bluuebunny/crossref_metadata_2025 huggingface dataset:
DETERMINATION OF ELECTRIC ROCK PARAMETERS FROM THE FIELD STRUCTURE OF A MAGNETIC DIPOLE*
Authors: BITTERLICH W. , KELLNER A. W.
Published: April 2006

Abstract

In the theoretical part of the present paper, formulas have been analyzed for a magnetic dipole in a homogeneous and unbounded medium. The magnetic field is elliptically polarized in the region between the quasistatic zone and the far field. Since the position and the shape of the polarization ellipses depend on the complex wave number, k, it is possible to determine k by measuring the polarization ellipses. From k, the conductivity and the dielectric constant of the medium are easily calculated. The functions required for the measuring method have been computed and plotted in graphs.

In the experimental part it was examined how far the theory may be applied to measurements of propagation through rock at frequencies ranging from 100‐1000 kHz. These measurements showed that reasonably defined mean values of rock parameters can be given only if the deviations of the field from the theoretically expected field are not too high. These deviations have been named field distortions and have been examined by means of statistical methods (variance ratio tests). Gallery cavity and inhomogeneity or anisotropy of the medium account for these distortions.
AN EXAMPLE OF CHROMITE PROSPECTION BY MAGNETICS*
Authors: BOSUM W.
Published: April 2006

Abstract

Results of rock magnetic investigations and magnetic field measurements in the Logar Valley, Afghanistan, are presented. Rock magnetic investigations on samples of the ore and the country rock have demonstrated that the chromite is strongly magnetic, whereas the country rock has proved predominantly non‐magnetic, partly weakly magnetic. The isanomalic maps show strong anomalies of some 1000 γ above the known chromite occurrences. Anomalies of the country rock amounting to some 100 γ have been observed only above basic dikes. The measured anomalies have been interpreted quantitatively by 2‐ and 3‐dimensional models. This interpretation is demonstrated and the question of chromite detection discussed. Model graphs are used to investigate in detail whether the extension in depth of ore bodies can be estimated.
APPLICATION OF GEOPHYSICAL METHODS FOR STUDYING THE TECHNICAL STATUS OF EARTH DAMS*
Authors: BOGOSLOVSKY V. A. , OGILVY A. A.
Published: April 2006

Abstract

Earth dams are constructed on a wide scale for hydrotechnical and reclamative purposes. However, the great water permeability of soils and their bad gradation are frequently responsible for the development of mechanical suffusion and other phenomena resulting in dam destruction.

Geophysical observations allow under certain conditions to forecast the development of adverse factors. They thus are, as practice shows, an important tool to observe the technical status of dams.

The difference in propagation velocities of elastic waves in the zone of aeration and full saturation determines the applicability of the reflected wave method for determining the depth of seepage flow in the dam supporting mass. This can be also achieved by electrical sounding and induced polarization methods since the resistivity and polarizability of air‐dry and water‐saturated grounds differ considerably.

Maps of equal potentials measured on the dam slope reflect the configuration of the contours of water table of the water flow passing through the dam. This makes it possible to obtain data on the direction and intensity of flow as well as on the presence of heterogeneous grounds filling the dam supporting mass. Besides, regime observations conducted according to the SP method allow to forecast the development of suffusion and observe the redeposition of grounds. If the SP anomalies do not increase with time, the suffusion processes can be regarded as fading out. Enhanced amplitude and increasing areas of anomalies show the intensification of these processes.

Combined application of some of the above mentioned methods allowed to obtain many practical results on a number of dams in Transcaucasia and Central Asia. The true position of the depression surface determined in the dam of one of the high‐mountain water reservoirs, for instance, considerably differed from the designed one. The configuration of the seepage flow surface and its fluctuation caused by the rise and drop of the water level in the reservoir have been studied in detail on another dam situated on a plain. The sites of the most intensive outwash of fine material have also been located and the processes of soil redeposition in the dam supporting mass characterized. The data of the geophysical investigations were used to plan hydroinsulation and repair work.
VELOCITY DISPERSION OF SEISMIC WAVES*
Authors: O'BRIEN P. N. S. , LUCAS A. L.
Published: April 2006

Summary

In well velocity surveys made to calibrate Sonic (CV) Logs the calibration survey uses frequencies around 50 Hz whereas the Sonic Logging tool uses frequencies around 20 kHz. There thus exists the possibility of making a direct measure of velocity dispersion. In any one survey the disturbing factors, both instrumental and operational, will often mask any dispersive effect that might exist. Consequently this paper reports on a statistical analysis of the velocity differences resulting from calibration surveys and Sonic logs. Only Borehole Compensated Sonic Logs were used. Four areas were investigated: the North Sea, Abu Dhabi, Libya and Alaska. After rejecting logs and calibration records which were obviously in error there remained 424000 feet (about 130 km) of usable log distributed throughout 66 wells. The four areas were analysed separately and in no case was the estimated dispersion significantly different from zero. However, the mean values did correlate with lithology from (− 0.17 ± 0.18)% for the essentially carbonate section in Abu Dhabi to (+ 0.45 ± 0.25)% for the sand‐shale section in Alaska, a positive sign meaning that the higher frequencies travelled faster. Except for Alaska the calibration surveys were made with a wall‐clamp geophone, and for these areas amplitude measurements were made. After suitable corrections estimates of the absorption parameter Q were obtained. These varied from 20 to 200 with mean values of 63 for Libya, 70 for Abu Dhabi and 88 for the North Sea (excluding the Tertiary). If, as is usually assumed, the absorption mechanism is linear and is described by a Q which is independent of frequency, then these values would necessarily imply dispersion of several percent. As instanced above no such dispersion was observed. It is possible that the expected dispersion was compensated for by invasion of the mud filtrate into the borehole walls, but it is more likely that the absorption mechanism was substantially non‐linear.
THE APPLICATION OF LINEAR FILTER THEORY TO THE DIRECT INTERPRETATION OF GEOELECTRICAL RESISTIVITY SOUNDING MEASUREMENTS*
Authors: GHOSH D. P.
Published: April 2006

Abstract

Koefoed has given practical procedures of obtaining the layer parameters directly from the apparent resistivity sounding measurements by using the raised kernel function H(λ) as the intermediate step. However, it is felt that the first step of his method—namely the derivation of the H curve from the apparent resistivity curve—is relatively lengthy.

In this paper a method is proposed of determining the resistivity transform T(λ), a function directly related to H(λ), from the resistivity field curve. It is shown that the apparent resistivity and the resistivity transform functions are linearily related to each other such that the principle of linear electric filter theory could be applied to obtain the latter from the former. Separate sets of filter coefficients have been worked out for the Schlumberger and the Wenner form of field procedures. The practical process of deriving the T curve simply amounts to running a weighted average of the sampled apparent resistivity field data with the pre‐determined coefficients. The whole process could be graphically performed within an quarter of an hour with an accuracy of about 2%.
MAPPING OF TECTONICS OF SEDIMENTS BY AIRBORNE MAGNETICS*
Authors: LINSSER H.
Published: April 2006

Abstract

The considerable increase of sensitivity of new magnetometers allows the elaboration of magnetometric maps with contour intervals of fractions of one gamma. On these maps, small anomalies appear which had never been visible before. They are caused by susceptibility contrasts within the sediments.

The areal distribution of these small undulations of the magnetic field depends on tectonics, therefore their analysis can give useful information about the tectonic pattern of the sediments. This paper discusses the automatic transformation of these small anomalies into tectonic units. The problem is solved by a method called Digital Template Analysis which has already yielded very useful results in gravity interpretation.

The application of the method described is restricted to surveys executed by high sensitivity magnetometers. Therefore it is supposed that in most oil exploration problems these magnetometers will replace instruments with lower sensitivity. The improved magnetometer surveys in combination with the interpretation method described represent a most efficient, fast and unexpensive reconnaissance method.
A SOLUTION OF THE DIPPING INTERFACE PROBLEM USING THE SQUARE ARRAY RESISTIVITY TECHNIQUE*
Authors: BROADBENT M. , HABBERJAM G. M.
Published: April 2006

Abstract

Apparent resistivity measurements with the square array technique in the vicinity of a dipping interface have the advantage over collinear array methods that they are less dependent on orientation of the array.

In order to exploit this, existing potential solutions for the dipping interface problem have been adapted for the computation of apparent resistivities over such a feature using a square array.

Comprehensive interpretation techniques covering this problem are given and the limitations imposed by residual array orientation effects are discussed.
COMPARISON OF “AQUAFLEX” WITH CONVENTIONAL EXPLOSIVE FOR SEISMIC REFRACTION MEASUREMENTS AT SEA*
Authors: BLUNDELL D. J. , HANDLEY R. L.
Published: April 2006

Abstract

A seismic refraction line fired using conventional explosive charges was repeated using a seismic cord, “Aquaflex”. It was found that travel times and frequencies were the same for both kinds of source and that amplitudes from “Aquaflex” were comparable with those produced using conventional explosives more than ten times the weight. This advantage carries with it gains in cost and efficiency and reduces damage to fishing.
POSITIONING OF MARINE SURVEYS WITH AN INTEGRATED SATELLITE NAVIGATION SYSTEM*
Authors: KRONBERGER F. P. , FRYE D. W.
Published: April 2006

Abstract

For almost two years, satellite navigation has been used in reconnaissance exploration. However, only in the last six months has it been demonstrated that an “integrated” system provides navigational accuracy for all geophysical prospecting. This paper describes the accuracy of the integrated system and illustrates its merits.

The primary objective of the integrated system is to provide continuous, accurate, geodetic position information. This is accomplished by integrating instantaneous position, velocity and heading from outputs of the Doppler sonar, gyrocompass, velocimeter, vertical references with periodic updates obtained from the Navy Navigation Satellite System. Each of the subsystems, their operations and functions will be discussed.

Satellite navigation offers many advantages for marine surveys. Features for point‐to‐point automatic steering, onboard plotting of shotpoint data, firing of seismic energy source as a function of exact distance and real‐time data logging onto digital tape have been implemented. With the onboard computer all navigation, gravity and magnetic information is recorded on tape and some preprocessing and filtering of gravity and magnetic data may be accomplished.
DIURNAL CORRECTIONS TO MAGNETIC SURVEYS—AN ASSESSMENT OF ERRORS*
Authors: RIDDIHOUGH R. P.
Published: April 2006

Abstract

The comparison of a group of continuous total field stations in Ireland with the record of the total field from Valentia Observatory enables the errors which would be involved in using the Observatory as a diurnal correction base to be examined. For the most part, for a single correction, these errors lie between ± 2 and ± 6 gammas. However, they have three notable characteristics: (i) they are not closely dependent on the magnetic ‘noisiness’ of the day, (ii) they are only marginally reduced by the application of station‐base time differences and (iii), they are strongly related to differences between the amplitude of the daily magnetic variation at the station and at the base. A permanent geographical basis for the latter differences enables a system for the estimation of the errors to be proposed.
TEMPERATURE EXTRAPOLATION TO INFINITE TIME IN GEOTHERMAL MEASUREMENTS*
Authors: PARASNIS D. S.
Published: April 2006

Abstract

A simple calculation procedure that gives the equilibrium temperature in geothermal measurements reduces the reading time in field work from reported 60 minutes to about 10 minutes.
GOSSES BLUFF AIRBORNE MAGNETIC SURVEY*
Authors: YOUNG G. A.
Published: April 2006

Abstract

An airborne magnetic survey of an area of 580 km2 centred on Gosses Bluff was flown in 1968. This survey formed part of a joint project by the Australian Bureau of Mineral Resources and the United States Geological Survey to evaluate the hypothesis that Gosses Bluff is of impact origin.

Analyses of the magnetic data show that shallow sources of magnetic disturbance flank the Bluff on its southern side. Interpretation of the magnetic anomalies indicates their source to be shock‐melted breccias of possible Jurassic age.
POTENTIAL FIELD CONTINUATION BETWEEN GENERAL SURFACES*
Authors: SYBERG F. J. R.
Published: April 2006

Abstract

The theory of potential field continuation is studied with a view of extending it to include continuation between non‐linear surfaces. This theoretical extension is regarded as useful in reducing observed potential fields along one general surface to another general surface.

It is demonstrated that the continuation operators considered in past geophysical literature are special cases of the generalized operators. In view of the tremendous growth in the art of high‐speed computing it is possible to consider applications of the generalized operators. In worked examples upward continuation between a general surface and a datum is considered for the gravity field due to geometrically simple sources; this in order to test the accuracy of the digital applications. It is indicated that for observations on an undulating surface it is possible to account for considerable errors when, during interpretation, the observation locations are taken to be along a datum. For aids to profile interpretation in areas of considerable topographic relief some simple convolution operators for continuation between linear, non‐parallel surfaces are proposed.
CONDUCTING SPHERE IN ELECTROMAGNETIC INPUT FIELD*
Authors: MALLICK K.
Published: April 2006

Abstract

A frequency‐domain analysis is outlined for a conducting sphere in a uniform Input field: inequispaced alternating half‐sine wave pulses. The Barringer Input air‐borne electromagnetic exploration system uses such source fields. Theoretical profiles of Hρ(t), the horizontal magnetic component over the sphere from different elevations and for various conductivity and geometrical factors are presented. Based on these results some useful features such as penetration and detectibility are discussed.
GEOPHYSICAL CATEGORIZATION OF THE PRECAMBRIAN IN SOUTH INDIA*
Authors: VOGELSANG D.
Published: April 2006

Abstract

The ground follow‐up of a magnetic and radiometric air survey had to cope with the usual dilemma to check a great number of anomalies within a short time. A limited aggregate of magnetic anomalies, expected to correspond to magnetite quartzites was statistically selected for ground identification by this method: the ratios length/width times amplitude were listed for all coherent contours and the calculation of the standard deviation per unit area resulted in different key‐numbers for a new lithological “Salem Unit” and for the charnockitic or gneissic environment. The ground work thus directed and reduced by 85% yielded a substantial potential of iron ore. This was supported by by abundant determinations of the magnetic susceptibilities, confirming the sources of anomalies and revealing the amenabilities of iron ores for the magnetic separation process.

The lines of truncation of anomalies were found to represent a system of local and regional faults and shear zones, which segregated the area into different tectonic blocks. These sutures have also provided the ways of intrusion for alkaline and basic magmae in the style of a “Rift” structure, housing several carbonatites and impregnations of metal sulphides.

The airborne radiometrics obtained many uranium indications by gamma ray spectrometry. However, they led only to disseminated uranium‐silicates, associated with syenites, granites and pegmatites, each emanating a characteristic photon energy spectrum. But in general the radiation of thorium prevails, marking northern Madras as a “Thorium Province”.
A TWO‐AND THREE‐DIMENSIONAL GRAVITY DOT CHART*
Authors: MORGAN N. A. , FAESSLER C. W.
Published: April 2006

Abstract

This paper presents a dot chart for calculating gravity effects from two‐dimensional bodies with the addition of a supplementary set of curves which can be used to determine the “end corrections” to modify the calculated effects when the length of the body is not relatively long with respect to the horizontal dimensions of the cross‐section. The use of the chart is demonstrated by different examples. The results of theoretical cases amenable to mathematical treatment and computer results both compare favorably to those computed using the present chart.

A fundamental incentive of constructing this chart, in a final form ready to use, is to make it available to gravity interpreters for utilization. Future effort in chart construction is thereby avoided.
COMPUTATION OF TYPE CURVES FOR ELECTROMAGNETIC DEPTH SOUNDING WITH A HORIZONTAL TRANSMITTING COIL BY MEANS OF A DIGITAL LINEAR FILTER*
Authors: KOEFOED O. , GHOSH D. P. , POLMAN G. J.
Published: April 2006

Abstract

The computation method described in this paper is based on the existence of a linear relationship between the mutual coupling ratio and the kernel function in the integral expression for it. Accordingly, the mutual coupling ratio can be determined by first computing sample values of the kernel function and then subjecting these to a digital linear filter. In the present paper the appropriate sampling distance is determined and the values of the digital filter coefficients are computed, both for electromagnetic sounding with horizontal coils and for electromagnetic sounding with perpendicular coils.
CALCULATION OF THE VERTICAL GRADIENT OF THE GRAVITY FIELD USING THE FOURIER TRANSFORM*
Authors: AGARWAL B. N. P. , LAL T.
Published: April 2006

Abstract

Fourier transform techniques have been used to calculate the theoretical filter (amplitude) response function of Nth order vertical derivative continuation operation. The amplitude response functions of the vertical gradient and its continuation follow from the same. These response functions are subsequently used to calculate the weighting coefficients suitable for two dimensional equispaced data. A shortening operator has been incorporated to limit the extent of the operator. For comparative study, some of the developed coefficient sets and the one presented in this paper are analysed in the frequency domain and their merits and demerits are discussed.
TURAM INTERPRETATION BY CURVE MATCHING USING A LINE CURRENT APPROXIMATION*
Authors: DUCKWORTH K.
Published: April 2006

Abstract

The interpretation of Turam data is treated by means of a development of the conventional approach to this type of data. The development consists of a curve matching technique based upon a line current approximation to the true induced current pattern.

The technique requires computation facilities which are readily available in modern programmable calculators.

Practical examples of the use of the method illustrate that it can reveal information concerning the geometry of a conducting body which would not be gained by the conventional approach.
THE KALMAN FILTER AS A PREDICTION ERROR FILTER*
Authors: OTT N. , MEDER H. G.
Published: April 2006

Abstract

In mathematical statistical filtering the deconvolution problem can be solved by two different methods:

by inverse filtering

by calculating the prediction error.

Both methods are well known in the theory of Wiener filters.

If, however, the generating process of the signal is known and can be described by a set of linear first order differential equations, then the Kalman filter can also be used to solve the deconvolution problem. In the case of the inverse filtering method this was shown by Bayless and Brigham (1970). But, while their method can only be used if the original signal is a colored random process, this paper shows that in the case of a white process the prediction error filtering method is a more appropriate approach. The method is extremely efficient and simple. This can be demonstrated by an example which maybe of special interest for seismic exploration.
APPLICATION DU FILTRE DE KALMAN A LA NAVIGATION*
Authors: FAIL J. P.
Published: April 2006

Abstract

This paper is concerned with an application of the Kalman filter to integrated navigation systems. It is shown that this filter is well adapted to resetting the dead‐reckoning position by means of a radio‐hyperbolic system or of a Doppler sonar. The algorithm can also be used for resetting the dead‐reckoning position by means of a satellite navigation system.

The first part is briefly concerned with stochastic processes and system theory. The canonical equations for navigation problems are written. The second part is devoted to the Kalman filter. The third part discusses the application of the algorithm to navigation problems and presents some simulation results.
PARAMETER OPTIMIZATION OF VELOCITY DEPTH FUNCTIONS OF GIVEN FORM BY USE OF ROOT‐MEAN‐SQUARE VELOCITIES*
Authors: MARSCHALL R.
Published: April 2006

Abstract

From seismic surveys zero offset reflection times and root‐mean‐square velocities are obtained. By use of Dix‐Krey's formula, the interval velocities can be calculated.

If no well velocity survey exists, the interval velocities and T(o) times are the only available information. The suggested way to get a regionally valid velocity distribution is to select N“leading horizons”, where a major change in the velocity parameters occurs and to compute the parameters of the selected velocity depth function (in most cases linear increase with depth) by a special approximation for the interval between two adjacent “leading horizons”.

Herewith all reflection horizons within the interval are taken into account.
A Ge(Li) BORE HOLE PROBE FOR IN SITU GAMMA RAY SPECTROMETRY*
Authors: LAUBER A. , LANDSTRÖM O.
Published: April 2006

Abstract

This report describes the construction and properties of a Ge(Li) bore hole probe for in situ gamma ray spectrometry. The probe has the form of a cylinder 8.9 cm in diameter and 160 cm long. It contains a 22 cm3 coaxial Ge(Li) detector, cooled by liquid nitrogen. Holding time of the assembled probe is in excess of ten hours, energy resolution is 2.3 keV for 122 keV photons and 2.6 keV for 1333 keV photons.

The probe has been used in field experiments in the Ranstad uranium mine and for the study of neutron activation spectra obtained in bore hole models. Results from these experiments are presented and discussed. It is stressed that the Ge(Li) probe gives promise to become a powerful tool in bore hole activation analysis.
ANALYSIS OF LEAST SQUARES SMOOTHING OPERATORS IN THE FREQUENCY DOMAIN*
Authors: CHAN S. H. , LEONG L. S.
Published: April 2006

Abstract

The process of data smoothing by least squares operators with real and symmetric coefficients is shown to be equivalent to linear digital filtering. Study of the frequency response functions of the least squares operators shows that these operators are essentially low‐pass filters. The frequency response functions are characterized by the presence of a main lobe centered about the zero frequency together with a number of minor lobes of smaller amplitudes at higher frequencies. The predominant band of frequencies which is passed by a least squares operator occurs within the main lobe of the frequency response function. The filtering characteristics of a least squares operator are dependent on the degree of the polynomial and the number of data points. Generally speaking, the higher the degree of the polynomial the broader the pass band of the filter will be. But the effect of the number of data points on the pass band is exactly opposite.

Several figures showing the plots of the half‐power frequency, the quarter‐power frequency, and the half‐width of the main lobe for a large number of least squares operators with different combinations of the degree of the polynomial and the number of data points are presented in this paper. These figures will be helpful to those people who wish to make a selection of least squares operators suitable for their purposes.
TIME DOMAIN ELECTROMAGNETIC RESPONSE OF A SHIELDED CONDUCTOR*
Authors: NEGI J. G. , VERMA S. K.
Published: April 2006

Abstract

Velekin and Bulgakov (1967) in an interesting model experiment while studying the transient electromagnetic response of a conductive sphere placed below a thin conductive sheet found that at the earlier stages of the transience, the composite system response corresponded to the response due to the overlying sheet alone and at the later stages, it corresponded to that of the sphere alone. To examine whether such a separation of responses due to individual components can be analytically studied and applied to other source configurations, we have analyzed an idealized model consisting of two spherical shells. We find that in corroboration with the above results, the general nature of the curve consists of two humps representing the responses dominated by the outer shell and the inner shell respectively. In addition, however, we find that the two humps gradually disappear to yield a smooth decay curve for increasing values of the ratio σd1b/σ2d2a (where σ1, σ2 are the conductivities, d1, d2 are the thicknesses of the outer and inner shells respectively, and b and a are their respective distances from the centre) and the effect of inner shell on the composite system response is considerably reduced.
TOPOGRAPHIC EFFECTS ON ELECTROMAGNETIC DEPTH SOUNDING SYSTEMS *
Authors: VERMA R. K.
Published: April 2006

Abstract

Computations have been made for the effect of ground slope in combination with transmitter coil misorientation on different electromagnetic depth sounding systems and sets of frequency sounding master curves are presented for different angles. The effect of a thin air layer due to the elevation of coils above ground surface has been studied for different systems. Also discussed are the effects due to an error in transmitter‐receiver coil separation.
THE USE OF OBSERVATIONAL SPACE IN SEISMIC REFRACTION TECHNIQUES *
Authors: HABBERJAM G. M.
Published: April 2006

Abstract

Attention is given in this paper to the display of intensive seismic refraction information in a convenient form similar to spatial display techniques being developed for resistivity methods.

An observational space for first arrivals is defined and illustrated with respect to some simple structures and the possible use of such a representation in diagnosis is discussed.

The spatial display is suitable both for steeply dipping and gently dipping structures. In the latter case, the low relief space may be subjected to further operations to produce apparent velocity distributions and values simply related to plus times.

A practical example of the latter process and the subsequent interpretation is given.
BASIC PRINCIPLES OF TWO‐DIMENSIONAL DIGITAL FILTERING *
Authors: CLEMENT W. G.
Published: April 2006

Abstract

The geophysicist involved in the analysis of two‐dimensional data should have an understanding of the two‐dimensional finite Fourier transform and the mechanics of two‐dimensional filtering. Frequency aliasing must be considered when working with sampled data. In two dimensions it is advantageous to consider aliasing in terms of the overlap of the repeating spectra inherent in the finite Fourier transform. Two‐dimensional filtering can be performed as a transient convolution in the space domain, as cyclic convolution utilizing the frequency domain or as the multiplication of polynomials using the z‐transform. If the “edge” effects are removed, the results of the three methods are identical.
ENHANCEMENT OF ELECTROMAGNETIC ANOMALIES BY A CONDUCTING OVERBURDEN II *
Authors: GAUR V. K. , VERMA O. P.
Published: April 2006

Abstract

The influence of a partially conducting overburden/host rock on the electromagnetic response of a horizontal, tabular conducting ore body, investigated with the aid of quantitative scale model experiments, was discussed in an earlier paper (Gaur, Verma and Gupta 1971), which will be referred to as I.

This paper presents the results of more comprehensive experiments subsequently carried out to study the combined effect of various geological parameters, namely: the dip and depth of burial of the ore body, its electrical conductivity and that of the zone surrounding it. These results obtained for four different transmitter‐receiver configurations confirm the general enhancement of response in varying degrees, brought about by a conducting overburden. However, the transformation of the shapes of the anomaly profiles with increasing overburden conductivity is observed to depend on the dip of the ore body, being more drastic for gently dipping ones.

Variations in the inphase and quadrature components as well as in the phase of the anomaly have been studied for varying depths of burial of the ore body and for a number of values of the solution conductivity. Anomaly index diagrams have been constructed with a view to predicting possible values of the geological parameters from a knowledge of the anomaly components. It is felt that the notable overburden effects are caused by a drastic redistribution and concentration of currents, mostly in a narrow loop at the top of the ore body, brought about by its galvanic contact with an extended medium of relatively poorer conductivity.
SOME RESULTS OF EXPERIMENTAL GEOPHYSICAL SURVEYS FOR LOCATION OF ANCIENT GOLD WORKINGS, KOLAR, INDIA *
Authors: JAIN SHIKHAR C. , KUMAR R. , ROY A.
Published: April 2006

Abstract

On an experimental basis, observations were made with various geophysical methods to locate the ancient gold workings in Kolar Gold Fields, Kolar, India. The results of experimental surveys indicated that two‐electrode resistivity surveys with two spacings followed by limited trenching would be able to locate about 70 percent of the ancient workings, at 55‐60 percent of the cost of conventional trenching.
ELECTROMAGNETIC RESPONSE OF TWO‐DIMENSIONAL INHOMOGENEITIES IN A DISSIPATIVE HALF‐SPACE FOR TURAM INTERPRETATION *
Authors: DEY A. , MORRISON H. F.
Published: April 2006

Abstract

An integral equation technique based on the integral formulation of the electric field of a harmonically excited line‐source in the presence of a two‐dimensional scattering in‐homogeneity is outlined. In this treatment the inhomogeneity is assumed to have an arbitrary cross‐sectional shape and to be located in a layered, dissipative half‐space. The scattering responses in terms of the horizontal and the vertical magnetic field components observed on the ground surface are studied for a variety of geologic models involving overburden layers, vertical or inclined veins and multiple inhomogeneities. The horizontal component of the total magnetic field appears to be the most diagnostic response parameter. A thin vein shaped target with a cross sectional area of 0.05 δ2× 0.50 δ2 (δ2 being the skin‐depth in the lower half‐space) located 0.50 δ2 away from the line‐source and under a highly conductive overburden layer (σ1/σ2= 300) of thickness 0.02 δ2 is easily detected with a moderate resolution. The scattering responses are also sensitive to changes in depths of burial, inclination and conductivity contrast with the surrounding half‐space of the target inhomogeneity. An interpretation scheme as well as a number of characteristic detectability parameters are developed for the conventional Turam method used in mineral exploration.
DIRECT INTERPRETATION OF TWO‐ELECTRODE RESISTIVITY SOUNDINGS *
Authors: KUMAR RAKESH
Published: April 2006

Abstract

This paper describes the procedure for interpreting the apparent resistivity data measured with the two‐electrode array directly with the help of kernel function. The calculation of kernel function from the observed resistivity curve is done by the method of decomposition. In the method of decomposition the resistivity curve is approximated by a sum of certain functions, whose choice is only restricted by the requirement that the contribution to the kernel function corresponding to them should be easily computable. A few such functions are classified. These, and the standard curves for corresponding kernel functions obtained by utilising an integral expression for two‐electrode array expressing the kernel explicitly in terms of the apparent resistivity functions, are plotted on log‐log scale. The determination of layer parameters, that is, the layer resistivities and thicknesses from the kernel function can be carried out by a method proposed by Pekeris (1940).
A MODIFIED GEOELECTRICAL PROCEDURE USING POLARDIPOLE ARRAYS—AN EXAMPLE OF APPLICATION TO DEEP EXPLORATION *
Authors: ALFANO L.
Published: April 2006

Abstract

Dipole soundings are more sensitive to noise caused by lateral and superficial inhomogeneities than Schlumberger soundings. However, the former are preferable for deep explorations in view of the relatively short cables required. The simple solution of carrying out the field work by means of dipole spreads, and to transform the dipole resistivity diagrams into Schlumberger ones by means of proper formulae would be valid only for smooth and regular curves; but often, owing to the presence of lateral noises, the dipole data show a considerable scatter. For such cases a “continuous dipole sounding” method is proposed for which all successive dipoles are contiguous, so that all parts of the profiles are covered and interpolation is not necessary. Obviously the moving dipoles have lengths proportional to their distances, so that they appear equal in the usual bilogarithmic scale. It follows that only polar‐dipole arrays may be used. The transition from a dipole to the corresponding Schlumberger apparent resistivity diagram requires an integration constant which is not unequivocally determined. Therefore, the solution is not unique, but all possible derived Schlumberger diagrams have a common part. Similarly, they have some common interpretative results, which may be referred to the original dipole diagram obtained in the field.

A special measurement technique is required since the dipole‐dipole voltages to be determined are noticeably smaller than the Schlumberger ones. This is true also because dipole soundings are used for great depths and for long distances between the two dipoles.
SEISMIC MODEL STUDIES ON DIFFRACTION OF WAVES BY EDGES OF VARYING RADIUS OF CURVATURE AND DEPTH *
Authors: DATTA S. , BHOWMICK A. N.
Published: April 2006

Abstract

Results of studies carried out with the help of a three‐dimensional seismic model on waves diffracted from edges of varying radius of curvature R and depth h with respect to wave length λ are described. The amplitude decay, travel time, and apparent velocity of the wave diffracted from a sub‐surface edge of semi‐infinite length are found to depend on the parameters R, h, and distance from the edge on the surface provided the ratio of the parameters to λ are less than some limiting values. The nature of the amplitude decay is independent of R when the depth exceeds 2λ, and independent of h when R exceeds 1.5λ. When these are below the limiting values (h= 2λ and R= 1.5λ), the nature of the decay depends appreciably on R and h. The apparent decay in amplitude on the surface due to geometrical spreading by the diffracting edge is less than that of a cylindrical secondary wave source and decreases with increase in depth of the edge. The nature of the travel time curves of the diffracted waves near the edge depend on R/λ when the depth is within about one λ. Apparent velocity of the wave depends largely on R/λ in the zone of diffraction up to a distance of about one λ from the edge on the surface. Beyond this distance the velocity is almost the same irrespective of R/λ and depend only on h/λ. The width of the zone of diffraction caused by an edge of finite length comparable to λ is more and more narrow as the ratio of the distance of the edge on the surface to its depth increases.
A HIGH RESOLUTION MARINE SEISMIC SURVEY *
Authors: LUCAS A. L.
Published: April 2006

Abstract

Sub‐bottom profilers achieve high resolution but limited penetration beneath the seabed. Marine seismic surveys with long streamers and conventional sources suffer from an incomplete CDP stack and an unnecessarily low signal frequency in the early part of the section. Consequently there is a depth interval over which the optimum resolution of the seismic reflection method is not achieved. This paper describes the design, conduct, and results of an experimental survey made to obtain high resolution sections (predominant frequency about 200 Hz) with reflection times up to one second in the northern North Sea. The prime motivation for the experiment was to assess the possibility of mapping a shallow gas sand which had been encountered by the drillers.

The design concept of the survey was to keep all the benefits of conventional marine seismic data gathering and processing, but to reduce their dimensions and time scales by a factor of five to ten. The survey in 1972 recorded a total of twenty‐four lines using twenty‐four fold cover with 5 m spacing between the reflector points, and a separation of 50 m between adjacent lines. Multi‐electrode sparkers were used with a shallow purpose‐built twenty‐four channel 240 m hydrophone streamer. The recording sample interval was 1 ms. The final sections, obtained after conventional data processing, were of good quality. The results of velocity analysis helped to distinguish primary and multiple energy. Reflections with large negative reflection coefficients were seen in some parts of the survey area, and these are thought to indicate thin gas sands.
TWO DIMENSIONAL MASS DISTRIBUTIONS FROM GRAVITY ANOMALIES: A COMPUTER METHOD *
Authors: JIN D. J.
Published: April 2006

Abstract

Qureshi and Mula have suggested a computer method for the calculation of the shape of a two dimensional body from the gravity anomaly profile associated with it. The application of the method, however, was restricted to those bodies which have their top or base horizontal and end sides vertical. In this paper study has been made to investigate the possibility of eliminating the above mentioned restrictions and has arrived at the affirmative conclusion.
CONTOURING AND THE CONTOUR MAP: A NEW PERSPECTIVE*
Authors: WREN A. E.
Published: April 2006

Abstract

With few exceptions, traditional approaches to contouring have been too subjective. Contouring and contour maps are too often discussed in terms more appropriate to art than to science. With hand contouring there is some justification for this attitude; with machine (i.e. programmed) contouring there is none.

Hand contouring is highly susceptible to interpretive judgement and the interpreter is not bound by rigid mathematical constraints. Hence, in allowing for the interpreter's “freedom of expression” it may be difficult to evaluate hand contouring in a totally analytical and objective manner. Machine contouring, however, is based upon mathematical formulation. It is therefore a consistent and objective procedure, ideally suited to objective definition and analysis.

It can be demonstrated that the combined process of sampling plus contouring constitutes a two‐dimensional filter. The contouring component is that part which introduces “distortion” or wavenumber discrimination. An ideal contour package is one that acts as an all‐pass filter where the distortion is zero.

The application of filter theory to the evaluation of a machine contour package and its performance permits description in the more convenient language and terminology of the wavenumber domain, rather than that of the space domain. A more important advantage is that the contour package can be subjected to the various standards of filter evaluation such as amplitude and phase response.

The practical application, as well as the benefit, of this approach is revealed through the comparison, in both the space and wavenumber domains, of contour maps generated from various machine contour packages.
APPLICATION OF A HILBERT TRANSFORM METHOD TO THE INTERPRETATION OF SURFACE‐VEHICLE MAGNETIC DATA*
Authors: GREEN R. , STANLEY J. M.
Published: April 2006

Abstract

The development of an alkali vapour vehicle borne magnetometer providing very high resolution and a high sampling rate has called for reconsideration of interpretation procedures. With continuous profiling at ground level, the close proximity to near surface structures requires that the precise interpretation of geological boundaries be of paramount importance. Rapid digital recording also demands efficient data processing. Both these requirements can be met by the method described. Essentially the method reduces to a simple Hilbert transform of the magnetic profile. The calculation provides an extremely well defined position of the contact and accurate specification of the dip, strike, depth of overburden, and magnetic susceptibility parameters. Particular advantages are tolerance to high frequency noise, independence from a predetermined origin and baseline, and freedom from subjective judgements.
A PRELIMINARY PROCESSING OF THE BOUGUER ANOMALY IN AREAS OF VERY COMPLICATED RELIEF*
Authors: IANAS M. , SAVA C. S.
Published: April 2006

Abstract

Strictly speaking, the Bouguer anomaly is the difference between the gravity observed at some point and the gravity to be expected at the same point for a “normal” earth corresponding to the same level. If the points of observation are located on a surface deviating significantly from a plane, a reduction of the data to a common plane is necessary before interpretation; otherwise spurious anomalies might occur. The processing is based on the solution of the generalized Dirichlet problem. Application of the proposed procedure to a synthetic, but realistic, model bears out the validity of the approach.
ELECTROMAGNETIC DEPTH SOUNDING USING A SOURCE ORIENTATION MODE*
Authors: DUCKWORTH K.
Published: April 2006

Abstract

Electromagnetic depth sounding using source orientation as the sounding variable provides advantages in instrumental simplicity and operational flexibility when compared with conventional modes of electromagnetic sounding. The ease with which the technique may be simulated in a scale model permits its application to sounding over localized structures. The theoretical approach to interpretation is at present limited to structures which approximate a perfectly conducting half space. However, scale model tests suggest that the perfect conductor theory may be applicable to many localized structures.
ITERATIVE GRAVITY MODELING BY USING CUBICAL BLOCKS*
Authors: MUFTI I. R.
Published: April 2006

Abstract

Approximation formulas for a cube's gravity field yield results which, for all practical purposes, are as accurate as those obtained by using the lengthy expressions defining the exact field. This fact is utilized in the development of a new gravity modeling scheme which uses cubes as building blocks.

A geologically feasible shape of the anomalous body is assumed and filled with cubes. To start with the largest cube which can be accommodated is placed inside the body. After that, the sidelength of the cube is halved, and as many cubes of that size as possible are placed into the remaining space. This procedure is repeated until the system of cubes has approached the shape of the body. The total gravity field due to all cubes placed represents the field of the body.

The placement of cubes is governed by an arrangement which is symmetrical with respect to the center of the first cube. Consequently, the data calculated for one cube may be used repeatedly for other symmetrically located cubes; this approach greatly reduces the number of calculations actually carried out. By reducing the size of the first cube and making it sufficiently small so that its density may be treated as uniform, the method can be used for evaluating the gravity field of an irregularly shaped body whose density varies in an arbitrary fashion.

The accuracy of the method is investigated by computing the gravity field of a sphere constructed from cubical blocks and buried at a shallow depth. A comparison of results with the analytical data obtained for the corresponding perfect sphere shows that a very high precision, of the order of microgals, is reached even at the end of the second cycle; moreover, depending on the accuracy desired, the method is three to five times faster than the Talwani‐Ewing algorithm. The method may be applied to borehole gravimetry, where very high precision is required, as well as to problems of topographic correction, where speed is the most important consideration.
LINEAR TRANSFORMATIONS OF GRAVITY AND MAGNETIC FIELDS*
Authors: GUNN P. J.
Published: April 2006

Abstract

The spectral representation of gravity and magnetic fields shows that the mathematical expressions describing these fields are the result of convolution of factors which depend on the geometry of the causative body, the physical properties of the body and the type of field being observed. If a field is known, it is possible to remove or alter these factors to map other fields or physical parameters which are linearly related to the observed field. The transformations possible are: continuation, reduction to the pole, converting between gravity and magnetic fields, converting between components of measurement, calculation of derivatives, and mapping magnetization and density distribution, relief on interfaces, and vertical thicknesses of layers.
ON THE POSSIBILITIES OF DETERMINING DIP IN MAGNETIC INTERPRETATION WITH THE INFINITELY DEEP PLATE MODEL*
Authors: HJELT S.‐E.
Published: April 2006

Abstract

Dip and magnetic susceptibility of very deep magnetic plates can be estimated approximately from either vertical, horizontal or total field measurements. A general accuracy of 2–5 degrees is easily obtained, if the other plate parameters, most notably horizontal position of the plate, are precisely determined. For reliable interpretation, measurements around the anomaly maximum or on the dip‐side flank of the anomaly should be preferred. The depth extent of the plates must be great, some ten times the plate width at least.

The method is best suited to form a part of a plate interpretation scheme, where the other plate parameters are found by some other suitable means. The method can be applied to a simultaneous determination of dips of several plates, but because of its error sensitivity an iterative formulation should then be preferred.
A STUDY ON THE POLES AND ZEROES OF STEFANESCU'S RAISED KERNEL FUNCTION
Authors: LOEWENTHAL D.
Published: April 2006

Abstract

The theory of geoelectric sounding exploration is based on Stefanescu's kernel. In order to study the analytic properties of the kernel, necessary and sufficient conditions on it's poles and zeroes are developed. A distinction is made between “physical” and “perfect” models.

It is shown that a physical model may be determined from either the set of poles or the set of zeroes of the raised kernel function at the top of the model. These roots may be of desired multiplicities.

For determining a perfect model, both sets of poles and zeroes are required and they all have to be simple ones.