bluuebunny/crossref_metadata_2025_split huggingface dataset:
MIGRATION STRATEGY*
Authors: HOSKEN J. W. J. , DEREGOWSKI S. M. 
Published: April 2006

ABSTRACTThe geophysical data processor today has on offer a great variety of tools for the inversion of seismic reflection data to estimate geological structure. The major subset of these comprises migration procedures, which span a wide range of sophistication and cost in terms both of computation time and manual effort on the part of interpreters and processing staff. The choice of an over‐powerful process can be very wasteful, but on the other hand too naive a migration procedure can lead to wrong interpretations which are much more costly still.Complete inversion procedures which aim to delineate all changes in rock densities and elastic properties in the subsurface are still in the imaginative stages of research. Not even the most sophisticated migration procedure in current use with real data, however, provides a complete inversion, but all depend in some measure on prior knowledge of the velocity structure of the section of the earth traversed by the seismic energy. Such knowledge may be very approximate at first, but each inversion should, through the skill of the interpreter, allow him to revise his velocity model and, up to some limit imposed by the quality and ambiguity of the original data, to improve the next inversion. Paradoxically, he can often be helped by using forward modeling procedures to check the implications of his ideas in the data domain, both in deciding how to update the velocity model and in selecting the most appropriate migration process to use next.We review here the currently available toolkit of migration and modeling processes and make suggestions as to how each process can fit into a learning strategy which can improve the interpretation as economically as possible and in as many iterative steps as the complexity of the earth's velocity structure makes necessary. An example is shown of the strategy being used in a complex overthrust region.The authors wish to thank the Chairman and Board of Directors of BP Exploration Co. for permission to publish this paper, and also make acknowledgment to our colleagues whose labours in research and development have made available to our use many of the essential tools required to implement the strategies we describe.
COMPUTING THE KERNEL FUNCTION IN RESISTIVITY SOUNDING WITH AN ARBITRARY ELECTRODE CONFIGURATION*
Authors: KOHLBECK F. 
Published: April 2006

ABSTRACTA power series expansion can be used to obtain the kernel from apparent resistivities for an arbitrary electrode configuration. Three types of function are most appropriate for this purpose. The expansion coefficients can be by a least‐squares method. In this case, ortho‐normalization of the functions is of great advantage. An example with the Wenner configuration is given.
THE USE OF VERTICAL LINE SOURCES IN ELECTRICAL PROSPECTING FOR HYDROCARBON*
Authors: ROCROI J. P. , KOULIKOV A. V. 
Published: April 2006

ABSTRACTConventional electrical prospecting can be extended to the search for deep‐seated hydrocarbon deposits, by using the steel casings of drill‐holes as vertical line sources. These sources produce at depth a density of current higher than the density created by point sources located at the ground surface. Several tests have shown that the contrast of conductivity between resistive hydrocarbon deposits and the surrounding salt water produces relevant anomalies on a resistivity map obtained with vertical line sources, especially where there exists a superficial masking effect caused by a highly resistive layer. In a survey carried out in the USSR, combined measurements were performed, both with line source and with surface point sources. The detected residual resistivity anomaly roughly delineates the contours of the known hydrocarbon deposit.
AN INVESTIGATION OF THE SPECTRAL PROPERTIES OF PRIMARY REFLECTION COEFFICIENTS*
Authors: WALDEN A. T. , HOSKEN J. W. J. 
Published: April 2006

ABSTRACTThis paper investigates the form of the nonwhiteness found in the reflection coefficients from a wide variety of rock sequences around the world. In all but one case densities are taken as constant due to the paucity of suitable density data. The reflection sequences are pseudo‐white only above a corner frequency, below which their power spectrum falls away according to a power law fβ, where β is between 0.5 and 1.5. This spectrum can be adequately modelled in practice very simply with an ARMA (1, 1) process which acts on a white innovation sequence. The corollary of this is that before wavelet estimation methods are applied (all of which‐except those based on synthetic seismograms—presuppose white reflection sequences) or deconvolution filters estimated, seismic traces should be filtered with the inverse of this process.Interestingly, the estimated ARMA processes group themselves into two clearly differentiated categories, having very different indices of predictability (or, strictly, indices of linear determinism). The two categories apparently correlate precisely with two kinds of sedimentation: one which consists largely of sequences of rocks with repeating properties, called “repetitive” in this paper but perhaps loosely describable as “cyclic”, and the other which is randomly bedded with no apparent pattern of components. The former has indices of predictability which are two to four times as great as those of the latter. Another, probably related, property is that β for the repetitive sequences tends to be greater than that for non‐repetitive rock columns.The observed power spectra are shown to be consistent with a simple model for the logarithm of acoustic impedance consisting of a mixture of processes where the distribution of (time) scale parameters is reciprocal.Detailed effects of block‐averaging and sampling the logs are shown to depend on the type of sequence under examination.
INVERSION METHODS FOR τ‐p MAPS OF NEAR OFFSET DATA—LINEAR INVERSION*
Authors: HARDING A. J. 
Published: April 2006

AbstractConventional velocity analysis, based on the ideas of rms velocity and hyperbolic reflection events in the x‐t domain, is restricted in validity to near vertical incidence. Thus analysis of near‐offset datasets usually requires the muting of wide‐angle reflections from shallow interfaces before the rms velocities are determined. The ray‐theoretical integral for the delay time τ, which depends on the slowness p and the velocity function, is valid for all angles. The wide‐angle reflections can be used to improve the accuracy of the derived velocity function in the near surface region, if the recorded x‐t data are mapped into the τ‐p domain. By representing the velocity function between reflectors as a series of gradient zones, i.e. regions with a uniform increase in velocity with depth, the recovery of the velocities may be posed as a matrix linear inverse problem for the slopes of the gradient zones. In order to convert the problem to a linear one, the velocity discontinuities at the reflecting interfaces must be fixed in advance. Their positions are based on the behaviour of the τ‐p map of the data. Finding a stable velocity model may require several iterations with the reflecting interfaces at different positions. An understanding of the workings of the inversion algorithm allied with an analysis of the causes of instability aids the search for a stable model.
DETERMINISTIC ESTIMATION OF A WAVELET USING IMPEDANCE TYPE TECHNIQUE*
Authors: LOEWENTHAL D. , LEE S. S. , GARDNER G. H. F. 
Published: April 2006

AbstractA new deterministic technique for estimating a wavelet suggested by Loewenthal and Jakubowicz requires measurement of both pressure and vertical particle velocity. Through construction of the impedance function a deterministic estimation of the wavelet and the reflectivity can be obtained.This idea is tested for a one‐dimensional model. The test is carried out by forming a synthetic seismogram of both pressure and particle velocity and checking the formulas for obtaining the estimated wavelet under noisy conditions.
IS IT POSSIBLE TO INCREASE THE RESOLUTION IN SEISMIC EXPLORATION FOR COAL BY USING HIGH FREQUENCY SIGNALS?*
Authors: RÜTER H. , SCHEPERS R. 
Published: April 2006

AbstractIn seismic exploration for coal data resolution is a fundamental problem. Modeling helps to understand those details of the geology that can be interpreted from the seismic image. For single seam exploration, the vertical resolution of a seismic section is defined by the bandwidth of the signals. If there are several seams, each seam acts as a high‐pass filter for reflections and as a low‐pass filter for transmitted waves. Synthetic seismograms show that reflections from deep seams have a low frequency content.Within a layered sequence of coal seams, many multiples are generated which disturb later primary reflections. The ratio of primaries to multiples depends on the frequency content of the seismic data and on the number of overlying seams. The multiple problem is more severe with high frequencies.Primary reflections from deep coal seams within a sequence can be detected only if low‐frequency signals are used. However, the use of low‐frequency signals reduces the resolution of the deeper data.
LOW‐PASS FILTERING OF NOISY SCHLUMBERGER SOUNDING CURVES PART 1: THEORY*
Authors: PATELLA D. 
Published: April 2006

ABSTRACTA contribution is given to the solution of the problem of filtering noise‐degraded Schlumberger sounding curves. It is shown that the transformation to the pole‐pole system is actually a smoothing operation that filters high‐frequency noise. In the case of residual noise contamination in the transformed pole‐pole curve, it is demonstrated that a subsequent application of a conventional rectangular low‐pass filter, with cut‐off frequency not less than the right‐hand frequency limit of the main message pass‐band, may satisfactorily solve the problem by leaving a pole‐pole curve available for interpretation. An attempt is also made to understand the essential peculiarities of the pole‐pole system as far as penetration depth, resolving power and selectivity power are concerned.
LOW‐PASS FILTERING OF NOISY FIELD SCHLUMBERGER SOUNDING CURVES PART II: APPLICATION*
Authors: GHOSH N. , WADHWA R.S. , SHROTRI B.S. , PATELLA D. 
Published: April 2006

ABSTRACTThe basic principles of the application of the linear system theory for smoothing noise‐degraded d.c. geoelectrical sounding curves were recently established by Patella. A field Schlumberger sounding is presented to demonstrate first their application and validity. To achieve this purpose, firstly it is pointed out that the required smoothing or low‐pass filtering can be considered as an intrinsic property of the transformation of original Schlumberger sounding curves into pole‐pole (two‐electrode) curves. Then we sketch a numerical algorithm to perform the transformation, opportunely modified from a known procedure for transforming dipole diagrams into Schlumberger ones. Finally we show a field example with the double aim of demonstrating (i) the high quality of the low‐pass filtering, and (ii) the reliability of the transformed pole‐pole curve as far as quantitative interpretation is concerned.
A LAYER‐STRIPPING TECHNIQUE FOR THE SUPPRESSION OF WATER‐BOTTOM MULTIPLE REFLECTIONS*
Authors: CARRION Ph.M. 
Published: April 2006

ABSTRACTA new method to suppress water‐bottom multiples (water‐bottom reverberations) uses the fact that in the domain of intercept time and ray parameter (τ–p domain) the water‐bottom reverberations are strictly periodical for a horizontal flat sea bottom.Using this property a comb filter can be designed. The window of the filter should be approximately equal to the duration of a source pulse. The algorithm finds the maximum of the periodical energy throughout the τ–p domain and then designs the comb filter which eliminates the water bottom reverberations from each trace in the τ– p domain. This process can be repeated for higher order reverberations. Finally the τ–p domain with attenuated multiples is transformed back to the conventional x ‐‐ t space.The method is illustrated on a variety of synthetic data and on a set of real marine CMP data acquired in the North Sea near the Norwegian shore.
OMNI‐DIRECTIONAL DOWNHOLE EM PROBES*
Authors: CULL J. P. , COBCROFT R. 
Published: April 2006

ABSTRACTBorehole surveys are usually commissioned to provide structural details as a guide to further drilling. Conductors remote from the hole are readily detected using EM probes. Vectors can be established by noting variations in wavelength, particularly for reversals of polarity. However, several holes in the same area are required to remove rotational ambiguities, and single axis systems may generate spurious anomalies related to errors in alignment. Consequently, a three‐component sensor has been designed to compare the orthogonal response at each depth. Data are expressed in terms of the invariant parameters of the polarization ellipse, and secondary fields are readily identified.
ELECTROMAGNETIC SOUNDING OF THE KAPURDI LIGNITE DEPOSIT IN WESTERN RAJASTHAN, INDIA*
Authors: PODDAR M. , DHANASEKARAN P.C. 
Published: April 2006

ABSTRACTIn the Kapurdi block of the Barmer lignite field in western Rajasthan, India, lignite occurs as a number of discontinuous seams of varying thickness up to a few m. The country rock is a succession of clay, sand and various members of the clay‐sand family. The overburden, comprising sand, Fuller's earth, bentonitic clay, clayey sand/sandy clay and clay, is thick and, electrically, highly conductive. Both its thickness (40–90 m) and conductivity (0.5–1.0 S/m) are highly variable. The lignite seams may occur anywhere in the clay‐sand sequence below this overburden.A ten‐layered electrical model of the Kapurdi area was parameterized on the basis of galvanic resistivity sounding and core drilling results. This model was then validated by electromagnetic sounding measurements made with a 200 × 200 m square loop transmitter and energizing current of variable frequency (80 Hz to 11 kHz), using central induction sounding techniques. With a loop of this size, only formations in the overburden could be mapped. Subsequent computer modeling has shown that a larger loop of size, say, 800 m and a frequency band of 30–500 Hz are required to map formations below–as well as those within–the overburden. Even then only various clay‐sand interfaces can be mapped, i.e., only over‐burden thickness and the likely depth range of lignite seams (if they occur) can be estimated. Direct mapping of lignite seams is not feasible in practice. This is true of loop‐dipole, as well as central induction sounding. It is interesting that skin depth need not always be a factor limiting depth of exploration in electromagnetic prospecting.
THE P400 WATERGUN SOURCE IN THE QUEST FOR IMPROVED RESOLUTION AT ALL DEPTHS*
Authors: LUGG R.D. , BRUMMITT J.G. 
Published: April 2006

ABSTRACTThe low‐frequency response of the P400 watergun is an improvement over that of the S80 version. It has been further enhanced by deployment in a vertically staggered array. The notch in the amplitude spectrum at about 35 Hz due to the interference between the precursor and main implosion pulse has been virtually eliminated, and depth notches due to the free surface interface at the upper end of the spectrum have been greatly reduced. The spectral band is thus very broad and well‐shaped and corresponds to an even shorter signature.After convolution with the streamer ghost, the amplitude of the implosion pulse from the composite far‐field signature is more than 30 times the composite precursor amplitude. Thus the signal approaches the minimum‐phase condition.Shot‐generated noise scattered back by diffractors located at or near seabed is the single greatest impediment to increased penetration, especially in high resolution surveys. Such noise decays much less rapidly than signal, particularly so with broad band data. Furthermore, back‐scattered interference coming from a cone of bearings between 15–45° with respect to the line is particularly damaging since in these directions the stack enhances the interference and confuses the primary velocity analysis. The watergun source patterns are therefore element‐weighted to maximize directivity within these critical directions in the frequency band 50–155 Hz.When spatial aliasing is reduced by increased streamer spatial sampling and by discrete anti‐alias filters during the record cycle,f‐k filtering can be applied to the field records as an effective supplement to the source and receiver patterns without the aliased ‘wrapped around’ noise alignments destroying the upper end of the spectrum of genuine reflectors. Furthermore, increased spatial sampling is a pre‐requisite in thef‐k‐migration process if the valuable higher frequencies of broad band data are to be migrated in steep‐dip situations without aliasing.Thus, spatial resolution is a necessary complement to temporal resolution if maximum advantage of the broad‐band P400 watergun source is to be realized.
GEOELECTRIC DETERMINATION OF QUALITY CHANGES AND TECTONIC DISTURBANCES IN COAL DEPOSITS*
Authors: CSÓKÁS J. , DOBRÓKA M. , GYULAI Á. 
Published: April 2006

ABSTRACTA new underground geoelectric method is presented for the determination of small tectonic disturbances and barrenings in coal seams. The distribution of the apparent resistivity can be mapped from the measured apparent resistance data by using a recently developed geoelectric imaging method. The applicability of the methods are proved by in situ measurements and by a model experiment.
EXTENDED MARINE ARRAYS VERSUS SIMULATED EXTENDED ARRAYS*
Authors: ROKSANDIý M.M. 
Published: April 2006

ABSTRACTTwo factors are responsible for the fact that an extended marine source array performs better than a point source:1. a higher degree of transmission of the radiated seismic energy through the water‐sediment interface providing a better penetration;2. filtering effects.The higher degree of transmission is due to: (a) the directivity of extended sources, (b) the lower reflection coefficient at the water‐sediment interface for seismic waves radiated from an extended source array than for spherical seismic waves radiated from a point source, (c) the lower amplitude decay of the pulses from an extended source than from a point source. In addition, signature characteristic of an extended source array and Fresnel zone of waves generated by such a source differ from those corresponding to a point source.The propagating wavelet radiated from a point source array may not be, in a sedimentary sequence below the sea‐floor, the linear combination of wavelets emitted from point sources. In such cases, there is a noticeable difference between the performance of a field‐implemented source array and that of the corresponding simulated source array. The performances of the field‐implemented and simulated extended receiver arrays can be identical if the recording system is adequate and the processing technique appropriate.
LOVE‐TYPE SEAM‐WAVES IN WASHOUT MODELS OF COAL SEAMS*
Authors: BREITZKE M. , DRESEN L. 
Published: April 2006

ABSTRACTThe propagation of Love seam‐waves across washouts of coal seams was studied by calculating synthetic seismograms with a finite‐difference method. Seam interruption, seam end and seam thinning models were investigated. The horizontal offset, the dip of the discontinuities and the degree of erosion served as variable parameters. Maximum displacement amplitudes, relative spectral amplitudes and phase and group slowness curves were extracted from the synthetic seismograms.Both seam interruption and seam thinning reduce the maximum displacement amplitudes of the transmitted Love seam‐waves. The degree of amplitude reduction depends on the horizontal offset and the degree of erosion. It is four times greater for a total seam interruption than for an equivalent seam thinning with a horizontal offset of four times the seam thickness. In a seam cut vertically, the impedance contrast between the coal and the washout filling determines the maximum displacement amplitudes of the reflected Love seam‐waves. They diminish by a maximum factor of four in oblique interruption zone discontinuities with a dip of maximum 27°, and by a maximum factor of ten in a seam thinning with a degree of erosion of at least 22%.The analysis of the relative spectral amplitudes indicates a preferential transmission of those phases with frequencies below, and a preferential reflection of those phases with frequencies above the first mode Airy‐phase. The relative spectral amplitudes of the reflected Love seam‐waves show a distinct interference pattern of the waves reflected at both interruption zone discontinuities.The dispersion analysis reveals a flattening of the phase and group slowness curves with increasing frequencies, horizontal offset and degrees of erosion.These results imply that a detection of washouts in‐mine will be possible in a frequency range including at least the first mode Airy‐phase. An interference pattern and a flattening of the dispersion curve indicate a washout rather than other seam obstructions and leads to an estimate of the washout dimension.
CALCULATION OF POTENTIAL FIELDS FOR STOCHASTIC MODELS OF HETEROGENEOUS GEOLOGICAL OBJECTS*
Authors: VAKHROMEYEV G.S. , DAVYDENKO A.Y. 
Published: April 2006

ABSTRACTThe solution of the direct problem of magnetic and gravity prospecting for heterogeneous objects is discussed from the view‐point of the probability description of object models and the anomalous effects caused by them. Three stochastic models for the determination of the basic characteristics of non‐stationary geophysical fields above the finite objects are suggested: linear, with a random distribution of the physical properties and fixed source geometry; non‐linear, of a single object with random parameters, and an ensemble of random sources. With the methods suggested for the formalized description of the structure of heterogeneous objects for those models, expressions are derived enabling the calculation of mathematical expectations, dispersions and correlation functions of anomalous effects.Model examples illustrate the application of stochastic modeling for the study of the influence of the character and dimensions of heterogeneities of the internal structure of the disturbing bodies on the probability characteristics of expected anomalous effects. It is shown that the assumption of the random character of the anomalous effect can be used for direct calculation of the optimum Wiener filter enabling the recovery of the shape of anomaly contaminated by random noise, as well as for calculating an observation step that is sufficient for recording the expected anomalous effects with prescribed accuracy.