bluuebunny/crossref_metadata_2025_split huggingface dataset:
RAPID DETERMINATION OF CUBE'S GRAVITY FIELD *
Authors: MUFTI I. R. 
Published: April 2006

AbstractGravity modeling involves the evaluation of gravity field due to bodies of irregular shape. This is commonly accomplished by dividing the body into smaller units of regular shape whose field can be determined analytically.Cubes are ideal building blocks for constructing bodies of irregular shape. However, the exact expression defining the gravity field of a cube‐shaped body is very long and requires tedious numerical work. Approximation formulas for rapid evaluation of the cube's gravity field are presented and discussed in detail. Their use for computing the field for small distances leads to large errors in the results; on the other hand, using the exact formula for large distances where approximation formulas yield practically identical results, is waste of time. After a detailed analysis about the nature of the cube's gravity field, certain criteria are established for suitably combining the two alternatives. This results in tremendous reduction in the task of computations without affecting the accuracy of data.
PHYSICAL AND COMPUTER MODELING OF INDUCED POLARIZATION *
Authors: AIKEN C. L. , HASTINGS D. A. , STURGUL J. R. 
Published: April 2006

AbstractThe Laboratory of Geophysics of the University of Arizona was presented with an exploration problem by Falconbridge, Ltd. of Canada. Massive mineralized hemispherical “pods” are in the vicinity of a tuff layer of high conductivity and induced polarization response, covered by large thicknesses of resistive volcanics. The initial approach was to utilize electrolytic tank modeling. The extreme resistivity and IP contrasts proved to be difficult to recreate. Two dimensional modeling was attempted next with conductive paper, using copper and silver paint for anomalous masses. This method also proved inadequate. Finally, mathematical equations were solved which could model any arbitrary anomalous body in any steady state electrical field. Plane waves as well as point current sources producing non‐plane waves are possible. Finite difference equations were derived for the non‐linear partial differential equations under consideration. The equations were solved using a digital computer. Initially, the boundary conditions had to be satisfied at the boundaries of resistivity changes, severely restricting possible geometric shapes for anomalous bodies. The final and successful solution was to apply numerical techniques to obtain solutions of equations which require only that the relative resistivities through the area be specified. The Falconbridge problem and its solution are analyzed.
NEW POSSIBILITIES FOR REFLECTION SEISMICS BY UNDERSHOOTING BODIES WITH COMPLICATED TECTONICS *
Authors: BADING R. , ECHTERHOFF J. , KREY TH. , MARSCHALL R. 
Published: April 2006

AbstractIn modern oil exploration layers of prospective interest with rather simple structural features are often overlain by very complicated bodies as e.g. saltdomes or other kinds of diapirs, olistostromes, or front zones of overthrusted blankets. In all these cases normal reflection seismic investigations, where downgoing and upgoing rays are rather close to each other, mostly fail, either because no reflections from underneath the complicated bodies are obtained, or because a reliable migrated depth presentation becomes practically impossible due to the inhomogeneity of the overlying bodies.The undershooting technique avoids these difficulties by using ray paths which do not traverse the complicated bodies e.g. by shooting on one side of a saltdome and recording on the other side. On account of the large shot‐geophone distances in this method special considerations and computer processes were developed concerning moveout corrections for common depth point stacking and migrated depth presentation.In many cases the location of the disturbing complicated bodies is known in advance. The shooting and recording program can then be adjusted to this knowledge and thereby kept to a minimum. If the location of the complicated bodies is unknown a more extended seismic program has to be carried out encompassing a great variety of shot‐geophone distances. But in this case the approximate location of the complicated bodies can be deduced from the survey too.Results are presented in order to give an idea of the efficiency of the new seismic tool.
THE STABILITY OF THE SOLUTIONS OF THE POTENTIAL FIELD TRANSFORMATIONS *
Authors: IANĂṠ M. , MOLDOVEANU N. 
Published: April 2006

AbstractThe quantitative analysis of the potential fields leads to the solution of some operational equations which sometimes have unstable solutions representing fictitious anomalies. A general method to find numerically stable solutions of such problems is presented in this paper. The subject of the downward analytical continuation is also discussed. The method has been checked on a theoretical model and applied to a gravity map.
PULSE CODING IN SEISMIC PROSPECTING SOSIE AND SEISCODE *
Authors: BARBIER M. G. , VIALLIX J. R. 
Published: April 2006

AbstractThe conventional seismic technique is subject to a recording time following each transmission of energy, in which it is forbidden to release any new pulse. The recording time depends on the deepest reflection of interest, and is often 10 seconds or more in actual practice. To each transmission corresponds one record, i.e. a fixed amount of data which cannot be increased in a given time.Pulse coding allows us to go beyond this limit, by transmitting several times during the normal recording time. The procedure gives as many records as there are pulses, but they overlap, each event being repeated every time there is a pulse. It is possible to process the composite record back to its usual appearance with all events in their proper place if the time breaks are accurately known and make up a code such that the unavoidable noise generated by the process be kept, on the final section, below the ambient noise. The processing is quite similar to that of records made from vibrating sources, though faster in practice.The additional information can be devoted to a saving of time and money as the same profile may be recorded in a shorter time; or to an improvement of quality of the section due to a higher order of coverage, a multiplication of the ray paths and a closer sampling of the reflectors. It is also possible to record information in several planes at the same time, and to work out a 3‐dimensional restitution, without loss of production.The process applies to all kinds of sources provided they can be triggered according to the code with sufficient accuracy. Depending on the source and conditions of implementation, the method benefits from other advantages such as better resolution, increased flexibility, and better coupling.Two different names have been given to the process, Sosie and Seiscode, which apply to slightly different parameters for the sequence of pulses. Sosie is more useful at sea, while the normal scope for Seiscode is onshore. Both names are trademarks for SNPA.
DEPTH OF INVESTIGATION IN RESISTIVITY METHODS USING LINEAR ELECTRODES *
Authors: APPARAO A. , RAO T. GANGADHARA 
Published: April 2006

AbstractDepths of investigation for normal (bipolar) and focusing (unipolar) linear electrode arrays have been computed following the method given earlier (Roy and Apparao 1971) both in homogeneous and layered earth. The focusing arrangement is found to be superior to normal arrangement both in regard to depth of investigation and zone of uniform depth of investigation. This analysis holds good in layered media too. Further, the depth of investigation increases with the increase in the length (2M) of the electrode and reaches a limit, 0.29L for normal and 0.50L for focusing, when the electrode attains an optimum length to satisfy the infinite condition. This optimum length of the electrode is six times the distance L of the observation point from the electrode for normal and about ten times for focusing. A contour diagram for contributions of individual ground elements to the total signal measured at the centre of focused system is also discussed.
TWO‐DIMENSIONAL POWER SPECTRAL ANALYSIS OF AEROMAGNETIC FIELDS *
Authors: MISHRA D. C. , NAIDU P. S. 
Published: April 2006

AbstractA study of probability distribution function and spectrum of the Airborne Total Intensity Map for an area of 3000 Squ. miles covered by the Bundelkhand granite in India has revealed a close correlation between the Gaussian or non‐Gaussian nature of distribution and the shape of unite value contour on the spectrum plot. Further studies of radial spectrum plots suggest that the surface magnetic sources are mainly responsible for the non‐Gaussian character of the distribution function and as well as for irregular spectrum shape. The histogram of depth values for magnetic sources obtained from radial spectrum plots show three horizons, namely (i) surface (ii) 410 m(1400′) and (iii) 1000 m (3400′).Based on these informations blocks with ferromagnetic composition at the surface can be identified from those which are non magnetic at the surface.
APPROXIMATE RULES FOR THE COMPOSITION OF APPARENT RESISTIVITY SECTIONS *
Authors: HABBERJAM G. M. , JACKSON A. A. 
Published: April 2006

AbstractConsideration is given to the form of apparent resistivity sections obtained by varying electrode array position and spacing over two dimensional features.Since basic solutions exist for horizontally layered media and for vertical or dipping single interfaces, simple rules are proposed whereby these solutions can be directly combined to give solutions for more complex sections. Substantial approximations may be involved as the rules do not fully allow for interactions between the constituents.The range of error incurred in application of the rules is explored, with particular reference to the square array system of resistivity measurement. The two extremes‐ of orientation of the square array with respect to the line of section are separately considered in this survey.Within appropriate limits, the rules appear extendable to a wide variety of simple structures for which rigorous solutions would be formidable. The rules are finally discussed in relation to practical applications.
TRANSFORMATION OF DIPOLE RESISTIVITY SOUNDING MEASUREMENTS OVER LAYERED EARTH BY LINEAR DIGITAL FILTERING *
Authors: DAS U. C. , GHOSH D. P. , BIEWINGA D. T. 
Published: April 2006

AbstractThe technique of linear digital filtering as developed for the direct interpretation of Schlumberger and Wenner soundings (Ghosh 1971) has been applied here for the derivation of the resistivity transform function from the field dipole measurements as the first step in directly interpreting dipole data. Filter coefficients for this transformation have been worked out for the radial‐polar, perpendicular and parallel (30°) arrays of dipole sounding. The procedure combines speed with accuracy.
NAVIGATION REQUIREMENTS FOR GEOPHYSICAL EXPLORATION *
Authors: SHERIFF R. E. 
Published: April 2006

AbstractMarine geophysical work requires accurate navigation on a continuous basis. A repeatability of 150 m CEP (circular error probable) with sequential positioning of 15 m CEP is usually acceptable. Quoted figures for many presently available navigation systems yield nearly the required accuracy. In actual operations, however, such accuracies may not be achieved, and usually the performance actually realized cannot be determined for want of independent measurements for comparison. If more redundancy in navigation measurements were available on an integrated basis, the limitations of individual systems would no longer be so critical and the accuracy of measurements could be assessed in real time. Failures in navigation continue to occur. Such failures usually result from ignorance of the capabilities of the navigation system and limitations and consequent choice of a system which is not appropriate to the demands of a particular survey. The points to be covered in work standards are listed.
A CONTRIBUTION TO THE QUANTITATIVE ERROR ANALYSIS IN MAGNETIC AND GRAVIMETRIC INTERPRETATION *
Authors: HJELT S. E. 
Published: April 2006

AbstractThe paper discusses the smallest obtainable parameter errors (variances) in the interpretation with the least‐squares method. Useful approximations of the sum of squares contained in the minimum error expressions are obtained using results of numerical integration. The approximations lead to especially simple results for long interpretation profiles, when the parameter errors are proportional to the square root of the point separation.Formulae are developed and examples shown for minimum error calculation in gravimetric interpretation with the cylindrical model and in magnetic interpretation with the two‐dimensional plate model. Smallest errors are obtained when the interpretation profile is chosen around the anomaly maximum except for dip and depth extent interpretation of magnetic plates.
STACKING FILTERS AND THEIR CHARACTERIZATION IN THE (f—k) DOMAIN *
Authors: HUBRAL P. 
Published: April 2006

AbstractThe implementation of a stacking filter involves the filtering of each trace with an individual filter and the subsequent summing of all outputs. The actual position of a trace in space as well as certain simultaneous shifts of traces and filter components in time do not influence the process. The resulting output is consequently invariant to various arbitrary coordinate transformations. For a certain useful class of ensembles of non‐linear moveout arrival times for signals a particular transformation can be found which transforms a given ensemble into one consisting only of straight lines. It is thus possible to reduce, for instance, the analysis of a stacking filter designed for hyperbola‐like moveout curves to the analysis of a velocity filter with linear moveout curves. As the (f—k) transform is a very useful concept to describe a velocity filter, it can consequently be applied to characterize a stacking filter in regard to its performance on input signals with non‐linear moveout.
THE INTERPRETATION OF GRADIENT ARRAY CHARGEABILITY ANOMALIES *
Authors: QUICK D. H. 
Published: April 2006

AbstractEmpirical equalities derived from time domain induced polarization scale modelling with the gradient array over simple geometries, and from the potential field functions for equivalent simple charge configurations are similar. The function for the dyke like body is analogous to the magnetic case allowing both total and vertical magnetic field interpretation techniques to be applied to gradient array chargeability anomalies.
THE DETERMINATION OF FILTER COEFFICIENTS FOR THE COMPUTATION OF STANDARD CURVES FOR DIPOLE RESISTIVITY SOUNDING OVER LAYERED EARTH BY LINEAR DIGITAL FILTERING *
Authors: DAS U. C. , GHOSH D. P. 
Published: April 2006

AbstractThe technique of linear digital filtering developed for the computation of standard curves for conventional resistivity and electromagnetic depth soundings is applied to the determination of filter coefficients for the computation of dipole curves from the resistivity transform function by convolution. In designing the filter function from which the coefficients are derived, a sampling interval shorter than the one used in the earlier work on resistivity sounding is found to be necessary. The performance of the filter sets is tested and found to be highly accurate. The method is also simple and very fast in application.
COMPATIBILITY OF EM AND IP SURVEYS *
Authors: VOGELSANG D. 
Published: April 2006

AbstractMethods and typical case histories of EM and IP surveys are compared in order to check their compatibility for the investigation of large areas for base metal sulphides. It is demonstrated that the fast, low cost EM may miss sulphide concentrations which cause no reduction of apparent resistivity, or which act like horizontal slabs, where EM anomalies occur marginally. In such cases only IP can recognize the whole extent of the mineralization. The misleading role of graphite exists for both methods.The conclusion is that EM remains the only economical method to survey whole ore districts, but that the shortcomings should always be taken into account in the interpretation. They can be overcome by employing geological, geochemical or other geophysical indicators in selecting promising targets for follow‐up IP. Even if less than 20% of the area is covered by the expensive IP, there is a fair chance that all sulphide bearing rocks will be found. Therefore, the question is not whether to apply EM or IP methods, but how to combine them best.
THE USE OF FOURIER SERIES METHOD IN UPWARD CONTINUATION WITH NEW IMPROVEMENTS*
Authors: TSAY L. J. 
Published: April 2006

AbstractThis paper shows how to reduce the errors near edges of potential field data that result from using Fourier series in the computation of upward continuation of potential field anomalies. This kind of error, if uncorrected, can lead to erroneous geological interpretation.The errors that occur at both edges of potential field data after upward continuation originate from the representation of data by using Fourier series from which data becomes periodic with discontinuities or sharp changes between each period. In order to reduce this type of error, we propose, either 1) to use only the cosine series, or 2) to add a certain number of constant data to both edges of the original data before continuation. Using these new schemes, we have demonstrated the improvement on the accuracy near edges of continued anomalies with profiles of magnetic anomaly computed from an assumed model.
EFFECTS OF DATA ERRORS AND ACCURACY IN SECOND DERIVATIVE COMPUTATION BY RATIONAL APPROXIMATION*
Authors: AGARWAL B. N. P. , SINGH J. 
Published: April 2006

AbstractThe effects of systematic (constant) and random errors in the observed data have been investigated analytically for rational approximation method of computing second derivative involving a summation of the products of the averages of the gravity field with the corresponding weight coefficients, both in numerator as well as in denominator. A theoretical gravity anomaly over three spheres has been analyzed to demonstrate the high accuracy in the approximation. Since the sums of the weight coefficients in numerator and denominator are zero and one respectively, the regional gravity anomaly, even though approximated by a constant value over the entire area under computation, can produce substantially large error in the calculated derivative value. This is happening because of the contribution of the regional field in the denominator. Thus, inspite of the high accuracy in rational approximation, the method has limited application to field cases where a combined gravity field consisting of regional and residual anomalies is usually used. Master curves are presented for the constant and random errors by which a rough estimate of the percentage of error in second derivative computation can be made provided one has some idea of the magnitudes of the regional field and random error.
ADVANCED METHOD FOR SELF‐ADAPTIVE ESTIMATION OF RESIDUAL STATIC CORRECTIONS*
Authors: SÁGHY G. , ZELEI A. 
Published: April 2006

AbstractThe estimation of residual static corrections is investigated as a linear system. For 1200% coverage shot one way offset weighting functions—or in other words, estimation error operators—are computed. Assuming zero time shift determination errors and zero RNMO it is seen that large residual corrections will distort the effectivity of the estimation.With an iterative procedure which includes the repeated averaging according to depth point, shot point, and geophone point the effectivity of the method becomes more favourable. This can be learnt from the evaluation of estimation error operators computed by recursive formulae.In practice, due to the time shift determination errors, the routine does not give improved results in the desired level. Errors can be reduced with a more reliable time shift determination routine.For detecting and suppressing the unfavourable effect of time shift errors still remaining in the data a self‐adaptive method was developed. In this non linear routine the data are evaluated at all three averaging processes according to limit values and weighting factors computed from the data itself.Field examples processed in different ways are presented.
NEW RESULTS IN RESISTIVITY WELL LOGGING*
Authors: ROY A. 
Published: April 2006

AbstractTwo independent theoretical analyses show that, spacing for spacing, the two‐electrode normal device investigates deeper than the seven‐electrode focused Laterolog 7. The first analysis, published earlier, compares the radii of investigation of the different sondes from the portion of the ground that contributes maximum to, or fifty percent of, the measured signal. The second relates to the nature of the departure curves for the two sondes for an infinitely thick resistive formation pierced by a bore hole. With increasing spacing, the apparent resistivity for the normal device rises much faster and asymptotically approaches the true resistivity much earlier than that for Laterolog 7. These two analyses also prove that, in a Laterolog 7, the radius of investigation increases as O1O2 decreases relatively to A1A2. Since any complex point electrode system is equivalent to a superposition of elementary dipoles or two‐electrode devices, it follows that no resistivity sonde of discrete point electrodes can have a radius of investigation larger than that for the normal device of the same spacing. Laboratory measurements in a model tank confirm these theoretical results.For thin highly resistive formations we find from theoretical and model studies that the normal device is markedly superior to Laterolog 7 as it gives apparent resistivity values much closer to the true. It has other advantages as well. Thus, our results on the comparative performances of the normal and Laterolog 7 devices are at variance with those published so far since the introduction of Laterolog 7 in 1951.For reasons given in the text, the chief characteristic of Laterolog 7—namely, focusing the current from the central electrode in a thin horizontal sheet into the target formation—does not endow it with any special advantage. The main reason is that the measured potential in a Laterolog 7 is caused not only by the current focused into the target formation but also by the currents from the outer two electrodes which flow, due to the same focusing process, entirely through the adjacent formations and the mud column. Indeed, the contribution to the measured signal by the outer two power electrodes is considerably larger than that from the central.
DETECTION OF CAVES IN A KARST FORMATION BY MEANS OF ELECTROMAGNETIC WAVES*
Authors: KAŠPAR M. , PECEN J. 
Published: April 2006

AbstractThe application of classical geophysical methods in locating karst caves did not yield always unambiguous results. It was our task to verify a new method that has not yet been tested. In investigating caves in a karst formation, electromagnetic waves in the band 2–10 MHz were applied.The physical analysis of the problem is carried out, the measuring device used for the perradiation of the rocks between the boreholes is described, and the results of the measurements made in the karst area of East Slovakia (ČSSR) are shown. The verification from mines is included. The results mentioned are demonstrated in a space model. The work performed in the mines verified unambiguously the interpretation of the indications measured.The results show the suitability of putting the electromagnetic method in question into the set of geophysical methods within the hydrogeological survey of karst areas.