bluuebunny/crossref_metadata_2025_split huggingface dataset:
RESOLUTION OF RESISTIVITY TOMOGRAPHY INFERRED FROM NUMERICAL SIMULATION1
Authors: SASAKI YUTAKA 
Published: April 2006

AbstractSome factors affecting the resolution and accuracy of resistivity tomography are examined using numerical simulation. The inversion method used is based on smoothness‐constrained least‐squares and finite‐element methods. An appropriate block discretization is obtained by dividing the target region into square blocks of size equal to half the minimum electrode spacing. While the effect of the damping factor on the resolution is significant, the resolution is not very sensitive to Gaussian noise as long as the damping factor is properly chosen, according to the noise level. The issue of choosing an optimum electrode array should be considered at the planning stage of a survey.When the instrumental accuracy is high, the dipole‐dipole array is more suitable for resolving complex structures than the pole‐pole array. The pole‐dipole array gives somewhat less resolution than the dipole‐dipole array but yields greater signal strength; thus, the pole‐dipole array may be a good compromise between resolution and signal strength. The effect of an inhomogeneity located outside the target region may be very small if block discretization is done so as to represent the resistivity variations in both the target and outside regions.
APPLICATION OF GEOELECTRIC METHODS USING BURIED ELECTRODES IN EXPLORATION AND MINING1
Authors: DRASKOVITS P. , SIMON A. 
Published: April 2006

AbstractVarious geoelectric methods which have been developed and applied in the last 10–20 years in ELGI are discussed. These methods which use buried electrodes are: hole‐to‐surface gradient mapping to detect bauxite deposits in sinkholes below a resistive screening layer; in‐mine gradient profiling to map the basement topography below galleries; and the hole‐to‐surface version of geoelectric layer tracing to find outcrops of mineralized zones penetrated by drillings.Data processing procedures have been developed on the basis of common concepts and hypotheses to link theoretical models with geological structures. The objects investigated are determined as the difference between the theoretical models and geological structures. The predominant part of the real electric field measured above the geological structures is the theoretical field related to the theoretical model. The effects of the objects (the anomaly) are superposed on the theoretical field but their extent is small compared with the values of the latter. The theoretical field and the anomaly depend strongly on the separation from the sources. For this reason the anomalies are difficult to recognize. Therefore the ratio of the theoretical field to the measured one is computed, since σa, the apparent specific conductivity, is proportional to this ratio.It is demonstrated that since the changes in the σa curve depending on the location of the observation point are small, the anomalies can easily be recognized on the curve. The σa, curve computed in the above way reflects the objects better than the originally measured electric fields.Examples illustrate the solution of the above‐mentioned geological problems by the practical application of adequate geoelectric methods using buried electrodes.
AUTOMATED FIRST ARRIVAL PICKING: A NEURAL NETWORK APPROACH1
Authors: MURAT MICHAEL E. , RUDMAN ALBERT J. 
Published: April 2006

AbstractA back‐propagation neural network is successfully applied to pick first arrivals (first breaks) in a background of noise. Network output is a decision whether each half‐cycle on the trace is a first or not. 3D plots of the input attributes allow evaluation of the attributes for use in a neural network. Clustering and separation of first break from non‐break data on the plots indicate that a neural network solution is possible, and therefore the attributes are suitable as network input.Application of the trained network to actual seismic data (Vibroseis and Poulter sources) demonstrates successful automated first‐break selection for the following four attributes used as neural network input: (1) peak amplitude of a half‐cycle; (2) amplitude difference between the peak value of the half‐cycle and the previous (or following) half‐cycle; (3) rms amplitude ratio for a data window (0.3 s) before and after the half‐cycle; (4) rms amplitude ratio for a data window (0.06 s) on adjacent traces. The contribution of the attributes based on adjacent traces (4) was considered significant and future work will emphasize this aspect.
SEISMIC MODELLING OF SEAM WAVES EXCITED BY ENERGY TRANSMISSION INTO A SEAM1
Authors: GRITTO R. , DRESEN L. 
Published: April 2006

AbstractThe traditional method of exciting channel waves in coal deposits underground consists of firing explosive sources in a mid‐seam position generating seam waves of the Rayleigh and Love type. We investigate various source positions and excitation mechanisms within the bedrock structure surrounding the seam to evaluate the effects of source positions adjacent to the seam. The investigation is based on analogue and numerical modelling of half‐ and full‐space cases, for which the excitation and the nature of Rayleigh channel waves are examined.In the analogue modelling, sources, located from mid‐seam out into the bedrock, along the edge of a 2D plate model, excited channel waves through a conversion of the free surface Rayleigh wave at the edge of the plate. The excited channel wave belongs to the normal mode range. Frequency‐wavenumber analysis shows that the symmetric 2nd mode of the channel wave is excited with frequencies comparable to the forcing frequency of the source signal. The polarization changes from retrograde to prograde, as the wave develops from the front to the rear of the seam, respectively. The amplitude‐depth distribution resembles that of an ordinarily excited seam wave, for the symmetric component. However, the antisymmetric component does not show the characteristic change of sign in amplitudes across the mid‐seam axis.Numerical modelling with sources located in the bedrock (full‐space case) shows that relocating the source away from the seam lowers the frequency content of the excited channel wave. Based on these investigations, the influence of a lower‐frequency source signal on the excitation of the channel wave is examined in an analogue experiment. Sources are sited in the bedrock adjacent to the seam at three locations. A lower‐frequency wavelet is calculated for each source location from the results obtained in the numerical analysis. For comparison, a higher‐frequency wavelet is also used which is known to be optimal for this model geometry when excited by a mid‐seam source location. It is found that in two cases the use of the lower‐frequency wavelet improves the channel wave excitation, while no amplification is achieved in one case.
DEPTH OF DETECTION OF BURIED CONDUCTIVE TARGETS WITH DIFFERENT ELECTRODE ARRAYS IN RESISTIVITY PROSPECTING1
Authors: APPARAO A. , RAO T. GANGADHARA , SASTRY R. SIVARAMA , SARMA V. SUBRAHMANYA 
Published: April 2006

AbstractDepth of detection of a target can be defined as that depth below which the target cannot be detected with a given electrode array assuming that the minimum detectable anomaly is 10%. Following this definition, physical modelling was carried out to determine depths of detection of conductive targets of limited lateral extent such as a vertical sheet, a horizontal cylinder and a sphere (infinitely conducting).It is seen that the two‐electrode array has the greatest depth of detection followed by the three‐electrode array, while a Wenner array has the smallest depth of detection, when the array spread is in‐line (i.e. perpendicular to the strike direction). On the other hand, the depth of detection with a Wenner array improves considerably and is almost equal to that of the two‐electrode array when the array spread is broadside (i.e. along the strike direction).With an increase in the depth extent of the vertical sheet from 10 to 20 times its thickness, there is an increase in the depth of detection with all arrays except for the three‐electrode array when the array spread is in‐line, and with the Wenner array when the array spread is broadside.
MULTIPARAMETER GEOPHYSICAL LOGGING AT THE YAVA LEAD DEPOSIT: A STATISTICAL APPROACH1
Authors: CINQ‐MARS ANNE , MWENIFUMBO C. J. , KILLEEN P. G. , CHOUTEAU M. 
Published: April 2006

AbstractThe present study describes multiparameter geophysical logging carried out at the Yava sandstone lead deposit, Nova Scotia, Canada. Statistical analysis of the multiparameter data set shows that the spectral gamma‐gamma ratio log (SGG ratio) is the most useful technique for characterizing the disseminated sulphide mineralization.Principal component analysis (PCA) indicates that the apparent chargeability (IP parameter) responds to the presence of clay minerals in the sandstone in addition to disseminated sulphides, so that the induced polarization method (IP) does not accurately delineate the disseminated galena content as was originally assumed in the preliminary log interpretation. PCA has also confirmed that the SGG ratio and density are related to lead content and that lithological variations can be delineated with natural radioactivity and resistivity.The zinc content of the deposit was poorly characterized by geophysical logs. Sphalerite occurrences seem to be localized as narrow bands (< 1 cm) which were not geophysically detectable.
APERTURE COMPENSATION TOMOGRAPHY1
Authors: CARRION P. , VESNAVER A. , BOEHM G. , PETTENATI F. 
Published: April 2006

AbstractConventional velocity analysis can handle a horizontally stratified medium well. There is no indication, though, that it will be as successful when applied to a more complicated geological structure. In fact, a small angle of incidence may transform to a wide‐angle reflection event for a dipping interface. In this case, conventional velocity analysis may lead to large errors and thus cannot be applied. Seismic tomography is attractive as it is virtually free from any restrictions imposed on the velocity distribution in the model space or on the setup of a seismic experiment. It is important, however, to recall that seismic tomography yields results of inferior quality compared to medical tomography. This paper investigates the reason for this and how to suppress a significant blurring of seismic tomograms. Unlike medical tomography, one cannot provide full angular coverage of the model space in a typical seismic experiment: the sources and the receivers cannot surround an unknown object inside the earth to provide a complete spectrum of view angles. Incomplete angular coverage may lead to the occurrence of large inaccuracies in the computed tomograms especially when the initial model is poorly chosen. We demonstrate a method of suppressing the adverse effects related to an incomplete angular recording. This is ‘compensation tomography’ which can be used efficiently in the case of a limited angular aperture. Numerical experiments illustrate the theory.
POROSITY AND PORE STRUCTURE FROM ACOUSTIC WELL LOGGING DATA1
Authors: TAO G. , KING M.S. 
Published: April 2006

AbstractWyllie's time‐average equation and subsequent refinements have been used for over 20 years to estimate the porosity of reservoir rocks from compressional (P)‐wave velocity (or its reciprocal, transit time) recorded on a sonic log. This model, while simple, needs to be more convincingly explained in theory and improved in practice, particularly by making use of shear (S)‐wave velocity. One of the most important, although often ignored, factors affecting elastic velocities in a rock is pore structure, which is also a controlling factor for transport properties of a rock. Now that S‐wave information can be obtained from the sonic log, it may be used with P‐waves to provide a better understanding of pore structure. A new acoustic velocities‐to‐porosity transform based on an elastic velocity model developed by Kuster and Toksöz is proposed. Employing an approximation to an equivalent pore aspect ratio spectrum, pore structure for reservoir rocks is taken into account, in addition to total pore volume. Equidimensional pores are approximated by spheres and rounded spheroids, while grain boundary pores and flat pores are approximated by low aspect ratio cracks. An equivalent pore aspect ratio spectrum is characterized by a power function which is determined by compressional‐and shear‐wave velocities, as well as by matrix and inclusion properties. As a result of this more sophisticated elastic model of porous rocks and a stricter theory of elastic wave propagation, the new method leads to a more satisfactory interpretation and fuller use of seismic and sonic log data. Calculations using the new transform on data for sedimentary rocks, obtained from published literature and laboratory measurements, are presented and compared at atmospheric pressure with those estimated from the time‐average equation. Results demonstrate that, to compensate for additional complexity, the new method provides more detailed information on pore volume and pore structure of reservoir rocks. Examples are presented using a realistic self‐consistent averaging scheme to consider interactions between pores, and the possibility of extending the method to complex lithologies and shaly rocks is discussed.
RESIDUAL STATICS ESTIMATION: SCALING TEMPERATURE SCHEDULES USING SIMULATED ANNEALING1
Authors: NØRMARK E. , MOSEGAARD K. 
Published: April 2006

AbstractLinearized residual statics estimation will often fail when large static corrections are needed. Cycle skipping may easily occur and the consequence may be that the solution is trapped in a local maximum of the stack‐power function. In order to find the global solution, Monte Carlo optimization in terms of simulated annealing has been applied in the stack‐power maximization technique. However, a major problem when using simulated annealing is to determine a critical parameter known as the temperature.An efficient solution to this difficulty was provided by Nulton and Salamon (1988) and Andresen et al. (1988), who used statistical information about the problem, acquired during the optimization itself, to compute near optimal annealing schedules.Although theoretically solved, the problem of finding the Nulton–Salamon temperature schedule often referred to as the schedule at constant thermodynamic speed, may itself be computationally heavy. Many extra iterations are needed to establish the schedule.For an important geophysical inverse problem, the residual statics problem of reflection seismology, we suggest a strategy to avoid the many extra iterations. Based on an analysis of a few residual statics problems we compute approximations to Nulton–Salamon schedules for almost arbitrary residual statics problems. The performance of the approximated schedules is evaluated on synthetic and real data.
ON THE COMPLETENESS OF DATA SETS WITH MULTIELECTRODE SYSTEMS FOR ELECTRICAL RESISTIVITY SURVEY
Authors: XU BIWEN , NOEL MARK 
Published: April 2006

AbstractThis paper describes how, using a surface linear array of equally spaced electrodes, potential data can be obtained for use in electrical resistivity imaging. The aim is to collect a complete data set which contains all linearly independent measurements of apparent resistivity on such an array using two‐, three‐ or four‐electrode configurations. From this primary data set, it is shown that any other value for apparent resistivity on the array can be synthesized through a process of superposition. Numerical tests show that such transformations are exact within the machine error for calculated data but that their use with real field data may lead to noise amplification.
A gravimetric 3D Global inversion for cavity detection1
Authors: Camacho A.G. , Vieira R. , Montesinos F.G. , Cuéllar V. 
Published: April 2006

AbstractA gravimetric survey, covering a site 200 m square, was carried out in order to locate karstic cavities. After eliminating the regional trend using a polynomial fit, the residual is modelled by least‐squares prediction. Correlated signals for several wavelengths are detected. The inversion of these anomalies is performed by a global 3D adjustment using spherical bodies as models. The adjustment is repeated in order to obtain a stable configuration. The results show the probable presence of a system of cavities and galleries. Data collected from boreholes and the subsequent appearance of sink‐holes are consistent with the results.
Definitions of apparent resistivity for the presentation of magnetotelluric sounding data1
Authors: Başokur A.T. 
Published: April 2006

AbstractA new definition of apparent resistivity for the presentation of magnetotelluric sounding data is proposed. The new definition is based on the frequency‐normalized impedance function. Both the existing and proposed definitions of apparent resistivity are analysed theoretically and are compared using model curves computed for a 1D earth model. Apparent resistivity curves computed using the proposed definition are a better approximation to the true resistivity values of the subsurface layers. In addition, the layers are more noticeable on the apparent resistivity curves, which is an advantage, especially for the ascending and descending type of apparent resistivity curve.
A method to estimate the total magnetization direction from a distortion analysis of magnetic anomalies1
Authors: Fedi Maurizio , Florio Giovanni , Rapolla Antonio 
Published: April 2006

AbstractKnowledge of the declination and inclination of the total and induced magnetization vectors is normally required for the interpretation and analysis of magnetic anomalies. A new method of estimating the direction of the total magnetization vector of magnetized rocks from magnetic anomalies is proposed. The unknown declination and inclination (D*T and I*T) can be found by applying a reduction‐to‐the‐pole operator to the measured anomalies for different couples of total magnetization direction parameters (DT and IT) and by observing the variation of the anomaly minimum as a function of both DT and I*T.and D*T are estimated using the maximum of this function. Comparing our method to previous methods, one advantage is that our estimates are not zero‐level dependent; furthermore, the method allows inclinations to be well estimated, with the same accuracy as declinations; finally declinations are not underestimated. Our method is applied to a real case and meaningful results are obtained; it is shown that the feasibility of the method is improved by removing the low‐frequency components.
An airborne tensor VLF system. From concept to realization1
Authors: Pedersen Laust B. , Qian Wei , Dynesius Lars , Zhang Ping 
Published: April 2006

AbstractRadio signals from very low frequency (VLF) transmitters distributed world‐wide have been used for several decades to study the lateral variations of the electrical conductivity in the upper few hundred metres of the earth's crust. Traditionally, in airborne applications, the total magnetic fields from one or two transmitters are measured to form the basis for construction of maps that primarily show those conductive structures that are parallel or subparallel to the direction to the transmitters. The tensor VLF technique described in this paper makes use of all signals available in a predefined frequency band to construct transfer functions relating the vertical magnetic field and the two horizontal magnetic field components. These transfer functions are uniquely determined for a particular measuring site and contain information about the lateral conductivity variations in all directions. First experiences with real field data, acquired during a test survey in Sweden, show that maps of the so‐called peaker, the spatial divergence of the transfer functions, give an image of the conducting structures. Most of the structures can be correlated to small valleys filled with conducting sediments or valleys underlain by conductive fracture zones in the crystalline rocks.
Transversely isotropic media equivalent to thin isotropic layers1
Authors: Schoenberg Michael 
Published: April 2006

AbstractAny set of isotropic layers is equivalent, in the long wavelength limit, to a unique transversely isotropic (TI) layer; to find the elastic moduli of that layer is a solved problem. The converse problem is to find a set of isotropic layers equivalent to a given TI media. Here, explicit necessary and sufficient conditions on the TI stiffness moduli for the existence of an equivalent set of isotropic layers are found by construction of a minimal decomposition consisting of either two or three isotropic constituent layers. When only two constituents are required, their elastic properties are uniquely determined. When three constituents are required, two have the same Poisson's ratio and the same thickness fraction, and even then there is a one‐parameter family of satisfactory minimal decompositions. The linear slip model for fractured rock (aligned fractures in an isotropic background) yields a restricted range of transverse isotropy dependent on only four independent parameters. If the ratio of the normal to tangential fracture compliance is small enough, the medium is equivalent to thin isotropic layering and in general its minimal decomposition consists of three constituents.
Velocity model updating using image gathers1
Authors: Eckhardt Wietze 
Published: April 2006

AbstractFor successful prestack depth migration an accurate velocity model is needed. One method for model updating is based on image gather analysis. In an image gather all reflectors line up horizontally if the correct velocities are used for the depth migration. This is also true for dipping reflectors, as all traces of an image gather belong to the same surface coordinate. The images of the reflector in an image gather curve upwards if the velocity used for the migration is too low, or downwards if the velocity is too high. This deviation can be used for model updating. Curves which depend on depth, offset and a parameter which relates the estimated to the true model are fitted to the image. By calculating the coherence along the deviation curves, this parameter can be estimated and hence an update can be calculated.Formulae are derived for the deviation curves and the update of the velocity depth model for a multilayered model for both shot and common‐offset migrated data, with and without gradients. The method is tested on synthetic data with satisfactory results.
Numerical tests of 3D true‐amplitude zero‐offset migration1
Authors: Filpo E. , Hubral P. 
Published: April 2006

AbstracrAn amplitude‐preserving migration aims at imaging compressional primary (zero‐or) non‐zero‐offset reflections into 3D time or depth‐migrated reflections so that the migrated wavefield amplitudes are a measure of angle‐dependent reflection coeffcients. The principal objective is the removal of the geometrical‐spreading factor of the primary reflections. Various migration/inversion algorithms involving weighted diffraction stacks proposed recently are based on Born or Kirchhoff approximations. Here, a 3D Kirchhoff‐type zero‐offset migration approach, also known as a diffraction‐stack migration, is implemented in the form of a time migration. The primary reflections of the wavefield to be imaged are describeda prioriby the zero‐order ray approximation. The aim of removing the geometrical‐ spreading loss can, in the zero‐offset case, be achieved by not applying weights to the data before stacking them. This case alone has been implemented in this work. Application of the method to 3D synthetic zero‐offset data proves that an amplitude‐preserving migration can be performed in this way. Various numerical aspects of the true‐amplitude zero‐offset migration are discussed.