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53rd EAEG Meeting
- Conference date: 26 May 1991 - 30 May 1991
- Location: Florence, Italy
- ISBN: 978-90-73781-03-0
- Published: 28 May 1991
1 - 100 of 315 results
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Interpolation beyond aliasing by (t,x)-domain PEFs
Authors J. F. Claerbout and D. NicholsA great many processes are limited by the simple statement of the spatial Nyquist requirement-that no wavelength be shorter than twice the sampling interval on the data wavefield. This condition forces costly expenditures in 3-D reflection data acquisition. The sirnple statement of the Nyquist requirement oversimplifies reallife. At the 1989 SEG meeting, S. Spitz showed astonishing 2-D interpolations that seem to violate the Nyquist requirement and so they give us a deeper understanding of it. The real Nyquist limitation is on the spectral bandwidth, not on the maximum frequency, and Spitz' results can be explained by saying that the bandwidth need not be contiguous. We often characterize data as "good" or "noisy" when we really mean it contains "few" or "many" plane wave events. For noisy data there is no escaping the simple form of the Nyquist limitation but with good data it seems we can.
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A semblance-guided median filter
Authors E. E. Reiter and M. N. ToksözWe describe and implement a slowness selective median filter based on information from a local set of traces. The filter is constructed in two steps, the first being an estimation of a preferred slowness and the second the selection of a median or trimmed mean value to replace the original data point. A symmetric window of traces is selected about each trace to be filtered and the filter repeatedly applied to each time point. The preferred slowness is determined by scanning a range of linear moveouts within the user specified slowness passband. Semblance is computed for each trial slowness and the preferred slowness selected from the peak semblance value.
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3D Reconstruction of irregularly sampled wave fields
Authors N. A. Kinneging, A. J. W. Duijndam and L. OnkiehongA complication in 3D marine acquisition is that streamer feathering causes a crossline distribution of the actual midpoints. In conventional processing the individual traces are gathered in bin sorts and stacked after NMO correction. The stacked traces are posted at the centre of the bin, thus causing a posting error. This approach has some disadvantages. Depending on the feathering conditions some bin sorts will be overpopulated, while others will be underpopulated producing inconsistent stacking responses. The binstack smears the data over the bin area and thus puts a limit on the resolution. On the other hand this latter effect may be considered as a kind of spatial anti-alias filter.
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The impact of processing on the APF.VO responses of a marine seismic dataset
Authors A. Mazzotti and G. RavagnanIt is weIl known that the diagnostic potential of seismic data may be severely dependent on the acquisition and processing procedures. This is certainly true for any kind of interpretation approach or inversion algorithm but it becomes of the utmost importance when we deal with prestack seismic data. Namely, the Amplitude Vs. Offset (AVO) and the cornbined Amplitude, Phase and Frequency Vs. Offset (APF.VO) analyses may be very sensitive to the previous acquisition and processing. The main purpose of this paper is to show the impact that processing procedures have on the APF.VO responses of reflected signals pertaining to an offshore seismic line. We focus on two separate targets that give rise to bright reflections on the stack section: a cineritic layer and a gas-sand bed.
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Integration of trend and detail
Authors N. P. van Dijk, G. J. M. Lörtzer and A. J. BerkhoutLinearized inversion of amplitude versus offset (AVO) from multi-offset data consists of weighted stacking. It yields the elastic parameters as traces in terms of bandlimited relative contrasts. (Smith and Gidlow 1987; Lörtzer and Berkhout 1989). The weighted stacking results are displayed as seismie traces. These traces may be considered as spiky relative contrasts convolved with the seismic wavelet.
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The signal coherence as a versatile diagnostic tool to improve seismic data processing effectiveness
Authors B. Boiardi and M. CardamoneThe increasingly large amount of seismic data involved in 20 and 30 seismic data processing projects requires the personal skill of the analyst to be assisted in the choose of algorithms and parameters. At the same time the increasing complication of nowadays exploration themes wants the seismic method to provide highly reliable and uniform results.
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The wave equation stack
By R. SilvaIt is generally known that common midpoint (CMP) stacking and Kirchhoff migration are similar operations. This similarity arises from the fact that they both perform summation over hyperbolic trajectories defined by two-way travel times. Since migration can be considered as a shifting of energy with respect to the midpoint axis, stacking can be described in terms of shifting energy with respect to the offset axis.
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New perspectives in inverse VSP method by using explosives in the open hole
Authors P. C. Layotte, C. Naville and P. FroidevauxA borehole seismic experiment was carried out in the southeast part of France in the Balazuc-1 well belonging to the B.R.G.M.'s scientific project G.P.F. (Géologie Profonde de la France: Deep Geology of France). This experiment was supported by the ARTEP* association.
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The influence of acquisition system; coupling environment and sensor type on VSP image quality
Authors K. J. Dodds, T. Spencer, G. Ballantyne and R. BuntThe influence of the acquisition system type and its coupling environment on the quality of a VSP reflector signature is a source of conjecture prior to any VSP survey. We present a case history data set that involves variations of both acquisition system and environment including two different types of sensors and evaluate the differences evident in both the raw data and final processed image.
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Secondary shear waves generated from source boreholes
Authors J. A. Meredith, M. N. Toksöz and C. H. ChengIn cross weIl tomography experiments there are two types of sources commonly used. The most common are point sources, such as the airgun or dynamite, and axial sources. In general, the point sources generate tube waves which travel up and down the borehole and these tube waves contain more than 99% of the energy generated by the source (Winbow, 1989). AdditionaIly, point and radial sources also generate shear waves (Heelan, 1953, Lee and Balch, 1982). Axial sources do not generate tube waves (Winbow, 1989; Meredith, 1990) and therefore will not be discussed in this abstract.
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Acquisition and processing of VSP data obtained in horitzontal wells drilled from floating platforms
Authors S. A. Petersen and R. HeggernesWithin the last couple of years the use of horizontal wells for oil production purposes has increased. Clearly this new acquisition environment has an impact on the logging tools and deployment methods employed and on the post-survey dataprocessing. This is also the case for the Vertical Seismie Profiling (VSP) method. In the following the problems met in collecting and processing VSP data from floating platforms are discussed for an example from the central part of the North Sea.
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Multicomponent VSP radon transform for transversely isotropic media
By J. E. GaiserShear-wave particle motion directions measured in vertical seismie profiles (VSP) are very sensitive to anisotropic properties of the medium; however, in the case of compressional waves this is not true (Crampin 1981). Analyzing polarization diagrams (holograms) to measure these directions can be inaccurate because individual 3-component geophones are subject to interference from ambient noise and ot her coherent signals. An alternative is to investigate polarization characteristics along wavefronts using multicomponent Radon transforms.
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VSP Noise suppression through spectral equalisation
By P. B. DillonSpectral equalisation or spectral balancing is the suppression of seismic noise through power-spectral processing only, the phase spectra remaining unchanged. Tufekcic et al (1981) demonstrated a multichannel scheme in which the power spectra were spatially smoothed, over a large number of traces. Coppens and Mari (1984) showed that selectively weighting the power spectrum over the useful frequency band also gave good results. Both these schemes target the distortion introduced by surface-seismic ground roll. Ground roll is usually characterised by high-amplitude narrow-band spectral interference. That is, the noise is non-Gaussian. Since the character of VSP noise is very of ten non- Gaussian, it would seem that spectral equalisation should solve some of the noise problems experienced in the borehole environment.
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VSP and acoustic logging. Case history on the Cere La Ronde structure
Authors J. L. Mari and F. VerdierThe comparison of seismic data recorded at very different frequencies (a few hundred Hz for VSP and a few ten KHz for acoustic logging) and of dipmeter information can help for the interpretation of seismic anomalies. The VSP can have a lateral investigation of several ten or even hundred meters and a 1 to 10 meters vertical resolution. The acoustic logging used in reflected or diffracted modes has a vertical resolution of a few ten centimeters an 1 to 20 meters lateral investigation.
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3D kinematic inversion from a set of line profiles
More Less2-D interval velocity analysis is inaccurate in the case of tree-dimensional structures. This is demonstrated on the isotropic two-Iayered model where two reflecting interfaces have conflicted dip azimuths (Figure 1). We consider the profile Y=5km. The stacked section (Figure 2) is obtained from CMP gathers calculated by 3-D ray tracing. Interval velocity analysis along this profile is carried out by the 2-D coherency inversion (Landa et al.,1989). This analysis provides a high coherency (normalized semblance is about 0.8) for both reflection horizons on the CMP gathers.
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Velocity analysis with tomographic inversion on shot record migration
More LessPrestack time migration can still be a suitable alternative to image complex structures with moderate lateral velocity variations and it is less sensitive to the knowledge of the migration-velocity model than prestack depth migration.
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Advanced 3D macro-model estimation: The Sistre Project
By J. L. GuizouGetting a reliable depth image of the subsurface requires the identification of the main geological bodies beneath the surface of the earth, the estimation of the wave propagation velocities through these bodies, and an accurate positioning in depth of the interfaces which separate the geological layers. The exploration industry needs in the field of macro-model estimation have recently evolved from the conventional 2D framework towards the more attractive 3D framework. As a consequence, our geophysical industry must he prepared to offer reliable tools which make it possible to estimate structurally complex 3D geological bodies from seismic data. Several authors have reported methods to carry out three-dirnensional macro-model estimation using ray-tracing techniques and traveltime inversion techniques (Fawcett (1983), Chiu and Stewart (1987), and Guiziou and Haas (1988) ). However, these methods are limited to layer-cake-type subsurface models.
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A review of surface consistent imaging and DMO for irregular topography
Authors A. N. Ellis, R. A. Bale and P. W. KitchensideIn the processing of land data, static time corrections are used for improving velocity analyses, the enhancement of stacked sections and correcting for long-wavelength structural distortion due to irregular topography. Staties can be split into three categories, namely short-, medium- and long-wavelength, where medium wavelengths are of the order of one spread length. However static corrections are only valid when ray paths are verticaI near the surface. Therefore when the ray paths deviate from the verticaI, owing to steeply dipping reflectors or high velocities in the near surface (such as in permafrost environments), they must be replaced by more accurate wave-equation techniques.
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Impedance inversion using constant-offset reflection seismic data
Authors D. Wang, A. B. Weglein and T. J. UlrychAperture migration (aperture-compensated migration inversion) [1,2] inverts the zero-offset reflectivity from prestack reflection seismic data correctly. This method accommodates the incident angle of the wave, the reflector's dip and the actual receiver apert ure and is superior to conventional migration methods.
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Computation of true-amplitude reflections from offset data
Authors J. Schleicher, M. Tygel and P. HurbalCompressional primary seismic non-zero offset reflections as, e.g., common-midpoint or common-offset reflections are frequently used in seismic parameter estimation and imaging. Seismic migration procedures oper- . ating on such data are currently employed with increasing success. However, the dynamic aspects of wavefield migration methods still pose basic unsolved problems. One of the the questions is how the observed amplitudes can provide a measure of angle-dependent reflection coefficients.
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Estimation of elastic parameters from AVO effects in t-p domain
Authors J. Helgesen and M. LandrøThe objective of AVO techniques is to extract elastic layer parameters and lithological characteristics of subsurface layers from seismic measurements. Many of the works reported in this field has been of qualitative nature, e.g. using AVO techniques to distinguish between hydrocarbonrelated and lithology-related bright spots.
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A contribution on the study of the production mechanism of solfide mineralization self potential
Authors G. A. Skianis and T. PapadopoulosThe production mechanism of sulphide mineralisation self potential is still open to question. The majority of geophysicists support basically the dipole model. According to this model electrochemical reactions take pIace in the system of the mineralized body and the surrounding medium and as a resuIt an excess of negative and positive charges (of the same magnitude) are formed at the upper and lower part of the ore body, respectively. Thus an electrical field (geobattery) is formed the potential of which can be measured at ground surface. There have been proposed various models to describe the electrochemical reactions which take place in and around the dipole (Sato-Mooney 1960, Habashi 1966, Thornber 1975, Bolviken 1979, Sivenas-Beales 1982.).
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A New approach to data acquistion processing and control for noisy geoelectric voltage recordings
Authors C. Satriano, V. Cuomo, V. Lapenna, M. Machiatto, D. Patella and C. SerioIn this paper we are concerned with the problem of picking out the useful signal from noisy voltage recordings in geoelectrical prospecting. The voltage time series, recorded at the measuring probes of the electrodic array is made up of the sum of a deterministic component, the response of medium to the energizing current, and a random noise component.
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Measurements of D.C. small signals masked by noises
By L. AlfanoThe measurement of a man made signal S, masked by natural noises of mean strenght N, is common to many geophysical methods; but the Author will take into account only the case of the geoelectrical D.C. current surveys about which he reached a sufficient experience. The problem arose when deep sounding where carried out by means of Dipole-Dipole arrays and with long distances between the two dipoles,up to twenty kilometers.
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Geoelectrical survey of structures closed in depth with two-dipping discontinuities
Authors E. Cardarelli and M. BernabiniIn a previous study presented to Geophisycal Prospecting we considered the case of bidimensional structures in which a conductive medium is between two resistive seminfinite media. The two dipping discontinuity surfaces were considered to be parallel (or divergent) planes (open structures).
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Interpretation of airborne geophysical measurements in sedimentary environments by image processing
Authors A. Gulyas, M. Bodrogt, R. Csatho, G. Y. Büttner and G. CsillacDetailed airborne EM, magnetic and radiometric mapping was carried out in the Transdanubian Central Range (Hungary) over an area of more than 240 km² in the past 3 years. The geology of the area is rather complicated. The Transdanubian Central Range is built up mostly Mesozoic and Tertiary sedimentary rocks, mafic volcanic rocks are also present. As the investigated geological objects are quite small e.g. bauxite lenses with a diameter of a few 10 m - the line spacing of the flying was 50 m.
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Analysis of D.C. logging through metal casing
Authors C. Schenkel and H. F. MorrisonThe measurement of formation resistivity through metal casing can aid in characterizing existing reservoirs for effective recovery of oil and gas, as well as geothermal heat, without the cost and time of drilling new wells. Measuring the resistivity of adjacent fonnations may be useful to locate oil that was missed during the original logging prior to insertion of the casing. The method is also useful to monitor changes in resistivity caused by subsurface processes such as injection or leakage of contaminants from a waste site, flooding operations for enhanced oil recovery, or extraction processes of geothennal production. Several patents have recently been issued which describe methods and devices that are capable of measuring formation resistivity through casing, (Kaufman, 1989; Vail, 1989a, 1989b; Gard et al., 1989). Currently, it is only known that Vail has developed and tested such a device, called Through-Casing Resistivity TooI (TCRT). Kaufman (1990) investigated the behavior of the potential and its derivatives for a borehole with casing based on models of an infinite-length conductive pipe in a homogeneous medium. Schenkel and Morrison (1990) presented numerical results for a point souree of current in or beyond the end of a fixed length of casing in a homogeneous half-space. This study has now been extended using a surface integral equation approach can calculate the potentials for various electrode configurations in the presence of a finite-length casing and a layered medium.
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First application of lP method in Iran to boundary detection of deeply buried salty and sweet water aquifers
Authors E. Yousefi and J. KimiaghalamThe induced polarization (commonly known as lP) method is one of the most effective tools in mineral exploration. As it is known the induced polarization is based on the electrochemical phenomenon of overvoltage, that is on the establishment and deection of double layers of electrical charge at the interface between ionic and electronic conducting materials when an electrical current is caused to pass across the interface (H.O. Seigel).
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Source signature estimation using plane wave decomposition
Authors V. Shtivelman and D. LoewenthalRecently a new deterministic technique for source signature estimation in one-dimensional models of media was introduced (Shtivelman and Loewenthal, 1988; Loewenthal and Shtivelman, 1989). The technique was developed for a general case of multilayered models with arbitrary locations of the source and two closely spaced receivers (provided the source is above the receivers) and can he applied to both marine and land data. The technique is based on the application of two sequential extrapolation processes and does not require explicit separation of the wave field into up and downgoing components. Lately, the method has been extended to the two-dirnensional problem (Loewenthal and Shtivelman, 1990).
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Multiple elimination via inversion velocity analysis and shifted hyperbolas
Authors G. Canadas, S. Desaunay and E. de BazelaireThe inversion processes introduced in the conventional velocity analysis yield better velocity stack gathers. Improved resolution means more accurate picking and better quality of the stacked signal. However, two problems are encountered when using these processes for multiple attenuation: 1) the high cost of inversion and 2) the construction of stacked traces from the inverted velocity-stack gather. Solutions are brought about by using shifted hyperbolas (de Bazelaire, 1988) and by implementing non-conventional stack methods.
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Wave equation multiple suppression using constrained cross equalization
By D. J. MonkA method for improving the attenuation of water layer multiple energy is suggested. The improvement is achieved using wave equation extrapolation to generate an initial model of the multiple energy, and then constraining the way the time variant wavelet is modified to fit the observed multiple energy. Reconciling the initial multiple model wavelet with the input data is a critical part of this process, and several techniques have been suggested previously by other authors. The approach used here is to fit time, amplitude and phase of the wavelets, by adapting the model trace using a weighted sum of four traces which can each be derived from the initial multiple model trace.
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Multiple suppression by multi-channel predictive deconvolution
Authors M. T. Taner, R. F. O‘Doherty and E. BaysalSince its inception deconvolution has become one of the standard tools of seismic data processing. It is used to suppress reverberations, wavelet shaping and suppression of short and long period multiples, most of the instances, all at the same time. Failure of single channel predictive deconvolution of suppressing multiples longer than 200 millisecond period led to developments of alternate methods, most of which involve with the multichannel processes. Prediction across the channels in the radial direction (Taner, 1980) and using slant stack traces (Alam and Austin, 1981) have been successful for sirnpler water bottom geometries. Of the multi-channel processes for general multiple suppression the ones using the wave equation continuation to predict the multiple arrival times (Morley, 1982; Wiggins, 1985) had a physical basis, hence more promise for success. The rest were various forms of multi-channel filters.
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Multichannel attenuation of high amplitude peg legs multiples. Example from the North Sea
More LessAstrong impedance contrast, such as the base of the Cretaceous in the North Sea, generates a train of high amplitude water bottom peg legs which can heavily obscure the underlying less energetic reflections, often of great petroleum interest. Attenuation of these peg legs, a prerequisite for defining the structural image, is a persistent and difficult problem in processing the seismic data.
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Multiple attenuation by predictive deconvolution in the p-t domain
More LessMany multiple attenuation or suppression methods depend on discrimination of the moveouts of reflections in unstacked data. Most of these methods, such as NMO stacks and f-k dip filters, are not very effective when primary and multiple reflections have similar moveouts and stacking velocities. Predictive deconvolution in the p-tau domain does not require such velocity discrimination. Land reflection seismic data from North Africa contains many strong multiples with essentially the same moveout as weak underlying primaries. Predictive deconvolution successfully attenuated these multiples and improved the interpretation of the subsurface.
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Deconvolution by the double-streamer experiments
Authors A. Matthies, H. Huck and M. TygelIn exploration seismics the geophysicist has to face two fundamental problems during data processing, namely the recovery of the source pulse, and thereafter the solution of the inversion problem, i.e. the extraction of the reflectivity series of the medium under investigation. This research adresses to both problems in a specific measurement configuration. We demonstrate the estimation of an arbitrary (i.e. mixed-delay) source wavelet and of the medium reflectivity from point-source seismograms obtained from a vertical double-streamer experiment. We assume the medium to be acoustic and horizontally stratified bounded or not bounded by a free surface above and a homogeneous acoustic half space below.
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High resolution processing of reflection data
Authors A. L. R. Rosa, T. J. Ulrych and W. S. LeaneyThe widespread occurrence of subtle trap accumulations offshore Brazil has led to the need for the development of a high resolution processing scheme that helps the delineation of these features. To be successful, a comprehensive wavelet processing technique should take into consideration the spectral properties of both the seismic pulse and the reflectivity function. In this paper we present such a technique, and we apply it to the processing of reflection data from the Campos basin offshore Brazil.
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Tutorial interpretive evaluation of migrated data
By O. YilmazIn practice, decisions with regard to migration include choice of migration strategies (2-D/3-D, poststack/prestack, time/depth), migration algorithms (integral, finite-difference, frequency-wavenumber) , parameters for a given algorithm (aperture width, depth step, stretch factor), factors related to input data (profile length, spatial sampling, noise), and most importantly, migration velocities (the weak link between seismic and geologic sections). By using a large number of field data examples, the problem of conflicting dips with different stacking velocities and that of strong lateral velocity variations associated with complex overburden structures will be discussed. Emphasis will be made on uncertainties in migration velocities and interpretive evaluation of the results we obtain from migration.
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True-amplitude finite-offset migration
Authors P. Hurbal, M. Tygel and J. SchleicherSeismic reflection data provide traveltimes and amplitudes of seismic waves. Current processing and interpretation are, however, almost totally based on traveltime measurements. This is easy to understand as traveltimes possess the robustness and stability attributes required to the implementation of most seismic data manipulations. Amplitudes of primary reflection arrivals are strongly related to angular dependent refiection coefficients and, if properly processed, may be of great interpretational value.
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Time migration imaging for complex overburden
Authors M. O. Marcoux, A. Downie and R. WesterbergLarge lateral velocity gradient in the overburden causes structural imaging by conventionally applied time migration to fail. Depth migration can image such structures correctly but depends on a modelling procedure which is more costly than time methods and which brings interpretation into the processing sequence at a relatively early stage. In this presentation, an extension to prestack time migration is used to image the well known Marmousi synthetic to gauge the effectiveness of the extended technique relative to prestack depth migration results.
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Migration to zero-offset in variable velocity media
More LessMigration to Zero-Offset (MZO) or Prestack Partial Migration (PSPM) transforms prestack data into zero-offset data and is equivalent in a constant velocity medium to the normal moveout correction (NMO) followed by the dip moveout correction (DMO) applied as a single step process. For media with lateral velocity variation or with sharp increase of velocity in depth the migration to zero-offset process cannot be split in NMO followed by DMO, but must be applied as a single process.
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Prestack frequency-wavenumber (f-k) migration in a transversely isotropic medium
Authors A. Gonzalez and W. LynnPrestack frequency-wavenumber (f-k) migration (Stolt, 1978) is an efficient way to do prestack time migration. This technique is also useful for migration velocity estimation, where it is expected to yield dipindependent velocities (Li et al, 1991). In practice, however, we often find that imaging velocities do exhibit dip-dependency. This is especially noticeable with fault-plane reflections which usually image best at velocities higher than those of surrounding flat events. In a companion paper (Lynn et al, 1991), it is shown that the dip dependency of migration velocities can be explained by assuming that waves have propagated in a transversely isotropic earth. Because anisotropy affects both processing and interpretation of seismic data, it is important to identify its presence and measure its strength. In this paper we develop the necessary modifications to prestack f-k migration that compensate for such media. Moreover, we find that the transverse-isotropy adjustment parameter can be used to estimate, in an average sense, the amount of transverse isotropy.
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A unified formulation of wave field decomposition: theory and examples
Authors P. Herrmann, C. P. A. Wapenaar and A. J. BerkhoutUsing a recording by multicomponent sources and receivers, it is possible to decompose the vector wave field (velocities and stresses) into its scalar components (P and S waves) at the source and receiver side. The fully decomposed data set can constitute the starting point of a new procedure to process multicomponent data, as has been described by Berkhout and Wapenaar (1990).
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Downward extrapolation of decomposed P and S wave fields
Authors G. Haime and C. P. A. WapenaarThe use of amplitude information in pre-stack seismic data to delineate and characterize reservoirs has shown a considerable growth in recent years. Therefore, in redatuming or prestack migration techniques it is required that the used extrapolation operator contains all the necessary characteristics to treat both the phase and amplitude information in a correct manner, even in cases where the medium is strongly inhomogeneous and/or anisotropic.
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Finite-difference prestack reverse time migration using P-SV wave equation
Authors M. A. B. Botelho and P. L. StoffaPrestack Reverse Time Migration (RTM) is performed using a first-order system of hyperbolic elastic equations of motion and constitutive laws expressed in terms of particle velocities and stresses. It is well known that the RTM procedure must contain three important steps, i.e., it must apply (a) the imaging condition, (b) the reverse time extrapolation and (c) the extraction of amplitudes from the seismic data in order to build the final images. Here the above mentioned steps are implemented using the same code, which is a two-dimensional explicit finite-difference scheme. It is of second-order accuracy in time and fourth-order accuracy in space.
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Extraction of accurate P- and S-wave velocities from fixed offset well seismic profiles, UK Central North Sea
Authors J. M. Reilly and M. IdreesThe acquisition and processing of both compressional and shear wave energy from Well Seismic Profiles (WSP) is an important source of data for the investigations of seismic Amplitude Versus Offset (AVO) and anisotropy. In the marine environment we are normally dependant upon the mode conversion of conventional compressional sources (air- or watergun) for the production of shear wave energy.
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Focusing P-SV converted waves
Authors P. Y. Granger, R. Garotta and F. BaixasWith the present state of the art in data processing, converted P-SV wave sections do not show as comprehensive and detailed an image as that obtained from compressional waves. This observation alone may explain why there are objections to the use of converted waves. Results are clearly disappointing because of inaccuracies in CRP gathering and the lack of dip moveout.
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The derivation of shear velocities from log and VSP data
Authors W. L. Nutt and K. DoddsA knowledge of formation Shear Velocities is essential for many state of the art computations in Petrophysics and Geophysics. In particular mechanical properties of formations can only be accurately predicted if the corresponding formation shear velocity is equally accurately known. The interpretation and prediction of Amplitude versus Offset dependence of seismic reflection strength, AVO, is also heavily dependent on the knowledge of formation shear velocities.
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3D Multicomponent seismic reservoir characterization of South Casper Creek field, Natrona Co.
Authors R. D. Benson and T. L. DavisThe South Casper Creek field provides a unique opportunity to study a reservoir in an established oil field. A large amount of geologie and geophysical data has been collected which include weIl logs, core analyses, multicomponent vertical seismic profiles, and 3-D multicomponent surface seismic data. Interpretation of these data have contributed to an improved understanding of the reservoir. The continuing objective of this work is to develop 3-D multicomponent seismic methods to better characterize reservoir heterogeneities.
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Using mode converted shear data to map conglomerates - A case history
Authors J. D. Boyd and M. P. HarrisonThe Carrot Creek field, of Alberta, Canada produces oil from sandstones and conglomerates of the Cardium formation, which is of Upper Cretaceous (Turonian) age. A strong impedance contrast between the conglomerates and surrounding shales has made it a successful target for seismic exploration, as a purely stratigraphic play.
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Advances in airborne gamma-ray system design for geophysical and environmental applications
Authors J. R. Cox, J. Masek and J. BartosekThe purpose of this paper is to acquaint current users of Gamma- Ray equipment with the latest technology in this specialised field. Wherever possible the inforrnation is presented with a bias to Airborne applications, but of course the techniques discussed are directly applicable to Ground or Borehole applications. The presentation tries to balance an accurate explanation of these methods, with time allotted, so out of necessity, a somewhat simplified description has been used where possible.
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Radon emanometry over some kimberlites and lamproites in Southern and Western Botswana
Authors G. McDowall and H. KoketsoRadon decay product measurements have been widely used in uranium exploration, but until now this method has not been used in the exploration for kimberlites and lamproites.
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The application of transient electromagnetics to an integrated mineral exploration program
Authors N. J. Cooper and G. MaliotisFor the past 30 years, considerable geophysical interest has been focussed at mineral exploration of the Troodos Ophiolite, Cyprus. The local mining environment broadly comprises medium to small scale sulphide lenses, hosted at intermediate levels of the extrusive section. Mineral concentrations result from post-extension secondary phase alteration along ridge-parallel and transcurrent faults. Complex vertical and lateral inhomogeneities in the ophiolite section have complicated geophysical ground studies. Conventional ground exploration techniques give both inconclusive and inconsistent results when used to delineate structural features and target anomalies. Some previous success has been achieved with the I.P. method for detailed target evaluation, but this labour- and time-intensive approach is not suitable for broader scale exploration of the Ophiolite.
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On the application of geophysics in the exploration for copper and chrome ores in Albania
Authors A. Frasheri, L. Lubonia and P. AlikajThe exploration of copper and chrome ores in Albania is carried out through a wide complex of geophysical, geochemical and geological methods, which are applied in coordination to each other, according to the geological problems to be solved and the exploration phases.
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Geophysical investigations for the troll sub-sea pipe tunnel
Authors O. Kr. Fjeld, H. Deisz and B. AagaardThe Troll Pase I Project is a part of NS Norske Shell's development plans for the Troll Gas Field located 80 km west - northwest of Bergen, the Troll field is the Europes largest offshore gas field and in full production will be capable of providing 10 % of Europes total gas requirement. The wellstream will be produced on a minimally manned production platform and lead ashore for processing at a plant 40 km north of Bergen.
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Resistivity measurements on the sea bottom
Authors O. B. Lile, K. R. Backe, H. Elvebakk and J. E. BuanIn connection with the construction of a sub-sea tunnel under the 1050m wide and 35m deep Maursund strait, the Norwegian Public Roads Administration had the foilowing preinvestigations carried out: 1. Refraction seismies on the sea bottom. 2. A cored borehole from the shore 290m along the tunnel trace. 3. Acoustic tomography between the sea bottom and the borehole. 4. Resistivity measurements on the sea bottom. 5. Resistivity tomography between the sea bottom and the borehole. 6. Resistivity logging in sounding drillholes in front of the tunnel face. This paper reports the results from the resistivity measurements on the sea bottom and between the sea bottom and the diamond drillhole.
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Very high resolution 3D reflection seismic imaging of small scale structural deformations
Authors J. -P. Henriet, M. Verschuren and W. VersteegA small clay diapir under the river Scheldt in Antwerp has been used as a target for testing the possibilities of very high resolution 3D reflection seismic imaging in geotechnical practice. Results of this test may be relevant to a number of site survey cases in water-covered areas, e.g. where it is essential to elicit and analyse the spatial distribution of structural defonnations.
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Stratigraphic processing techniques for digital narrow-beam echosounding data from deep sea sediments
Authors M. Breitzke and V. SpieszContinuous high-resolution echosounding of deep sea sediments up to a maximum penetration of 200 m images sedimentation patterns and processes related to climatic and oceanographic changes. A new technical concept was applied to transmission and reception of high-frequency signals and requires arevision and adaption of classical stratigraphic processing techniques and interpretative approaches with respect to signal length and bandwidth. Selected methods are discussed in comparison to the frequency range and signal shape used in conventional (marine) seismics.
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3D Monte Carlo residual statics
By R. E. HaleStacking of common mid-point gathers assumes that reflectors corrected for normal move-out (NMO) and dip move-out (DMO) will align across the gather. If reflections do not align due to incorrect statics the stacked traces will be degraded. Static corrections for land seismic data calculated from measurements made in the field have limited accuracy so it is standard practice to pass all land data through a residual statics process before stack. The residual statics process estimates the difference between the field statics and the "true" statics required to align the reflectors. Wiggins, Larner and Wisecup (1976) described how the residual statics could be resolved into surface consistent components and this method is now called conventional residual statics. Ronen and Claerbout (1985) showed how surface consistent components could be calculated directly from stacked correlations and allowed larger residual statics to be estimated. Rothman (1985) extended this work by describing a method of estimating residual statics using simulated annealing (Monte Carlo residual statics). All three techniques were originally designed for 2-D seismic data. The conventional residual staties method generalised to 3-D seismic data with only minor modifications. The upgrade of the Monte Carlo method to accept 3-D data was more difficult and is the subject of this presentation. Real 2-D and 3-D data examples are shown which demonstrate the effectiveness of this technique.
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Attenuation of land multiple energy case studies from the Saudi Arabian Peninsula
Authors P. G. Kelamis and E. F. ChiburisThe use of Radon transforms for multiple suppression has recently gained popularity in the processing of seismic reflection data. The increased separation of events in the model space, obtained through the forward transform, is the cornerstone for multiple elimination. The application of the forward Radon transform effectively deconvolves the smearing and sharpens the image of the events. Thus, by inverse mapping the multiple energy and subtracting it from the original CMP gather. a primartes-only gather is obtained. In this paper, we first describe a simple, cost-effective methodology for multiple elimination via Radon transforms. Then, we show two distinct, case histories from the Saudi Arabian peninsula. Our processing scheme combines partlal stacking and the application of the forward/inverse Radon transform. Partial stacking just prior to the forward transform improves S/N and regularizes the geometry of the data. Thus, the Radon operators are computed only once, i.e., for the first gather. Finally, we examine the proposed Radon multiple elimination scheme in conjunction with AVO studies. We show that amplitudes and phases may be altered after Radon multiple elimination. Thus caution is advised in the application of Radon transforms for AVO studies.
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Migration of refraction events - A combination of travel time and wavefield techniques
By E. P. BrücklArbitrary lateral changes of thickness and velocity are allowed for. First order discontinuities are assumed as dominant features. Vertical velocity gradients if present should be positive and comparatively small. Topographic irregularity and layer thickness may be of comparable magnitude. The "plane layer and constant velocity"-model is considered inadequate.
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Cooling schedules for simulated annealing in the residual static estimation problem. Construction of master schedules
By E. NormarkVarying thicknesses and velocities of the surface layers in reflection seismic surveys on land induce different delays on the seismic recordings, which make static corrections necessary. If the a priori information in terms of the field statics is insufficient, residual static corrections are needed. Normally, residual statics are estimated by traveltime picking methods. An alternative technique is to maximize the stack-power function (Ronen et al., 1985). Originally, the solution was found by linearized optimization along the axis in the parameter space, defined by the source and receiver static parameters. However, if large residual statics are required the objective function, the stack-power, has more than one maximum. Therefore, using linearized optimization the solution might easily be trapped in a local maximum.
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Feed-back stacking and applicatoin for velocity analysis
Authors O. E. Næss and L. BrulandIn the present work a new algorithm, Feed-Back Stacking (FBS), to discriminate better between primary and other interfering events, is presented. The method has been developed especially in order to improve stacking velocity determination of difficult data. For velocity analysis a new version of Constant Velocity Stack (CVS) which incorporates the FBS method is proposed. with the new type of analysis a stepwise relative reduction or change of noise events will occur. The changes itselves are used as a criterium to discriminate between the different events. The new algorithm (FBS) shares the principle of a stepwise change of events with the Superstack method (Næss, 1979), but is otherwise different. Especially, the FBS is a linear method while Superstack is not.
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Model-based stack: A method for constructing the true zero-offset section
More LessUsually it is assumed that a stacked time section represents a zero-offset section. But in many areas with complex structures or streng lateral variations it is not valid. In this case a poststack depth migration gives a wrong image of the model. We propose an 'optimal' way to produce the 'true' zero offset section. We call this section the Model-Based Stack. This method consists in calculation the actual traveltime trajectories by ray tracing for each CMP and for each time sample according to the velocity-depth model obtained from the coherency inversion, and stacking the traces within CMP gather along these trajectories. Now a poststack depth migration applied to Model-Based Stack (MBS) produces a better image of the subsurface because MBS represents a zero-offset section even in cases of complex geology. We believe that the processing procedure including Model-Based Stack followed by poststack migration is a fast and reliable alternative to prestack migration.
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Improving the performance of F-X prediction filtering at low SNR
Authors P. E. Harris and R. E. WhiteAs commonly forrnulated, the method of random noise reduction by linear prediction in the f-x domain (Canales, 1984) suffers from three problems. These are (White and Sumrners, 1989): 1) The wavenumber response of the prediction filter does not peak exactly at the wavenumbers of the signal components. 2) The amplitude of the filter response is less than one at the signal component wavenurnbers, leading to attenuation of the signal. 3) Sidelobes in the filter response cause noise at wavenumbers weIl separated from the signal wavenumbers to leak through into the filtered data.
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Planning a marine 3D survey in front of Rotterdam harbour
Authors J. J. Nooteboom, J. A. Bandell and C. J. A. CorstenDuring 1989 the acquisition department of NAM was asked to prepare a 710 km2 marine 30 seismic program. to be recorded in 1990. This marine 30 program was part of a total of 3100 km2 which had to be recorded by GECO between April and October using the twin vessel technique which was developed during previous years.
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How to fight a monster
By F. KochMONSTER" is a village near Rotterdam, which gave its name very symtomatically to a NAM-survey area, in which a complicated 3D-survey was to be conducted.
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Enhanced servovalve technology for seismic vibrators
By D. ReustA criticaI component of a servohydraulic vibrator is the servovalve. The servovalve meters the flow of hydraulic oil into and out from the actuator, producing a force which is applied by a baseplate, to the earth's surface.
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The seismic signals generated by the bit and the drillstring in a horizontal well in the Larderello geothermal field
Authors J. -J. Chameau, F. Batini and G. OmnesThe Larderello geothermal reservoir is locally situated in Paleozoic phyllites and quartzites. Permeability is controlled by fractures. The reservoir is identified with a fractured zone usually not more than 150 meters thick and located at the top of the Paleozoic basement.
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Planning and field techniques for 3D land acquisition in highly tilled and populated areas. Today's results and furure trends
Authors L. Bertelli, B. Mascarin and L. SalvadorIn recent years, 3D Seismic Techniques have acquired an increasing importance in field appraisal and development. The results obtained, in terms of greater knowledge of the geological setting, more accuracy in field development and finally in the rate of success, have spurred oil companies to move quickly towards 3D Exploration Surveys, in spite of the considerable investments involved. Once again, the outcome of 3D surveys has confirmed that the rate of success in properly locating wells on the basis of 3D seismie data has increased dramatically, especially in areas characterized by a complex geological setting.
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A moving coil dynamic accelerometer
Authors A. Obuchi and T. FujinawaThis paper describes a new type of seismic sensor that enables flat frequency response from 3 Hz to 200 Hz with any angle of tilt.
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Safety in seismic - exploring the myths
More LessSafety has long suffered from being seen as necessary but a nuisance. Reduced production, unnecessary cost, and "insurance will pay" are all reasons why safety is of ten dropped down the list for discussion.
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Porosity and pore structure from acoustic well logging
Authors G. Tao and M. S. KingWyllie's time average equation and subsequent refinements have been used for over 20 years to estimate the porosity of reservoir rocks from compressional (P)-wave velocity (or its reciprocal, transit time) recorded on the sonic log. This model, while sirnple, can no longer be convincingly explained in theory or satisfactorily used in practice.
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Wave propagation through fluid saturated porous media, energy losses (permeability) incorporated
More LessIn the oil industry there is a need for obtaining physical properties of potential reservoir rock from in-situ measurements. Acoustic measurements can be executed as borehole refraction measurements in the petrophysical case and as crosswellshooting in borehole seismics. A quantitative determination of permeability from acoustic measurements cannot be made because a suitable model is not yet available. It is suggested that, by including the permeability as an energy loss term in the mathematical model, this determination might become possible.
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Elastic anisotropy due to aligned cracks in porous rock
By L. ThomsenThe theory of the effects of cracks upon acoustic velocity in rock is founded upon the classic paper of Eshelby (1957). The effects of a dilute set of aligned ellipsoidal cracks (cf e.g., Hudson, 1980) are well established, although finite concentrations or crack-shape distributions are problematic (Thomsen, 1986b). Despite a long history, there has been essentially no experimental confirmation of this theory. The difficulty has been in fabricating samples containing cracks of controlled number, location, orientation, shape, and size. The new technique discussed by Rathore, et al. (1991) appears to have solved this problem, at least for cracks in porous rock.
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Effect of fluid micro-distribution on velocity and attenuation in porous rock
Authors A. L. Frisollo and L. ThomsenWhen two phases of fluid are present within a porous rock, there is an a priori possibility that the distribution (on the intergranular scale) of the fluid phases may affect the elastic and attenuative properties of the rock. If this were true, it could have important implications for the interpretation of seismie veloeities, especially since the microscopic distribution (as weIl as the macroscopic distribution) may vary during production of a hydrocarbon reservoir, and since the production process may be monitored remotely by seismic means.
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Critical porosity, elastic bounds, and seismic velocities in rocks
By A. NurData for cornpressional Vp and shear Vs velocities versus porosity in sands show that none of the existing models are adequate predictors of velocities over the entire range of porosity: The Voigt and Reuss average are too far apart and too far from the data. Although self-consistent approximations for spherical pores and especially needles come closer to the data, they rail to account Ior the critical porosity Фc = 38% below which grains are load-bearing and above which grains are in suspension.
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Seismic wave propagation in uncracked and cracked carrara marble
Authors S. Peacock, C. McCann and J. SothcottWe have used a simple rock, Carrara marbIe, to test theories of seismic wave propagation through media containing suites of small cracks. The theories (Hudson 1980, 1981, 1986) predict the effects of the shape, alignment, filling and number density of the cracks on seismic velocity, attenuation and anisotropy. They are used in specialist seismic interpretation to evaluate cracks and crack alignment for determining the potential of oil and geothermal reservoirs and indicating stress regimes.
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Seismic attenuation and permeability in clay rich sandstones
Authors A. I. Best, J. Sothcott and C. McCannThe attenuation and velocities of compressional and shear waves are important parameters for predicting the nature of porous rocks. Intrinsic attenuation gives us information on the nature of the pore fluid, whilst seismic velocities teIl us about the rock matrix. Surprisingly, little is known of the relationships between the seismic attenuation of sedimentary rocks and their petrophysical and geotechnical properties such as porosity, permeability and clay content. Part of the work of the Geophysics Research Group at P.R.I.S. is to study these relationships by making small scale laboratory measurements under simulated reservoir pressures.
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Joint inversion of seismic and geoelectric data recorded in an underground coal mine
Authors L. Dresen, M. Dobroka, A. Gyulai, T. Ormos and J. CsokasUntil now, the layer sequence "rock-coal-rock" and faults in coal seams in underground hard coal mines have been detected by seismic waves and geoelectric measurements. To determine the geometrical and petrophysical parameters of bedrock and coal, the data recorded using seismic and geoelectric methods have been inverted independently. In consequence, the inversion of partly inaccurate data necessarily resulted in a certain degree of ambiguity.
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Shallow fault location in coal measures using offset wenner resistivity profiling
Authors B. A. Hobbs and A. M. ReadingA region within the Westphalian Lower Coal Measures of the Midland Valley of Scotland is currently under consideration as a site for open-cast mining. Mining will only be viable to one side of a fault running through the region and drilling to determine the exact position of the fault is not possible. A technique employtng nondestructlve surface measurements will therefore have to be used for precise fault location. The coal measures are overlain by 10-20m of boulder clay above which lies peat which has been worked commercially.
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Hole-to-surface seismic reflection surveys for opencast coal exploration
Authors E. Kragh and N. R. GoultyOpencast coal mines in such a densely populated country as the UK are mostly of small size by world standards, but they are highly profitable. Annual production of opencast coal is currently about 17 million tonnes, constituting some 20% of the UK coal industry's total output. Transport costs are low as the coal is produced for domestic consumption, and consequently opencast coal is a very important national resource.
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Self-potential survey for a preliminary geothermal research in a Southern Italy coastal area
Authors R. Quarto and D. SchiavoneSystematic self-potential exploration of large areas proved to be effective in geothermal research. In this field, SP measurements directly depend on the fluid flow through thermoelectric and electrokinetic phenomena. Thus, due to its low prospecting costs, a preliminary geophysical survey with the SP method can be particularly useful in new areas of geothermal interest.
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Crustal imaging of the Larderello geothermal field, Italy, from teleseismic P waveforms
Authors J. E. Foley, M. N. Toksöz and F. BatiniThe seismic velocity structure of the Larderello Geothermal Field of central Italy is investigated by inversion of seismic travel travel-time and waveform observations made on the 26-station seismic monitoring network operating in that area. We have divided the program of imaging the velocity structure of the Larderello area into two separate stages. The first stage utilizes abundant low frequency (0.5 to 3.0 Hz) teleseismic P-waveform data. Both teleseismic travel-time residual and waveform analysis techniques are used to produce a broad picture of the crust to depths of about 40 km. The second stage utilizes higher frequency (4.0 to 12 Hz) P and S wave travel-time observations from local earthquakes, and images the shallow and mid-crust to depths of about 15 km with higher resolution. The results of the first stage techniques are presented in this paper and the results of the second stage inversion in the following report (Block et al.).
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Crustal velocity structure of the Larderello geothermal field determined from local earthquake arrival time data
Authors L. Block, M. N. Toksöz and F. BatiniA joint hypocenter-velocity inversion of local earthquake arrival time data has been performed in the Larderello Geothermal Field in Tuscany, Italy. More than two thousand local earthquakes have been recorded in the Larderello area since 1977. Most of the earthquakes occur shallower than 8 km depth, although a few occur as deep as 30 km. The corner frequency of the data is typically 25 Hz. P wave and S wave arrival times from 269 of these earthquakes were used to simultaneously determine the hypocenter parameters and the three-dimensional P wave velocity and Vp/Vs structures. Due to a variety of rock types present in the uppermost 2 km, as well as a complex tectonic history, the shallow velocity structure is very complicated. This shallow structure was determined from seismic reflection and vertical seismic profiling (VSP) data and kept fixed during the joint hypocenter-velocity inversion.
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3D Surface seismic and well seismic applied to the delineation of geothermal reservoirs in metamorphic formations in the Larderello area
More LessThe delineation of deep (>3000m) reservoirs is a primary objective in the exploration of the LardereIlo and Mt Amiata (Italy) geothermal fields. Drilling wells at such depths in relatively hard rocks is expensive, therefore risk must be minimized through optimum application of surface exploration techniques. Since 1974 ENEL has performed extensive reflection seismic surveys. They made it possible to identify well defined groups of reflectors within a metamorphic basement composed of phyllites, micoschists and gneisses (Batini et at, 1978). An exploratory well, the San Pompeo 2 well, revealed the presence of a permeable zone filled with geothermal fluids at very high temperature (>400°C) and pressure (>240 bars) and corresponding to the main reflector or K marker. Subsequent deep drilling targeted at reflections with lateral continuity over 1-2 km above the K marker demonstrated the existence of productive zones associated with these reflections. Because these reflections are less continuous, their delineation is a major objective. A 3D seismic reflection survey combined with multi-offset well seismic profiles, well logs and measurements on core samples (ERL/MIT, 1989) was therefore planned as a pilot study. It was carried out at the Monteverdi test site where an extensive drilling programme was to take place.
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Inversion of combined data of the CRE nad CEE imaging
Authors A. Berkovitch, B. Chelchinsky, S. Keydar and V. ShtivelmanThe present paper is devoted to solution of an inverse problem - construction of 2D "true model" of a studied medium, using data obtained by the Common Reflecting (CRE) and Common Evolute Element (CEE) methods. The CRE and CEE methods are components of a novel system of processing and interpretation of seismic data, which is based on fundamental topological ideas, new different types of stack and homeomorphic images of studied media. The mentioned and other components of the homeomorphic imaging system were presented on Adelaide ASEG, Dallas and San Francisco SEG, Berlin EAEG, Istanbul IASPEl and Jerusalem Mathematical Geophysics Meeting and several journal papers.
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Depth focusing analysis: The effects of elevation corrections and structural DIP
More LessPrestack depth migration, combined with depth-focusing analysis (DFA), is a useful method for imaging and velocity estimation in areas of complex structure. However, such regions often possess significant topographic relief that leads to large elevation-static corrections prior to migration. The result is a distorted wavefield that yields erroneous depth-focusing information. Of further consequence are the limits imposed by the zero-dip approximation to the wave equation used for velocity estimation.
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Migration velocity computation using plane wave decomposed data
By M. T. TanerIn general, seismic data is acquired and processed as point source illuminated reflections. CDP or CMP processing is the central point of the processing sequence. Imaging is referred to as pre- or post- stack migrated data. Conventional CDP processing requires several velocity analyses, each for a different stage of the processing sequence to produce the final subsurface imaging. These velocities do not strictly relate to the subsurface physical parameters. However, the pre-stack shot domain imaging is the only procedure where the data satisfies the wave equation with no simplifying approximations and the velocities used for image construction are directly related to the subsurface physical characteristics.
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Migration velocity analysis by migration of velocity spectra
Authors C. J. Beasley and R. KlotzMigration of seismic data is required to collapse diffractions and position reflectors in the correct spatiallocation. Today's accurate migration algorithms applied to high-resolution seismic data can, in theory, produce accurate images of nearly vertical structures. However, in practice, migration accuracy depends not only on the accuracy of the migration algorithm, but also on the accuracy of the migration-velocity field. Indeed, as dip increases, accuracy in migration becomes critically dependent on the migration velocity.
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Velocity model development for prestack depth migration at different geological environments
Authors P. Farmer, S. Eidsvig and A. HeggernesIn the presence of lateral velocity variations depth migration rather than time migration should be used to obtain a proper image of the subsurface. If the velocity model is known and if the stack is a good estimate of zero-offset section, post-stack depth migration often gives good results. Normal common-midpoint stacking relays on the assumption of hyperbolic moveout curvature. Prestack depth migration, however, is not dependent on the validity of the common-midpoint assumption, and therefore it should be used in complex areas with strong lateral velocity variations to develope the velocity model and to image the structure properly.
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The hyperbolic radon transform
Authors E. de Bazelaire, S. Desaunay and J. R. ViallixReducing to a point the moveout curve of the reflections in common midpoint is the oldest problem facing seismic processing. This is currently achieved through a dynamic correction and multicoverage stacking. The process is costly, both in computing and preparation time, and of limited efficiency for multiple attenuation. So much so that, in recent years, new methods have been sought.
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Multiple suppression in the Barents Sea using the generalised radon transform as a notch velocity filter
Authors R. G. Williams, A. Haugen and N. CooperThe generalised Radon transform has been proposed as a means of multiple attenuation on CDP records. Initial results, especially on synthetic data, suggested an improvement in both velocity resolution and amplitude preservation compared to conventional FK techniques. The generalised Radon transform discriminates between primaries and multiples by measuring their curvature with respect to offset unlike the FK transform which discriminates on the basis of gradient. The use of curvature offers the possibility of separating primaries from multiples even at very short offsets. It is also possible to separate multiples with smaller velocity differences than is possible with FK methods. However, despite the initial optimism, generalised Radon transform techniques have failed to replace FK methods as the industry standard. This is partly because of the increased cost and partly because of technica1 prob1ems.
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A t-p inversion technique including amplitude
More LessTo improve the reliability of a velocity-depth function v(z), it is desirabie to use the amplitude information throughout the entire inversion process instead of interpretating it only in the x-t-domain ( x-distance, t-tirne ), which is of ten arnbigous because of crossing traveltime curves and shadow zones.
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Angular coverage compensation in transmission tomography
Authors G. Boehm, P. Carrion, F. Pettenati and A. VesnaverIn seismic tomography, the quality of reconstruction depends on different parameters such as angular coverage, signal-to-noise-ratio, local ray geometry, imposed constraints, etc. For the basic definitions and references on geotomography we send the reader to Nolet (1987) and Carrion (1987). In this study, we pursue the following goals: to give a detailed account on recoverability versus angular coverage, to study image blurring due stochastic characteristics of noise and to show the role of constraints in those cases when angular coverage is incomplete.
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Acoustic tomography for enhanced oil recovery and reservoir description
Authors J. H. Justice, A. A. Vassiliou, M. E. Mathisen, W. H. Troyer and P. S. CunninghamAs large commercial oil fields become increasingly difficult to find, attention has turned to the extensive reserves which still remain unproduced in our existing oil fields. Many of these fields are in decline and continued economic production depends on secondary (waterflood) or tertiary enhanced oil recovery (EOR) procedures. EOR programs are generally expensive and are often carried out in complex reservoir environments where even small improvements in efficiency may carry major economic benefits.
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Imaging reflectors in a crosswell seismic experiment
Authors M. Becquey, J. O. Bernet-Rollande, J. Raulent and G. NoualCrosshole seismic surveys can give improved resolution because the source and receivers can be lowered as close to the target as possible. CrossweIl tomography is now extensively used. But, to image reservoir layers, this technique needs wells drilled rather far below the reservoir, an uncornmon configuration for production wells. The crosshole reflection method does not entail such a limitation.
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High resolution cross-hole seismic tomography at the Backhorn test facility in Illinois, USA
Authors H. Saito, T. Takahashi, T. Imayoshi and S. SakashitaHigh resolution cross-hole seismic experiments were conducted at the Backhorn Test Facility in Illinois, U.S.A., where we drilled five 270 m boreholes over the oil producing field for the purpose of various borehole geophysical experiments. According to the exploration and production histories of the field, the oil reservoir is the porous zone of Silurian Kankakee "White Lime" carbonate which is horizontally distributed at the depth of about 200 m, and has no more than 10 m thickness. In fact, two boreholes out of five we drilled produce oil from the Kankakee formation. The results of several kinds of logging measurements conducted just after drilling five boreholes indicate a good correlation between formations from borehole to borehole. However, they also suggest significant lateral changes in porosity within the Kankakee formation.
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New developments in wave equation tomography
Authors Y. Luo and G. T. SchusterA new methodology is described which allows for wave equation inversion of skeletalized parts of data records. Skeletalized data is a reduced subset of the original raw data that is mainly influenced by only one type of model parameter; e.g. first arrival traveltimes skeletalized from seismograms are mainly influenced by the velocity (not density) parameters.
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Iterative geophysical diffraction tomography
Authors L. -J. Gelius and J. J. StamnesA new iterative diffraction-tomography method, tailored for geophysical applications, is described. The main problem in geophysics is the complicated background medium. Therefore, we have derived a reconstruction algorithm which take into account that the point-spread function is space variant. Using this algorithm, we compensate for the limited-view problem in an iterative manner.
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Traveltime tomography in anisotropic media: some examples
Authors R. G. Pratt, C. H. Chapman and W. J. McGaugheyTravel-time tomography is often applied to cross-hole seismic data and images of the velocity distribution are used to provide stratigraphic and/or structural interpretations. The technique is often applied in regions where one might expect significant anisotropy (for example in complex, layered stratigraphy or in fractured rocks). Without accounting for anisotropy, the tomographic images suffer from distortions that can render the results un-interpretable.
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