bluuebunny/crossref_metadata_2025_split huggingface dataset:
INTEGRATED INTERPRETATION, 3D MAP MIGRATION AND VSP MODELLING PROJECT, NORTHERN U.K. SOUTHERN GAS BASIN1
Authors: REILLY J. M. 
Published: April 2006

AbstractDepth conversion in the northern part of the U.K. Southern Gas Basin is complicated by the presence of Zechstein (Permian) salt swells and diapirs. In addition, the post‐Zechstein (post‐Permian) section displays large lateral velocity variations. The primary agents which control the velocity of this stratigraphic section are: (1) depth of burial, (2) lithological variation within individual formations, and (3) the effects of subsequent tectonic inversion. An integrated approach which combines well velocity, seismic velocity and seismic interpretation is required for accurate depth estimation.In 1988 Mobil and partners drilled an exploratory well in the northern part of the U.K. Southern Gas Basin. This well was located near the crest of a Zechstein salt diapir. Over 2000 m of Zechstein was encountered in the well. The Permian Rotliegendes objective was penetrated at a depth of over 3700 m.The initial delineation of the objective structure was based on the results of 3D map migration of the seismic time interpretation. Spatially‐variant interval velocity functions were used to depth convert through five of the six mapped horizons. Both well and model‐based seismic interval velocity analysis information was used to construct these functions.A moving‐source well seismic survey was conducted. The survey was run in two critical directions. In conjunction with presurvey modelling, it was possible to confirm immediately the structural configuration as mapped to a distance of 7 km from the well. Post‐survey 3D map migration and modelling was employed to further refine the structural interpretation. Although the question of stratigraphic anisotropy was considered in the evaluation of the long offset modelling, no evidence was found in the field data to support a significant effect.Finally, comparisons were made of: curved‐ray versus straight‐ray migration/modelling, midpoint‐depth velocity versus (depth‐dependant instantaneous velocity functions, and Hubral‐ versus Fermat‐based map depth migration algorithms. Significant differences in the results were observed for structural dips exceeding 15o and/or offsets exceeding 6 km. Map depth migration algorithms which employed both curved rays and spatially‐variant instantaneous velocity functions were found to best approximate the ‘true’ geological velocity field in the study area.
PRESTACK INVERSION OF GROUP‐FILTERED SEISMIC DATA1
Authors: HELGESEN JAN 
Published: April 2006

AbstractThree methods for least‐squares inversion of receiver array‐filtered seismic data are investigated: (1) point receiver inversion where array effects are neglected; (2) preprocessing of the data with an inverse array filter, followed by point receiver inversion; (3) array inversion, where the array effects are included in the forward modelling.The methods are tested on synthetic data generated using the acoustic wave equation and a horizontally stratified earth model. It is assumed that the group length and the group interval are identical. For arrays that are shorter than the minimum wavelength of the emitted wavefield, and when the data are appropriately muted, point receiver inversion (first method) gives satisfactory results. For longer arrays, array inversion (third method) should be used.The failure of the inverse array filter (second method) is due to aliasing problems in the data.
AUTOMATIC INTERPRETATION OF GRAVITY GRADIOMETRIC DATA IN TWO DIMENSIONS: VERTICAL GRADIENT1
Authors: KLINGELÉ E. E. , MARSON I. , KAHLE H.‐G. 
Published: April 2006

AbstractThe magnetic and gravity field produced by a given homogeneous source are related through Poisson's equation. Starting from this consideration, it is shown that some 2D interpretation tools, widely applied in the analysis of aeromagnetic data, can also be used for the interpretation of gravity gradiometric data (vertical gradient). This paper deals specifically with the Werner deconvolution, analytic signal and Euler's equation methods. After a short outline of the mathematical development, synthesized examples have been used to discuss the efficiency and limits of these interpretation methods. These tools could be applied directly to airborne gravity gradiometric data as well as ground gravity surveys after transformation of the Bouguer anomalies into vertical gradient anomalies. An example is given of the application of the Werner deconvolution and Euler's equation methods to a microgravity survey.
GRAVITY GRADIENT TENSORS DUE TO A POLYHEDRON WITH POLYGONAL FACETS1
Authors: KWOK YUE‐KUEN 
Published: April 2006

AbstractUsing the conjugate complex variables formulation, closed‐form formulae for the gravity gradient tensors of the gravitational potential due to a homogeneous polyhedral body composed of polygonal facets are derived. The treatise considers the cases of the observation point being inside the polyhedron, on the surface of a facet, or outside the polyhedron.
MEASURED REFLECTION STRENGTHS OF UNDERWATER PIPES IRRADIATED BY A PULSED HORIZONTAL DIPOLE IN AIR: COMPARISON WITH CONTINUOUS PLANE‐WAVE SCATTERING THEORY1
Authors: WENSINK W. A. , HOFMAN J. , VAN DEEN J. K. 
Published: April 2006

AbstractAt Delft Geotechnics the technique of ground‐penetrating radar is in use for the detection of buried objects such as pipes. In order to give our ‘measurements in the field’ a more quantitative interpretation, a series of experiments has been started under well‐defined conditions.A cylindrical vessel contains water, simulating wet soil. A pulsed transmitting antenna (TA) is mounted above the water‐surface irradiating horizontal underwater pipes. The reflected pulses are detected by a horizontal receiving dipole (AP, i.e. ‘air‐probe’) in the air. The reflecting objects used in the experiments are steel pipes, PVC rods and hollow PVC pipes filled with air or water. The depth of the pipes varies from 0.25 to 2 m. The strength of the reflected pulse depends on the type of pipe, its diameter, its depth, the electromagnetic properties of the water and also on the strength and polarization of the incident E‐field. The latter is (mainly) parallel to the axis of the pipe in the present experiments.The experimental results have been compared with calculated results using the theory of a continuous plane wave, incident on an infinitely long cylinder in a homogeneous dielectric medium with the same dielectric constant as water. In a previous paper an experiment was described in which a movable receiving dipole in the water measured the transverse, mutually orthogonal Eø‐ and Eθ‐components of the pulses emitted by the TA. The amplitudes of Eθ versus depth, measured in that paper, are used in the calculations as amplitudes of the incident field. The attenuation of the scattered field is accounted for by assuming exponential damping. The relative amplitudes of the scattered field for different pipes lying at the same depth are reproduced very well by this simple theory. Also the relative values of the scattered amplitudes for a given pipe lying at different depths are described neatly, provided the refraction of the scattered field at the water‐air interface is accounted for.
A SEARCH FOR DIRECT HYDROCARBON INDICATORS IN THE FORMBY AREA1
Authors: BUSBY J. P. , PEART R. J. , GREEN C. A. , OGILVY R. D. , WILLIAMSON J. P. 
Published: April 2006

AbstractIt has been proposed that the high‐frequency, low‐amplitude magnetic anomalies found over some hydrocarbon deposits are due to long‐term microseepage of hydrocarbons into iron‐rich sedimentary roof rocks, with subsequent precipitation of diagenetic magnetite or pyrrhotite at or near the water‐table. Aerogeophysical data sets, comprising both magnetic and gamma‐ray spectrometer measurements, over the Formby oil‐field, Lancashire, U.K., have been analysed for hydrocarbon‐related anomalies. Detailed ground magnetic traverses were also made to investigate some of the aeromagnetic anomalies.No hydrocarbon‐induced magnetic anomalies were detected. The majority of the high‐frequency events occurring in the aeromagnetic data correlated with cultural features, others were attributed to artefacts of the data processing. In particular there were no extensive areas of high‐frequency, low‐amplitude anomalies as might be expected from authigenic magnetic minerals.Borehole chippings from inside and outside the oil‐field were examined. High magnetic susceptibilities were due to contamination during drilling. Magnetite, which was identified by mineralogical analysis, was found to be of detrital origin. If any diagenetic magnetic material was present it was in such small quantities as to be insignificant.The oil‐field did not possess a characteristic gamma‐ray spectrometric signature. Anomalies were found to be related to hydrological conditions and to the distribution of surficial deposits. Attempts to suppress the influence of the drift geology had only limited success.It is concluded that the effectiveness of high‐resolution aeromagnetic onshore surveys for direct hydrocarbon detection has yet to be established. In particular, the anomaly found over the Formby oil‐field was caused by the cumulative effect of borehole casing. Similar cultural contamination by oil‐field equipment may explain some of the anomalies discovered over hydrocarbon deposits in North America. It is also unlikely that the spectrometric method can be applied diagnostically in any but the most simple and drift‐free geological environments.
CHEBYSHEV EXPANSIONS FOR THE SOLUTION OF THE FORWARD GRAVITY PROBLEM1
Authors: ZHAO S. K. , YEDLIN M. J. 
Published: April 2006

AbstractChebyshev expansions are used to solve the 3D forward gravity problem. Since the matrix factorization method is used to solve the coefficient equation system and the fast Fourier transform (FFT) technique is used to compute the forward and backward Chebyshev expansions, this method is very fast. Multipole expansions are used to calculate approximate boundary conditions (BCs) for realistic problems. When the length of any source‐body dimension is less than 70% of the minimum dimension of the computational domain, the relative error caused by the approximate BCs is about 1%. A cell‐average discretization method is suggested. The accuracy obtained by the cell‐average discretization is much better than that obtained by the traditional point‐injection discretization. The Chebyshev expansion technique was applied to four density models including a complex geological structure consisting of two normally faulted layers. Models which were finely sampled had a maximum relative error of about 1%.
ASPECTS OF CHARGE ACCUMULATION IN d. c. RESISTIVITY EXPERIMENTS1
Authors: LI YAOGUO , OLDENBURG DOUGLAS W. 
Published: April 2006

AbstractWhen an electric current is introduced to the earth, it sets up a distribution of charges both on and beneath the earth's surface. These charges give rise to the anomalous potential measured in the d. c. resistivity experiment. We investigate different aspects of charge accumulation and its fundamental role in d. c. experiments. The basic equations and boundary conditions for the d. c. problem are first presented with emphasis on the terms involving accumulated charges which occur wherever there is a non‐zero component of electric field parallel to the gradient of conductivity. In the case of a polarizable medium, the polarization charges are also present due to the applied electric field, yet they do not change the final field distribution. We investigate the precise role of the permittivity of the medium. The charge buildup alters the electric fields and causes the refraction of current lines; this results in the well‐known phenomenon of current channelling. We demonstrate this by using charge density to derive the refraction formula at a boundary. An integral equation for charge density is presented for whole‐space models where the upper half‐space is treated as an in‐homogeneity with zero conductivity. The integral equation provides a tool with which the charge accumulation can be examined quantitatively and employed in the practical forward modelling. With the aid of this equation, we investigate the effect of accumulated charges on the earth's surface and show the equivalence between the half‐space and whole‐space formulations of the problem. Two analytic examples are presented to illustrate the charge accumulation and its role in the d. c. problem. We investigate the relationship between the solution for the potential via the image method and via the charge density. We show that the essence of the image method solution to the potential problem is to derive a set of fictitious sources which produce the same potential as does the true charge distribution. It follows that the image method is viable only when the conductivity structure is such that the effect of the accumulated charge can be represented by a set of point images. As numerical examples, we evaluate quantitatively the charge density on the earth's surface that arises because of topography and the charge density on a buried conductive prism. By these means, we demonstrate the use of the boundary element technique and charge density in d. c. forward modelling problems.
AMPLITUDE, PHASE AND FREQUENCY VERSUS OFFSET APPLICATIONS1
Authors: MAZZOTTI A. 
Published: April 2006

AbstractIn a layered earth the seismic reflection response for incidence at non‐normal angles is dependent upon the elastic constants and thicknesses of the layers.The possibility is investigated of increasing the diagnostic value of seismic data by using the phase and the frequency v. offset information in addition to that from the amplitude v. offset.The combined amplitude, phase and frequency versus offset (APF.VO) analysis is carried out through the computation of amplitude, phase and frequency indicators based on the analytical description of seismic traces.Both synthetic and actual data are examined.From the analyses of synthetic seismograms, it is shown that modifying the velocities and thicknesses of a given target layer, by introducing different pore fluids or lithological conditions, produces changes in APF.VO plots. In particular, the effects related to interference among reflections and to critical angle phenomena are clearly detected by both the phase and amplitude v. offset indicators in terms of phase shifts and amplitude variations. The frequency indicator is mainly controlled by the spectrum of the propagating wavelet.Since the basic synthetic model is derived from an existing well, located close to a seismic line, some actual CDP gathers are analysed. Features related to interference and critical angle are again evident in the APF.VO plots of the actual data. The amplitude indicator appears to be reasonably stable while the phase shows a higher spatial variability and a stronger sensitivity to noise.Differential interference with offset often occurs in actual layered structures and distorts seismic data significantly. Therefore AVO interpretation and AVO inversion procedures must also tackle this problem. Knowledge of phase and frequency variations v. offset may help classical AVO interpretation and yield further information for use in inversion techniques.
SOME REMARKS ON NOISE STABILITY IN DYNAMIC INVERSION OF REFLECTION SEISMIC DATA1
Authors: MEYER H. G. 
Published: April 2006

AbstractThe dynamic inversion of reflection seismic data is investigated with reference to the influence of noise on pseudo‐impedance logs. Model traces are calculated with 0, 5, 15 and 50% noise, respectively. In solving the inversion problem, the algorithm of Marquardt and Levenberg is used in connection with singular value decomposition (SVD). The results are within a 1% error range so that there was no visible change in the logs. Further signal analysis show that there is no dependence on the phase content of the wavelet used if all other parameters of the model are known.
RESISTIVITY OF ROCK‐SALT IN ASSE (GERMANY) AND PETROPHYSICAL ASPECTS1
Authors: YARAMANCI U. , FLACH D. 
Published: April 2006

In the Asse salt‐mine (Germany), where extensive research is carried out on various aspects of nuclear waste disposal in rock‐salt formations, the resistivity of a future construction site for a test seal at a depth of 945 m has been investigated. Measurements have been conducted, using various types of the four‐point electrode configuration, on a network of 180 electrodes permanently installed in boreholes. A fully computer‐controlled measurement system has been built consisting of a resistivity meter, a switchbox for electrodes and a computer with special control software. The system has interactive and programmable automatic working modes with an extensive capability for documentation and processing including the immediate evaluation of apparent resistivities and is suitable for long‐term continuous observations.The average resistivity at the site which consists of almost homogeneous Stassfurt rock‐salt, is approximately 0.6 × 106Ωm with little variation. There is a small anomaly due to a conductive inclusion, probably a local anhydrite band. There is no indication of anisotropy so far and no significant change with time. The specific amount of water in the effective pores and its change can be estimated from the observed resistivities using the extended form of Archie's equation with pore saturation included. In this case data for the specific internal surface from samples are available and the order of permeability may be estimated using the formation factor derived from resistivities.
BOREHOLE RADAR APPLIED TO THE CHARACTERIZATION OF HYDRAULICALLY CONDUCTIVE FRACTURE ZONES IN CRYSTALLINE ROCK1
Authors: OLSSON OLLE , FALK LARS , FORSLUND OLOF , LUNDMARK LARS , SANDBERG ERIC 
Published: April 2006

AbstractThe borehole radar system, RAMAC, developed within the framework of the International Stripa Project, can be used in three different measuring modes; single‐hole reflection, cross‐hole reflection and cross‐hole tomography. The reflection modes basically provide geometrical data on features located at some distance from the borehole. In addition the strength of the reflections indicate the contrast in electrical properties. Single‐hole reflection data are cylindrically symmetrical with respect to the borehole, which means that a unique fracture orientation cannot be obtained. A method has been devised where absolute orientation of fracture zones is obtained by combining single‐hole reflection data from adjacent holes. Similar methods for the analysis of cross‐hole reflection data have also been developed and found to be efficient. The radar operates in the frequency range 20‐‐60 MHz which gives a resolution of 1–3 m in crystalline rock. The investigation range obtained in the Stripa granite is approximately 100 m in the single‐hole mode and 200‐‐300 m in the cross‐hole mode.Variations in the arrival time and amplitude of the direct wave between transmitter and receiver have been used for cross‐hole tomographic imaging to yield maps of radar velocity and attenuation. The cross‐hole measurement configuration coupled with tomographic inversion has less resolution than the reflection methods but provides better quantitative estimates of the values of measured properties.The analysis of the radar data has provided a consistent description of the fracture zones at the Stripa Cross‐hole site in agreement with both geological and geophysical observations. Comparison of the radar results with seismic cross‐hole data showed excellent agreement with respect to shape and location of the fracture zones in space. Comparison with hydraulic data shows that the features identified by radar are of hydrogeological significance.
INVERSION OF GRAVITY AND MAGNETIC DATA FOR THE LOWER SURFACE OF A 2.5 DIMENSIONAL SEDIMENTARY BASIN1
Authors: MICKUS KEVIN L. , PEEPLES WAYNE J. 
Published: April 2006

AbstractGravity and magnetic data have been inverted to obtain the continuous lower surface of a 2.5 dimensional sedimentary basin. The non‐linear problem is linearized and a solution is calculated through a recursive process until the predicted data matches the observed data. An average model is then calculated and a resolution analysis shows which features are uniquely determined. The results of individual inversion indicate that a final solution is initial model dependent but the average models are independent of the initial model except at the margins. The average model for the magnetic solutions have uniformly smaller spreads than the gravity solutions.The algorithms were applied to data from the Sanford Basin in North Carolina. The results indicate that the basin is asymmetrical in shape with a maximum depth of 3.2 km. Comparing these results with those obtained from a generalized linear inverse (GLI) algorithm indicate that the higher‐frequency features determined from the GLI algorithm are not resolved.
SHEAR‐WAVE VELOCITY ESTIMATION IN POROUS ROCKS: THEORETICAL FORMULATION, PRELIMINARY VERIFICATION AND APPLICATIONS1
Authors: GREENBERG M. L. , CASTAGNA J. P. 
Published: April 2006

AbstractShear‐wave velocity logs are useful for various seismic interpretation applications, including bright spot analyses, amplitude‐versus‐offset analyses and multicomponent seismic interpretations. Measured shear‐wave velocity logs are, however, often unavailable.We developed a general method to predict shear‐wave velocity in porous rocks. If reliable compressional‐wave velocity, lithology, porosity and water saturation data are available, the precision and accuracy of shear‐wave velocity prediction are 9% and 3%, respectively. The success of our method depends on: (1) robust relationships between compressional‐ and shear‐wave velocities for water‐saturated, pure, porous lithologies; (2) nearly linear mixing laws for solid rock constituents; (3) first‐order applicability of the Biot–Gassmann theory to real rocks.We verified these concepts with laboratory measurements and full waveform sonic logs. Shear‐wave velocities estimated by our method can improve formation evaluation. Our method has been successfully tested with data from several locations.
PLANE‐WAVE CONSTRAINTS IN 2D FILTER DESIGN1
Authors: WALTHAM D. 
Published: April 2006

AbstractA method is presented for developing and/or evaluating 2D filters applied to seismic data. The approach used is to express linear 2D filtering operations in the space‐frequency (x–ω) domain. Correction filters are then determined using plane‐wave constraints. For example, requiring a vertically propagating plane wave to be unaffected by migration necessitates application of a half‐derivative correction in Kirchhoff migration. The same approach allows determination of the region of time‐offset space where half‐derivative corrections are correct in x–t domain dip moveout. Finally, an x–ω domain dip filter is derived using the constraint that a plane wave be attenuated as its dip increases. This filter has the advantage that it is significantly faster than f–k domain dip filtering and can be used on irregularly spaced data. This latter property also allows the filter to be used for interpolation of irregular data onto a regular grid.