bluuebunny/crossref_metadata_2025_split huggingface dataset:
IMPROVED PHASE‐ELLIPSE METHOD FOR IN SITU GEOPHONE CALIBRATION*
Authors: LIU H.‐P. , PESELNICK L. 
Published: April 2006

ABSTRACTFor amplitude and phase response calibration of moving‐coil electromagnetic geophones two parameters are needed, namely, the geophone natural frequency, f0, and the geophone upper resonance frequency fu. The phase‐ellipse method is commonly used for the in situ determination of these parameters. For a given signal‐to‐noise ratio, the precision of the measurement off0 andfu depends on the phase sensitivity, f(δφ/δf) For some commercial geophones f(δφ/δf) atfu can be an order of magnitude less than the sensitivity atf0. In this paper we present an improved phase‐ellipse method with increased precision. Compared to measurements made with the existing phase‐ellipse methods, our method shows a 6‐ and 3‐fold improvement in the precision, respectively, on measurements of f0 andfu on a commercial geophone.
MIGRATION OF VSP DATA BY RAY EQUATION EXTRAPOLATION IN 2‐D VARIABLE VELOCITY MEDIA*
Authors: HU LIANG‐ZIE , McMECHAN G.A. 
Published: April 2006

ABSTRACTThe standard Kirchhoff algorithm can be generalized for migration of pre‐stack finite‐offset data from variable‐velocity media. The concentric ellipses over which the data are spread in constant velocity media become significantly distorted (even multi‐valued) in the variable velocity case. The specific shapes can be explicitly defined by kinematic extrapolation of the source and recorded wave fields with the ray equation.The use of Kirchhoff migration with a surface source and a subsurface recorder requires that two sets of Kirchhoff loci be superimposed. For each trace, the first set of loci is computed with the source and the actual recorder position as foci; the second set is computed with the source and the virtual recorder position as foci. This dual procedure explicitly incorporates the primary diffracted energy and the free‐surface reflections, respectively.Implementation involves the construction of a virtual medium, lying above the free surface, with a velocity distribution that is the mirror image of the actual distribution below the free surface. Ray‐equation extrapolation is performed through the real/virtual boundary. The resulting image is produced in a split form, with all the contributions of the primary reflected and diffracted energy lying in the lower ‘real’ half and all the contributions of the energy that was reflected at the free surface lying in the upper ‘virtual’ half. The final image is produced by folding the split image about the free surface and adding the two halves. A practical advantage is that the origin of various contributions (and artifacts) can be more readily identified (for interpretation or removal) in the split images.The ray‐equation pre‐stack migration algorithm is very general. It is applicable to all source‐recorder geometries and variable velocity media and reduces exactly to the standard Kirchhoff algorithm when applied to zero or finite‐offset surface survey data. The algorithm is illustrated by application to VSP data. For the VSP geometry, the algorithm does not require any specific trace spacing (in depth) and can be used for data from deviated as well as vertical holes.
ON THE UPWARD CONTINUATION OF POTENTIAL FIELD DATA BETWEEN IRREGULAR SURFACES*
Authors: IVAN M. 
Published: April 2006

AbstractThe potential field and its derivatives at points above an irregular surface can be approximately obtained from the sampled potential field data acquired on that surface.A method of minimizing the truncation effect, which appears when gravity and magnetic maps are processed with the aid of surface integrals, is derived.The results are compared with those of the most relevant similar methods by using a theoretical, but realistic, model.
COMPLEX RESISTIVITY OF SYNTHETIC SULPHIDE BEARING ROCKS*
Authors: MAHAN M.K. , REDMAN J.D. , STRANGWAY D.W. 
Published: April 2006

ABSTRACTThe complex resistivity of synthetic samples has been measured over the frequency range 0.01 Hz to 1.0 MHz. The synthetic samples were made of a quartz matrix and sulphide particles. Grain sizes, volume fractions, and sulphide particles of different nature were used. The samples were saturated to different degrees, with different types of electrolyte. The results obtained were used to check the validity of the theoretical electrochemical model developed by Wong (1979). This model fits the experimental data reasonably well in the frequency range where the dispersion and the phase maxima occur. Against the model expectations, the experiment did not duplicate the Debye‐form of spectra because most of the common metals are oxidized to the ionic form and an electrochemical charge transfer reaction occurs at the interface. The model prediction regarding the effect of the distribution of the grain radii on the complex resistivity could not be duplicated experimentally.
AN ALGORITHM FOR FAST TIME‐DOMAIN COMPUTATION OF ONE‐DIMENSIONAL SYNTHETIC VERTICAL SEISMIC PROFILES*
Authors: THYBO H. 
Published: April 2006

AbstractThe normal incidence seismic wavefields in a horizontally layered model of the earth have a well‐known matrix description if the earth model has been transformed into a layer stack built up by thin layers, all of equal traveltime. To compute a Synthetic Vertical Seismic Profile (SVSP) the wavefields are iterated through the whole layer stack by means of the matrix description, either expressed in the time domain or in the frequency domain. In conventional methods a single layer is added to a resulting stack of layers at each recursion step. However in the present algorithm, a whole layer stack is added to another layer stack at each recursion step. Improvement in computing time obtained by doing so is approximately linearly dependent on the typical number of layers between receivers. This makes the method very suitable for detailed modeling wherever high resolution of the model is desired.
A PORTABLE LOCAL LOOP VLF TRANSMITTER FOR GEOLOGICAL FRACTURE MAPPING*
Authors: HAYLES J.G. , SINHA A.K. 
Published: April 2006

ABSTRACTA portable low‐power Very Low Frequency (VLF) transmitter using a large square loop antenna has been designed, assembled and tested by the Geological Survey of Canada (GSC) for geological studies of fracture patterns in igneous rock masses. Standard laboratory equipment, consisting of a signal generator, a 1100‐W power amplifier and several high‐power tuning capacitors, was used for the purpose.Field tests at the Chalk River facilities of Atomic Energy of Canada Limited have demonstrated a remarkable similarity between survey results obtained using the VLF signals from the local loop transmitter and from distant US Navy VLF transmitters. The local loop was used to simulate the fields from navy stations NAA in Cutler, Maine and NSS in Annapolis, Maryland. Conductor axes, mapped by using these navy stations, and by using the loop antenna yielded almost identical results. A survey was also done in the same area with the local loop placed in such a manner that the direction of the VLF field was at 45° to the field directions from NAA and NSS. In this case, the same conductor axes were located with only minor shifts in position, indicating that conductors whose axes lie within 45° of the direction of the primary horizontal magnetic field are mapped. Thus, it is probably sufficient to have two sources with orthogonal VLF fields to map all VLF conductors in an area. Since in most areas at least one navy VLF station can be used, the local loop transmitter can be used to generate a signal at right angles to the direction from the navy transmitter to allow a more complete VLF survey coverage.
AN OPTIMIZED DIGITAL FILTER FOR THE FOURIER TRANSFORM*
Authors: NISSEN J. , ENMARK T. 
Published: April 2006

AbstractShort filters for calculating Hankel‐transformations, with special attention to the d.c.‐sounding problem, have been published in recent years. These filters, with a typical length of less than 25 coefficients, have made it possible to implement, e.g., VES‐interpretation programs on microcomputers and 3‐D electric and electromagnetic modeling programs on minicomputers. Initially the performance of the short filters was rather poor, but with the introduction of short optimized filters there has been a considerable improvement in the accuracy.An optimization procedure is applied to design a 20‐point filter for the Fourier sine‐transformation. This filter may be useful in electromagnetic prospecting theory, e.g., in the calculation of the electric and magnetic field from a line source.
ON THE TIME‐VARYING WIENER FILTER*
Authors: DIMRI V.P. 
Published: April 2006

ABSTRACTFor a new approach to designing the time‐varying Wiener filter, the input is first divided into sections and then the time‐varying filter is determined from the entire input and the desired output. The technique differs from the existing one in which the time‐invariant filter is determined from each section. Hence, the main difference, between the proposed and the existing technique lies in the arrangement of input data. The proposed technique requires fewer computational operations and performs better than the time‐invariant Wiener filter, as illustrated by numerical examples.
SOME IMPROVEMENTS IN THE INTERPRETATION OF VERTICAL ELECTRIC SOUNDING CURVES*
Authors: CHYBA J. 
Published: April 2006

ABSTRACTThe process of VES interpretation is discussed, including the following points.(a) Preliminary interpretation by means of master curves. It is shown that the positions of the auxiliary points K and Q depend on the resistivity of the substratum. The interpretation is improved if the auxiliary points are determined separately for each master curve.(b) The individual parts of the measured curves are shifted in overlapping MN electrode positions so that the total sum of squares of the shifts is minimal.(c) The ambiguity may be reduced by means of supplementary information or assumptions on the resistivities. Fixing the resistivities is not always possible because discrepancies may arise between the ground measurement and the well‐logging data. The simultaneous interpretation of several VES curves is recommended assuming constant resistivities. This assumption may be subsequently verified by means of the F‐test.(d) A nonlinear algorithm is proposed for the determination of confidence intervals. As the multi‐dimensional confidence intervals are often very complicated, it is recommended to construct only one‐dimensional confidence intervals for the estimable parametric functions.(e) A ‘double‐least‐squares’ optimization technique is presented. The optimization is performed on the estimable parametric functions, and the individual parameters are determined so that the solution remains near the initial guess. This technique is faster than the singular value decomposition.
TUTORIAL THE PROGRESSIVE ATTENUATION OF HIGH‐FREQUENCY ENERGY IN SEISMIC REFLECTION DATA*
Authors: ZIOLKOWSKI A. , FOKKEMA J. T. 
Published: April 2006

ABSTRACTSeismic reflection data always exhibit a progressive loss of high‐frequency energy with time. This effect is partly attributable to irreversible processes such as the conversion of elastic energy into heat (commonly known as absorption), and partly to reversible processes associated with interference between reflected waves arriving at different times. This paper looks only at reversible linear elastic effects at normal incidence and asks the following question: if there were no such absorption, would there still be a progressive loss of high‐frequency energy?Using normal incidence and a layered elastic earth model we prove the following results.1. The normal incidence response of a sequence of plane parallel elastic layers is non‐white.2. The pressure wave reflected by a layer that is thin compared with a wavelength is differentiated with respect to the incident wave.3. The transmission response of a thin layer is consequently low‐pass and the transmission response of a sequence containing many thin layers is very low‐pass.4. The well‐known effect of the transport of acoustic energy by peg‐leg multiples within thin layers is identical with this low‐pass transmission response.5. It follows that the high frequency energy is reflected back early in the seismogram.6. By comparison, very low‐frequencies are transmitted through the layered sequence easily and are reflected with difficulty. There is probably a lack of low‐frequency energy in the reflection seismogram, by comparison with the spectrum of the incident plane wave.It follows that any meaningful evaluation of frequency‐dependent absorption in seismic data cannot take place unless the frequency‐dependent linear elastic effects are taken into account first.
SEISMIC MEASUREMENTS FOR SAFETY IN MINES*
Authors: KÖRMENDI A. , BODOKY T. , HERMANN L. , DIANISKA L. , KÀLMÀN T. 
Published: April 2006

ABSTRACTThe study of rock stresses and their changes is of great importance for safety in mines. To detect dangerous stress accumulations in coal mines an empirical method, Jahn's drilling test, is generally used. An experimental survey to solve the same problem by geophysical measurements was undertaken in a Hungarian coal mine. The basic idea was to determine the easily measurable seismic velocities instead of the more difficult to measure stresses in the rocks, since there is a monotonic relation between them.During the survey seismic transmission‐type measurements were carried out in the fore‐field of longwall faces between the top and tail roads. The seismic velocity data obtained were processed using an iterative algebraic reconstruction technique to determine the ‘velocity field’, i.e., the seismic velocity distribution, of the area covered by the ray paths. By periodically repeating the measurements in the same area, it was possible to follow the changes in the stress conditions caused by mining operations.
GEOTHERMAL PROSPECTING BY GEOELECTRIC SOUNDINGS IN NE GREECE*
Authors: THANASSOULAS C. , TSELENTIS G.‐A. , KOLIOS N. 
Published: April 2006

ABSTRACTIn total 77 direct current resistivity soundings were carried out during a geothermal exploration survey of the Genisea, NE Greece, geothermal field. The data revealed a high electrical conductivity zone at the center of the investigated area and suggested that an anomalous heat source lay beneath the study area.This was confirmed by subsequent drilling data. Temperature measurements, from 11 boreholes, were used for the construction of isotherms that correlated very closely with the geoelectric data.
GEOPHYSICAL MAPPING BY STATIONARY ELECTRIC AND MAGNETIC FIELD COMPONENTS: A COMBINATION OF POTENTIAL GRADIENT MAPPING AND MAGNETOMETRIC RESISTIVITY (MMR) METHODS*
Authors: SZARKA L. 
Published: April 2006

ABSTRACTA combined application of two independent direct current geoelectric methods, potential gradient mapping (PM) and magnetometric resistivity (MMR), is proposed for the investigation of high‐resistivity basement structures.From a one‐dimensional relation between horizontal electric and magnetic fields (due to currents originating from point electrodes placed on the surface of a two‐layered half‐space having a high‐resistivity basement) several advantages of the combined PM‐MMR method are shown for two‐dimensional and three‐dimensional basement structures. Three‐dimensional models have been studied by analogue modeling experiments.This new mapping method embraces two parameters: the horizontal electric conductance map, derived from the ratio of the related electric and magnetic components, and the gradient map of their product. A close relation between structural forms and anomaly maps is obtained by using two orthogonal source‐electrode layouts.Based on model results a two‐channel measuring instrument was developed. The method and the instrument were tested near Sopron where a gradual deepening of, and faults in, the Paleozoic basement were found.
INVERSE Q‐FILTERING. A SPECTRAL BALANCING TECHNIQUE*
Authors: GELIUS L.J. 
Published: April 2006

ABSTRACTInverse Q‐filtering (IQF) is a technique designed to correct for transmission losses due to inelastic attenuation. It is based on the constant‐Q model and is derived from a Taylor series solution of a standard convolutional‐trace model (primaries only). To avoid a non‐causal solution, the attenuation is assumed to be minimum‐phase. Band limitation is introduced to make IQF a stable process in the presence of noise. The main features of IQF are demonstrated using both synthetic and field data.
CORRECTIONS FOR CONDUCTIVITY ESTIMATES IN INDUCTION PROSPECTING OF SULPHIDE DYKES IN A LAYERED ENVIRONMENT*
Authors: NEGI J.G. , GUPTA O.P. , JOSHI M.S. 
Published: April 2006

ABSTRACTIn a weathered environment estimates of depth and conductance of metallic sulphide dykes from conventional anomaly index diagrams for a vertical half‐plane in air have to be corrected, besides the usual corrections, for:1. moderate conductivity of the host rocks, and2. finiteness of strike length S and depth extent D.Model experiments have been carried out to evaluate the response variation of a vertical planar conductor with varying depth extent and strike length for both insulating and conductive surroundings. The results indicate:1. A conductor with finite depth extent (D/L < 2.5) or strike length (S/L < 5.0) in an insulating medium yields a lower estimate of conductance (mineralization) and a greater depth.2. A moderately‐conductive host rock enhances the anomaly and rotates the phase so that the conductor appears to be more resistive (less mineralized) and shallower.The results have practical significance since in weathered surroundings a highly‐mineralized body of finite size could be missed, or misjudged, because of low estimates of conductivity and depth.
TOWARDS OPTIMUM ONE‐WAY WAVE PROPAGATION1
Authors: HOLBERG O. 
Published: April 2006

ABSTRACTNumerical wavefield extrapolation represents the backbone of any algorithm for depth migration pre‐ or post‐stack. For such depth imaging techniques to yield reliable and interpretable results, the underlying wavefield extrapolation algorithm must propagate the waves through inhomogeneous media with a minimum of numerically induced distortion, over a range of frequencies and angles of propagation.A review of finite‐difference (FD) approximations to the acoustic one‐way wave equation in the space‐frequency domain is presented. A straightforward generalization of the conventional FD formulation leads to an algorithm where the wavefield is continued downwards with space‐variant symmetric convolutional operators. The operators can be precomputed and made accessible in tables such that the ratio between the temporal frequency and the local velocity is used to determine the correct operator at each grid point during the downward continuation.Convolutional operators are designed to fit the desired dispersion relation over a range of frequencies and angles of propagation such that the resulting numerical distortion is minimized. The optimization is constrained to ensure that evanescent energy and waves propagating at angles higher than the maximum design angle are attenuated in each extrapolation step. The resulting operators may be viewed as optimally truncated and bandlimited spatial versions of the familiar phase shift operator. They are unconditionally stable and can be applied explicitly. This results in a simple wave propagation algorithm, eminently suited for implementation on pipelined computers and on large parallel computing systems.
DETECTION AND RESOLUTION OF THIN LAYERS: A MODEL SEISMIC STUDY1
Authors: KROLLPFEIFER D. , DRESEN L. , HSIEH C. H. , CHERN C. C. 
Published: April 2006

ABSTRACTThe detection and resolution of a thin layer closely situated above a high‐impedance basement are predominantly determined by both the frequency content of the incident seismic wavelet and the existence of the nearby high‐impedance bedrock.The separation of the thin layer and the basement arrivals is investigated depending on the low‐frequency content of the wavelet. The high‐frequency content of the wavelet is kept constant. The initial wavelet spectrum with low frequencies has a rectangular shape. All wavelets used have zero‐phase characteristics. Numerical and analogue seismic modelling techniques are used. The study is based on the geology of the Pachangchi Sandstone in West Taiwan.Firstly the resolution of a thin layer between two half‐spaces is examined by applying the Ricker and De Voogd‐Den Rooijen criteria. The lack of low‐frequency components of the incident seismic wavelet reduces the shortest true two‐way traveltime by about 20%. In addition, low‐frequency components of the wavelet diminish the deviation between true and apparent two‐way traveltime by about 65% for layer thicknesses in the transition from a thick to a thin layer.The second step deals with the influence of a high‐impedance basement just below a thin layer on the detection and resolution of that thin layer. Reflected signal energies and apparent two‐way traveltimes are considered. The reflected signal energy depends on the low‐frequency content of the incident wavelet, the layer's thickness and the distance between the basement and the layer. This applies only to layers with thicknesses less than or equal to one‐third of the mean wavelength in the layer, and a distance to basement in the range of one to one‐half of the mean wavelength in the rock material between layer and basement.The minimum thin‐layer thickness resolvable decreases with increasing distance to the basement; i.e. for a layer thickness of one‐third of the mean wavelength in the layer the relative error of the two‐way traveltime increases from 5% to 30%, if the distance is reduced from one to one‐half of the mean wavelength in the material between the basement and the thin layer.Finally, a combination of vertical seismic profiling and downward‐continuation techniques is presented as a preprocessing procedure to prepare realistic data for the detection and resolution investigation.
VELOCITY ANALYSIS OF THE SH‐CHANNEL WAVE IN THE SCHWALBACH SEAM AT ENSDORF COLLIERY1
Authors: COX K. B. , MASON I. M. 
Published: April 2006

ABSTRACTAn in‐seam fan shoot was conducted in 1981 over a 300 m × 500 m panel of the Schwalbach seam at Ensdorf by a team from Prakla‐Seismos AG of Hannover under contract to Saarbergwerke AG, Saarbrücken. The object was to study SH‐mode propagation in the coal seam waveguide. The high quality dataset retrieved provides a general and valuable test bed with which to compare the performance of in‐seam seismic velocity analysers.Five different dispersion analysers are demonstrated using the Schwalbach data. They are all based on the a priori assumption of coal seam homogeneity and isotropy. Space or time windows limit the resolution of the Fourier moving‐window analyser, the migration based phase‐velocity analyser, and the double Fourier transformer. The other two analysers, the maximum entropy moving‐window analyser and the phase‐moveout analyser, achieve noise‐limited super‐resolution by predicting the probable behaviour of the wavefield outside the window.The coal seam's characteristics conform to those predicted for a simplified model based on proposals by Elsen, Rüter and Schott of Westfälische Berggewerkschaftskasse, Bochum. The slight discrepancy between theoretical and actual dispersion characteristics could be reduced by increasing the model's complexity. However, there would be no material gain without testing the validity of the signal processing assumptions of seam isotropy and homogeneity.