bluuebunny/crossref_metadata_2025_split huggingface dataset:
A FAST METHOD FOR DETERMINING THE LAYER DISTRIBUTION FROM THE RAISED KERNEL FUNCTION IN GEOELEGTRICAL SOUNDING*
Authors: KOEFOED O. 
Published: April 2006

AbstractIn 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

AbstractNon 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

AbstractA 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

AbstractThe 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

AbstractModern 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

AbstractDuring 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

AbstractA 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

AbstractConsideration 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

AbstractThe 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

AbstractIn 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.ZUSAMMENFASSUNGIm 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

AbstractThe 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

AbstractThe 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

AbstractThe 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

AbstractVarious 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.
COMPARATIVE FIELD PERFORMANCE OF ELECTRODE ARRAYS IN TIME‐DOMAIN INDUCED POLARIZATION PROFILING *
Authors: ROY A. , JAIN SHIKHAR C. 
Published: April 2006

AbstractFive test profiles in time‐domain induced polarization measurements over conducting sulphide and graphite mineralisations were run with the two‐electrode, three‐electrode, modified unipole, pole‐dipole, dipole‐dipole, Schlumberger, and Wenner α and β configurations. The results show that, compared to the other electrode systems, the simplest two‐electrode array produces the largest anomalies with the smallest of spacings.
INLINE AND BROADSIDE EM DIPOLE DIPOLE PROFILING OVER A THIN VERTICAL INFINITELY CONDUCTING VEIN *
Authors: JAIN SHEEL C. 
Published: April 2006

AbstractMaster diagrams for electromagnetic responses of nine dipole dipole systems are presented for various depths of a vertical infinitely conducting vein. The YY system gives the minimum anomaly for both the inline and broadside arrays. Among other inline systems, it is difficult to decide the clearcut superiority of one system over the other, whereas the XX system in broadside array gives maximum anomaly.
THE ESTIMATION OF SIGNAL SPECTRA AND RELATED QUANTITIES BY MEANS OF THE MULTIPLE COHERENCE FUNCTION *
Authors: WHITE R. E. 
Published: April 2006

AbstractA seismic trace recorded with suitable gain control can be treated as a stationary time series. Each trace, χj(t), from a set of traces, can be broken down into two stationary components: a signal sequence, αj(t) *s(t—τj), which correlates from trace to trace, and an incoherent noise sequence, nj(t), which does not correlate from trace to trace. The model for a seismic trace used in this paper is thus χj(t) =αj(t) * s(t—τj) +nj(t) where the signal wavelet αj(t), the lag (moveout) of the signal τj, and the noise sequence nj(t) can vary in any manner from trace to trace. Given this model, a method for estimating the power spectra of the signal and incoherent noise components on each trace is presented.The method requires the calculation of the multiple coherence function γj(f) of each trace. γj(f) is the fraction of the power on traced at frequency f that can be predicted in a least‐square error sense from all other traces. It is related to the signal‐to‐noise power ratio ρj(f) by
image
where Kj(f) can be computed and is in general close to 1.0. The theory leading to this relation is given in an Appendix.Particular attention is paid to the statistical distributions of all estimated quantities. The statistical behaviour of cross‐spectral and coherence estimates is complicated by the presence of bias as well as random deviations. Straightforward methods for removing this bias and setting up confidence limits, based on the principle of maximum likelihood and the Goodman distribution for the sample multiple coherence, are described.Actual field records differ from the assumed model mainly in having more than one correctable component, components other than the required sequence of reflections being lumped together as correlated noise. When more than one correlatable component is present, the estimate for the signal power spectrum obtained by the multiple coherence method is approximately the sum of the power spectra of the correlatable components. A further practical drawback to estimating spectra from seismic data is the limited number of degrees of freedom available. Usually at least one second of stationary data on each trace is needed to estimate the signal spectrum with an accuracy of about 10%. Examples using synthetic data are presented to illustrate the method.
DÉCONVOLUTION RAPIDE *
Authors: BOIS P. 
Published: April 2006

AbstractDuring the last ten years data processing has enabled the geophysicist to seriously attack the problem of how to improve the quality of seismic sections. For instance new processing methods have been proposed for improving the resolving power of the seismic pulse. These methods are based on deconvolution and they require lengthy computations.The method discussed here for pulse contraction is based on the use of Hadamard transforms. It is very fast but nevertheless the quality of the result stands comparison with that of conventional methods.