE OF EXPECTED ERROR IN THE DESIGN OF LEAST‐SQUARES OPTIMUM FILTERS*
Authors: WIGGINS RALPH A.
Published: April 2006

The design of least‐squares optimum filters is based upon minimizing a suitably defined error criterion. The expected value of this error is easily computable after the coefficients of the filter have been determined.

When a particular filtering problem is specified, there are several parameters which are specifically not included in the optimization procedure. However, the magnitude of the expected error may be quite sensitive to these parameters. The examination of the relative values of the expected error for variations of these unspecified parameters may lead to a better definition of the filter problem.

The parameters which are left unspecified by the general least‐square filter definition include:

1. The addition of white noise to the signal autocorrelation to stabilize the filter behavior.

2. The specification of the shape of the desired output of the filter.

3. The specification of the lag between the desired output and the input.

Examples are given showing the relationship between these parameters and the value of the expected error.
A METHOD OF TACKLING SOME SEISMIC PROBLEMS AND ITS APPLICATIONS *
Authors: ALFANO L.
Published: April 2006

The theory is explained and practical applications are shown for a numerical procedure in seismology. Particularly the problems concerning the generation of waves under the action of external pressures, and their propagation, in non‐homogeneous, both elastic and absorbing media, have been carried out. These problems have been assumed mono‐dimensional and refer to plane and spherical waves.

The procedure is based on the solution, by means of series, of the wave differential equation, non‐homogeneous, and with non‐constant coefficients. It is a direct numerical method whose advantage is, mainly, the possibility of tackling, without great difficulties, problems regarding non‐homogeneous elastic and absorbing media.

On the contrary the methods which require the theoretical expression of the solutions by means of formulae, generally, present conceptual and numerical difficulties.

As examples of application of this procedure, the following cases have been carried out by means of numerical calculations.

a) Propagation of a wave, initially of symmetrical shape, in a viscoelastic medium; from the results it appears that the wave propagates without losing its symmetry, i.e. without sensible dispersion. A theoretical analysis has been carried out to justify this result, showing that the dispersion in viscoelastic media is noticeable only for relatively high frequencies. It seems that the practical absence of dispersion in field experiments do not exclude necessarily the viscoelastic character of absorption.

b) Generation of plane waves under the effect of a uniform pressure distributed on the plane surface of a medium. The way the length and the shape of the generated wave depends not only on the type of pressure acting on the surface but also on the near surface impedance variations has been studied.

c) Generation of a spherical wave under the action of a pressure in a spherical hole.

The examples treated show how the length and shape of the wave depends on the radius of the hole. Particularly the frequencies of the wave spectrum are proportional to this radius, for a given type of pressure acting in the hole.

The characteristics of this procedure would also permit the study of media for which the stress‐strain relations are not univocal and linear (non linear absorption). This study, interesting for the wave propagation in rocks, is worth while to be carried out in a special paper.
THE USE OF A SQUARE CONFIGURATION IN RESISTIVITY PROSPECTING *
Authors: HABBERJAM G. M. , WATKINS G. E.
Published: April 2006

Consideration is given to the use of a configuration of four electrodes set in a square array for resistivity measurements.

It is found that, by passing current successively between different pairs of electrodes, an apparent resistivity can be determined which is both more sensitive to the position of the array centre and less dependent on orientation than the measures usually obtained with colinear arrays of electrodes. At the same time the observations made enable the degree of the departure of local conditions from conditions of lateral homogeneity to be assessed.

Theoretical and practical examples of the use of this electrode system are given and the use of the system both as a tool in mapping and in depth investigations is considered.

It is shown that provided electrode spacings are suitably arranged the results of a probe carried out using the square array can be interpreted by conventional methods.

The system is shown to have particular advantages in the investigation of lateral resistivity variations and the reduced dependence on orientation makes possible the recasting of interpretation data in an orientationally invariant form with a consequent drastic reduction in the number of type curves required for a particular problem.
ELASTIC WAVES IN LAYERED MEDIA *
Authors: BORTFELD R.
Published: April 2006

The problem of elastic waves–from point sources–in layered media is solved in a general and elementary way. Only the simplest principles of the Theory of Functions are necessary. The solutions can immediately be used for numerical applications.
SOME ASPECTS OF INDUCED POLARIZATION (TIME DOMAIN)*
Authors: BERTIN JACQUES
Published: April 2006

In the first part, the author, recalling the principle of d/c current measurements, shows by means of some examples, different experimental results relative to the induced polarization phenomenon. He presents the case of the negative induced polarization which can be explained by geometric effects. He gives some examples of saturation curves and discusses the problem of the linearity of the I.P. phenomenon. Then he shows some aspects which demonstrate that I.P. decay curves do not always conform to the general law. He concludes by showing the influence of the current electrodes, the position of which, relative to the polarizable bodies, may alter the shape of the I.P. anomalies.

In the second part, the author presents different case histories of I.P. mineral surveys where I.P. is compared to other geophysical methods and drilling results.
AN UNDERWATER SOUND SOURCE WITH HIGHER SEISMIC EFFICIENCY *
Authors: EDELMANN H.
Published: April 2006

In order to increase the seismic efficiency of the Sparker‐system developments were made in 1966/67. Results illustrating the main steps of this development are discussed. A new type of electrode called Transploder electrode was field‐tested and proved to be promising.
REALIZATION OF SHARP CUT‐OFF FREQUENCY CHARACTERISTICS ON DIGITAL COMPUTERS
Authors: MEYERHOFF H. J.
Published: April 2006

It was found in Part I of this paper that approximating the sharp cut‐off frequency characteristic best in a mean square sense by an impulse response of finite length M produced a characteristic whose slope on a linear frequency scale was proportional to the length of impulse response, but whose maximum overshoot of ±9% was independent of this length (Gibbs' phenomenon). Weighting functions, based on frequency tapering or arbitrarily chosen, were used in Part II to modify the truncated impulse response of the sharp cut‐off frequency characteristic, and thereby obtain a trade‐off between the value of maximum overshoot and the sharpness of the resulting characteristic. These weighting functions, known as apodising functions, were dependent on the time‐bandwidth product Mξ, where 2ξ, corresponded to the tapering range of frequencies.

Part III now deals with digital filters where the number 2N–1 of coefficients is directly related to the finite length M of the continuous impulse response. The values of the filter coefficients are taken from the continuous impulse response at the sampling instants, and the resulting characteristic is approximately the same as that derived in Part II for the continuous finite length impulse response. Corresponding to known types of frequency tapering, we now specify a filter characteristic which is undefined in the tapering range, and determine the filter coefficients according to a mean square criterion over the rest of the frequency spectrum. The resulting characteristic is dependent on the time bandwidth product Mξ= (N–1/2)ξ up to a maximum value of 2, beyond which undesirable effects occur. This optimum partially specified characteristic is an improvement on the previous digital filters in terms of the trade‐off ratio for values of maximum overshoot less than 1%. Similar to the previous optimum characteristic is the optimum partially specified weighted digital filter, where greater “emphasis is placed on reducing the value of maximum overshoot than of maximum undershoot”. Such characteristics are capable of providing better trade‐off ratios than the other filters for maximum overshoots greater than 1/2%. However these filters have critical maximum numbers 2.NC–1 of coefficients, beyond which the resulting characteristics have unsuitable shapes. This type of characteristic differs from the others in not being a biassed odd function about its cut‐off frequency.
TRANSFORMATION OF MAGNETOMETRIC DATA INTO TECTONIC MAPS BY DIGITAL TEMPLATE ANALYSIS*
Authors: LINSSER HELMUT
Published: April 2006

Results of gravimetric surveys can be interpreted by comparing the Bouguer field values with master curves based on simplified geological models. It has been shown in a previous paper how this procedure can be transformed into routine processes which can be computerized. The application of this method has yielded useful results in detailed gravity surveys.

The present paper discusses the application of the same interpretation principles to magnetic data. After some modifications, the method elaborated for the gravimetric data can be used for the interpretation of magnetometric survey results. Magnetometric‐tectonic maps are obtained which show the structural picture by common geological symbols. In the case of faults, the dimensions of depth of burial and throw are indicated on the maps.

The method is illustrated by an example where these procedures have been applied to gravimetric and magnetometric data of the same area. Two different maps are obtained: One shows the tectonics according to density contrasts and the other map depicts the tectonic situation on the basis of magnetic susceptibility contrasts.
REALIZATION OF SHARP CUT‐OFF FREQUENCY CHARACTERISTICS ON DIGITAL COMPUTERS*
Authors: MEYERHOFF H. J.
Published: April 2006

In Part I of this paper, we examined the properties of the best mean square approximation to the sharp cut‐off frequency characteristic by an impulse response of finite length. It was found that the sharpness of cut‐off for the resulting frequency characteristic depended on the length of the impulse response–but because of the discontinuous nature of the specified frequency characteristic, this best mean square approximation always had a maximum overshoot of ± 9%, independent of the length of the impulse response (Gibbs phenomenon).

In Part II, we investigate ways of reducing this ± 9% overshoot at the expense of a reduced sharpness of cut‐off. The discontinuous frequency characteristic is first approximated by a continuous characteristic with linear or cosine frequency tapering. The impulse response for such tapered characteristics consists of the impulse response of the discontinuous frequency characteristic weighted by a certain function corresponding to the type of tapering employed. The best mean square approximation to the tapered characteristic by an impulse response of finite length M will produce a frequency characteristic whose properties are now dependent on the time‐band width product Mζ, where 2ζ is the tapering range.

A trade‐off exists between the maximum overshoot and the sharpness of cut‐off for the resulting characteristic for both forms of frequency tapering. Instead of considering other forms of tapering in the frequency domain, we now investigate arbitrarily chosen weighting functions in the time domain to determine the minimum length of impulse response for a minimum value of maximum overshoot and a maximum value of sharpness of cut‐off.

Part III will discuss the digital realization of the above finite length impulse responses together with the optimum partially specified digital filter approximation to the desired frequency characteristic.
VERFAHREN ZUR AUSWERTUNG REFLEXIONSSEISMISCHER UNTERTAGEMESSUNGEN NACH DER SPIEGELPUNKTMETHODE*
Authors: SCHMIDT EDGAR
Published: April 2006

Bei reflexionsseismischen Untertagemessungen werden die Geophonaufstellungen und die Schußpunkte im Bergwerk angelegt. Die Möglichkeiten hierzu sind durch das Grubengebäude und durch die Meßmethode eingeschränkt. Ungeachtet dessen werden oft Aussagen über die Lage eines Reflektors im dreidimensionalen Raum benötigt. Es müssen daher hohe Anforderungen an die Konstruktionsmethoden gestellt werden können.

Zunächst ist es im Hinblick auf die Meßgenauigkeit erforderlich, zwischen günstigen und ungünstigen Anordnungen von Aufstellungen und Schußpunkten zu unterscheiden. Diese Frage wird in der Einleitung kurz behandelt.

Die Beschreibung von allgemeingültigen Konstruktionen ist das eigentliche Ziel dieser Arbeit in ihrer hier vorliegenden Fassung. Es wird gezeigt, wie mit Hilfsmitteln der Darstellenden Geometrie beliebig im Raum gelegene ebene reflektierende Flächen‐elemente nach der Spiegelpunktmethode konstruiert werden können. Dies kann, je nach der Korrelation, auf zwei verschiedenen Wegen geschehen, die in den Abschnitten I und 2 getrennt beschrieben werden.

Im ersten Abschnitt wird der Fall zweier Geophonaufstellungen behandelt, welche wie die Schenkel eines Winkels aneinandergefügt sind. Sie können von einem beliebig gelegenen Schußpunkt aus beschossen werden (siehe den linken Teil der Abbildung I). Anhand eines Beispiels aus der Kaligrube Salzdetfurth bei Hildesheim wird dies etwas näher erläutert. Bei bekannter Streichrichtung oder bei bekanntem senkrechten Einfallen des Reflektors führt bereits eine Geophonaufstellung zum Ziel und es vereinfacht sich die Konstruktion.

Der zweite Abschnitt bringt den Fall zweier nicht notwendig zusammenhängenden Aufstellungen, die nicht in einer gemeinsamen Ebene zu liegen brauchen. Durch Anwendung der Technik des Gegenschießens gelingt es auch so, ein reflektierendes Flächen‐element zu erhalten (siehe den rechten Teil der Abbildung I). Naturgemaß ist die Lage der beiden Schußpunkte hierbei nicht beliebig.
HEAD WAVES IN TWO‐DIMENSIONAL SEISMIC MODELS*
Authors: DATTA S. , BHOWMICK A. N.
Published: April 2006

Studies made on head waves in a large number of two‐dimensional horizontal and inclined layer seismic models are described. The assumption that the amplitude of head wave decreases exponentially with distance is found to be valid in horizontal layers. For the correct estimation of the head wave decay coefficient, the decrease of amplitude due to the divergence of wave energy should always be considered.

The variations of head wave amplitude at large distances from the source appear to follow the theory of Heelan (1953). It is, however, concluded that the theory can only be verified from the measurements in models which have large velocity contrast and thick layer at the top.

A prominent interference phenomenon between the direct and head wave has been observed near the critical distance. This is more marked for up‐dip profile in the case of inclined layer models. The rate of head wave amplitude decrease with distance beyond the zone of interference is found to depend on dip angle, direction of the dip, and velocity contrast.
VELOCITY SPECTRA AND THEIR USE IN STRATIGRAPHIC AND LITHOLOGIC DIFFERENTIATION*
Authors: COOK ERNEST E. , TANER M. TURHAN
Published: April 2006

Velocity Spectra which were originally developed for the optimum stacking of seismic data have been found to give considerable information concerning lithologic and stratigraphic changes in the geologic section. In the Gulf of Mexico shale sections and sand bodies have been recognized on the Velocity Spectra display, and in the Caribbean last year a first attempt was made to utilize Velocity Spectra information for the determination and mapping of lithology. Since that time, a new program has been developed which takes dip into account when computing the interval velocity. This program has been applied to a seismic section in the North Sea which has resulted in a geologic model derived from interval velocities which were found to be quite consistent. Such a model can be of great value in geological interpretation.
QUANTITATIVE INTERPRETATION OF TRANSIENT ELECTROMAGNETIC FIELDS OVER A LAYERED HALF SPACE*
Authors: MORRISON H. F. , PHILLIPS R. J. , O'BRIEN D. P.
Published: April 2006

Abstract

The transient fields from a finite horizontal loop excited by a half sine wave current pulse have been computed numerically for a particular source receiver configuration at a height of 100 meters above a layered ground. The amplitude of the vertical component of the magnetic field has been chosen for the interpretation. Curves of apparent conductivity vs. time, plotted during the off‐time of the signal, show that layering is easily resolved, that resonance effects are present and that polarization effects are detectable for certain types of polarization.
OPTIMUM DIGITAL FILTERS FOR SIGNAL TO NOISE RATIO ENHANCEMENT*
Authors: TREITEL SVEN , ROBINSON ENDERS A.
Published: April 2006

Summary

One of the main objectives of seismic digital processing is the improvement of the signal‐to‐noise ratio in the recorded data. Wiener filters have been successfully applied in this capacity, but alternate filtering devices also merit our attention. Two such systems are the matched filter and the output energy filter. The former is better known to geophysicists as the crosscorrelation filter, and has seen widespread use for the processing of vibratory source data, while the latter is. much less familiar in seismic work.

The matched filter is designed such that ideally the presence of a given signal is indicated by a single large deflection in the output. The output energy filter ideally reveals the presence of such a signal by producing a longer burst of energy in the time interval where the signal occurs. The received seismic trace is assumed to be an additive mixture of signal and noise. The shape of the signal must be known in order to design the matched filter, but only the autocorrelation function of this signal need be known to obtain the output energy filter. The derivation of these filters differs according to whether the noise is white or colored. In the former case the noise autocorrelation function consists of only a single spike at lag zero, while in the latter the shape of this noise autocorrelation function is arbitrary.

We propose a novel version of the matched filter. Its memory function is given by the minimum‐delay wavelet whose autocorrelation function is computed from selected gates of an actual seismic trace. For this reason explicit knowledge of the signal shape is not required for its design; nevertheless, its performance level is not much below that achievable with ordinary matched filters. We call this new filter the “mini‐matched” filter.

With digital computation in mind, the design criteria are formulated and optimized with time as a discrete variable. We illustrate the techniques with simple numerical examples, and discuss many of the interesting properties that these filters exhibit.
ESTIMATION OF SPECTRUM AND CROSS‐SPECTRUM OF AEROMAGNETIC FIELD USING FAST DIGITAL FOURIER TRANSFORM (FDFT) TECHNIQUES*
Authors: NAIDU PRABHAKAR S.
Published: April 2006

Abstract

An efficient method of computing spectrum and cross‐spectrum of large scale aero‐magnetic field (or of any other two‐dimensional field) has been developed and programmed for a digital computer. The method uses fast Fourier transform techniques. Briefly, the method is as follows: a digitized aeromagnetic map is divided into a number of rectangular blocks. Fourier transforms of these blocks are computed using a two‐dimensional fast Fourier transform method. Finally, the amplitude of the Fourier transforms is averaged to give the desired spectrum. Computation of cross‐spectrum follows the same lines. In fact, the same programme may be used to a compute the spectrum as well as cross‐spectrum. The method has a number of computational advantages, in particular it reduces greatly computational time and storage requirements. The programme has been tested on synthetic data as well as on real aeromagnetic data. It took less than 30 seconds on an IBM 360/50 computer to compute the spectrum of an aeromagnetic map covering an area of approximately 4500 square miles.
THE ASSOCIATION OF RESISTIVITY SOUNDINGS*
Authors: HABBERJAM G. M.
Published: April 2006

Abstract

A simple measure, the association parameter, is proposed for directly comparing the results of two electrical soundings. The use of this measure to classify field results and to gain some insight into geological structure before extensive depth interpretation is discussed. In particular it is shown that when used with soundings conducted using the tripotential technique the combined use of association parameter arid lateral inhomogeneity index can allow structural patterns to be discerned where otherwise they might be obscured.

Possible extension of the technique is considered.
SIMULTANEOUS ESTIMATION OF PARAMETERS OF REFLECTION EVENTS (DEPTH, DIP, VELOCITY) AND RELATIVE STATIC CORRECTIONS*
Authors: LASKI J. D.
Published: April 2006

Summary

It is pointed out that after identifying reflection events from the same horizon on two records obtained on the same spread from two different shotpoints, one can simultaneously estimate parameters of reflection events and relative static corrections. The parameters of reflection events are treated as quantities to compute whereas relative static corrections are treated as quantities to minimize by the least squares method. Static corrections obtained from different horizons for the same point on the spread are averaged.

The case of more than one pair of records for the same spread (or part of the spread), vital for multiple coverage, is also discussed.
AN INVESTIGATION OF THE RELATIVE ACCURACY OF THE MOST COMMON NORMAL‐MOVEOUT EXPRESSIONS IN VELOCITY ANALYSES*
Authors: ROBINSON JOHN C.
Published: April 2006

Abstract

Three common expressions for the normal moveout of recorded seismic events are investigated by numerical simulation procedures for accuracy in predicting the root‐mean‐squared (RMS) or mean, as the case may be, subsurface velocity function from seismic data. The principal investigation, for which detailed error curves are shown, was derived for a stochastic subsurface model composed of strata with thicknesses ranging up to 91.4 m (300 ft) and boundary velocity contrasts ranging up to 45.7 m/sec (150 ft/sec); there was a 95 percent chance of velocity increase with increased depth. The effects of changes in the basic statistical subsurface model are discussed.

The results appear to confirm the judiciousness of the choices of to and (x/z') as plotting parameters to be used with the respective percent errors in the three expressions, where are, respectively, the zero‐offset arrival time of, the offset distance of, and the mean‐squared velocity encountered by a seismic ray. Out of the three normal‐moveout expressions examined, the “straight‐raypath” expression with the RMS velocity substituted as its velocity term proved to be the most accurate in the determination of velocities.
NUMERICAL INTERPRETATION OF RESISTIVITY SOUNDINGS OVER HORIZONTAL BEDS*
Authors: MEINARDUS HANS A.
Published: April 2006

Abstract

The digital computer technique described for interpreting resistivity soundings over a horizontally stratified earth requires two steps. First, the kernel function is evaluated numerically from the inverse Hankel transform of the observed apparent resistivity curve. Special attention is given to the inversion of resistivity data recorded over a section with a resistant basement. The second step consists in the least‐squares estimation of layer resistivities and thicknesses from the kernel function. For the case of S or T‐equivalent beds only one layer‐parameter can be obtained, either the longitudinal conductance, or the transverse resistance respectively.

Two examples given in the paper show that a wide tolerance is permitted for Choosing the starting values of the layering parameters in the successive approximation procedure. Another important feature for practical applications is good convergence of the iterations. The method is probably best suited for interpreting profiles of electrical soundings with the purpose of mapping approximately horizontal interfaces at depth.
METHODE DE FILTRAGE DU FANTOME: APPLICATION A DES CAS REELS*
Authors: FAIL J. P. , LAYOTTE P. C.
Published: April 2006

Résumé

Après avoir essayé?analyser, par voie optique, ľeffet de la réflexion fantôme sur des enregistrements sismiques, on montre ľapplication de deux méthodes de filtrage de ce fantôme sur des exemples réels.

La première méthode utilise là mise à feu séquentielle de charges situées à différentes profondeurs dans un même trou; il s'agit ?un filtrage réalisé sur le terrain. Le deuxième procédé consiste à filtrer, au central, des enregistrements réalisés à des profondeurs de tir différentes avant de les sommer.
A FAST METHOD FOR DETERMINING THE LAYER DISTRIBUTION FROM THE RAISED KERNEL FUNCTION IN GEOELEGTRICAL SOUNDING*
Authors: KOEFOED O.
Published: April 2006

Abstract

In a previous publication (Koefoed 1968) a function called the “raised kernel function” has been introduced as an intermediate function in the interpretation of resistivity sounding data, and methods have been described both for the determination of the raised kernel function from the apparent resistivity function, and for the determination of the layer distribution from the raised kernel function.

In the present paper a procedure is described by which the second step in this interpretation method–i.e. the determination of the layer distribution from the raised kernel function–is considerably accelerated. This gain in interpretation speed is attained by the use of a standard graph for a function which defines the reduction of the raised kernel function to a lower boundary plane.
A PNEUMATIC SEISMIC ENERGY SOURCE FOR SHALLOW WATER / MARSH AREAS*
Authors: BREDE E. C. , JOHNSTON R. C. , SULLIVAN L. B. , VIGER H. L.
Published: April 2006

Abstract

Non dynamite seismic energy sources have shown only limited success in difficult shallow‐water or marsh prospect areas, and none have applicability to both environments. Recently, a method has been devised to adapt a marine pneumatic source for operation in abrasive surroundings. A system composed of four of these modified sources together with associated emplacement and retrieval mechanisms has been developed and has proven to be a reliable and efficient seismic energy source for both shallow‐water and marsh applications.
ULTRA DEEP SCHLUMBERGER SOUNDING AND CRUSTAL CONDUCTIVITY STRUCTURE IN SOUTH AFRICA*
Authors: VAN ZIJL J. S. V. , HUGO P. L. V. , DE BELLOCQ J. H.
Published: April 2006

Abstract

A second deep Schlumberger sounding has been carried out in South Africa, this time using a newly completed power transmission line, not yet in sevice, as emission line. The maximum current electrode separation attained was 600 km. The sounding was situated on a shield area with a thin cover of sedimentary rocks (Karroo System) overlying the archaean granite. The center was chosen near to the village of Dealesville.

It is shown that lateral effects at large AB spacings are unlikely, but due to numerous dolerite intrusions in the sediments surface electrode effects must be guarded against; thus AB positions were chosen by geologists. Because of a low signal/noise ratio at large AB spacings the measurements were treated statistically; in particular, the elimination of non‐random events improved the measuring accuracy.

Although a thin conducting layer at the base of the sedimentary succession decreased the resolution of the sounding curve, the results, which were interpreted with the aid of a computer, show positively that

the crust is multi‐layered with a highly resistant portion having a minimum resistivity of 15000 ohm.m and

a good, thick conductor appears as the final layer.

The Dealesville results are in complete agreement with those obtained from the first deep Schlumberger sounding at Pofadder (Van Zijl 1969) except that the transverse resistance of the lowermost resistant crust is about 100% larger at Pofadder due probably to a second period of metamorphism. Both electrical studies support the view of a dehydrated lower crust (granulite facies).
DELINEATION OF SHALLOW SALT DOMES AND SURFACE FAULTS BY TEMPERATURE MEASUREMENTS AT A DEPTH OF APPROXIMATELY 2 METRES*
Authors: POLEY J. PH. , VAN STEVENINCK J.
Published: April 2006

Abstract

The feasibility of using temperature measurements at a depth of about 2 m for locating and delineating salt domes and faults has been investigated both theoretically and in experimental field surveys.

It is shown that measurable temperature anomalies in the soil are to be expected over shallow salt domes.

In a field survey over a salt‐dome area bordering the Groningen gas field, a large number of temperature measurements were made in small holes (2 m deep, 3 cm in diameter) within a relatively short time (some weeks). The results clearly indicate several temperature anomalies with differential temperatures of about 1°C. Comparison of our thermal contour map with interpretations of available seismic or gravity data, or with direct evidence from wells, showed an excellent correlation. Seismic data even support the shape of the thermal contours.

Results in similar agreement with gravity or well data were obtained over salt ridges in a tropical area. Experiments showed that the technique worked as well in lakes and marshes as on dry land.

In addition, some experimental evidence collected so far over shallow and surface faults is presented. In several cases, strong thermal anomalies coincided with known surface faults. A thermal model for a surface‐fault zone is suggested which accounts satisfactorily for the observed thermal data. It suggests some diagnostic value for the fault's geometry.

For shallow faults, however, lack of knowledge of subsurface detail prevented any unambiguous correlation with observed thermal anomalies. Accordingly any geological use of thermal analysis over shallow faults remains debatable.

The field technique is simple, needs little correction and can, where useful, easily be included in routine gravity work to provide additional local information.
REFLECTIONS ON AMPLITUDES*
Authors: O'DOHERTY R. F. , ANSTEY N. A.
Published: April 2006

Abstract

Modern seismic recording instruments allow precise measurements of the amplitude of reflected signals. Intuitively we would expect that this amplitude information could be used to increase our knowledge of the physical properties of the reflecting earth.

The relevant factors defining the amplitude of a reflection signal are: spherical divergence, absorption, the reflection coefficient of the reflecting interface, the cumulative transmission loss at all interfaces above this, and the effect of multiple reflections.

Of these factors, three—spherical divergence, the reflection coefficient and the transmission loss—are reasonably clear concepts (though the estimation of transmission loss from acoustic logs caused some difficulties in the hey‐day of synthetic seismograms). Absorption still presents considerable problems of detail, but our understanding has increased significantly in recent years.

The factor least well understood is undoubtedly the effect of multiple reflections. Multiple paths having an even number of bounces can have the effect of delaying, shaping and magnifying the pulse transmitted through a layered sequence. Simple demonstations of this phenomenon can be made using elementary thin plates, and these can be presented for various synthetic and real sequences of layers. Such demonstrations lead one to explore the relation between the spectrum of the transmitted pulse and the spectrum of the reflection coefficient series.

If it were possible to isolate the amplitude and shape variations imposed by absorption within a layer, there would be a chance that this measure of absorption would be useful as a correlatable or diagnostic indication of rock properties. If it were possible to isolate the amplitude and shape variations imposed by multiple reflections, there would be a chance that this measure would be useful as an indication of cyclic sedimentation and of the dominant durations of the sedimentary cycles. However, the separation of these two effects constitutes a formidable challenge. The very difficulty of this separation suggests that it may be opportune to review the quantitative estimates of absorption made by field experiments.
PHYSICAL ASPECTS OF THE “AIRPULSER” AS A SEISMIC ENERGY SOURCE*
Authors: SCHULZE‐GATTERMANN R.
Published: April 2006

Abstract

During the last few years the airpulser, or air gun, has become very common as an energy source for marine seismic surveys. This paper describes the physical processes which take place during the operation of the pulser and develops theoretical results concerning the energy and frequency of the radiated signal and the amplitude decay of the secondary bubble pulses. The theory takes into account the presence of the airpulser itself which is assumed to be a rigid sphere within the bubble of released air. The theoretical results are combined and compared with measurements made of the pressure within the airpulser, the acceleration of the body of the pulser, and the amplitude and frequency of the signal radiated into the surrounding water. A formula for calculating the bubble frequency is given and a diagram made of the energy partition between mechanical losses, radiated energy, etc. Finally, a comparison is made of the energy release from the airpulser with that from TNT.
INDUCTION ANOMALY DUE TO AN ELONGATED COVERED ORE‐ZONE EXCITED BY A LONG CURRENT CARRYING CABLE*
Authors: NEGI J. G. , GUPTA C. P. , RAVAL U.
Published: April 2006

Abstract

A detailed analysis of the electromagnetic anomaly due to an elongated mineralized zone of low resistivity in presence of a fixed‐transmitter (a long cable carrying an, alternating current IeIωt) is presented where (i) the host rock is not highly resistive and might contain some disseminated mineralization, (ii) the ore‐body has an inhomogeneous conductivity, (iii) there is a contrast in magnetic properties of the ore‐body and the host rocks, and (iv) source is close to the conducting system i.e., anomalous zone is lying shallow.

The numerical computation of the generalized analytical expressions have been made. The selective screening behaviour of the cover is delineated and suitable frequency ranges for maximum detectability of a covered conductor have been obtained. A paradoxical decrease of the in‐phase component of the response function with the increase of the core conductivity has been found for large conductivities of the cover. For moderate values of response parameter the inhomogeneity in the conductivity is found to significantly affect the induced field. The present analysis of the various features of the secondary field will aid to the existing interpretation sensitivity of the induction prospecting data for porphyry conducting ore deposits with zonal wall‐rock alteration and sulphide distributions.
APPLICATION OF WIENER FILTERS TO TRANSFORMATIONS OF GRAVITY AND MAGNETIC FIELDS*
Authors: GUNN P. J.
Published: April 2006

Abstract

Consideration of the spectral representation of gravity and magnetic fields shows the field to be the result of the convolution of factors depending on the parameters of the field. Wiener filters, calculated using model transformations, provide an optimum method for altering these factors and hence effecting field transformations.
MAGNETOTELLURIC MODELLING BY THE FINITE ELEMENT METHOD*
Authors: SILVESTER P. , HASLAM C. R. S.
Published: April 2006

Abstract

The finite element method, here viewed as a special case of the Galerkin projective method, is applied to the modelling of magnetotelluric problems, and its adaptation to geological profiles is outlined. A novel method for obtaining surface field values, involving matrix representation of the normal derivative operator, is presented in detail.

Results obtained by this method are compared with well‐known infinite series solutions for the vertical fault and the outcropping dyke. Two profiles containing sulphide zones are also modelled, the results being compared with field data; satisfactory agreement is obtained.
A COMBINED INTERPRETATION OF A GEOELECTRICAL AND AN AIRBORNE‐MAGNETIC SURVEY IN PAKTIA, E AFGHANISTAN *
Authors: BOSUM BY W. , HOMILIUS J.
Published: April 2006

Abstract

In the region of Paktia (SE Afghanistan) geoelectrical resistivity measurements and airborne‐magnetic surveys were carried out. Although the aims of the two investigation methods were different, it was possible to draw new conclusions by combining the results obtained so far. The problems were: 1. Location of aquifers and determination of their structural position. 2. General survey to determine the crystalline basement as a basis for oil exploration.

Using geoelectrical methods, the depths of magnetic bodies were determined precisely, whereas the combination with airborne‐magnetic methods enabled an extrapolation of the geoelectrical results from profiles to the subsurface. By geoelectrical means the fill of intermontane basins was classified and sedimentary thicknesses of at least 1400 m were identified.

The magnetic anomalies are divided into both broad anomalies with small amplitude, caused by the crystalline basement, and narrow anomalies with large amplitude, caused by basic volcanics (ophiolites). The magnetic anomalies show that the occurrence of ophiolites is restricted to faults. The upper surface of the magnetized crystalline basement rises from 3000 m b.s.l. in the W to o m in the E. Thus, the concept of the geological structure of this region (trough to the W, swell to the E) is confirmed.

Im Gebiet von Paktia (SE‐Afghanistan) wurden geoelektrische Widerstandsmessungen und airbornemagnetische Messungen durchgeführt. Obwohl die Fragestellung für beide Untersuchungsmethoden unterschiedlich war (einmal: Ortung von Grundwasserleitern und Klärung ihrer strukturellen Lage, zum andern: Übersichtsvermessung zum Nachweis des Kristallins als Grundlage der Erdölexploration), konnten durch eine Kombination der Ergebnisse neue Aussagen gewonnen werden.

Mit Hilfe der Geoelektrik wurde die Tiefenlage der magnetischen Störkörper präzisiert, umgekehrt erlaubte die Verbindung mit der Airbornemagnetik die Ausdehnung der Geoelektrik‐Ergebnisse von Profilen in den räumlichen Untergrund. Durch die Geoelektrik wurden die Füllungen der intromontanen Becken gegliedert und Sedimentmächtigkeiten von wenigstens 1400 m nachgewiesen.

Die magnetischen Anomalien gliedern sich in breite großräumige Anomalien mit geringer Amplitude, deren Ursache im kristallinen Basement, und engräumige Anomalien mit großer Amplitude, deren Ursache in basischen Vulkaniten (Ophioliten) zu suchen ist. Die magnetischen Anomalien zeigen, daß die Ophiolithe an Störungen gebunden sind. Die Oberfläche des magnetischen Kristallins steigt von 3000 m b.s.l. im W auf o m.s.l. im E an und bestätigt damit die Vorstellung vom geologischen Bau des Gebietes (Trog im W, Schwelle im E).
SOME COMMENTS ON THE CALCULATION OF THE GRAVITATIONAL AND MAGNETIC ATTRACTION OF A HOMOGENEOUS RECTANGULAR PRISM *
Authors: GOODACRE A. K.
Published: April 2006

Abstract

The use of arctangents rather than arcsines in the expression for the gravitational attraction of a homogeneous rectangular prism reduces computational difficulties. Once a subroutine is available to compute one component of attraction in a Cartesian coordinate system, the other components may be obtained by cyclic permutation of the field point and body coordinate parameters. This technique also readily provides derivatives of the gravitational attraction and hence forms a compact method for the calculation of a magnetic anomaly due to a homogeneous rectangular magnetic prism.
A NOTE ON THE LINEAR FILTER METHOD OF COMPUTING ELECTROMAGNETIC SOUNDING CURVES *
Authors: VERMA R. K. , KOEFOED O.
Published: April 2006

Abstract

The values of the filter coefficients used for the computation of electromagnetic sounding curves are studied in conjunction with the values of the input function to the filter, or the range of values which the input function may assume, and the filters are broken off at such a place that the error in the sum of the products of filter coefficient and input function does not exceed a prescribed value. This analysis is carried out for the horizontal coils system, the perpendicular coils system, and the vertical coplanar coils system. The lengths of the filters so derived depend on the layer parameters, the frequency and the coil spacing. Even in the most unfavourable cases the filters are shorter than the filters published by Koefoed, Ghosh, and Polman (1972).
PATTERNS—WITH A PINCH OF SALT *
Authors: NEWMAN P. , MAHONEY J. T.
Published: April 2006

Abstract

The time‐honoured method of attenuating coherent noise in the seismic record is by the use of source and geophone arrays. In theory, and using methods familiar in the synthesis of digital frequency filters, arrays can be designed having virtually any desired response in the wavenumber spectrum.

In practice, arrays cannot be implemented with the same precision that is applied in design. The response actually achieved must be compromized by a number of factors. These include inaccuracies in the effectiveness or positioning of individual array elements, variations in ground coupling, and the effect of local heterogenities in the environment of the array. We have no reliable way of knowing how well a particular array will perform from one location to the next.

Statistical modelling methods have been applied to examine the effects of implementation errors. Experimental results, supported by statistical theory, show that errors are expected to impose a limit upon the rejection capabilities of an array. The expected limiting value of attenuation due to errors in element weights is inversely proportional to the standard deviation of errors and directly proportional to the square root of the number of array elements. Position errors exert a limiting influence which is wavenumber dependent such that attenuation decreases with increasing wavenumber. For arrays of common dimensions, Gaussian random errors of 10% standard deviation in element weights and positions result in an expected attenuation limit of about 30 dB.

It follows that the more ambitious array designs are less tolerant of errors, and must be implemented with greater care and precision in the field. The present work enables us to specify tolerances for any particular array design.

Ultimately, the degree of pattern refinement which may be effectively employed in any area is determined by errors which are associated with the array environment. Complex arrays are expensive to operate. In order to avoid over‐design it would be useful to establish the magnitude of errors to be expected under different terrain conditions.
EXPERIENCES WITH AUTOMATIC MAGNETIC INTERPRETATION USING THE THICK PLATE MODEL *
Authors: HJELT S. E.
Published: April 2006

Abstract

Various experiments are described in designing two‐dimensional magnetic interpretation algorithms using computer curve fitting techniques.

For a single anomaly the position of the anomaly maximum and the half‐width of the anomaly give good initial estimates of the plate position and thickness. A nomogram and formulae for improving these estimates is given. Curves and estimates for the effects of finite depth extent of a plate show the limits, when the lower surface of the plate can be neglected in curve fitting.

The combined anomaly of two parallel plates can be separated into partial anomalies with no common points using the horizontal derivative of the anomaly. The changes of the anomaly maxima and changes in anomaly half‐widths are studied as a function of plate separation. The position of the maxima and the half‐widths can be corrected before applying the one‐plate procedure for obtaining initial estimates of plate positions and thicknesses.

The performance of standard optimization methods of Powell, Davidon, and Marquardt in improving the values of the plate parameters are compared. The Powell method seems to be the most reliable for both single and multi‐plate anomalies. All methods become unacceptably slow when the number of plates is greater than 2 or 3. In these cases feasible interpretation times are obtained using the partial anomalies and sequential parabolic search of the parameter values as tailored specially to the thick plate model.

Experiments with three different error norms, the classical least squares, weighted least squares and minimax, show that the first norm gives the best overall performance in automatic interpretation. The behaviour of the classical least squares norm as a function of the plate parameters is also briefly described.