bluuebunny/crossref_metadata_2025_split huggingface dataset:
USE OF THE RESISTIVITY METHOD IN GEOLOGICAL MAPPING—CASE HISTORIES FROM RANIGANJ COALFIELD, INDIA*
Authors: VERMA R. K. , BANDYOPADHYAY T. K. 
Published: April 2006

AbstractThe effectiveness of the electrical resistivity method has been studied using various configurations with different spacings over the Southern Boundary Fault in the northwestern part, and across a dolerite dyke named “Salma dyke” in the central part, of the Raniganj Coalfield, India. It has been observed that the delineation of the fault and the dyke was made possible under differential tropical weathering conditions. Geoelectric sections across the fault and the dyke have been prepared on the basis of Schlumberger sounding results. In profiling, Wenner, two‐electrode, half‐Schlumberger and part of Al‐Chalabi's configurations were applied over the Southern Boundary Fault. Across the dyke, alpha‐, beta‐, and gamma‐Wenner, Schlumberger, half‐Schlumberger and two‐electrode configurations were employed. Azi‐muthal two‐electrode sounding was also studied over the fault.The interpretation of the results of apparent resistivity profiles across the Southern Boundary Fault suggests that the Wenner and two‐electrode configurations possess certain diagnostic features which help in mapping a single lithological contact, provided sufficient resistivity contrast exists. Although Schlumberger configuration seems to be quite suitable, other configurations may also be usefully employed over the dyke.
AIRBORNE RESISTIVITY MAPPING USING A MULTIFREQUENCY ELECTROMAGNETIC SYSTEM*
Authors: SINHA AJIT K. 
Published: April 2006

AbstractAirborne electromagnetic methods are most commonly used in mineral exploration. However, new developments, such as multifrequency capability and digital on‐board field recording, as well as improvements in instrumentation resulting in high signal‐to‐noise ratios in recorded data, have made their application in geological mapping possible.A three‐frequency airborne EM survey carried out over an area northwest of Timmins, Ontario, was interpreted in terms of thickness and resistivity of the layers of a two‐layer earth section. Since both in‐phase and quadrature components are measured, this provides six independent parameters at each point in space. Based on prior geological information and a preliminary interpretation of the field records, two two‐layer models of the subsurface seemed to be appropriate for most of the survey area. An automatic computerized interpretation procedure was devised to interpret the field data at each point in terms of thickness and resistivity parameters of those two models. When the geology is more complex, the data do not fit the models and no interpretations are made. Two maps illustrating the variation of the resistivity and the thicknesses of the layers were constructed from the interpreted data. These maps agree with the known geological information about the distribution of glacial clay in the area. Areas where the layered models do not fit are known to be areas where the geology is complex with a large number of dykes and other lateral inhomogeneities. The study shows that multifrequency airborne EM surveys can be very useful in geological mapping over inaccessible terrain and can significantly help the mapping geologist where outcrops are scarce.
AUTOMATIC PICKING OF FIRST ARRIVALS AND PARAMETERIZATION OF TRAVELTIME CURVES*
Authors: GELCHINSKY B. , SHTIVELMAN V. 
Published: April 2006

AbstractOne of the methods for studying the near‐surface low‐velocity zone and for subsequent determination of static corrections is the technique of employing first arrivals. During the past few years several computer algorithms, based on some simplifying assumptions, have been suggested for automatic determination of first arrivals.This paper suggests a new method for automatic picking of first arrivals, which works under quite general assumptions concerning the character of the data. The method is based on the use of correlation properties of signals and the application of a statistical criterion for the estimation of first arrivals time. A number of dimensionless parameters is used in the algorithm making it possible to regulate the level of reliability and the resolution of the picking procedure.The second stage of the algorithm is the parameterization of the traveltime curve, that is a division of the previously obtained t—x curve into separate rectilinear segments. The suggested algorithm of parameterization is based on an heuristic use of some properties of maximum likelihood estimates. This permits location of the breakpoints of the t—x curve and the estimation of the parameters of each rectilinear segment.A computer program has been written based on the picking and parameterization algorithm. This program has been tested on a large amount of field data and the results show that it works at least as well as the hand procedure.
TWO STRUCTURAL MODELS FOR THE WESTERN FLANK OF THE BRABANT MASSIF*
Authors: DE MEYER F. 
Published: April 2006

AbstractThis paper is concerned with an interpretation of the Bouguer anomaly on the Western flank of the Brabant Massif (Belgium). The position, shape, and density contrast of elementary bodies in the upper part of the earth's crust are determined in a purely numerical manner. A batholith‐like body and a basin‐type structure both adequately account for the observed surface field, at least from a one‐sided geophysical and mathematical point of view. Since seismological control is lacking the two models are representative of the ambiguity and indeterminacy of the definitions of the geologic cause of the anomalous features, if the data are restricted to gravity information alone.
ANISOTROPY DUE TO BEDDING—A COMPUTER STUDY*
Authors: LEVIN F. K. 
Published: April 2006

AbstractBedding on a scale small in comparison to the wavelength results in transverse isotropy. On the other hand, anisotropy has been observed in the field, particularly with horizontally polarized shear waves. In this article velocity data from 23 wells are used to estimate the resulting transverse isotropy and to compare these data with anisotropy observations reported in the literature. Since the well data were compressional velocity (or rather transit time) only, the corresponding shear velocity data were estimated on the basis of a reasonable—though arbitrary—assumption of Poisson's ratio. Actual densities were used wherever they were available; for the remainder of the data, density was assumed to be constant throughout the averaging interval.The anisotropy data estimated on this basis were generally considerably smaller than the observational data that have been reported in the literature. Although both the above assumptions—functional dependence of Poisson's ratio on transit time and constant density—tend to underestimate anisotropy, and although 23 wells is but a small subset of all available data, it appears unlikely that bedding alone could cause anisotropy of the magnitude reported.
STATISTICAL EVALUATION OF MT AND AMT METHODS APPLIED TO A BASALT‐COVERED AREA IN SOUTHEASTERN ANATOLIA, TURKEY*
Authors: ILKISIK O. M. , JONES A. G. 
Published: April 2006

AbstractThe efficacy of the magnetotelluric and audiomagnetotelluric (MT/AMT) methods for detailing the structure of a hypothetical geological section is investigated by using the singular value decomposition (SVD) technique. The section is representative of southeastern Turkey, which is mostly covered by basalt and is a prime area for oil exploration. One of the geological units, the Germav shale at a depth of 600 m, is a problem layer for electromagnetic surveys because of its very low resistivity (on average 3 Ωm) and highly variable thickness across the area (200–900 m). In the MT frequency range (0.0004–40 Hz) its total conductance—or, since its resistivity is known from resistivity log information, its thickness—is the best resolved model parameter. The total depth to the Germav shale and the resistivity of the Cambrian/Precambrian basement are the marginally resolved parameters. In the AMT frequency range (4–10000 Hz) the resistivity of the surface basalt layer strongly affects the resolution of the other, less important, model parameters which are the total depth to the Germav shale and the total conductance of the Germav shale. The errors in the measurements determine the number of model parameters resolvable, and are also important for interpretation of the geological model parameters to within a desired accuracy.It is shown that statistical evaluation of the MT and/or AMT interpretations by using an SVD factorization of the sensitivity matrix can be helpful to define the importance of some particular stage of the interpretation, and also provides a priori knowledge to plan a proposed survey. Arrangements of MT and AMT observations, together with some Schlumberger resistivity soundings, on a large grid will certainly provide three‐dimensional detailed information of the deep geoelectric structure of the area.
SIDESWIPE REFLECTIONS AND OTHER EXTERNAL AND INTERNAL REFLECTIONS FROM SALT PLUGS IN THE NORWEGIAN‐DANISH BASIN*
Authors: HOSPERS J. 
Published: April 2006

ABSTRACTSideswipe, foreswipe and backswipe reflections are described from salt plugs situated in the Norwegian‐Danish Basin (southern part of the Norwegian sector of the North Sea). These external reflections can be mapped and are shown to define the shoulders of a salt plug.Physical three‐dimensional models are described which aid in visualizing the various aspects of the external reflection process. Refracted waves which have been internally reflected in a salt plug have also been identified.
ON THE CALIBRATION OF THE WATER GUN PRESSURE SIGNATURE*
Authors: SAFAR M. H. 
Published: April 2006

ABSTRACTA simple field method was proposed by the author in 1976 for measuring the absolute amplitude of the pressure pulse radiated by marine seismic sources which radiate a bubble pulse. The proposed method involves the recording of the near‐field pressure signature radiated by the water gun using a wide‐band hydrophone. The key feature of the proposed method is that a knowledge of the hydrophone sensitivity and its distance from the water gun are not required.It is shown that the absolute amplitudes of the pressure pulses radiated by the S80 and P400 water guns obtained using the proposed method are in agreement with those obtained using a Ref‐Tek hydrophone.
EFFECT OF INCLINED ANISOTROPIC SUBSTRATUM ON MAGNETOTELLURIC RESPONSE*
Authors: SINGH RAMESH P. 
Published: April 2006

ABSTRACTIn tectonically active regions electrical conductivity anisotropies are the dominating features. The importance of conductivity anisotropy in the interpretation of magnetotelluric data is well known. In the present study numerical results presented which show the effect of a substratum with inclined anisotropy on the magnetotelluric response. The pronounced change on the magnetotelluric response is found for the models in which the substratum underlies (i) conductive and (ii) resistive overburden.
DC GEOELECTRIC SOUNDING TO DETERMINE WATER CONTENT IN THE SALT MINE ASSE (FRG)*
Authors: KESSELS W. , FLENTGE I. , KOLDITZ H. 
Published: April 2006

ABSTRACTIn the course of investigations concerning disposal of radioactive waste, DC‐geoelectric soundings were performed in the salt mine Asse in 1982. The survey resulted in the determination of resistivities of various salt formations. A comparison of various resistivities obtained in salt formations in the mine shows that it is possible to estimate the content of free water in salt using Archie's equation.The significant result of this survey is that salt formations in the mine can be divided into two categories according to their resistivities:1. Salt with resistivity between 107and 108Ωm (particularly Na2Sp and Na3β). The particularly pure halitic formations are to be found within this resistivity range.2. Salt with resistivity between 103and 105Ωm (Na2Tl, K2, transition salt). The salt formations with thin layers of anhydrite and a high content of hygroscopic salts are classified in the paper.
THE COMPLEX RESISTIVITY SPECTRA OF MODELS CONSISTING OF TWO POLARIZABLE MEDIA OF DIFFERENT INTRINSIC PROPERTIES*
Authors: SONG LIU , VOZOFF K. 
Published: April 2006

AbstractTheoretical model study shows that when an earth model is composed of two (Cole‐Cole) polarizable media, its normalized complex resistivity spectrum is approximately a multiplicative combination of the contributions of the two media. This also applies on inversion, but the two dispersions thus obtained are apparent rather than intrinsic dispersions.In models consisting of two media, either a multiplicative or an additive combination of Cole‐Cole functions fits the complex apparent resistivity spectrum. On inversion each combination gives similar parameters except for the apparent chargeability of the component with the shorter time constant. However, this can be compensated simply. In this sense the two representations are almost equivalent.We show that the apparent spectrum due to a finite polarizable body is actually a true Cole‐Cole dispersion, as is usually assumed in practice.The behavior of a complex apparent resistivity spectrum and its corresponding apparent Cole‐Cole dispersion parameters is influenced by the variation of the dilution factor with frequency. Hence when estimating intrinsic parameters from the nomogram, based on constant (frequency‐independent) dilution factors, particular care is required to correctly relate apparent parameters to intrinsic parameters.
WAVE FIELD EXTRAPOLATION TECHNIQUES FOR INHOMOGENEOUS MEDIA WHICH INCLUDE CRITICAL ANGLE EVENTS. PART I: METHODS USING THE ONE‐WAY WAVE EQUATIONS*
Authors: WAPENAAR C. P. A. , BERKHOUT A. J. 
Published: April 2006

AbstractPart I of this series starts with a brief review of the fundamental principles underlying wave field extrapolation. Next, the total wave field is split into downgoing and upgoing waves, described by a set of coupled one‐way wave equations. In cases of limited propagation angles and weak inhomogeneities these one‐way wave equations can be decoupled, describing primary waves only. For large propagation angles (up to and including 90°) an alternative choice of sub‐division into downgoing and upgoing waves is presented. It is shown that this approach is well suited for modeling as well as migration and inversion schemes for seismic data which include critical angle events.
COMPUTER REPRESENTATION OF COMPLEX 3‐D GEOLOGICAL STRUCTURES USING A NEW “SOLID MODELING” TECHNIQUE*
Authors: GJØYSTDAL H. , REINHARDSEN J. E. , ÅSTEBØL K. 
Published: April 2006

AbstractDuring our recent work with 3‐D dynamic ray‐tracing and velocity inversion problems, a new 3‐D model generation system has been developed using a so‐called “solid modeling” technique. The term “solid modeling” refers to the fact that the logical system governing the internal geometrical properties of the model describes the model as a combination of “solids” or “volumes” in 3‐D space. In each of these volumes the physical parameters (such as seismic velocity, density) vary continuously. Discontinuous changes occur only across the model interfaces separating the volumes.The model is constructed by firstly forming a number of “simple volumes” from the given interfaces and then combining these simple volumes into more complex volumes which represent the physical volumes of the model. It is easy to make changes to the model, by adding volumes or subtracting volumes and perform more composite operations, all by simple use of Boolean expressions. Every time a model has been specified (or changed), the internal logic automatically carries out a check of physical consistency of the 3‐D model space (no overlapping volumes, no holes).By including various types of coordinate transformations, different kinds of complex structures can be handled, such as salt domes and vertical and near vertical faulting.
WAVE FIELD EXTRAPOLATION TECHNIQUES FOR INHOMOGENEOUS MEDIA WHICH INCLUDE CRITICAL ANGLE EVENTS.
Authors: WAPENAAR C. P. A. , BERKHOUT A. J. 
Published: April 2006

ABSTRACTIn one‐way wave field extrapolation downgoing and upgoing waves are treated independently, which is allowed if propagation at small angles against the vertical in (weakly) inhomogeneous media is considered. In practical implementation the slow convergence of the square‐root operator causes numerical deficiencies. On the other hand, in two‐way wave field extrapolation no assumptions need to be made on the separability of downgoing and upgoing waves. Furthermore, in practical implementation the use of the square‐root operator is avoided. To put the two‐way techniques into perspective, it is shown that two‐way wave field extrapolation could be described in terms of one‐way processes, namely: (1) decomposition of the total wave field into downgoing and upgoing waves; (2) one‐way wave field extrapolation; (3) composition of the total wave field from its downgoing and upgoing constituents. This alternative description of two‐way wave field extrapolation is valid for media which are homogeneous along the z‐coordinate as well as for small dip angles in arbitrarily inhomogeneous media. In addition, it is shown that this description is also valid for large dip angles in 1‐D (vertically) inhomogeneous media, including critical‐angle events, when the WKBJ one‐way wave functions discussed in part I of this paper are considered.For large dip angles in arbitrarily inhomogeneous media the two‐way wave equation is solved by means of Taylor series expansion. For practical implementation a truncated operator is designed, assuming gentle horizontal variations of the medium properties. This operator is stable and converges already in the first order approximation, also for critical‐angle events.
NATURAL POLARIZATION STUDIES AT BALCOVA GEOTHERMAL FIELD*
Authors: ERCAN A. , DRAHOR M. , ATASOY E. 
Published: April 2006

AbstractContrasting resistivity, temperature, pressure, porosity and fluid migrations are the main causes of ion accumulations along the discontinuities which create coupling‐induced natural current flow which is observed as the Natural Polarization Potential (NPP) on the surface. Natural Polarization Electric Field (NPE) variations were recorded along 13 profiles about 2.5 km each in a north‐south direction. Interpretation of the coincident anomalies of the NPP and the NPE field resulted in determination of the polarization angle, polarization focal depth and the azimuths of the polarized interfaces. Considerable agreement between faults delineated by previous geological and geophysical investigations and polarization discontinuities was observed. The polarization plane was observed to be horizontal in high temperature areas but steepened gradually in relatively colder regions in the northern coastal area. From the polarization depth distribution two depression zones were observed, separated by an uplifted section elongated in a north‐south direction. The basin at the eastern side had an estimated polarization depth of 1.1 km and was limited by the Agamemnon‐2 and ‐1 faults, while the western basin had an estimated depth of 1.3 km and extended in the east‐west direction not previously reported. The eastern basin extends on the north side around the town of Inciralti, the western basin appeared to be elongated towards the town of Cesme.
SLANT STACKING AND ITS SIGNIFICANCE FOR ANISOTROPY*
Authors: HAKE H. 
Published: April 2006

ABSTRACTSlant stacking transforms seismic data, recorded as a function of source‐receiver offset and traveltime, into the domain of intercept time τ and ray parameter p. The shape of the τ‐p‐curves thus obtained is closely related to the slowness surfaces of the layers. A layer‐stripping operation in the τ‐p‐domain removes all effects of the layers above the target layer. The resulting curve is equal to the slowness surface of the layer except for a scaling factor containing the thickness and dip of the layer. The slowness surface is a characteristic surface for anisotropic media. This makes the τ‐p‐domain very suitable for detecting and describing anisotropic layers. The relationship between the shape of τ‐p‐curves, the slowness surfaces, and the geometry of the layers is derived. Synthetic τ‐p‐curves calculated with the reflectivity method show some difficulties that can arise in determining the shape of the curves and in applying the stripping operation. It is shown that the effects of vertical inhomogeneity on the interpretation of τ‐p‐curves in terms of anisotropy are small.
THE NATURE OF THE NON‐GAUSSIANITY OF PRIMARY REFLECTION COEFFICIENTS AND ITS SIGNIFICANCE FOR DECONVOLUTION*
Authors: WALDEN A. T. , HOSKEN J. W. J. 
Published: April 2006

ABSTRACTOne of the important properties of a series of primary reflection coefficients is its amplitude distribution. This paper examines the amplitude distribution of primary reflection coefficients generated from a number of block‐averaged well logs with block thicknesses corresponding to 1 ms (two‐way time). The distribution is always essentially symmetric, but has a sharper central peak and larger tails than a Gaussian distribution. Thus any attempt to estimate phase using the bi‐spectrum (third‐order spectrum) is unlikely to be successful, since the third‐order moment is almost identically zero. Complicated tri‐spectrum (fourth‐order spectrum) calculations are thus required. Minimum Entropy Deconvolution (MED) schemes should be able to exploit this form of non‐Gaussianity. However, both these methods assume a white reflectivity sequence; they would therefore mix up the contributions to the trace's spectral shape that are due to the wavelet and those that are due to non‐white reflectivity unless corrections are introduced.A mixture of two Laplace distributions provides a good fit to the empirical amplitude distributions. Such a mixture distribution fits nicely with sedimentological observations, namely that clear distinctions can be made betweensedimentary bedsandlithological unitsthat comprise one or more such beds with the same basic lithology, and that lithological units can be expected to display larger reflection coefficients at their boundaries than sedimentary beds. The geological processes that engender major lithological changes are not the same as those for truncation of bedding. Analyses of sub‐sequences of the reflection series are seen to support this idea. The variation of the mixing proportion parameter allows for scale and shape changes in different segments of the series, and hence provides a more flexible description of the series than the generalized Gaussian distribution which is shown to also provide a good fit to the series.Both the mixture of two Laplace distributions and the generalized Gaussian distribution can be expressed as scale mixtures of the ordinary Gaussian distribution. This result provides a link with the ordinary Gaussian distribution which might have been expected to be the distribution of a natural series such as reflection coefficients. It is also important in the consideration of the solution of MED‐type methods. It is shown that real (coloured) primary reflection series do not seem to be obtainable as the deconvolution result from MED‐type deconvolution schemes.
A VERSATILE ELECTROMAGNETIC MODELING PROGRAM FOR 2‐D STRUCTURES*
Authors: NISSEN J. 
Published: April 2006

ABSTRACTA new integral‐equation program for calculation of the E‐polarization response allows modeling of VLF for plane wave or line source input, e.g., magnetotellurics and Turam responses. The anomalous conducting body is modeled by a number of square cells, each of individual size and resistivity, and with arbitrary position in the host medium. This provides a high degree of flexibility and allows for simulation of rather complex conductivity structures. The computation time has been drastically reduced by using techniques such as digital filtering and fast Fourier transformation.The interpretation of a measured Turam profile and the influence of galvanically channelled currents in the conducting body is discussed.