bluuebunny/crossref_metadata_2025_split huggingface dataset:
MEASUREMENTS OF EARTH ATTENUATION FROM DOWNHOLE AND SURFACE SEISMIC RECORDINGS*
Authors: RAIKES S. A. , WHITE R. E. 
Published: April 2006

AbstractFrequency‐dependent attenuation of compressional waves within the earth has been estimated in the vicinity of wells from

spectral power ratios of the coherent events in separate time gates on the seismic section
matching a broadband synthetic trace with seismic data at the well, and
determining the operator that transforms one down(up) going pulse recorded in the well into another recorded at a deeper (shallower) level.

The accuracy of estimation of all three methods was insufficient to estimate attenuation over small depth intervals, and it was not possible to distinguish between the contribution due to internal multiples and that of genuine absorption with much confidence. Spectral ratios from (1) showed a smoother variation with frequency—and one more consistent with other estimates—when they were compensated for the spectra of the reflectivities over the time gates employed, but they did not provide more than a broad indication of attenuation over a substantial depth interval. Approach (2) was hampered by the restricted durations over which synthetic trace and seismic data can be reliably matched; approach (3) gave the best results. Here matching is a much more powerful tool than the spectral‐ratio techniques that are commonly applied since it can yield the form of the attenuation operator, i.e., both its amplitude and phase response, together with properly defined measures of its accuracy, while at the same time it minimizes the influence of noise and local interference effects at each recording level.For seismic target depths where internal multiple activity was low the logarithms of the amplitude responses of the estimated attenuation operators decreased approximately linearly with frequency and the phase responses showed no significant dispersion. Application of approach (3) to downgoing and upgoing waves estimated from a vertical seismic profile revealed the importance of changes in frequency‐dependent geophone coupling and their effect on values of Q determined from downgoing pulses only.
INTERPRETATION OF AEROMAGNETIC DATA FROM THE NORWEGIAN SECTOR OF THE NORTH SEA*
Authors: HOSPERS J. , RATHORE J. S. 
Published: April 2006

AbstractTotal magnetic intensity contour maps for the study region (between 2°E to 10°E and 56°N to 60°N) were digitized and converted to a regular grid of 285 × 285 points. The study area measures approximately 444 km × 444 km and the grid spacing is thus 1. 56 km. The International Geomagnetic Reference Field for 1975 was gridded for the above‐used net, and from the two data sets a further grid of the ▵T field was generated. A large number of profiles were constructed which were suitable for depth determinations. The regular grid ▵T data is also convenient for the computation of the second vertical derivative. Using the method of vertical prisms of Vacquier et al. (1963), a large suite of curvature‐depth indices was measured to complement the depths obtained from the intensity slopes and from boreholes which reach the crystalline basement. The depth to the magnetic basement has been contoured, and the resulting map is shown to be in good agreement with what is known about the deeper geology of the study area.The work reported here is part of a research project supported by Amoco Norway, BP Petroleum Development Ltd, Elf Aquitaine, Esso Exploration and Production, Norwegian Gulf, Norsk Hydro, Mobil Exploration Norway, Norwegian Petroleum Directorate, Royal Norwegian Council for Scientific and Industrial Research (NTNF), Norske Shell, and Statoil.
HIGH‐FREQUENCY SHALLOW SEISMIC REFLECTION MAPPING IN TIN MINING*
Authors: SINGH SURENDRA 
Published: April 2006

AbstractField results of shallow seismic reflections obtained with a propane‐oxygen detonator (POD) are presented. The survey site was in a tin‐mining area of the Kinta Valley in Malaysia. The shallow and irregular limestone bedrock is overlain by alluvial “tailing” and virgin sediments. Sizes of such mining areas can range from about 320 ± 320 m2 to 900 ± 900 m2. The survey was intended to delineate the topography of the bedrock, which is of vital importance in tin ore exploration and exploitation. The equipment included single‐ and 12‐channel signal enhancement seismographs, the POD, a hammer and thumper. The inexpensive and portable POD generates directional waves of reproducible form, variable energy of high frequency, and only a few surface waves at short offsets. Reflections at around 200 Hz were obtained from the shallow bedrock at about 25 m as well as from very shallow lithological interfaces. The interpretation of seismograms is supported by drill‐hole lithological sections and synthetic seismograms. The data illustrate the successful use of shallow reflections for mapping irregular bedrock. Reflection seismics can provide better horizontal and vertical details than the refraction method. Further improvements based on the data‐processing flexibility of new signal enhancement seismographs and synthetic seismograms are suggested.
INDUCED POLARIZATION IN ELECTROMAGNETIC INDUCTIVE SCHEMES*
Authors: WAIT J. R. , DEBROUX P. 
Published: April 2006

AbstractUsing homogeneous full‐space and half‐space models, we show that induced‐polarization properties of the medium influence the inductive coupling between two circuits. It is suggested that existing methods to interpret electromagnetic sounding data should be viewed with caution if the electro‐chemical dispersion is not taken into account.
A FIELD EXAMPLE OF THE USE OF ANISOTROPY PARAMETERS DERIVED FROM RESISTIVITY SOUNDINGS*
Authors: MATIAS M. J. S. , HABBERJAM G. M. 
Published: April 2006

AbstractIn areas where steep dips are encountered conventional practice in resistivity work has involved orienting arrays favourably in relation to the geological strike. In concealed conditions, however, the geological strike may not be known; moreover, strike may change with depth. Considerable advantage is to be gained, in such circumstances, by the use of crossed square arrays in that these yield orientationally insensitive resistivity measurements and also allow strike determinations and measurements of the effective vertical anisotropy.Two traverses of crossed square array observations are presented, together with one deeper sounding. The results show that, in favourable circumstances, reliable data on concealed strike directions can be obtained, and that the anisotropy findings greatly assist the subsequent interpretation. Model results pertinent to the field material are presented and discussed.
COMPARISON OF METHODS TO DETERMINE Q IN SHALLOW MARINE SEDIMENTS FROM VERTICAL REFLECTION SEISMOGRAMS*
Authors: JANNSEN D. , VOSS J. , THEILEN F. 
Published: April 2006

AbstractFour methods for the determination of Q in marine sediments are compared: two traditional methods, i.e. the risetime and the spectral ratio method, and two newly established ones, the spectrum modeling and the wavelet modeling method. In the latter one Q and the reflection time T are determined simultaneously, which gives a much better accuracy for T than reading it from the seismogram. The risetime and the spectral ratio methods are used for obtaining Q directly from the data. The principle of the modeling methods is to calculate the effect of absorption and dispersion on a reference wavelet or its spectrum for various values of Q, and the best fit between the observed and the calculated data leads to the optimum result. Numerical tests on synthetic data show that a precision of more than 25% for data containing noise or superposed arrivals can hardly be achieved; in any case, wavelet modeling is the superior method. Application to data from a vertical reflection profile in the Baltic Sea yields Q in the range of 15–100 for different layers, which is to be expected in the sedimentary environment of this area.The computations were performed in the Computer Center of Kiel University. The authors thank R. Meissner for his comments on the manuscript.
NORMAL‐INCIDENCE WAVEFIELD COMPUTATION USING VECTOR‐ARITHMETIC*
Authors: FERBER R.‐G. 
Published: April 2006

AbstractThe time‐domain discrete state‐space models for lossless layered media, characterized by equal one‐way traveltimes and normal‐incidence reflection coefficients, can be formulated in a vector‐arithmetic notation. This approach allows the computation of the seismic wavefield for arbitrary source and sensor location and is well suited for implementation on modern array processors. Included is an extension to a vector‐arithmetic notation for the computation of synthetic vertical seismic profiles.
A SYSTEMATIC APPROACH FOR EVALUATING AIRBORNE ELECTROMAGNETIC SYSTEMS*
Authors: BEST M. E. 
Published: April 2006

AbstractAirborne electromagnetic methods have been in use for more than 30 years. The selection of the most effective airborne system for a given geological/geographical environment is difficult since there are only a limited number of theoretical comparisons and very few comparative field evaluations of the systems.Shell Canada Ltd carried out a field evaluation using several systems to provide such a comparison. For this evaluation five different airborne systems were flown over 22 conductive features. Some lines were flown at various directions to strike, some at various elevations, and some with different line spacings. The airborne systems were then evaluated target by target for navigational control, line‐to‐line correlation, resolution, lateral coverage, signal strength, noise etc.The paper examines the philosophy behind our field evaluation and outlines steps required for setting up this comparison. Pitfalls are discussed as well as examples which illustrate the methodology. Two field examples are given from the Bathrust mining camp in New Brunswick and the Timmins Clay belt in northern Ontario (both in Canada). Some general conclusions which help our understanding of present day airborne electromagnetic systems are discussed at the end of the paper.
SOME REMARKS ON THE BEHAVIOR OF THE MAGNETOTELLURIC PHASE*
Authors: FISCHER G. 
Published: April 2006

AbstractThe magnetotelluric phase in the medium of incidence over a good conductor varies from its value at the conductor surface, usually close to 45°, to the characteristic standing‐wave phaseshift of 90° over a distance of a few conductor skin‐depths, much shorter than the vacuum wavelength at the frequencies considered. Over the top of a graben or at the surface of a two‐layer structure the basement can be looked upon as the matrix. If the graben or the overburden is a better conductor it will carry an extra current density in phase with the electric field, thus giving rise to an additional magnetic field also in phase with the electric field. This drives the argument of the surface impedance toward smaller values. For a more resistive graben or overburden the current is depressed, corresponding to a reduction of the surface magnetic field by an amount in phase with the electric field. This effect drives the phase of the impedance to values above the characteristic 45° of a uniform conductor.
EM AND IP RESPONSE OF A STEEL WELL CASING FOR A FOUR‐ELECTRODE SURFACE ARRAY. PART I: THEORY*
Authors: WAIT J. R. , WILLIAMS J. T. 
Published: April 2006

AbstractThe basic formulation for the mutual coupling between grounded circuits in the presence of a vertical conductor is used to examine the full electromagnetic and induced polarization response for a homogeneous half‐space model. Both interfacial polarization at the steel/electrolyte boundary and induced eddy currents in the casing are accounted for in the analysis. It is shown that for mild steel casings the eddy currents, hitherto neglected, play an important role because of their strong frequency dependence. Calculated curves of the appar‐ ent complex resistivity as a function of frequency show that the overall response can be influenced by the state of corrosion at the surface of the casing.
A NOTE ON THE INTERPRETATION OF MAGNETIC ANOMALIES OF INFINITE THIN SHEETS BY USING RELATION FIGURES*
Authors: CH. RAO RAMA , VIJAYAKUMAR V. , VIRUPAKSHI G. , RAO M. B. S. V. 
Published: April 2006

AbstractThe concept of relation figures is utilized in the case of a thin infinite sheet for identification and determination of the causative source geometry, and its various parameters. The plot of ▵T versus ▵V is a conic, the plots between the respective symmetric components and the respective asymmetric components are straight lines, and the resulting diagram between the first horizontal versus the first vertical derivative of either ▵T or ▵V is a cardioid.The authors are grateful to Shri V. Babu Rao and Dr R. K. Verma for their encouragement. They would like to thank the Director of the N. G. R. I. for permission to publish this paper. The neat drawing of Sri P. Sundara Rao is appreciated.
IDENTIFICATION OF SEISMIC REFLECTIONS USING SINGULAR VALUE DECOMPOSITION*
Authors: URSIN B. , ZHENG Y. 
Published: April 2006

AbstractSingular value decomposition (SVD) is applied to the identification of seismic reflections by using two different models: the impulse response model, where a seismic trace is assumed to consist of a known signal pulse convolved with a reflection coefficient series plus noise, and the delayed pulse model, where the seismic signal is assumed to consist of a small number of delayed pulses of known shape and with unknown amplitudes and arrival times.SVD clearly shows how least‐squares estimation of the reflection coefficients may become unstable, since a division by the singular values is required. Two methods for stabilizing this procedure are investigated. The inverse of the singular values may be replaced by zeros when they are less than a given threshold. This is called the SVD cut‐off method. Alternatively, we may use ridge regression which in filter design corresponds to assuming white noise. Statistical methods are used to compute an optimal SVD cut‐off level and also to compute an optimal weighting parameter in ridge regression. Numerical studies indicate that the use of SVD cut‐off or ridge regression stabilizes the least‐squares procedure, but that the results are inferior to maximum‐likelihood estimation where the noise is assumed to be filtered white noise.For the delayed pulse model, we use a linearization procedure to iteratively update the estimates of both the reflection amplitudes and the arrival times. In each step, the optimal SVD cut‐off method is used. Confidence regions for the estimated reflection amplitudes and arrival times are also computed. Synthetic data examples demonstrate the effectiveness of this method. In a real data example, the maximum‐likelihood method assuming an impulse response model is first used to obtain initial estimates of the number of reflections and their amplitudes and traveltimes. Then the iterative procedure is used to obtain improved estimates of the reflection amplitudes and traveltimes.
INSUFFICIENTLY SAMPLED Q: A CAUSE OF WAVELET DISTORTION IN THE GENERATION OF SYNTHETIC SEISMOGRAMS?*
Authors: MOSEGAARD K. 
Published: April 2006

AbstractThe effect of sampling of Q‐values on the generation of one dimensional synthetic seismograms including absorption and dispersion is investigated. For the well data considered, a decrease in the Q‐sampling interval results in an elongation of the estimated reflection waveform.Two explanations are given. The first refers to the way multiple energy is absorbed in the stratified medium. The second is based on the general observation that geological media with high velocity (e.g. limestones) often have high Q‐values, whereas media with low velocity (e.g. shales) often have low Q‐values.
SEISMIC SOURCES FOR SHALLOW REFLECTION SURVEYING*
Authors: McCANN D. M. , ANDREW E. M. , McCANN C. 
Published: April 2006

AbstractA standard seismic reflection profile was shot along a disused railway track at Onley, near Rugby, U. K. Four different seismic sources including explosives, the propane/oxygen gas‐gun, the Bolt airgun, and the borehole sparker were used and compared with each other in terms of output energy, penetration and resolution. The results indicated that the resolution of the borehole airgun and the gas‐gun was slightly higher than that possible with gelignite. Both these sources had an output energy which was equivalent to 30 g of gelignite.The borehole sparker was only useful for obtaining seismic information on the nearsurface weathered layer, since its output power (1 kJ) was very limited. However, McCann and McCann (1982) used a high‐power sparker source (14 kJ) on the nearby Grand Union Canal for a wide angle seismic reflection survey and achieved a maximum penetration of 250 m, which is comparable with the results obtained on land with the seismic sources mentioned above.The seismic reflection profile, which was interpreted in the light of borehole information in the area and the results of McCann and McCann (1982), successfully identified the surface of the Palaeozoic rocks. The problem of detecting the presence of thin, high‐velocity layers in a seismic refraction survey without the availability of other information to calibrate the seismic section is also highlighted.
AN EXTENSION OF THE BORN INVERSION METHOD TO A DEPTH DEPENDENT REFERENCE PROFILE*
Authors: BLEISTEIN N. , GRAY S. H. 
Published: April 2006

AbstractAn extension of the multidimensional Born inversion technique for acoustic waves is described. In earlier work, a perturbation in reference sound velocity was determined by assuming that the reference velocity was constant. In this extension, we allow the reference velocity to be a function of the depth variable z. The output of this method is a high‐frequency bandlimited reflectivity function of the subsurface. The reflectivity function is an array of bandlimited singular functions scaled by the normal reflection strength. Each singular function is a Dirac delta function of a scalar argument which measures distance normal to a reflecting interface. Thus, the reflectivity function is an indicator map of subsurface reflectors equivalent to the map produced by migration. In addition to the assumption of small perturbation, the method requires that the reflection data reside in the high frequency regime in a well‐defined sense.The method is based on the derivation of an integral equation for the perturbation in sound velocity from a known reference velocity. When the reference velocity is constant, the integral equation admits an analytic solution as a multifold integral of the reflection data. Further high frequency asymptotic analysis simplifies this integral considerably and leads to an extremely efficient numerical algorithm for computing the reflectivity function. The development of a computer code to implement this constant‐reference‐velocity solution is published elsewhere.For a reference velocity c(z) we can no longer invert the integral equation exactly. However, we can write down an asymptotic high‐frequency approximation for the kernal of the integral equation and an asymptotic solution for the perturbation. The computer implementation of this result is designed along the same lines as the code for constant background velocity. In tests the total processing time for this algorithm with depth‐dependent background velocity is usually considerably less than that required by a standard Kirchhoff migration algorithm. The method is implemented as a migration technique and compared with alternative migration algorithms on the flanks of the salt dome.
A CROSS‐HOLE AND FACE‐TO‐BOREHOLE IN‐SEAM SEISMIC EXPERIMENT AT INVINCIBLE COLLIERY, AUSTRALIA*
Authors: GREENHALGH S. A. , BURNS D. , MASON I. 
Published: April 2006

AbstractAn experimental cross‐hole and face‐to‐borehole in‐seam seismic survey was carried out at Invincible Colliery in the Western Coalfield of New South Wales. Objectives were to determine propagation characteristics of the Lithgow seam and to establish the infrastructure for seismic mapping hole‐to‐hole in Australia.The seam supports leaky P‐, S‐ and P‐SV‐modes. These modes propagate with group velocities (at 60 Hz) of 3.1, 1.5 and 1.2 km/s respectively. Particle motion polarization is well developed, as is dispersion of the SH‐mode. Attenuation rates are high. The seam is lossy (Q of approximately 20). Two prominent structures were mapped by mode conversion. One is believed to be a fracture zone, the other a zone of intense roof thrusting.The old workings and a minor strike‐slip fault, which intersected raypaths, were found to be relatively transparent to P‐ and S‐waves at 60 Hz.Telemetry delay and shot‐break timing errors of the exploder box are significant. The resulting traveltime scatter is reduced by means of a least‐squares “statics” procedure. The group velocity estimation algorithm (based on Fourier transform) yields dispersion characteristics which can be matched with theoretical results for a simple model of a coal seam waveguide.The experiment demonstrates the capability to retrieve in‐seam seismic data of diagnostic quality over an appreciable distance (2 km). The experience gained in both survey layout and data processing will be beneficial to future seam wave surveying of Australian coal mines.
DETECTION OF GAS BUBBLE BOUNDARY MOVEMENT*
Authors: BLONDIN E. , MARI J.L. 
Published: April 2006

ABSTRACTHigh‐resolution seismic surveys were performed on an underground gas storage of Gaz de France at Gournay‐sur‐Aronde (Oise) in order to obtain information about gas bubble boundaries within the reservoir. For that purpose, a light seismic source Soursile and spreads with small spacing between geophone groups (10 m) and small geophone group extension (10 m or 20 m) were used. In December 1982, a line recorded on the top of the anticline structure of the gas reservoir provided an estimation of the lateral extension of the bubble based on an amplitude anomaly (dim spot) associated with reservoir seismic horizons. In order to estimate the displacement of the gas bubble within the reservoir, three lines were recorded on the southeastern part of the anticline at different time periods (April 1981 and January 1982) with different volumes of gas in the reservoir. It is assumed that a variation of gas volume introduces a seismic velocity variation and as a result a travel‐time variation for the seismic waves that pass through the reservoir. A method based on the residual time shifts observed on reflectors below the reservoir from one survey to the other was developed and implemented. This method permitted a detection of a movement gas bubble boundary between April 1981 and January 1982 which agrees with drill hole data (water level measurements).
ANISOTROPY AND TRANSVERSE ISOTROPY*
Authors: CRAMPIN S. 
Published: April 2006

ABSTRACTA number of authors in the exploration literature have written about anisotropy, but have restricted their discussions to wave propagation through rock having transverse isotropy with a vertical symmetry axis. This note shows that there are fundamental differences between transverse isotropy when the symmetry axis is vertical (normal to the free surface) and more general anisotropy with an azimuthal variation of properties. These differences are important now that effective azimuthal shear‐wave anisotropy resulting from aligned cracks and pores is becoming recognized as a significant property of crustal rocks.