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56th EAEG Meeting
- Conference date: 06 Jun 1994 - 10 Jun 1994
- Location: Vienna, Austria
- ISBN: 978-90-73781-05-4
- Published: 10 June 1994
1 - 100 of 537 results
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A Discussion on the depth of dynamite source
Authors H. Amano, K. Watanabe and T. OtsukaThe dynamite has been used as the seismic source in many surveys. It has been reported that the amplitude and signature of seismic waves generated with the dynamite are affected not only by the charge size but also by the depth of the energy source (e.g., Winkel, 1989). The depth of the dynamite source is usually selected based only on the preliminary shots which are examined as one of the field tests. The amplitude and signature variation with the source depth is considered to be mainly dependent on the difference of the surrounding conditions of the energy source. In the field of civil engineering, the "soil test" has been caried out as a conventional method to investigate the condition of the ground. In this paper, we discuss the relationship between the result of soil test and the signature of seismic waves generated with the dynamite.
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Computerized refraction seismic analysis (RSA) and its application to static correction calculation
Authors R. K. Fruhwirth, A. Kogler and R. SchmoellerThe refraction seismic method was introduced into geophysics in 1920 by L .Mintrop. The Orchid salt dome was the first oil field discovered 1924 by this method. The success of the seismic method in petroleum exploration and the financial background of this branche encourages the development of more and more detailed analysis methods in both, the reflection and refraction seismic method. The rather simple RSA methods using critical distance or intercept time are nowadays in the era of computerization replaced by more sophisticated methods like the Wave Front Method, the Plus Minus Method or the Generalized Reciprocal Method.
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Dual source vibroseis acquisition for 2D survey by using conventional and E-code sweeps
Authors R. Servodio and L. BertelliAGIP has successfully transferred in the 2D acquisition the Dual source Vibroseis technique to achieve high resolution, low cost, vibroseis 2D data, following the fruitful experiences carried out in 3D acquisition (L. Bertelli, R. Servodio, 1991 and L. De Luchi, R. Marshall, H. Werner, 1987). As well known the aim of the dual source technique is to improve field efficiency by increasing the production and reducing the survey duration and costs. As a matter of fact the application of the dual source methodology in our 2D survey allowed a cost saving of about 40% of the final acquisition costs without smearing final data quality.
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Measuring the performance of deployed geophones
Authors R. A. Brook, M. L. Abrams, P. E. Carrol, G. D. Fisseler and D. F. KratochvilOver the last three years there has been a significant improvement in the precision of seismic acquisition systems. The Instantaneous Floating Point amplifier a standard in the exploration industry for over 25 years, has been succeeded by the 24-bit converter. The introduction of high performance geophones with reduced harmonic distortion has continued to improve data precision.
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Testing a new kind of explosive for seismic acquisition in hard ground
Authors P. Fink, E. Herndler, A. Kogler and Ch. SchmidAustrian alpine hydrocarbon exploration was motivated by exploration successes within Triassic dolomites under the Vienna Basin as those dolomites are also present in the overthrusts of the Northern Calcarous Alps (Wessely 1988).
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Recording reliability in seismic exploration as influenced by geophone-ground coupling
Authors P. W. Maxwell, H. A. K. Edelmann and K. FaberThe large amount of information carried by the seismic signal can only be brought into play for seismic interpretation if geophones are able to follow the movement of the ground faithfully. This means that the bandwidth of the transfer function of the geophone and the ground must be as large as possible. Latest developments in data acquisition instruments open new perspectives towards larger bandwidths.
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Correction of nonlinear and phase distortions of vibroseismic signals
Authors M. B. Shneerson, K. L. Mitchel and A. P. ZhukovSeismic signals excited by vibrational sources are subjected to nonlinear and phase distortions decreasing effectiveness of vibrational seismic prospecting. The distortion intensity is the largest at low frequencies due to the fact that relative movements of vibrator baseplate exceed by far the magnitude of ground elastic deformation.
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Beyond conventional 3D - 3D Undershoot
Authors W. Apel, H. -J. Koch, Th. Krey, L. Florchinger and R. MarschallIn the year 1968 the first 2D - undershoots (lines 62A, 64) were acquired in the concession Rotenburg - Wedehof for DEUTSCHE TEXACO AG. The acquisition geometry was as follows: - ∆g = 25m on top of dome and 50m at each side, 24 channels, fold = 6
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Extending the macro model by incorporating fine-layering propagation effects
Authors R. E. Slot and C. P. A. WapenaarIn seismic migration the elimination of propagation effects from the seismic data plays a major role. A wave propagating through the earth will encounter 'large contrast' boundaries which are separated by sequences of thin layers with smaller contrasts. Usually the propagation effects are quantified by a macro model, which accounts for the large contrast boundaries but suffices with the average velocities -(and densities) of the thin-layering between these boundaries. Hence, the angle-dependent dispersion effects due to internal multiple scattering in finely layered media, which have been studied extensively (O'Doherty and Anstey, 1971 ; Burridge and Chang, 1989; Herrmann and Wapenaar, 1992), are neglected in a macro model. We investigate the possibility of replacing a finely layered medium by a homogeneous, anisotropic, 'effective' medium with anelastic losces, thus mimicking the angle-dependent dispersion effects, and allowing for these effects to be incorporated in a 2-D or 3-D extended macro model for true amplitude migration.
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The generalized primary representation of 3D seismic data from highly heterogeneous media
More LessIn seismic modeling as well as in seismic imaging it is common use to ignore the rapid spatial variations of the medium parameters at a scale much smaller than the seismic wavelength. In particular, for 2-D or 3-D heterogeneous media the following approach is frequently used: - Replace the true medium by a 'macro model' that contains piecewise smoothly varying medium parameters and a 'contrast model' that is a spatially band-lirnited of the difference between the true medium and the macro model. - Formulate an implicit integral equation for the wave field in which the macro model describes the 'propagation effects' (in terms of Green's functions) and in which the contrast model describes the 'scattering effects'. - Solve this equation iteratively. This yields an explicit integral representation of the wave field that is non-linear in the contrast parameters.
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Limitations of elastic wave rayleigh scattering by an inclusion
Authors R. Gritto, V. A. Korneev and L. R. JohnsenScattering of seismic waves is a fundamental process in the propagation of waves through the Earth. In recent years, numerous authors have turned to the theory of scattering to describe the complicated nature of seismograms that occur in various places, believed to be caused by inhomogeneities and sequences of layering within the structure of the Earth. Recently, Korneev and Johnson (1993a, 1993b) derived a solution for the scattering of an elastic P wave by a spherical inclusion of arbitrary contrast and developed asymptotic solutions for this problem.
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Reflection points and surface points
By F. K. LevinThe reflection point on the plane - x sin θ + z cos θ = D (1) and the corresponding midpoint of a surface spread are connected by the relation - (x´p - xc cos θ) / X cos Ф cos θ = (yp - y,) / X sin Ф = X cos Ф / 4(D-xc sin θ). (2) In equation (1), D is the depth to the reflector measured perpendicular to the reflector from the origin and θ is the dip angle. In equation (2), X is the source-to-receiver separation and Ф is the profile direction measured from the x axis. The center of the spread is at (xc, yc, 0) is a coordinate system with the z axis perpendicular to the surface. The reflection point is (x´p, yp, D) in a coordinate system with the z axis perpendicular to the reflector.
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Finite difference modelling of faults and fractures
Authors R. T. Coates and M. SchoenbergA fault or fracture can be modelled as a surface across which stress traction is assumed to be continuous, yet displacement is allowed to be discontinuous. The simplest model that accounts for the effect of such a surface on seismic waves takes the traction vector and the displacement discontinuity vector to be linearly related by a `fracture compliance matrix'.
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Building complex geologic models by recursively merging the parts
Authors W. Wiggins and U. AlbertinA geologic model of an exploration target is usually complex, containing spatially variant velocities, and surfaces across which the lithology may be discontinuous. These complex features are not revealed en masse, but are discovered individually as seismic imaging uncovers them. For example, a salt structure is usually imaged first with a sediment-only model, then with the top-of-salt added, then with the bottom-of-salt added. Correspondingly , a geologic modeling facility should accommodate this piecemeal addition of detail to the model.
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Ray field segment computation - An efficient and flexible wave propagation technique
By K. AstebolThere is an increasing interest in integration of ray-based wave propagation in different seismic procedures. This calls for efficient and robust ray modelling techniques. with flexibility to be integrated in a wide variety of applications. Traditionally, the ray calculation techniques have focused on the calculation of single rays. The two common techniques, ray shooting and ray bending, both basically search for a single ray between one shot and one receiver, and the search is repeated to simulate surveys. A more recent approach, the wavefront construction [1],[2], scans time step by time step through the rayfield, the entire field composed by the rays. Arrivals are recorded as the wavefront passes receivers. Ray modelling works on a geometrical model of the subsurface. It is assumed that the model consists of continuons, smooth velocity fields, separated by continuous and smooth interfaces. There are no restrictions on model geometry.
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Event-oriented velocity estimation based on pre-stack data in time or depth domain
Authors E. Iversen and H. GjoystdalIn this paper a new method of the reflection tomographic kind is described, with the purpose of estimating an improved macro-velocity field for pre-stack depth migration. We have implemented an event-oriented local approach of the `layer-stripping' type, in which the event representation is referring to the unmigrated time domain. Each event is defined in terms of a few characteristic parameters, depending on the sorting criterion applied to the seismic data.
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Seismic processing with consistant modules
Authors M. T. Taner and E. BaysalDigital processing of seismic data had its commercial beginring in early 1960's. Processing steps were gradually developed to inciude more sophisticated and computationally more expensive processes as computer capabilities were expanded. Reviewing the last two decades of processing steps we have seen that while some new processes were added to the stream, most of the processes were kept the same. In fact there were a number of processes which were left over from analog days. They are still in use in the field and as part of the processing sequence. Since the laws of evolution dictates that everything must change with time, we expect to have some evolutionary if not revolutionary changes in the seismic exploration technology. In order to have an overall realistic view of the processing sequence, we have to start from the objectives and work al/ the way back to the field data specifications.
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Detailed reservoir description by integration of well and 3D seismic data
Authors R. Allred, E. Poggiagliolmi and K. E. SvelaTemporal and lateral resolution of seismic data is principally determined bv the shape and duration of the propagating wavelet. Its accurate estimation and the ability to change its shape are both of paramount importance to achieve the maximum resolution afforded by the available bandwidth. An integrated multi-line, multi-well wavelet estimation and shaping procedure was performed at Heidrun field, located offshore Norway and operated by Conoco Norway, Inc. The procedure, a proprietary technique known as space adaptive wavelet processing, involves borehole analysis, extraction of wavelets at well locations and performs implicit spectral inversion to condition the seismic pulse to a common, broad band, zero phase wavelet.
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3D AVO Processing - Evolution of a processing sequence
Authors I. F. Jones, V. Mandanche, S. Campbell and S. LancasterThe use of seismic amplitudes has long been recognized as an important aid in the location of hydrocarbons. More recently, this has been extended to include the study of the variations of amplitude as the shot-receiver offset is changed.
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Zero-phasing marine data using one-parameter phase filters and kurtosis maximization
Authors G. Cambois and N. D. HargreavesA.purely empirical approach that has been used to the seismic and well data is to apply a set of trial phase rotations to the seismic data. With certain provisos on data requirements and available bandwidth such an approach can be automated by using a maximum kurtosis criterion to select the desired phase correction (White, 1986). Whilst there is no apparent theoretical reason why phase rotations alone should be sufficient to zero-phase processed seismic data, this very simple procedure can be surprisingly effective in many cases. In this paper we explore the physical justification for such a one-parameter phase-scan in terms of the nature of the error arising out of conventional deconvolution processing. We derive another set of one-parameter phase filters that are physically more meaningful and also allow zero-phasing of marine data through kurtosis maximization.
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Characterising seismic time series using the discrete wavelet transform
More LessWe discuss an approach to summarising the features of seismic time series using the discrete wavelet transform (DWT). The aim is to provide several informative "attributes" which characterise lateral changes in the seismic traces. These attributes can be mapped for visual interpretation (Fig. 3) and also used to group together or to separate traces in regions from a survey, by application of statistical clustering techniques. Such clusters can inform extrapolation of observed characteristics from wells, or point to regions for more detailed examination.
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Wavelet transform (WT) in geophysical signal processing - An application to velocity analysis
Authors G. Olmo, L. Lo Presti and U. SpagnoliIn geophysics the reflected signals are often processed using stationary signal approximation witkin a timespace window. The Wavelet Transform (WT) represents a powerful analysis tool for nonstationary signals (e.g., Rioul and Vetterli, 1991); in this context the reflection seismology represents a promising field of application.
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Wavelet estimation via fourth-order cumulants
More LessA recent paper by Lazear (1993) holds out the possibility of directly measuring wavelet phase from seismic data, with no restrictive assumptions on the nature of the phase spectrum. His approach is "4th-order cumulant matching", in which an initial guess for the wavelet is iteratively updated until its 4th-order statistics match those of the data. A relatively large number of traces, several hundred at least, are needed to obtain reliable results, so the process does not readily lend itself to a trace-by-trace or even a shot-by-shot deconvolution. Even so, if reliable wavelet estimates can be obtained in typical circumstances this would be a considerable breakthorough, cutting across a number of existing uncertainties and difficulties in this area. In this paper I investigate the accuracy and the limitations of the 4th-order cumulant approach to wavelet estimation. I propose a procedure that can be used to assess the reliability of the derived wavelets, and I investigate the preprocessing necessary to improve their reliability. I demonstrate the technique as part of a quality control procedure that tracks the seismic wavelet through the different stages of a processing sequence.
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Successful estimation of Q from surface seismic data - A case study
Authors R. Dasgupta and R. A. ClarkAttempts have been made to estimate seismic quality factor, Q. of different rocks in the laboratory as well as from different forms of seismic data and the efforts have met with varied degree of success. The computation of Q from VSPs and check-shot surveys is quite successful (Ganley and Kanesewich,1980. Jannsen et al.1985 ; Stainsby and Worthington, 1986; Tonn,1991) whereas there exists no reported evidence of its successful estimation from surface seismic data. The main reason this is attributed to the multiplicity of the ray-paths associated with surface seismic data, where every sample represents a different ray-path. Apart from this, even for a CMP, path lengths and the reflectivity/transmissivity vary from one trace to another. Dasgupta and Clark (1993) proposed a method of computing Q from surface seismic data which solves the problems arising due to multiplicity of the ray-paths, and tested the technique using model data. In this paper, the method is reviewed and results of computing Q from surface seismic data from the central part of the southern UK North Sea are discussed. The method is further validated by comparison of the surface seismic results to estimates from VSPs both adjacent to the seismic line and in similar lithologies elsewhere in the North Sea.
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Spectral induced polarization studies of a Ni-Cu ore deposit
Authors H. Soininen and H. VanhalaLaboratory and field spectral IP results are presented from the Keivitsa Ni-Cu deposit in northern Finland. The deposit is related to basic-ultrabasic intrusive rocks (a layered basic intrusion), and has been estimated to contain more than 1000 Mt of low grade sulphide ore. Low sulphide content, high Cu/Ni ratio, and enrichment in platinum group elements are other characteristic features.
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Rapid least-squares inversion of apparent resistivity pseudosections
Authors M. H. Loke and R. D. BarkerA fast iterative inversion technique for the interpretation of data from resistivity imaging surveys using a microcomputer has been developed. This technique is based on the smoothness-constrained least-squares method and it produces a two-dimensional subsurface model directly from the apparent resistivity pseudosection. The inversion method is completely automatic and it does not even require the user to supply a starting model. In the initial iteration, a homogeneous earth model is used as the starting model for which the apparent resistivity partial derivative values can be calculated analytically. For subsequent iterations, a quasi-Newton method (Broyden, 1965) is used to estimate the partial derivatives. By avoiding the direct calculation of the partial derivatives, the computer time and memory space required by the finite-difference method to calculate the apparent resistivity values can be reduced by about eight and ten times respectively. The time taken to solve the system of normal equations for the least-squares method is also reduced by using matrix updating techniques associated with the quasi-Newton method (Golub and van Loan, 1989). Overall, this quasi-Newton inversion technique is about eight times faster than the conventional Gauss-Newton least-squares method.
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Modelling self potential anomalies from electronic conductors
By O. B. LileThe theory on the mechanism of SP anomalies over electronic conductors which Sato and Mooney published in 1960 has, until now, not been seriously challenged in its basic principles. The SP anomalies are explained by the shortcircuiting of the high redox potential in the pore electrolyte at the surface and the low redox potential deeper down. The conductor itself is inert and functions only to transport electrons from the low redox to the high redox environment. The redox potential is controlled by the oxygen activity. Close to the surface the pore water will regularly be supplied with rain water rich in oxygen. Especially in the porous and permeable overburden the pores will often be flushed by water coming from the surface. Beneath the grundwater table there is a lateral flow of groundewater in the permeable ocerburden with little exchange of water with the underlying bedrock. While the overburden may have a porosity of 30%, the porosity of a metamorfic bedrock may be 1-5%. On the basis of this discussion, a profile of the redox potential distribution in the ground can be drawn.
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SP Monitoring during the hydraulic fracturing using the TG-2 well
Authors N. Kawakami and S. TakasugiAs a part of the "Technology for Increasing Geothermal Energy Recovery" project being undertaken by the New Energy and Industrial Technology Development, Organization. (NEDO), a hydraulic fracturing test was carried out at the TG-2 well in Yunomori area, Iwate prefecture, in November 1992.
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A Study on nonlinear spectral IP phenomenon
By P. AlikajThe Spectral Induced Polarization (SIP) method according to the Cole - Cole model proved to be an interesting tool in metallic mineral exploration, particularly in texture discrimination. It opened a new scope in scientific studies and practical exploration on both the Frequency and Time Domains. However, while the mathematical treatment of this method was highly developed, the same thing did not happen in the explanation of its physical (electrochemical) basis. Such was the goal of the study presented in this paper, which led a new treatment called Nonlinear Spectral Induced Polarization (NSIP).
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Airborne electromagnetic anomalies related to low-conductivity 3D bodies
Authors M. Peltoniemi, R. Bars and G. A. NewmanResponses of a multifrequency, multicoil AEM system were modeled numerically for poor, 3D electrical conductors embedded in a resistive bedrock and overlain by an overburden of poor to moderate conductivity. Results were computed for horizontal coplanar coil configuration with coil separation L = 6.45 m and two frequencies, 7837 Hz and 51250 Hz. The EM3D code (based on Newman et al. 1989) used in the modeling is a set of programs, which takes into consideration both the induction currents, and the galvanic currents in the host rock and overburden. The separate programs are necessary to facilitate the solving of models with high or low conductivity contrasts and different primary-field sources.
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Multidimensional interpretation of TDEM data collected for mineral exploration in Israel
Authors L. Hellwig, M. Goldman and A. HoerdtDuring a feasibility study survey in 1987 thirteen central loop time-domain electromagnetic soundings were carried out over the sediments in Makhtesh Ramon, Israel. The aim of these measurements was to trace the subsurface extension of known quartz-syenite intrusions.
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Negative response of coincident loop above layered polarizable ground
Authors H. M. El-Kaliouby, S. A. Hussain, E. A. Eldiwany and E. A. HashishTransient Electromagnetic (TEM) response of Coincident loop above dispersive conductive layered ground shows an evidence of Induced Polarization (IP) effect which appears in what is termed as Negative-Response (NR) phenomenon. This phenomenon may be used as a simple guide for the detection of under-ground polarizable targets (Lee 1981 and Smith and West 1988a).
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Seismic processing on single- and multi-offset ground penetrating radar
Authors M. Grasmueck and H. HorstmeyerA comparison study of single- and multi-offset ground -penetrating radar (GPR) has been conducted across a crystalline terrane in central Switzerland. Electromagnetic waves can travel relatively long paths in crystalline rock, thus providing an optimum media for multi-offset GPR surveys with long transmission ranges.
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Resonance based target recognition for ground penetrating radar
Authors T. Campbell and J. M. TealbyThe ability to detect voids with Ground Penetrating Radar (GPR) has been recognised as one of its strengths [Campbell et al., 1993]. This is of importance in civil engineering, building assessment, and archaeology. The dielectric difference between an air void and its soil/ground surround (dielectric half space) is sufficient to enable comparatively easy detection.
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3D Processing of GPR data
By M. FritscheWith the application of GPR becoming more and more established in environmental engineering tasks, there is an increasing need for 3D data acquisition and processing capabilities. Making use of the full information present in data volumes that have been acquired along parallel profiles, a 3D diffraction stack will deliver better results in terms of signal-to-clutter ratio and positioning accuracy of buried objects. The most important factors, governing the output quality are the aperture width used for the stack, inline and crossline sampling intervals and possibly soil attenuation.
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Application of ground penetrating radar to geotechnical problems
Authors R. Fruhwirth and R. MuellerEspecially in the field of Geotechnics and environmental research there is an increasing demand for a method, which is applicable for the analysis of the subsurface and materials embedded therein in a flexible and rapid way and without too big a technical or financial effort. Among various geophysical surface and well logging methods the Ground Penetrating Radar (GPR) has turned out to be the one, that meets most of the previous requirements.
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Ground Penetrating Radar data interpretation in the Radon space
Authors P. A. Vercruijsse, M. Vermaas, J. W. A. W. Schoolmeesters and M. CsonkaIn recent years Ground-penetrating Radar (GPR) techniques have gained interest for the characterization of shallow subsurface properties. The areas of application of GPR in shallow subsurface characterization are numerous: e.g. mapping ice thickness, water depth, bedrock depth, soil stratigraphy, and groundwater depth (Davis and Annan, 1989) . Fisher, McMechan and Annan (1992) argued that multichannel recording and standard seismic processing can be used to improve the quality of GPR data.
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Eccentricity-migration for the imaging of pipes in radar reflection data
Authors C. Stolte, D. Ristow and K. -P. NickHyperbolas are a widely observed feature in reflection data aquired with ground probing radar. Zero-offset migration of these hyperbolas rarely improves the subsurface image. Often energy is smeared out and weak reflections disappear. The reasons are perturbations in the local geology influencing the velocity field, uncertainties introduced by uneven movement of the antennas, the bistatic character of the antenna as well as the geometry of the buried objects. In this work the target geometry is incorporated into migration to enhance the imaging of pipes and tables.
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Monitoring of a water well field by georadar
More LessGeoradar has been successful in detecting sudden steps in the water table which are created by thin, steeply inclined clay layers in push moraines in the Netherlands (Van Overmeeren, 1993). An example is shown in the georadar section of Figure 1, where an abrupt offset of 30 ns in the observed water-table reflector. (Fig. 1a) has recently been confirmed to be a groundwater step of considerable size (1.7 m) by test drilling (Fig. 1b). The jump in the reflector was traced along a number of parallel lines, where a further series of 9 horeholes confirmed the presente of important steps in the water table and proved the georadar predictions on depths and offsets to be remarkably close to reality.
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Georadar surveys in ice-pushed moraine areas in The Netherlands - Processing and interpretation
Authors B. C. Scheffers and M. CsonkaGeoradar becomes more and more common in hydrogeological and environmental studies as a cheap continuous profiling tool. In sediment filled basins with low conductivity and deep water table the maximum penetration achievable is approximately 40-50 m. Georadar, therefore, can be applied as a general geological reconnaissance tool. This paper will discuss the application of georadar for mapping the structural geological setting in a region of glacial deposits and icepushed moraines. Processing is carried out with seismic processing software. Interpretation is performed using all information available.
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Porosity prediction in Zechstein carbonates by analyzing tuning effects
By U. M. SchulzThe studied KALLE-Prospect is located in the Emsland area of NW-Germany. The prime exploration target is the gas bearing main dolomite of the Permian Zechstein at about 3000 m depth. Porosities in that layer are varying between 4 and 12 percent yielding to a significant reduction in acoustic impedance when being compared with the light facies. Thickness of the porous dolomite in this region can range between 10 and 40 m.
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3D Seismic inversion to explore for upper jurassic sands, North Viking Graben
Authors P. A. Hansen, D. J. Lowden and J. PaulsenLocalized deposits of Upper Jurassic sands are an important exploration target in the North Viking Graben of the North Sea. These excellent reservoir quality Munin sands can be found encased within the widespread Upper Jurassic shales, preserved in places on the flanks of eroded paleo-highs. The challenge is to be able to locate these sands which are often situated in structural lows and poorly represented by the well control. The ability to use seismic data to differentiate these sands from the more common shales provides a powerful exploration tool.
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Moosbrunn field revisited - An integrated approach to reservoir delineation
Authors R. V. Singh and E. RieserAn integrated approach of combining well-log data, core data, seismic rock properties, seismic attributes and 3 component VSP were utilized to delineate reservoir distribution in the Moosbrunn field in the southern Vienna basin of Austria.
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Evaluation of log velocities and densities for application in seismic modelling - A Zechstein case history
By K. G. MaverThe Danish Zechstein evaporites and carbonate deposits of the Southem Zechstein Basin, which have been the target of intense hydrocarbon investigation, are characterized by distinct velocities and densities and by a cyclic distribution (Fig . 1). This distinct distribution is utilized to calculate unique average velocities and densities for the individual lithologies by subdividing according to thickness, facies association and porosity. By assigning the calculated values to lithological data from wells and calculate synthetic seismograms, that correlate with both the synthetic seismograms calculated from the original logs and the corresponding seismic sections, the validity of the calculated average velocities and densities are found to be good. The values have a wide range of applications in seismic modelling and significantly enhance the seismic interpretation and prediction of porous carbonate intervals in southem Jylland, Denmark.
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Prospect definition by pre-stack depth migration of a grid of seismic lines - A case history
Authors G. C. Robinson, J. P. Tung, H. J. Tieman and G. R. SavageThe reservoir section in the Red Sea coastal plain of Saudi Arabia is often overlain by evaporites. Within these strata, halokinesis produces a lateral juxtaposition of evaporites and clastic sediments. The significant lateral velocity contrast generated by this juxtaposition of clastics and evaporites produces a corresponding distortion in the time image. As a result, structure maps of sub-salt horizons produced from depth conversion of time picks are grossly inaccurate in the vicinity of the salt diapirs.
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Determination of porous zones in lower Permian-carboniferous carbonates in the Pechora Sea
Authors I. Kerussov, V. Martirosyan, A. Simonov, S. Chernikov and O. ChernovaLower Permian-Carboniferous carbonates are the main hydrocarbon sequence in the offshore area of the Timan-Pechora oil and gas province (Pechora Sea). Their- prospectivity has been proved by drilling results. The reservoirs are vuggy cavernous and fractured limestones. Their development is related to: - zones of high porosity in areas of long-lived reverse, overthrust and strike- slip faults; - bioherms; - zones of redeposited detrital limestones related to local uplift and erosion of some offshore areas; - zones of secondary leaching of carbonates.
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Complex interpretation of seismic and nonseismic data over gas-bearing structures in the Molassezone (Alt Prerau Gasfield)
Authors N. H. Blaumoser, G. Fitzket, E. Libowitzky, W. Seiberl and D. ZychThe gasfield Alt Prerau is situated 80 km NE of Vienna near the Czechian boarder in the perialpine Molassezone, the southeastern part of the Carpathian Foredeep. The basis is formed by Mesozoic, particularly Jurassic sediments (Malm). The upper formation consists of Eggerian, Eggenburgian, Ottnangian, Carpathian and Badenian sediments. The gas accumulation occurs in Oncophora Beds - Ottnangien (Aniwandter, Bimka, Zych, 1990).
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Field evaluation of 3D seismic acquisition parameters - A case study
Authors G. P. Lourantos and A. M. LewalleDuring the design stage for the acquisition of 3D seismic surveys all essential parameters are tested and evaluated using available software packages. However, some key acquisition parameters can be objectively assessed only through field tests. Results from such tests assist in optimising the acquisition parameters and minimise the operational costs.
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A Case history of a six streamer high resolution 3D survey off NEW Australia
Authors M. Boardman and R. WalkerThis paper is a case study of the adaptation of conventional multi-streamer marine 3D acquisition techniques to provide extremely dense sampling over a shallow-medium depth target. The availability of a six-streamer vessel meant that high fold in 12 .5x12.5m CDP bins could be achieved, in a cost and time effective manner. The results of this survey have already resulted in a significantly altered shallow reservoir map and a complete re-appraisal of the complex faulting at deeper target levels under the shallow reservoir itself. The survey also provides insights into the potential of improving the resolution of 3D over more conventional targets.
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Processing marine 3D seismic data on board during acquisition
Authors D. T. Johnson, D. G. Bradshaw, R. G. Early and W. J. DoneA transportable computer system for field processing of 3D marine seismic surveys has been developed by Amoco Production Company Research, and used aboard contractor vessels since November 1992. Through January 1994, full fold 3D binned stack volumes have been processed for ten 3D surveys, with deployments on five different vessels.
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The changing role of on-board processing
More LessDuring the last two or three years the rapid growth in the power of the small workstation type of computer has brought about a revolution in the capabilities and applications of seismic data processing on board ever more complex 3D marine acquisition vessels. In this paper we use data examples recorded over the last four years to demonstrate how the new role of on-board processing benefits the cycle of acquisition, processing and interpretation. Processing now forms the constructive link in this cycle, rather than the isolated specialised activity it became during the days of main-frame domination. In particular, the interpreter is brought into the action at an early stage.
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Real time offshore 3D processing - A case history
Authors R. I. Thornton, J. M. Reilly, P. Millard and M. L. JohnsonWe are to describe the method used to produce onboard the survey vessel, a full-fold 3D DMO stack data volume immediately available at the completion of the acquisition of a 3D seismic survey The Camelot gas field is located in the southernmost portion of the U.K. southern gas basin. The trapping structures are a complex of rotated fault blocks. The reservoir is the Permian Rotliegendes formation. Reservoir depths average 1850m (1.2 TWT). Seismic imaging of the reservoir is complicated by the Rotliegendes fault pattern, lack of 2D seismic coverage over shallow sand banks, and the presence of a major Jurassic graben in the southern half of the area of interest. Production from the field began in 1988. Initial delineation of the field was based upon numerous vintages of 2D seismic program.
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Dual sensor attenuation of flexural ice waves and random noise
Authors F. J. Barr, D. L. Nyland and G. A. SittonSurface energy sources generate good quality reflection data in Arctic regions where the seasonal ice is in direct contact with the land beneath it. However, they generate a very high-amplitude, dispersive noise wave in the expansive regions where there is a layer of water beneath the seasonal ice. This noise, the flexural ice wave, efficiently propagates from the sources to the receivers and severely masks the weaker reflections arriving simultaneously.
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Multiple attenuation using buried vertical hydrophone arrays
Authors N. Moldoveanu and M. ChangTypical near surface geology in a transition zone area consists of a water layer, a section of muddy deposits and underlying layers of more consolidated sediments (Fig. 1). Depending upon the water depth and the water bottom reflectivity, multiply reflected and refracted energy can be trapped in the water layer and degrade the seismic data quality. This trapped energy is traditionally dealt with in the data processing, which, depending upon the severity of the problems, may or may not be sufficient.
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Application of linearized inversion on field data
Authors F. Coppens, A. Bourgeois, D. Mace and V. RichardThere has been a considerable amount of work to solve the linearized inversion of seismic data (Ikelle et al.,1988, Beydoun et Mendes, 1989, Lambare et al., 1992, Jin et al., 1992). We have proposed a 2D target-oriented prestack linearized inversion techniques using finite differences (Mace et al., 1989, Bourgeois et al., 1993). It considers that the actual medium is formed by small perturbations from an input plane stratified (1D) reference model, such that the Born approximation holds.
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Linearized elastic inversion in the k-w domain
Authors G. Bernasconi, G. Drufuca and F. RoccaWe examine the implementation aspects of the elastic linearized inversion in the k-w domain introduced by Rocca et. al (1993) and De Nicolao et al. (1993). The method is based upon the extension to the elastic case of the diffraction tomography technique of Wu and Toksoz (1987) and the singular value decomposition of the transfer function between data and parameters. The conceptual steps are: - Linearization of the forward problem through the Born approximation and linearized scattering of point diffractors (Wu and Aki, 1985). - Analysis of the response of the medium in the k-w domain (decomposition in sinusoidal perturbations of a uniform background). This leads to a diagonal relation between data and parameters as the incident and reflected waves are linked to the medium wave number by the Bragg resonance condition. - Inversion and analysis of the information of the reflections by means of singular value decomposition technique.
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Bayesian inference, Gibbs sampler and uncertainty estimation in nonlinear geophysical inversion
Authors M. K. Sen and P. L. StoffaThe Bayes or the Tarantola-Vallette formulation of the geophysical inverse problem describes the solution of the inverse problem as the a posteriori probability density (PPD) function in model space. Since the complete description of the PPD is impossible in the highly multidimensional model space of geophysical applications, several measures such as the highest posterior density regions, marginal PPD and several orders of moments are often used to describe the solutions. Calculation of such quantities requires evaluation of multi-dimensional integrale. A faster alternative to enumeration and blind Monte-Carlo integration is importance sampling which may be useful in several applications. Importance sampling can be carried out most efficiently by a Gibbs sampler (Geman and Geman, 1984). We introduce here a new method called the parallel Gibbs sampler (PGS) based on genetic algorithms and show that the results from the two samplers are nearly identical.
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Comparison of steepest descent and conjugate gradient algorithms in 2D finite-difference migration/inversion
By E. CausseElastic inversion of seismic data by modeling and data fitting with finite-difference modeling is attractive, as no restriction is put on the model geometry, and because information is naturally gained from all types of waves propagating in the subsurface model to be recovered.
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Genetic algorithms for inversion of single channel seismic data
More LessMany inverse seismic problems are non-linear problems and involve some kind of optimization. Often least squares or conjugate gradient methods, so called local techniques, are used to solve such an inverse problem. A disadvantage of these local techniques is that a relative good starting model, close to the global optimum, is needed to get a good solution. An other approach is to use non-linear inversion techniques like Genetic Algorithms (GA), Monte Carlo, Simulated Annealing or use iterative linear techniques with some 'build-in' randomness (Verbeek). These methods do not require a 'good' starting model. The randomness in these methods makel sure that the whole model space is sampled for good solutions. The advantage of GA is that it combines local and global search mechanisme.
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Velocity determination near a bottom simulating reflector using post-stack generalized linear inversion
By J. LeinbachBottom simulating reflectors (BSRs) occur within oceanic sediments of the shelf, slope and rise systems of the worlds oceans. These reflectors mimic the water bottom reflection, and are thought to represent the phase boundary between solid gas hydrate above and methane below. Research into these gas deposits has been motivated by their potential as an energy resource. Because BSRs are pressure and temperature dependent, they are found in water depths greater than about 500 meters. These reflectors occur in most marine environments. However, due to gas expulsion associated with sediment compression, BSRs commonly occur near convergent margins. They represent isotherms and have been used to determine heat flow (Yamano and Uyeda, 1982).
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Migration and multi-parameter inversion in transversely isotropic elastic media
More LessTransverse isotropy is a common form of anisotropy encountered in many physical applications. It can be intrinsic to a material or rock type, occurring due to thin layering or also by alignment of pores and fractures due to the stress conditions affecting a medium. When such a medium is encountered, conventional isotropic inversion methods must be abandoned for anisotropic imaging and multi-parameter inversion methods. This requires development of new imaging algorithms and gaining an understanding as to the nature of the inverse problem.
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Confidence measures for AVO inversion
Authors W. Beydoun, S. Jin and C. HanitzschAmplitude Versus Offset (AVO) inversion attempts to estimate multiparameter elastic information from prestack P-P reflection data (e.g., P- and S-wave impedance relative changes) prior to petrophysical/lithological characterization. Much more information can be conveyed regarding these parameters by specifying confidence measures in addition to parameter estimates. The term confidence is purposefully used to reflect our inability to provide "true" uncertainties. However, the intent is to indicate in some relative sense when or where criteria measuring processing quality are violated. This can be achieved in three ways: (1) the reflectivity versus angle domain, (2) the depth image (parameter) domain, and (3) the data (seismogram) domain. The emphasis is to demonstrate the usefulness of confidence measures by using consistently the three domains.
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From seismic surface measurements to pseudo VSP data
Authors R. Ala‘i and C. P. A. WapenaarVertical seismic profile (VSP) has been accepted as an important tool for the calibration of seismic surface data processing and for the improvement of geological interpretation in the direct vicinity of the borehole . In this paper we describe a new technique for the transformation of surface data into VSP data, being referred to as Pseudo VSP (containing the same information but presented in a different format) . The nucleus of pseudo VSP generation is downward extrapolation of a. wave field from the surface into the subsurface . The pseudo VSP will improve the integration of surface data with real VSP. The generation of pseudo VSPs along a line , where well information is not available, will allow us to extend the geological knowledge in all lateral directions . The pseudo VSP generation technique requires high quality shot records, a description of the source properties and a macro model of the subsurface . For extensive discussion of different extrapolation operators we refer to Berkhout (1982) and Wapenaar and Berkhout (1989) .
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Source and pseudo-logs estimation from VSP inversion
Authors J. L. Petit, V. Richard and M. CuerThe well seismic profiling has been widely used for a long time, first contributing to the definition of a velocity depth model, second producing a reference trace for accurate stratigraphic calibration of surface seismic events. For zero-offset vertical seismic profiles (VSP), a processing technique by inversion of the acoustic wave equation has been proposed by Lefebvre (1985), Macé and Lailly (1986). This VSP inversion method carries out in one step what would have required three steps in standard processing (up - and downgoing wavefield separation, deconvolution and corridor stacking). It has been widely used for estimate an acoustic impedance pseudo-log in travel time similar to the one obtain from sonic and density logs (Blanco and Canadas, 1992). However, the method deals with the simplest problem which relies on the one dimensional propagation model considering plane compressional waves travelling at notmal incidence. This technique has some limitations: 3D geometrical spreading is not taken into account, VSP in deviated wells and offset VSP cannot be inverted. In this paper, we propose a new approach to overcome these limitations: source and pseudo-logs estimation from VSP inversion in cylindrical coordinates.
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AVO Calibration using borehole data
Authors P. Armstrong, B. Chmela and S. LeanyAVO analysis of surface seismic data has become a widely used technique in recent years. Its success has varied greatly and the use of log data to model anticipated AVO anomalies bas not always succeeded in explaining the AVO response observed on CMP gathers. The reasons for this are many and include reflectivity mismatches between the surface seismic and log data, tuning effects, geometric effects, processing-related issues and anisotropy. Without an independent measure of the seismic AVO response and the ability to include both anisotropy and tuning effects in the forward modelling, a satisfactory explanation of the origins of the surface seismic AVO response is very difficult to achieve. This paper addresses both of these requirements, investigating the use of a Walkaway VSP to measure both AVO and anisotropy and employing anisotropic plane-wave modelling which retains the full detail of the log information in the reservoir. These techniques have great potential for improving the interpretation and calibration of AVO responses at well locations.
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An example of walkaway processing based on spectral matrix filtering
Authors J. L. Mari, F. Huguet and J. LaurentTwo walkaway seismic profiles were recorded in the vicinity of a vertical well. The first profile, oriented north-south, was composed of 103 offset VSPs. The maximum source-offset was 800 m. The average distance between two source positions was 10 m. The second profile, oriented east-west, was composed of 60 offset VSPs with a maximum offset of 500 m. The source was a vibroseismic source. The receivers in the vertical well were 15 permanent vertical sensors, clamped in the 540-680 m depth-interval. The distance between two permanent sensors was 10 m.
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Wavefield decomposition on offset VSP data using parametric separation
Authors P. Gavin, J. Marc and A. EssabarSeparation of Upgoing and Downgoing waves is an essential and important process in study of Vertical Seismic Profile. Commonly used two dimensional transform techniques like FK or t-p, model implicitly input data as the superposition of a large number of plane waves. The idea of modelling array waveform data as the small number of direction-dependant wavefied has been developed by Seeman and al [1] to separate Upgoing from Downgoing waves. In 1990, Esmersoy [2] proposed a multicomponent separation (including optimization for non linear parameters) by introducing parametric approach to model Downgoing P waves and SV wavefield. In this way, we use this techniques and we propose to include a measure of the quality of the wave separation at each depth and each frequencies by studying the condition number.
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Reducing smiles in a wavefield propagator VSP migration
Authors M. Williams and G. M. JacksonThe migration of well seismic data (VSPs and Walkaways) is typically dominated by large ambiguity arcs or smiles. These are much more dominant than on surface data migrations because of the narrow zones of illumination and variable fold of coverage within these zones. Normal practice is to limit the aperture of the migration around an expected dip (e.g. v.d. Poel and Cassels, 1989). This is easily done in a Kirchhoff or GRT type migration (integral solutions) in which the summation of data samples is limited to selected traces. The calculation of the Green functions is typically ray-based. When the overburden geology becomes complex, Green's functions become multivalued and ray methods become less stable. This makes diffraction stack (integral) migration methods less attractive and wavefield propagators a more viable option. Unfortunately the flexible implementation of aperture limitation to reduce smiles is more difficult. The propagation direction of energy at the depth migration bin is not explicitly contained in the formulation. Here we present a modified version of the split-step Fourier shot gather migration of Lee et. al. (1991) which allows the selection of particular propagation directions at the level of interest. Forward modelling using rays can be used to deduce the necessary propagation directions for the selection of reflectors of the expected dip.
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A Neural network picker for VSP data
Authors H. Dai and C. MacBethArtificial neural networks (ANN) are presented as a way of automatically picking shear-wave arrival onset times in VSP data. Picking is achieved by utilizing the relative vector modulus of two horizontal component recordings as the neural network input. A discriminant function, F(t) , determined by the output of the trained ANN, is then employed to pick the arrival onset. The results demonstrate this neural network architecture is successful after training by a few selected datasets.
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Zero moveout (ZMO) stacking
Authors P. Bunchen and A. KraganskyThis paper describes a simple stacking procedure based on summing the traces of a common shot or common mid-point gather, without the usual relative time shifts. We refer to this procedure as zero moveout or ZMO stacking. The stacking procedure may include an offset dependent weighting function, which is determined by the dimensionality of the data. It is shown that for the correct choice of weighting function, the resultaat stacks are exact zero-offset one dimensional seismograms for horizontally layered media. We explore the practical implementation of ZMO stacks towards source estimation. The principle can also be used for inversion.
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Multi-dip radon stack as an alternative to conventional stacking
Authors M. M. N. Kabir and D. J. VerschuurConventional velocity stack is performed after a hyperbolic moveout correction and straight summation of the data (CMP gathers). This process is equivalent to summing the data along hyperbolic paths. Although the Radon transform (RT) has never been designed to perform such operation as stacking, it is very interesting to note the basic definition of the forward generalized Radon transform (GRT) which is obtained by summing the original data along a path. If this path happens to be a hyperbolic one, then in principle, the forward GRT domain would indeed be equivalent to a velocity stack domain. And picking suitably in this transform domain should enable us to obtain an equivalent of a velocity stack. This is the idea that lays the foundation of the proposed method.
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From well logs to surface seimics using an equivalent medium
More LessMeasurements carried out at different stales may not agree in their observations. An equivalent medium approach can link measurements carried out at different scales. I apply high frequency (Dix) and low frequency (Schoenberg&Muir) averages to a well log and compare the results to a conventional velocity analysis of surface seismic data.
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Velocity analysis incorporating 3D pre-stack time migration
Authors R. G. Williams and N. J. CooperConventional industry processing of seismic data includes an approximate NMO and DMO prior to velocity analysis. The NMO and DMO imaging steps are approximate only in the sense that an approximate (simple) velocity field is used; the algorithms themselves are accurate. After velocity analysis, NMO is re-applied with the more detailed velocity field derived from the analyses. In the constant velocity case, DMO is almost independent of velocity and the iterative procedure described above plus post stack migration yields an accurate image.
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Velocity analysis as shape parameter estimation of wideband events
Authors U. Spagnolini and F. RoccaGeophysical processing requires the evaluation of the kinematical parameters of reflected wavelets (wideband events). Reflection tomography requires the estimation of traveltime and ray parameters; velocity analysis estimates the curvature (Shape Parameter: SP) from reflected or migrated wavefronts. In any case, SP estimation entails die definition of a reliable coherence measurement of wideband events along a pre-assigned time-space window. Better algorithms allow a simplification of the SP picking and reduces the spurious peaks due to incoherent and interfering noise. This paper introduces and compares the joint use of matched filter and Karhunen-Loeve (K-L) decomposition for high resolution SP estimation of wideband events.
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The application of stacking and migration in refraction seismology
Authors E. Bruckl and F. KohlbeckFor decades the conceptions of stacking und migration have been welt established processes in reflection seismology. These processes utilize the information contained in the total wavefield. However, in contrast to this evolution, most modern developments in refraction seismology restrict themselves to the evaluation of traveltimes. Although the way of the implementation of the complete refracted wavefield is indicated by the wavefron- or ray-methods, applications still remain the exception. Based on these classical approaches and some more recent research work (Hill 1987, Bruckl 1987, Bruckl 1991) a method for processing and interpretation of refracted wavefields inciuding stacking und migration has been developed. These new wavefield techniques offer to the user a range of facilities which comprise optimization of the interpretation model, quality control, improved accuracy, and resolution.
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Determination of shallow velocity-depth model from seismic refraction data by coherency inversion
Authors E. Landa, S. Keydar and A. KravtsovThe reflection survey is the dominant method of seismic prospecting. However, reflection waves clone are not always effective for studying the upper part of the subsurface. The strong surface waves and non-ray effects do not give stable and reliable results in the upper part of the reflection seismic section. Unlike reflections, refraction waves can be observed outside the zone of interference with surface and reflection waves. Seismic refraction waves have different applications in seismic prospecting.
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Evaluation of sea bottom features from seismic calibration data - A case history
Authors D. Monk, R. Flud and M. PlumleeDuring 1992 a seismic survey was conducted in the West Delta area of the gulf of Mexico. The water depth in the area of the survey varied from approximately 20 feet to over 200 feet, and for this reason, seismic acquisition using a bottom cable technique was selected. Additionally the use of hydrophones and geophone was chosen, because of the potential to minimize ghost and reverberation effects associated with the variable water layer. It was known prior to the commencement of the survey, that in some areas, the "hard" water bottom was overlain by soft mud flows with remarkable physical properties including velocity in some cases of less than 1000 feet/sec. (ref. Meeder et.al. (1988)). It was anticipated that the mud flows may present significant problems in both acquisition and processing of the seismic data, and an attempt was made prior to the seismic survey, to map the sea floor features as part of a hazard survey.
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Ray-mapped focusing - A migration velocity analysis for Kirchoff pre-stack depth imaging
Authors J. P. Jeannot and I. BerrangerFocusing analysis was first described for time migration by Yilmaz and Chambers (1984) and adapted to depth migration by Jeannot and al. (1986). Although the focusing equations were established for a constant velocity medium and a flat, horizontal reflector, focusing analysis has been used extensively for velocity model-building, even in the presence of severe lateral velocity variations. One of the main flaws of conventional focusing analysis is its sensitivity to dip. Audebert (1993) proposed residual zero-offset migration of the focused energy so that the same CRP will be imaged as downward continuation proceeds. We show how this concept can be implemented in Kirchhoff migration.
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Kirchoff migration/inversion in thinly layered media
Authors M. T. Widmaier, S. A. Shapiro and P. HubralAmplitude-preserving prestack Kirchhoff migration (Schleicher et al., 1993) is a well discussed tool to image reflector positions and to analyse offset-dependent reflectivity. Additionaly to the known weighting function of Kirchhoff-summation, we introduce a new correction to take into account the influence of thinly layered media on wave propagation.
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Anti-aliased Kirchoff migration
Authors D. E. Lumley, J. E. Claerbout and D. BevcDiscrete space-time aliasing of Kirchhoff migration images arises when the migration operator summation trajectory is too steep for a given seismic trace spacing and frequency content. This operator aliasing is independent of data aliasing, and can seriously degrade the focusing quality of subsurface reflectivity images, especially in the presente of large reflector dip and sparse or irregular 3-D acquisition geometries. We have developed a method based on local time- and dip-variant temporal filtering to suppress Kirchhoff migration operator aliasing. In a numerically efficient implementation, we apply local N-point anti-aliasing triangle filters as three-point filter operations after a step each of causal and anticausal temporal trace integration. Our approach is very conservative in its memory requirements, as demonstrated by a 3-D implementation on a massively parallel Connection Machine CM-5. We compare our new anti-aliased Kirchhoff migration to a conventional aperture-weighted Kirchhoff migration, in applications to both 2-D and 3-D marine seismic data. The results indicate that our anti-aliasing method enhances the resolution of steep salt-sediment interfaces and faults, and suppresses false reflections caused by conventional Kirchhoff migration operator-aliasing artifacts.
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Finite difference depth migration with micro spatial filters
More LessFinite difference operators that are accurate and very spatially narrow may be constructed by optimizing the implicit 45° finite difference operator, analytically computing the required inverse operator for an inhomogeneous medium, and then truncating the resulting operator in a way which preserves accuracy and stability.
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A Unified approach to seismic reflection imaging
Authors M. Tygel, P. Hubral and J. SchleicherGiven a seismic record of either displacement or pressure seismograms and assuming a laterally inhomogeneous, isotropic three-dimensional (3-D) macro-velocity model, we provide a complete theory of seismic reflection imaging. It consists of (a) a Kirchhoff-type weighted diffraction stack to transform (migrate) the seismic data from the record space (i.e., the time-distance domain) into the image space (i.e.. the depth-distance domain) and (b) a weighted isochrone stack to transform (demigrate) the migrated seismic inlage back into the record space.
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Imaging complex structures using band-limited Green's functions
By D. NicholsMany geophysical processes can be formulated in terms of the Green's functions of the wave equation. A process such as pre-stack Kirchhoff migration requires the estimation of the Green's function for every surface (source or receiver) location. The cost of calculating and storing the full Green's function is often prohibitive. Calculating one Green's function involves solving the full two-way wave equation for an impulsive source at each surface location. The storage required for the full Green's function is very large, the Green's function has an extra dimension, depth, compared to surface seismic data.
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Pre-stack depth migration with compensation for absorption
Authors R. Sollie, R. Mittet and K. HokstadThe propagation of seismic waves in real media is in many respects different from propagation in an ideal solid. The anelasticity of the medium will cause dissipation of seismic energy, thus decreasing the amplitude and modifying the frequency content of the propagating wavelet. We present a method for including absorption effects into prestack finite-difference migration schemes. We will outline the theoretical foundation for the method, describe the design of the extrapolation filter, and thereafter test the method on synthetic data. The aim of this work is to show that a prestack depth migration scheme that compensates for absorption is both feasible and gives improved images.
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Wave field extrapolation in the space-wave number domain
Authors F. J. Dessing and C. P. A. WapenaarThe forward model of seismic data that is used within our seismic research project DELPHI [1] is based on a number of operator multiplications.
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Sampo EM studies of Keivitsa nickel-copper deposit
Authors T. Jokinen, T. Valli and T. MatunenThe Geological Survey of Finland (GSF) has found and is exploring the Keivitsa' ore deposit in northern Finland, maybe the largest sulphide deposit ever found in Finland. This multimetal deposit contains nickel, copper, platinum group elements and gold.
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Underground tunnels detection and location by high resolution seismic reflection
Authors B. Piwakowski, J. C. Tricot and B. DelannoyThe seismic techniques are applied successfully for seismic imaging of geological structures having great dimensions compared to the wavelength (layers), but their application for detecting smaller objects as tunnels and cavities remains still limited. This paper presents the successful application of the high resolution seismic reflection for the detection and localisation of the remains of the old chalk-pits, appearing in the north of France and being now unexploited. Same positions of these voids which form a network of tunnels are mapped (Fig.1 .a), whereas others are unknown and represent a real danger for both population and building activities. General geological well logging data in this region is shown in, Fig. 1.b. The the tunnels occur typically at a depth of 8 - 15 m and are followed by a water table.
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Autonomous underwater vehicle - A new tool for ocean exploration
Authors B. Sichler, D. G. Masson, J. L. Michel and V. RenardFour main kinds of tools are used in the study of oceanic environments surface instruments, deep towed instruments at a' variety of depths (including ROVs), manned submersibles which are free from the surface influence, and bottom stations which are particularly useful for long-term monitoring. Autonomous Underwater Vehicles (AUVs) are relatively new to the marine scientific community: their independence from the surface and their unmanned nature gives them both advantages and disadvantages over more conventional tools. Because they are not linked to a ship their energy supply is restricted, but at depth, their operation is not disturbed by the sea state. Because they are unmanned, they need many automatic control systems, but all the features linked with the safety and comfort of observers may be safely omitted from their design . The speed of the vehicle is two to four times higher than that of conventional deep-tows, but one consequence is that the data collected must be largely processed after recovery. Many kinds of AUVs already exist. Generally they have been built for very specific tasks and their mass ranges from a few kilograms to several tons. A few of them have reached operational status in deep sea applications (e.g. Epaulard in France, AUSS in USA, MT88 in Russia). More recently, a small low cost AUV has been used for CTD measurements under the Arctic ice.
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High resolution seismic methods applied to subsidence evaluation and solution mine design
Authors J. A. C. Meekes, L. J. J. van Vliet and S. P. WijnSolution mined cavities in sedimentary deposits can cause serious subsidence problems. The occurence of subsidence, espcially sinkhole formation and ground collapse, is partly due to insufficient knowledge of the local geology and of the subsidence mechanisms. Susbsidence prediction improves safety and reduces environmental risk of existing solution mines.
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The use of geophysical techniques for the detection of - Partially collapsed - Mine shafts
Authors J. Brouwer and V. NijhofDuring the past centuries the Southernmost part of the Netherlands has been known for extensive marl recovery. Systems of galleries and mine shafts exist over - assumed - areas of several hundred km². At some locations mine galleries have recently collapsed causing severe damage to the roads and buildings in the surrounding urban areas. The reason for the sudden collapse of parts of the mining system was supposed to lie in leakage of drainage pipes, causing a decrease in carrying capacity of the marl. Due to the collapsed galleries parts of the system of mining shafts have become unreachable making it but impossible to determine the exact lateral exterit of the collapse features and the galleries from within the mining system and leaving only surface methods to locate the shafts and to determine the potential hazard for similar collapse features.
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Multichannel processing of very high resolution seismic
Authors Y. -H. de Roeck, B. Marsset, J. Meunier, M. Noble and R. GiraultThe installation of offshore structures requires a fine survey of the near subsurface to detect the nature of the ground and eventual obstacles. Seismic reflection methods are the most commonly used. But up to now at least, these methods were limited to single channel acquisition, sometimes digital (in the best cases) due to technology limitations. A fire cycle of 0.5 or 1 sec and a sampling rate of 5kHz could not allow multichannel recording. The advent of new technologies allows the use of the multichannel concept for superficial subsurface exploration and brings: (1) a significant improvement of signal to noise ratio, (2) real depth information of the interfaces using velocity analysis, (3) the possibility to acquire data in rougher sea conditions because of its multiplicity.
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Simultaneous inversion of group- and phase-velocity data in engineering seismics
Authors R. Misiek, L. Dresen and Ch. SchneiderIn engineering seismics it is important to get complete information about the lithostructural parameters of near surface layers. By a simultaneous inversion of the groupand phase-velocity dispersion data of Rayleigh surface waves it is possible to determine these parameters and, in contrast to refraction seismics, especially those of low velocity layers and the S-veloctities of all near surface layers. Using both standard P-wave refraction seismics and surface wave methods, only a few changes in the refraction field layout are needed to get high quality information about the lithostructural parameters of the near surface layers. A separate S-wave refraction survey is unnecessary.
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Highly reliable broadband three-component piezoseismometers for engineering and mining digital observation systems
By V. M. FremdDigital PC-based systems of seismic observations are now widely used. In these systems the signals from remote seismometers are transmitted to central point equipped with a universal PC for information recording, acquisition, processing and storage. Such third generation digital system provides necessary seismometric channels parameters, efficient data processing and good operation and financial characteristics. In the presente of accessible PC and telemetric devices the main problem is the optimal matching highly reliable seismometers.
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Sampo EM interpretation in 3D structures
Authors M. Oksama, T. Jokinen and H. SoininenSampo is a wide-band electromagnetic method. Transmitter is a vertical magnetic dipole and receiver measures three perpendicular magnetic components. Standard interpretation of Sampo results is based on a layered earth or thin plate model (Soininen and Jokinen, 1991 ; Dyck et al., 1981).
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Induced polarization effects in transient EM data for structural mapping
Authors B. A. James and A. ZerilliInduced Polarization (IP) effects in transient electromagnetic (TEM) measurements have not been extensively addressed by practitioners of TEM methods for structural mapping. This discussion focuses on illustrating IP effects pertinent to central loop sounding measurements. Examples of field data from several localities are given to illustrate these effects.
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A Rapid solution for electromagnetic scattering - The first step to 3D imaging
Authors P. W. Walker and R. W. GroomOur ability to understand and interpret the Earth's conductivity structure has been limited by our ability to model it. Modelling the electomagnetic response of complex structure can be computationally very intensive, and because of this, such structure is often represented by models that are too simpte to be realistic. Examples of this are common. One is in the helicopter electromagnetic (HEM) method, where the ground is assumed to be either a homogenous halfspace, or at best, horizontally stratified. Another example is in magnetotellurics, where the ground is often assumed to be either one or two dimensional, and the source excitation a plane wave.
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System design for acquisition of precise electromagnetic data
More LessThe advent of new, sophisticated inversion techniques for interpreting crosswell subsurface electromagnetic data brought with it stringent demands for improvements in the accuracy and fidelity of the measurements. In particular, it is now possible to form a tomographic image of the subsurface section bounded by two vertical boreholes if the acquired data is noire free over a large bandwidth. Theoretical considerations and the results of numerical experiments presented by Lee and Xie (1993) indicate that representative tomograms can be constructed using high accuracy, wideband, time domain electromagnetic data which is transformed to a wave domain prior to analysis. The image is constructed using a non linear ray tracing technique normally reserved for seismic velocity tomography. In the present case however, this technique can be used to obtain an image of the subsurface distribution of electrical conductivity.
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Controlled source EM tomography - An improved imaging technique for magnetic susceptibility and resistivity
Authors S. Sakashita, H. Shima and S. GasnierHigh resolution imaging technique has been required in environmental and engineering studies as well as in reservoir characterization and mineral exploration. Geotomography methods have been studied since 1980's to obtain higher resolution.
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Development of multi-component multi-frequency data acquisition system for borehole EM tomography
Authors S. Gasnier, H. Shima, S. Sakashita and A. AbdelhadiIn the framework of the geophysical investigations between two drilt holes, a new EM instrument has been developed in the 4Hz to 18KHz frequency range. This system consists of a downhole transmitter probe located in one drill hole and a three-component receiver probe located in the second drilt hole. A reference cable coming from the transmitt er probe enables to measure the in-phase and the out-of-phase parts of each three components with respect to the primary field (fig. 1).
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Automatic inversion of AEM data into multilayer parameters and its application to groundwater exploration in the Namib Desert
Authors K. P. Sengpiel, B. Fluche, R. Lenz, B. Rottger and W. VossAs a contribution to the Water Master Plan of Namibia, the Federal Institute for Geosciences and Natural Resources (BGR) conducted a helicopter-borne geophysical survey in an area of 5300 km2 in the Namib Desert in 1992. The survey area is bounded in the North by the ephemeral Kuiseb River and in the West by the Atlantic Ocean.
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