bluuebunny/crossref_metadata_2025_split huggingface dataset:
TOPOGRAPHIC EFFECTS ON ELECTROMAGNETIC DEPTH SOUNDING SYSTEMS *
Authors: VERMA R. K. 
Published: April 2006

AbstractComputations 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

AbstractAttention 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

AbstractThe 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

AbstractThe 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

AbstractOn 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

AbstractAn 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

AbstractThis 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

AbstractDipole 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

AbstractResults 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

AbstractSub‐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

AbstractQureshi 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

AbstractWith 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

AbstractThe 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

AbstractStrictly 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

AbstractElectromagnetic 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

AbstractApproximation 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

AbstractThe 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

AbstractDip 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

AbstractThe 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.
THERMAL INFRA‐RED TECHNIQUE APPLIED TO MODERN INVESTIGATION*
Authors: THOMAS J. , PEDEUX J. P. , ARNAUD C. 
Published: April 2006

AbstractDuring the last few years a substantial improvement has been made in the development of the thermal infra‐red scanner so that 1.5 milli‐radian resolution is now commonly obtained. Direct transfer of information on 70 mm film through fiber glass transcoder allows immediate and refined interpretation.Although the collected data represent the image of thermal anomalies within a pellicular section of the earth's crust, it is proved that because of “per ascensum” phenomena, deep buried sources induce surface thermal changes. The difficulty lies in showing an absolute relationship between these deep phenomena and thermal surface occurrences.A careful examination of several cases related to hydrology, karstic geology, civil engineering, pollution, hot springs should help the geophysicist by providing him a new tool for shallow investigations. Thermographics with interpretation are shown to support this statement.
INTERPRETATION OF MAGNETIC ANOMALIES USING SPECTRAL ESTIMATION TECHNIQUES*
Authors: CASSANO E. , ROCCA F. 
Published: April 2006

AbstractThe depth of causative bodies may be derived from the power spectrum of their magnetic anomalies. After a short review of the theoretical basis of the method, several examples of its application to synthetic cases are shown. Disturbing effects due to improper choice of the sampling interval and to anomalies only partially contained in the examined segment of the profile are studied.The spectral method is then applied to real cases; the tapering of the anomalies, the application of non‐linear filters and the effects of anomaly superpositions are investigated.In conclusion, an appraisal of the method and of its possible practical impact has been given.