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- Conferences
54th EAEG Meeting
- Conference date: 01 Jun 1992 - 05 Jun 1992
- Location: Paris, France
- ISBN: 978-90-73781-04-7
- Published: 01 June 1992
1 - 100 of 405 results
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3D Seismic: A strategic tool in exploration and production
More LessSince the first 3D survey seventeen years ago, Companies of the Shell Group have operated nearly 250 such surveys outside North America and, as non-operating partners, have been involved in many others . Managing safety and paying proper attention to environmental impact is in increasing challenge.
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Comparison of seismic results after Dip and Strike acquisition
More LessModel studies have shown the advantage of acquiring seismic data parallel to the main tectonic directions or to lithological variations . For instance, for an imaging problem below a fault, if the acquisition is orthogonal to the fault plane, the seismic ray paths are crossing the fault and will be affected if the acquisition is parallel to the fault plane, all the-ray paths are in the same panel and wil] not be distorted . In the Jatter case, we therefore have better conditions for applying traditional processing tools and, more especially, for applying Normai (hyperbolic) Move Out .
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Seismic acquisition in the vicinity of a large compressor plant
Authors R. A. Brook, D. L. Bremner and R. P. DartThe use of 3-D seismic in the exploitation of mature fields is betoming increasingly common . However, in many cases noise generated by oil field equipment compromises data quality . This paper describes a prospect that was considered a no record area within a 3 km radius of a large compressor plant . A noise study shows that witti careful parameter design and new recording techniques useful data can be collected witpin 200 m of the plant .
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Design of 3D land acquisition in a structurally complex environment
Authors J. M. Mougenot, E. Robein, J. Ravat and A. MorashSince 1987, 3D seismics have been the main tool used by ELF AQUITAINE for exploration as well as for field development in the Pyrenees foothills (Aquitaine basin of South France) . The geological context made of very tectonized structures and the . rather poor signal/noise ratio at target depth (4500 m to 5500 m) have led to develop specific design tools for these 3D surveys .
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Efficient 3D surveys in a jungle environment
Authors M. F. Bee, H. Supiyanto, J. M. Bearden and E. F. HerkenhoffFor the last fourteen years , Caltex Pacific Indonesia (CPI) has been acquiring 3-D data over existing production fields in the jungle and/or swampy environment of Central Sumatra , Indonesia . The recent years have seen an increase in 3 -D activity in CPI which has forced an evolution towards cost reduction without jeopardizing data quality. CPI has successfully developed 3-D acquisition techniques which are operationally feasible and economically attractive . This achievement is the result of careful planning to optimize field parameters, the use of a gapped inline swath geometry, an increase in the number of channels available for recording , an increase in the rintis (Jungle cutting for acces) and drilling effort, surveying with GPS, and radio shooting . This paper wilt describe the above techniques and their advantages it will also demonstrate that the 3-D data acquired is of high quality with uniform fold , good offset and azimuth distributions, and economically reasonable for the difficult environment .
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Multidimensional seismic exploration and its first application results
Authors A. S. Kashik, V. KH. Kivelidi, D. P. Zemtsova and V. A. MilashinA further enhancement in the accuracy of the seismic explóration methods is associated witti the development of 4D and 5D technologies based on a rational integration of profile data witti borehole measurements for more detailed study of geologie features and for monitoring the wavefield parameters in the course of the reservoir development . Witti regard to onland operations, concurrent observat.ions in a number of Wells, with the shot point locations scattered over an area, will permit : a) depth of the subsurface survey to be enhanced by deeper placing both source and especially receiver locations, below the near-surface section which is narmally a severe filter for waves reflected from deeper interfaces ; b) velocity determination accuracy to be mach improved by processing data from individual Wells and by comparing the results in overlap zones ; c) time sections to be obtained from various depth levels ; d) reflection horizons to be correlated along profile lines and over an area
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Luna-Hera Lacinia 3D case history: solving complex structures in presence of heavy operative constraints by combining different acquisition techniques
Authors L. Bertelli, A. Nicora and L. SalvadorThe Luna - Hera Lacinia - 3D survey (Offhore Calabria, Southern Italy) covers an area of about 180 square Kms and represents an excellent example of the integration of deep, shallow marine and land 3D acquisition techniques . The survey has been acquired as a "full 3D" on an area characterised by a complex marine and land morfology with serious land operating constraints and complex subsurface structures at a commercially acceptable cost. The target of the survey was to define the structural setting of the miocene reservoirs of Luna and Hera Lacinia gas-fields in order to optimize their development stage. Considering the difficulties, the survey initially required an adequate planning of the various stages of operation and following this a numerical simulation to evaluate its effectiveness .
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Velocity errors in linearized elastic inversion
Authors A. de Nicolao and G. DrufucaErrors in velocity on the overburden produce depth errors in the target and also a distortion of behaviour of amplitude and phase versus offset . The paper presents a quantitative description of these effects for a simple Barth model using linearized roversion (De Nicolao et al ., 1991) . Even few percents of relative velocity error are very damaging to the results .
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Velocity discrimination by differential semblance
Authors W. M. Symes and M. KernThis paper presents an algorithmfor estimation of seismic p-wave velocity from multioffset reflection seismograms . The algorithm optimizes a cost function, part of which is the mean-square error between predicted and data seismograms . It also includes a, differential measure of event semblance or coherente, whence its name : differential semlance optimization . Unlike other implemations of the mean-square error criterion, the differential semblance version retains sensitivity over a wide range of velocity models . We present some numerical evidente of this sensitivity, using an example drawn from the Marmousi model [1] . A previous implementation of differential semblance optimization for acoustic p-tau data and layered models has been applied with success to both synthetic and field data sets and has a complete mathematical justification [4] . Therefore a, subsidiary purpose of this paper is to demonstrate an implementation of differential semblance in a non-layered ("complex structure") context . We employ the perturbational (generalized Born, primaries-only) approximation to the 2D constant density acoustic model . The velocity is split into a smooth background velocity v and a rough (oscillatory) reflectivity r = ðv/v, the latter regarded as a perturbation of the former . Denote the seismogram (i .e . suite of shot gathers corresponding to shot positions x,) by S(v)(x,t, x). Note that S depends linearly on R and nonlinearly on v . Let Sdata devote a, data set to be inverted.
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Linearized elastic inversion for multioffset marine seismic data
Authors D. Mace, A. Bourgeois, G. Etienne and V. RichardFor each shot s, the linearization consists in substituting the observed field Ps by the sum of the primary field Ps and of the scattered field ðPs where Ps corresponds to the non linear propagation in the reference medium m0 and ðPs is a linear function of the perturbation ðm which is computed from the Jacobian Js of the non linear operator at m0 . The process of linearization is associated with smalt perturbations.
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Paraxial wave equation inversion with geometric constraints
By C. BunksThe paraxial wave equation is one of the most accurate pre-stack imaging tools for seismic data . It has been shown that pre-stack paraxial wave equation migration correctly images seismic data even for very complex velocity models (such as those containing caustics). Furthermore, the fact that the paraxial approximation yields a one-way wave equation is ideal for migration and imaging since in principle it does not generate reflections as do the acoustic and elastic wave equations. Nevertheless, the formulation and implementation of the paraxial wave equation give rise to some undesirable numerical artifacts.
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Calculation of the differential semblance gradient
Authors M. Kern and W. M. SymesA major computational task in implementing the Differential Semblancce Optimization method (see the companion paper [2] for a presentation of the method) is an accurate computation of the gradient of the objective function. In this paper , we show how such a computation may be accomplished, and assess its accuracy on a representative example.
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Determination of fresnel zones by traveltime inversion
Authors P. Hurbal, J. Schleicher, M. Tygel and C. HanitzschSeismic wave propagation in the high-frequency range is usually described by (zero-order ) ray theory. Its validity conditions are extensively investigated, e.g., by Kravtsov and Orlov (1990) . For finite frequenties a ray can only be viewed as a mathematical concept . There is a ( frequency-dependent) region in its vicinity that influences the wavefield received at the end of the ray . This region is the so-called (first) Fresnel volume of the ray . Any cross-section through it is a Fresnel zone. (Gelchinsky, 1985; Cerveny and Soares, 1992; Knapp, 1991) . In other words, the Fresnel volume is the envelope of all Fresnel zones along the ray. For seismic stratigraphic modeling, usually the Fresnel zone at the reflector is of interest, so as to know which part of it contributes to the reflected field . Fresnel zones are computed very efficiently by forward dynamic ray tracing through a known velocity model (Cerveny and Soares, 1992) . However, exploration geophysicists involved in a stratigraphic analysis would prefer to know the Fresnel zone of a reflecting interface without knowing the details of the reflector overburden, i .e., they are interested in solving an inverse problém . For a plane layered Barth model, it is possible to solve this inverse problem (i e., to calculate the Fresnel zone directly from measured traveltimes, e.g., Erom the rms-velocity) .
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Nonlinear inversion of seismic reflection data: state-of-the-art
Authors A. Tarantola, M. Noble, C. Barnes, M. Charara, H. Igel, J. Lindgren, G. Röth and N. RoxisThere is little doubt that to extract a maximum of information from seismic data, one must try to fit the observed waveforms by synthetic waveforms, obtained through careful modeling of the propagation of seismic waves.
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Nonuniqueness in nonlinear traveltime tomography
Authors J. R. Matarese, W. L. Rodi and M. N. ToksozMethods of crosswell tomography have focused largely on the construction of images of seismic velocity and little on the uncertainty, or nonuniqueness, of such images . In this paper we address the Jatter problem using a Backus-Gilbert methodology adapted to nonlinear inverse problems.
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Borehole coupling effects on clamped geophone responses: experiments and theory
Authors M. H. Worthington, C. K. Jituboh, C. Chang and S. K. ChangSeismic waves detected by clamped geophones in a borehole can be affected by the borehole at high frequenties (Chang et al, 1989). A 20 cm diameter water filled borehole resonates in anti-syminetric mode at approximately 4 kHz and in symmetric mode at approximately 9 kHz. Below 2 kHz, the clamped geophone response is a combination of monopole, dipole and quadrapole vibirations in the borehole. To illustrate these effects, a, crosshole experiment was perfomed at the Imperial College borehole test site. Figure 1 Shows veldcity and gamma logs from the site and the source and receiver distribution in two, vertical holes. A three component clamped accelerometer was positianed at. 65m and 105m depth and detonators were fired at 5m intervals from 30m to 105m in the other Hole. The inter-hole spacing is 25m . The accelerometer was specifically designed to be free of resonances up to at least 2.5 kHz . Figure 2 (a.) and (b) show the recorded waveforms and the corresponding first arrival P wave hodograms from the radial and axial components of the accelerometer when positioned at 65m and 105m depth respectively. Depths in these figures refer to the positions of the source . The dominant frequency of these data. is at 1 kHz. Note that there is very little axial vibration when the source and the receiver are at approximately the same depth where as at, larger depth offsets, the hodograms are elliptical. It is conceivable that some hodogram ellipticity might result from interference between the direct and reflected or refracted waves within the multi layered medium. Full elastic wave finite difference modelling was perfomed with a model based on the borehole sonic and density logs. Hodograms calculated from these synthetic seismograms are shown in figure 2 (b) (labelled MODEL) and are seen to be essentially linear.
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Borehole effects on downhole seismic measurements
Authors C. Peng, C. H. Peng and M. N. ToksozThere are increasing interests in acquiring crosshole and VSP surveys at frequency up to 1 kHz or more, with the intention to resolve the fine details of structures and lithology between Wells (Bregman et al ., 1989; Harris, 1988; Tura, 1991) However, at frequencies on the order 1 kHz, the existence of the fluid-filled borehole has a strong influence on the downhole measurements. Depending on the frequency and angle of incidence, as well as the formation properties, the measured displacements on the borehole Wall or pressure at the center of borehole may be significantly different from those of the incident wave. Without taking proper care of this effect, any imaging or inversion algorithm which utilizes both the amplitude and phase information of recorded energy will produce erroneous results.
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Reservoir definition using triaxial walkaway VSPs
Authors K. J. Dodds, M. A. Jones, M. Idrees and W. S. LeaneyA gas discovery was made by British Gas with an exploration well in block 47/4b of the UK continental shelf. It was important to define the extern of the reservoir particularly to the North West . The structural history of this location is extremely complex, panicularly with respect to the near surface . Ray-path distortion through the complex overburden means conventional surface seismic data is of extremely poor quality, making interpretation difficult . British Gas embarked on an extensive walkaway and offset VSP program on well 47/4b- 5 The success of these surveys to image the Rotliegende azimuthally from the well was dependent on the ability of the processing to compensate for ray - path distortion together with a confident separation of shear and compressional components. The key step involved the extension of parametric inversion developed for VSP to walkaway geometry which resolved the raw triaxial data info its constituent wavefields (Fig 2). This allowed conventional migration with the 3D-lattice geometry to define the extent of the field (Fig 1 ).
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Practical aspects of processing crosshole seismic reflection data
Authors P. S. Rowbotham and N. R. GoultyA crosswell seismic dataset was acquired by SERES at Scheemderzwaag in the Groningen gas field in late 1990 (see news item in First Break, September 1991, p401) . Recordings were made in Triassic and Zechstein strata around 2200-2600m depth, where the Wells are just over 300m apart. Stanford University's acquisition system was used, which has cylindrical piezoelectric Bender transducers as sources and receivers.
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Reservoir monitoring using the VSP technique. A case study in the Oseberg field
Authors S. E. Johnstad and H. AhmedThe Oseberg Field is located in the Norwegian North Sea blocks 30/6 and 30/9, approximately 140 km northwest of Bergen with a maximum extension of 26 km north-south and a maximum width of 6 km east-west . The Field Game on stream in December 1988 . The Field comprises three major easterly rotated fault blocks Galled Alpha, Gamma and Alpha North . Hydrocarbons occur within the sandstones of the deltaic Middle Jurassic Brent Group . The reservoir sands exhibits excellent reservoir quality with porosities around 20-25% and permeabilities ranging up to several darcys . The structural dip is normally 6-10 ° east-northwest and the gross Brent thickness varies between 40 and 200 m .
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3D survey design and processing of a walkaway seismic line
Authors A. J. Calvert, J. Blanco and S. GarcanskiIn 1990, a walkaway borehole seismic survey was carried out over a producing field in Gabon with the objective of imaging a very specific section through the reservoir . In order to do this, and also to account fully for the three-dimensional nature of the target, three-dimensional raytracing was employed at the design stage to determine precisely the downhole tool position, the surface shot points and the location of the reflection points on the reservoir such that they all lay in a single, non-vertical plane. It was thus possible to treat the subsequent processing of the data as a conventional survey with the simple modification that the plane of propagation of the direct P-wave and the upgoing reflections was not vertical . A key element in the processing was the reorientation of the data from the borehole reference frame to that of the slanted processing plane. This increased the signal-to-noise ratio of the vertical component and oriented correctly the horizontal components, permitting, after velocity filtering and deconvolution, the use of both components in the final migration .
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Acoustics and very high resolution seismics in a highly deviated well
Authors J. L. Mari and P. GavinTo day horizontal Wells have been proven to be an efficient way of producing hydrocarbon reservoirs . Know-how on horizontal well drilling makes this technique very atractive considering the gain in productivity and recovery. In order to optimize production, one of the most important needs is to evaluate the reservoir in the horizontal welt area. Among possible reservoir characterization techniques, the available tools for horizontal wells are the vertical seismic profile and the full-waveform acoustic technique (J.L. Mari and J. Laurent, 1991). Full waveform acoustic data can be used to obtain acoustic sections, with a lateral investigation of several meters trom the borehole and vertical resolution of several lens of centimeters.
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SH-wave generator consisting of a vertical force and a pit in its vicinity
Authors C. Kitsunezaki, T. Yokio, K. Okuzumi and N. GotoContrary to conventional idea, predominant SH-waves are sometimes radiated from vertical impacts on the ground surface. Systematic field experiments were carried out to investigate the mechanism. The experiments revealed that the SH-waves are excited by waves incident on the boundary of a lower rigidity portion in the near-surface ground close to the impact positions. Accordingly inhomogeneities in the ground are considered to be the main cause of such SH-wave radiation that we encounter incidentally in usual seismic works using vertical forces as their sources.
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Anisotropy and crustal structure off Lofoten, Norway, from vertical incidence and wide-angle seismic data
Authors R. Mjelde, M. Sellevoll and E. W. BergA 15 sec TWT high-quality multichannel seismic reflection line was acquired in 1987 along a 175 km long NW-SE trending profile on the continental shelf across the Lofoten Islands, N. Norway. A seismic wide-angle experiment was performed in 1988 along the same profile as well as along a perpendicular profile, using 10 three-component Ocean Bottom Seismographs (OBS) with 20-25 km spacing .
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Design of acquisition systems for crustal studies using the reflectivity method
More LessWe have set up a code for the generation of synthetic seismograms which includes modeling of different acquisition systems and noise. It is based on the well-known reflectivity method because it allows a rigorous description of all the propagation phenomena that affect the spectrum of the signal. Some experiments on crustal models are presented. As a result, we can evaluate the offset range where the reflection acquisition systems conventionally used in hydrocarbon exploration improve the signal to noise ratio with respect to the methods traditionally applied to crustal studies (wide angle reflection seismics). We can also investigate the impact of different acquisition schemes on the success of single processing steps such as the velocity analysis of deep events. Moreover, during the survey, the noise generated by the program can be set at the level observed on the field records so that we can predict the expectable quality of deep reflections and we can perform a real time optimization of the acquisition parameters .
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The ecors 2.5D experiment: an attempt to image deep crustal structures with a sparse 3D recording geometry
Authors J. -M. Marthelot, M. Diagnieres, A. Hirn, A. Paul, D. Rapping, R. Silioi, B. Damotte, M. Layotte, M. Specht, J. Ravat, E. de Bazelaire and M. van ReesIn order to determine the 3D structure of crustal scale volumes by deep reflection profiling at a reasonable cost , the acquisition and processing parameters used in industrial exploration must be modified. A balance has to be found between the necessity to obtain data on a large su rfacé (for Jeep events migration distances can be quite large) and the necessity to use spatial sampling parameters appropriate for processing (taking into account the inherent loss of lateral resolution with depth) .
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Lower crustal reflectivity from waveform inversion
Authors M. Failly and S. C. SinghDeep seismic reflection profiles show that the lower part of the continental crust often contains many strong sub-horizontal reflections. These reflections are interpreted as a sequence of finely layered structuren. To confirm or invalidate this interpretation, we can use a seismic waveform inversion to estimate the reflectivity in the lower crust. However, it is very difficult to invert for the whole data set, partly because of large computation time in modelling and partly because the reflected energy in the upper crost dominates the misfit function.
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Two-dimensional modelling of resistivity and magnetotelluric data from the Belvedere Spinello salt mine, Italy
Authors G. Santarato and V. IlicetoMining the Earth's resources may often cause serious problems to the environment . In this study, mining of an evaporitic layer particularly rich in salt has caused the formation of underground cavities filled with brine . This brine has spread in all directions and probably upwards too, in largely unknown ways .
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Investigation for geothermal sources on Lesvos Island (Greece) by means of resistivity and P-wave seismic techniques
Authors J. Louis, T. Papadopoulos, N. Delibasis and N. BoulgarisCombined geophysical investigations including deep Schlumberger resistivity soundings, microearthquake monitoring and velocity inversion of local earthquake arrival time data have been performed on Lesvos island . Lesvos island, situated in the northeastern part of Aegean Sea, belongs to a Tertiary island arc system and is certainly affected by a regional anomaly of geothermal gradient, whose extent and importance are as yet unknown . The tectonics of the island, together with the occurence of some thermal springs, indicate that geothermal conditions could exist on it . The areas between villages Petra Árgenos, Kalloni - Stipsi and Polychnitos area can be considered of potential interest . The joint geophysical survey aimed to delineate areas through the tectonic structure of which there are indications of thermal fluid circulation .
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Seismic refraction and resistivity soundings as indicators of intricate properties of glacial deposits in North Germany
Authors M. G. El-Behiry, A. I. Bayoumi, R. Meissner, Y. E. Abdelhady and A. S. Aby El-EttaSeveral compressional and shear wave seismic refraction profiles together with, some full schlumberger array electrical resistivity soundings were conducted on glacial deposits in an area in the Northern part of Germany with the purpose of defining the petrophysical and mechanical properties and the structural setting of the subsurface layering.
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Structural mapping in 'basin and range-like' geology by electromagnetic methods - a powerful aid to seismics
Authors P. Y. Galibert and P. AndrieuxA large-scale magnetotelluric (MT) survey was designed to supply additional information to seismic studies for petroleum exploration in an area characterized by Cenozoïc normal faulting with graben-like structures filled up with clastic rocks . Seismic data is of high quality within the valleys and of very poor quality in between, due to outcropping hard limestones and/or igneous rocks. Initial drilling results show that even in high--quality seismic data areas , interpretation was erroneous because of unexpected thick interbedded igneous rocks .
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Concerning the digitization of high-resolution seismic signals
By T. M. McGeeSome commercially available marine seismic sources produce very broad-band signals. A high-resolution source signature can have a rise time of six-hundredths of a millisecond and a duration lens than two-tenths of a millisecond. Such sources, capable of penetrating tens of meters of sea-floor sediment with resolution on the order of a decimeter, are used extensively for engineering and environmental purposes. The digital recording and processing of data provided by them is increasingly common as faster and cheaper elcetronics become available . Thus two questions arise; how fast should they be sampled and to how many bits resolution? These questions have been addressed experimentally by field observation, but complete answers require a reconsideration of some traditional assumptions. That leads to conclusions that leem to fly in the face of conventional wisdom .
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Small vibrator development
More LessThe use of vibrators, as sources for seismic exploration of the Barth, have in the past been limited to large expensive surveys . To provide a broader range of exploration witti the advantages of a vibratory source, we have developed a small economical vibrator. Sources for use in shallow seismic surveys have in the past been impulsive . The acoustic energy was released in a short time at a high amplitude. The largest amounts of energy veere generated witti explosiees. Explosives are becoming more difficult to obtain and provide minimal control over the produced spectra. With a small vibrator acoustic energy can be released slowly, to sum to a large quantity, causing minimal environmental damage with maximum spectra control .
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The use of sigma-delta A/D convertors in modern engineering seismic data aquisition systems
Authors F. B. Michelsen, T. Kanemori, A. Peter and J. H. MimsSeismic surveys for engineering applications require relatively inexpensive seismic acquisition systems that are compact and rugged. As electronic technology has evolved, systems that meet these requirements have progressed from simple analog devices to computer controlled instruments that can record more channels and store the digital data.
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2-D FEM rock failure modeling in the vicinity of mining excavations
Authors Z. Mortimer and A. CichyThe process of rock failure due to mining exploitation is correlated with variations of geophysical fields. Some models of brittle failure of rocks in the Alain strain conditions have been tested on the base of stresses, seismic waves velocities and gravity anomaly distributions in the vicinity of the cicrular opening . It was assumed that in zones of rock failure the rock fracturing developes . Non-elastic behaviour of the rockmass in these zones is approximated by some changes of elastic parameters according to different failure models. Interaction between shear crack and working face was tested for their different configurations. Empirical Hoeck & Brown strenght criterion and the threshold of 50 % of this criterion for the begining of the dilatancy process have been used .
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A joint inversion algorithm to process geoelectric and subface wave seismic data
Authors M. Dobroka, L. Dresenz, A. Gyulai, A. Hering and R. MisiekIn the exploration of near surface structures geoelectric and seismic methods are commonly used. The independent inversion of the data is well-established. Surface wave data are usually considered as noise, however they contain valuable information about the geological structure which can be extracted by inverting the data. The independent inversion of both surface wave seismic and geoelectric data show internal ambiguity and nonuniqueness. Our aim in this paper is to show that these problems can sufficiently be reduced using the geoelectric and surface wave data together in a joint inversion algo rithm.
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Time domain electromagnetic, seismic refraction and potential field exploration of the Troodos ophiolite spreading structure
By N. J. CooperMineral exploration programs in complex geological environments require a clear understanding of both regional structure and local factors influencing mineralisation. Recent submarine images of active spreading environments have shown striking parallels between present ridge structuren and ancient ophiolitic terrasnes. Geological models now present a viable framework that requires evaluation in the structural and urineral environment by geophysical means .
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Recent geophysical surveys for massive sulphides in the Pyhasalmi area Central Finland
By T. RekolaThe search for economic Zn-Cu-S- deposits has been going on in the Pyhäsalmi area, central Finland for nearly 30 years . In 1958, when the Pyhäsalmi ore deposit was discovered the studies undertaken by Outokumpu Exploration in the area were given a big boost. immediately after discovery of the deposit, systematic magnetic, EM and gravimetric surveys were started over and in the vicinity of the orebody . The massive pyritic Pyhäsalmi Zn-Cu-S ore causes a distinct gravimetric and electrical slingram anomaly . The ore contains 2 - 4% pyrrhotite ; which greatly improves its electric conductivity . The results of the geophysical survey had a decisive impact on the instruments and methods used in the following search for a massive pyritic zinc ore of the Pyhäsalmi type .
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Soil investigation by radar techniques: first experiments by the Italian telecommunications operating company
Authors F. Esposto, F. Roscini and Gede PasqualeEvery year SIP, the Italian Telecommunications operating Company, performs thousands of kilometers of excavation, mainly open, for cable installation . Three kinds of excavation machinery are used : traditional, special and microtunnelling machines . In all cases preliminary investigations are needed before actual digging commences in order to détermine the position of any utilitys (pipes and cables) in the excavation cross section, not only in order to avoid damaging them, but also to plan the optimal excavation lay-out (for instante the road side less laden with pipes). Such investigations, useful in the case of traditional digging methods, become essential when using special or microtunelling machines, which characteristically breakthrough any utility in their path .
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Application of GPR to the construction of utilities
Authors J. M. Baltassat and G. PottecherThis paper presents how Ground Probing Radar can be used before laying shallow utility lines. The first reconnaissance problem is the selection of a path avoiding the other utilities present . The traditional methods have shortcomings which can be overcome by GPR : plans are inaccurate in location and depth, electromagnetic detection is inefficient for non-Metallic utilities . Another reconnaissance issue is geology, which governs the choice of laying technology, either open trench, impact moling or directional drilling . Trenchless technologies cannot work in some geological media and are sensitive to ground inhomogeneities. Geological raps are often meaningless at shallow depth in urban areas, while checking holes cannot provide continuity. GPR can detect boulders and underground masonry remains, as well as characterize the geologic media .
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Use of GPR to obtain layers thickness variation in a test track
Authors E. Lorenzo, I. Fernandez and V. CuellarA test track built by the official research center of the Spanish Ministry of Public Works (CEDER) came into operation in January 1988 . The track constituted by two 75 m long straight sections connected by two curves has an oval shape and a total length of 304 m . It stands on a concrete box 8 wide and 2 .60 m deep .
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Ground penetrating radar for roads and bridges: strengths and limitations
Authors N. S. Parry, J. les Davis and J. R. RossiterGround Penetrating Radar (GPR) has been used increasingly on roads and bridges over the last five years. GPR offers non-destructive evaluation of the subsurface along continuous profiles, providing unique information on the structural layers and material properties of roadways . Given that roads have a relatively simple geometry and are usually composed of homogeneous, engineered materials, this application would appear ideally suited to SPR . However, there are several pitfalls associated with this application . Some of these pitfalls are generated by equipment limitations, such as pulse length, sampling rates and fitter . Others are introduced when standard assumptions, commonly applied to GPR data from geotechnical environments, are used . In addition, the particular Weeds of the end client must be considered .
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Ground penetrating radar applications for nondestructive inspection of construction work
Authors M. Bashforth and S. KoppenjanThe nondestructive inspection of construction work has gained significant interest in the United States in the 1940's. The decire to verify building standards and codes has become an increasing problem. In many cases there - is no existing nondestructive method to accomplish this. Knowledge of the location and depth of building support mate rials is greatly needed .
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Groundradar applications for road constructions
Authors V. Hopman, A. J. M. Peters and J. W. de FeijterIn the Netherlands a large amount of infrastructural elements are concentrated on a relative small area . Highways, railways, waterways and all kinds of roads intersect each other at many locations . High investments are necessary to maintain these infrastuctures. Also cables, pipes and many cycle-paths cross main infrastructuren.
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Save Venice. A multidisciplinary investigation in the Rio Nuovo collapsing embankments
Authors M. Picollo, A. Fio, G. Niccolini and G. SpangheroBecause of erosion problems in the foundation wall of Rio Nuovo, probably caused by the ferry boat traffic, some collapse phenomena were observed on the surface of the embankment . A diagonal fracture in the asphalt surface interested a corner of the ferry boat station showing a collapse of about 15 cm. The interest was to understand the geometry of the collapsing structure, the depth limit of the event, the possible relationships with the morphology of the channel bottom and the identification of other hazard areas in west and east embankments .
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Radar techniques for the evaluation of masonry structures
More LessCivil engineers have a need to evaluate the condition of masonry structures such as bridges and tunnels in order to classify the condition of the structure . Impulse radar techniques have been used in the past, but interpretation of the data has been difficult as the radar data does not necessarily correspond with the geometrical structure . British Rail Research commissioned ERA Technology, over several years, to carry out a detailed investigation of the correlation between the radar image and the physical structure, in order to improve the clarity of the radar image .
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Subsurface imaging with EMAP - an aid to seismic processing and interpretation
By R. K. WarrenThe EMAP (Electromagnetic Array Profiling) exploration method has been combined with a direct inversion process for conversion to depth to produce a subsurface resistivity cross-section. This cross-section is interpreted in parallel with a seismic cross-section to enhance the prediction of rock type and structure. In complex thrust environments, areas of shallow volcanics and/or carbonates the EMAP method is used to provide additional data to either help the seismic processing and/or aid the seismic interpretation . In particular, the EM data are used to build an independent seismic velocity file for depth migration .
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Electromagnetic mapping of electrical conductivity beneath the Columbia basalt
Authors Y. Shoham, F. Morrison and C. Torres-VerdinThe Colombia basalts cover a petroliferous sedimentary section and they present a representative example of the challenging problems of trying to determine sedimentary thickness and structure in situations where seismic methods do not provide adequate depth of exploration . Between 1979 and 1986 Shell conducted a series of controlled source (CSEM) and natural field (MT) surveys at several sites in the Columbia basalts and the results for a profile near Boylston are described in this study .
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Advances in the integrated interpretation of seismics with magnetotellurics (illustrated by practical examples from the Pannonian basin in Hungary)
By Z. NagyIntegrated interpretation of seismic reflection and magnetotelluric (MT) surveys is a new approach to improve the results in prospecting for hydrocarbons. Recent, successful applications of MT method to the hydrocarbon exploration are discuseed by Christopherson (1991). However, there are few published case histories on the integration of seismic and MT data. The main purpose of this paper is to demonstrate that the interpretation of seismic sections can be improved by incorporating results from magnetotelluric surveys . In Hungary, the Hungarian Oil and Gas Co. is prospecting for hydrocarbons . Most of the work is in several subbasins of the Pannonian basin (Silverman and Barton, 1991) . Although the seismic reflection method is the primary tool employed, non-seismic methods are also applied, including magnetotelluric techniques (Ádám, Nagy and Varga, 1989) and other controlled source EM methods (Nagy et .al, 1991) . MT soundings are used for integrated interpretation purposes.
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Transient electromagnetic and seismic prospecting - an analysis of their similarities
More LessA fundamental introduction is presented to the similarities that exist between the transient electromagnetic and the seismic prospecting methods.
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Structure, stratigraphy, fluid content and field procedures for seismic and EM
Authors A. M. Ziolkowski, W. Peet, K. -M. Strack, P. Andrieux and K. VozoffThis paper looks at the problem of joint inversion of seismic and electromagnetic data and points to ways in which the acquisition of data in both methods should be improved. The sonic and resistivity logs from the same well often show a marked resemblance. Rocks with high compressional wave velocities, such as carbonates, are often very resistive. Porous permeable rocks with a large fluid content are often very conductive and have lower compressional wave velocities. The conductivity can vary enormously within a porous medium depending on the fluid content. Salt water is much more conductive than gas, for example . This suggests that electromagnetic methods should be used to determine fluid content within a reservoir and thus locate the reserves. The problem with electromagnetic methods is that they have poor resolution when used on their own .
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Integrating long-offset transient electromagnetics (LOTEM) with seismic in an exploration environment
Authors K. -M. Strack and K. VozoffDuring the past decade long-offset transient electromagnetics (LOTEM) was applied to many exploration problems in Africa, Australia, China, Europe, India and the USA . Initially, the technique was applied in areas where Bood seismic data was difficult to obtain. With the experience of ongoing surveys and an improvement in instrumeritation, the main application which is emerging is the combination with seismics to map porosities in carbonate reservoirs. The major problems overcome along the way included acquisition difficulties (strong cultural noise levels, not enough data), data processing difficulties (low signal-to-noise ratios), and interpretation limitations (restricted 3-D modeling capabilities). LOTEM is now a vérsatile and reliable exploration tool .
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Integration of MT, seismic, gravity, and magnetic data for reconnaissance of the Colombian Amazon
Authors W. A. Burgett, A. Orange and R. F. SigalAmoco Production Company integrates the interpretation of magnetotelluric (MT) data with seismic, gravity, and magnetic data for regional basin reconnaissance and prospect evaluation worldwide. Experience shows that : MT data acquisition can be fast and affordable; high quality data suitable for relatively high resolution interpretation can be acquired in most parts of the world; and in many instances MT data has made a critica) contribution to the final interpretation. One case history that demonstrates some of these advantages is described in this paper. MT was used as part of a very broad regional program to study a large, remote, and essentially unexplored area . A seismic program and stratigraphic well tested the interpretation of gravity, magnetic and MT data and the correlation was excellent .
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Papua New Guinea MT: looking where seismic is blind
More LessHydrocarbon exploration in the Papuan fold belt is made extremely difficult by mountainous terrain, equatorial jungle and Chick kanstic Miocene limestones at the surface. The high velocity karstic limestones at or near the surface often renden the seismic technique useless for imaging the subsurface . In such areas Magnetotellurics (MT) provides a valuable capability for mapping subsurface structure. The environment of the Papua New Guinea (PNG) highlands presents a wide range of MT data acquisition and interpretation problems, the most important of which, from an interpretational point of view, are the rugged topography, severe electric staties, and complex two and three dimensional stroclone. The main structural interface which can be mapped with MT, due to the large electrical contrast, is the contact between the resistive Darai limestone and the underlying conductive sediments of the Ieru Formation. Within the Ieru the reservoir of Toro Sandstone does not have sufficient electrical contrast with the surrounding shales to allow its direct mapping. Nevertheless the ability to map the base of the Darai accurately allows the mapping of the Toro by inference since the leru Fonnation maintains a relatively constant thickness. BP, as part of its ongoing exploration program in PNG, pas acquired more than 700 MT sites to aid in structural interpretations.
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Synergetic interpretation of well test and seismic data for reservoir characterization
Authors S. Ronen, C. Economides and G. VarvikA pressure buildup well test is a measurement of bottomhole pressures in a wellbore beginning while the well is flowing and continuing for a period of several hours or days after the well is shut in . After the well is shut in, no force maintains the low pressure at the well and the tendency toward equilibrium results in a buildup of pressure in the wellbore. The pressure perturbation initiated by shutting in the well, expands about the well, and trends in the measured wellbore pressures may be altered whenever the pressure wave encounters a change in rock or fluid properties .
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Lateral prediction of reservoir properties using seismic attributes with well log data
Authors S. Ronen, M. Hattori, S. Geoltrain and P. SchultzSeismic attributes are mathematical transformations of seismic data, typically dove after stacking and imaging (Taner et al, 1979) . Examples are amplitude, instantaneous frequency, P and S velocities, band-limited impedance, apparent dip or azimuth, and others . Rock or petrophysical propertjes (e .g ., porosity, Poisson's ratio, relative shale volume, fluid content, and others) describe the medium itself, and are measured or computed in situ from logs. Seismic attributes carry the promise of indicating such real propertjes, since in some circumstances certain attributes can be expected to correlate highly with specific rock properties (Sonneland et al, 1990)
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Improved reservoir description by use of an integrated approach: application to a gas storage reservoir
Authors P. Y. Dequirez, F. Fournier and V. RichardThe integration of all available information is a key point to improve the description of a reservoir . In particular, seismic data are essential to complete our knowledge of the reservoir between the wells. We illustrate this point through an integrated procedure applied to a gas storage reservoir. This procedure combines different techniques using post-stack seismic data, impedance well logs and geological information. It is divided in three mais steps 1. waveform calibration of the stacked seismic data using wen log-baserf reflectivity 2. generation of inter-well impedances using stratigraphic modelling and inversion 3 . inter-well impedance facies recognition using multivariate statistical techniques and geological interpretation .
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Seismic blind zone detection by image processing techniques
By G. DarcheSeismic blind zones (weak or incoherent reflections) are an important source of mistakes for automatic 2D/3D interpretation softwares, and their detection would save these softwares a substantial computation time. To isolate these blind zones, we apply a recent image processing technique, the SNAKE method, traditionally used for edge detection, and we adapt it to the particular case of seismic images, to find smoothly shaped blind zones . After a description of the SNAKE method, we apply it to our problem, and illustrate the method on a 2D seismic section.
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3D Seismic: An exploration & production tool for the Abu Gharadig Basin, Egypt
Authors M. Nessim, S. M. H. Abedi and M. el ToukhyThe Abu Gharadig Basin is well established as a prolific hydrocarbon bearing province of the Western Desert, Egypt. However, the oil and gas fields discovered so far are moderate to small in site and are trappel in a complex stratigraphic and structural setting. Postulated hydrocarbon generation models indicate that significantly more hydrocarbons (up to 3 billion bbls) have been generated in this basis and have migrated into the available traps. However, the relatively remote location of this area requires careful economic considerapons before commitment to any potential development. Therefore, better means of trap definition is highly desirable .
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Prestack seismic signatures of actual and synthetic reflections from different petrophysical targets
Authors G. Ravagnan, A. Mazotti, A. MMelis and G. BernasconiWe describe the reflections from gas sands, lignitic sands and cinerites of a marine seismic dataset in terms of their Amplitude Vs .Offset (AVO) responses . As an additional aid to the interpretation we also make use of the Phase and Frequency Vs . offset (PVO and FVO) indicators . These offset dependent signatures are related to the different petrophysical, and thus elastic, situations or to peculiar interference patterns and may help to distinguish the amplitude anomalies observed in the stack sections . It is in fact well known that the presence of volcanic sediments or of lignites is a common cause of "false" bright spots .
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High resolution seismic stratigraphy: a new approach showing new possibilities of seismic profiling
Authors N. Ya. Kunin and A. G. BudagovSeismic sequences analysis elaborated by the US scientists (P. Vail et al) has been promoted in many countries . In the Russian Federation this analysis is widely applied. In this presentation we shall discuss some results of theoretical and practical investigations . High resolution (fine-layer) seismic stratigraphy operates with objects smaller than a seismic sequence . According to their size magnitude the odjects are classified as sesmic packages and seismic quanta, the explanation of these terms being Biven .
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Seismic stratigraphy or structural formational interpretation of seismic data, or both?
Authors I. A. Mushin, E. A. Kozlov and L. Yu. BrodovEvidently, there are now two approaches in geologic interpretation of geophysical data that can be considered as dominating ones . One of them is the well known and widely used seismic stratigraphy (SS) . Another one is the structural-and-formational interpretation (SFI) elaborated by Russian geophysicists. Up to now, the application of the SFI was restricted to oil and gas provinces of Russia, Kazakhstan, Turkmenia, and recently - Pakistan and Ethyopia .
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Recent advances in onboard seismic QC systems
Authors D. Pajot, H. Bragstad and J. H. BarschModern marine seismic acquisition techniques have drastically reduced the acquisition colt per unit of subsurface coverage . Today modern seismic versels typically record 6 subsurface lines in one vessel traverse; the seismic data volume acquired every 8 seconds may be in exces of 20 Mega bits . However the complexity of the most advanced configurations and the amount of gear deployed at sea have kept the high acquisition costs per unit of time . This lead rome operators to re-think the way seismic data quality must be assessed in the field, evolving from a basic signal to noire ratio evaluation done on single fold data to more sophisticated analysis dove on stacked/filtered data . In the case of bad weather operators sometimes hold the decision to suspend the acquisition until the noise becomes unacceptable on stacked sections processed in a commercial data processing center .
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Evaluation of 3D coverage specs - a case study
Authors M. Brink, B. Ursin-Holm and K. KolbjørnsenIt is common practice in 3-D seismic surveying to specify criteria for CMP coverage in terms of minimum acceptance levels fór different parts of the streamers . In case these minimum levels are not met, infill seismic data are acquired in addition to the socalled primary seismic data . Depending on sea currents the infill percentage is often between 10 and 25 percent . The need for infill is also depending on the extent of flexible binning which is considered acceptable . For different levels of infill in combination with fixed and flexible binning the seismic results of a 3-D seismic survey have been analysed, in order to assess the relevante of these infill specifications . The main results of this analysis are presented in this paper . It comprises full 3-D seismic processing including 3-D migration of the primary dataset with and without the infill dataset . For this processing botte fixed binning and flexible overlapping binning has been used . In addition the effects on seismic data quality of less accurate navigation by omitting the acoustic network, has been investigated .
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An examination of tapered binning in 3D marine data acquisition
Authors B. O‘Sullivan and L. LuIn this paper we will examine different techniques used during 3D marine data acquisition to reduce the amount of additional "infill" data required. In particular, we will look at how tapered binning is applied, its underlying assumptions, and the geophysical and economic concerns resulting from using tapered binning. We will also examine how tapers are applied in the field, the consequences and subsequent effects on processing techniques. In relation to this, the effects on stacking, DMO, AVO, and migration of a data set before and after tapering process are discussed .
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Source array design to attenuate multiples in a hardwater bottom area - conclusions from an experimental seismic survey carried out on the North-West shelf of Australia
Authors C. R. T. Ramsden and C. CooperThe North West Shelf of Australia is a difficult data area . The seafloor and near surface comprises a Tertiary carbonate wedge which thickens away from the coast. Strong multiple sequences are generated between the sea surface, the seafloor and other strong reflectors in the near surface. The absorption of the primary energy by the carbonate wedge and the low reflectivities in the zone of interest results in the masking of these reflectors by the multiple train. Back scattered energy is not a severe problem because the seafloor is generally smooth and flat .
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Field testing of a seismic G.I. gun array
More LessDuring the last decades the airguns have become a standard energy source for seismic surveys in the industry. Sleeve guns were introduced a number of years ago and are widely used today. This paper aims at comparing the results of a testline shot with a sleeve gun array to the results of a testline shot with the newly developed G.I. gun array . These data were shot in the summer of 1991 . The data were acquired in Norwegian waters in quadrant 16. An airgun shot line from the same area, acquired in 1989, is also available for comparison .
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A demonstration of efficient exploitation of airgun interaction
Authors M. H. Safar and V. B. HewettIn a paper presented at the 60th Annual International SEG Meeting in San Francisco, 1990, and also at the 53rd EAEG Meeting Florence Italy, 1991, Safar et al showed from the measured far field signatures radiated by a 4 x 40 cu.in. compact sleeve gun array (cluster), that higher values of Primary-to-Bubble ratio are achieved at the expense of radiating considerably lens energy flux. Consequently, it was suggested that the importante of the Primary-to-Bubble ratio as a design criterion should be reconsidered and the radiated energy flux should be given a serious consideration in the design of compact gun arrays. This is because the strength of the reflected signal is essentially determined by the radiated energy flux.
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Experimental study of temperature effects on air gun signatures
Authors J. Langhammer and M. LandroMarine seismic sources have been studied extensively during the last two decades. Both experimental and theoretical studies of air guns and water guns have been performed. One of the main theoretical issues have been to onderstand the damping mechanisme for the bubble oscillations in an air gun pressure signature . Both Ziolkowski (1990) and Laws et al . (1990) pointed out that condensation and evaporation of water vapour across the bubble wall are essential procesnes involved in the damping mechanism. Since these processen are driven by pressure and temperature gradients across the bubble wall, we decided to investigate the influence of changing the temperature of the water surrounding an air gun . From measurements it is well known that the bubble time period changes when the sea temperature changes . However, this knowledge is of qualitative nature, and how other signature characteristics like primary to bubble ratio is influenced by a change in the water temperature is not so well known. In order to quantify these effects, we performed a controlled tank experiment . We used a water tank of approximately 850 litres . In order to attenuate reflections From the walls and the bottom of the tank, absorbers of 5cm thickness were placed inside the tank . The source was a BOLT 6 00 B air gun with a chamber volume of 1.9 cu.in. In all the experiments the firing pressure was 100 bar . The gun was placed at 0.5 m depth, and the hydrophone was placed 0 .22 m from the gun ports.
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Screw seismic sources
Authors K. Davies, G. Hampson, H. Jakubowicz and J. OdegaardIn this paper we propose a novel method of acquiring marine seismic data in which screw noise is used as the seismic source. This approach has many potential advantages and might be suitable for seismic acquisition beneath the ice-cap in polar regions .
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Layer-based oriented full wave inversion
Authors F. Jurado, V. Richard and M. CuerWe present a 3-D inverse method for wave propagation in horizontally layered elastic isotropic media. The direct modeling uses a Hankel transform in the horizontal direction. We have previously introduced in the inversion problem a parameter reducing formulation based on a layered medium assumption (Jurado et x1,1991). In this paper we present a travel-time formulation of the problem to reduce coupling between velocity and depth. These two formulations are viewed as a change of variable or reparameterization on the usual depth formulation in the roversion problem. A synthetic example illustrates the interest of botte formulations .
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Statistical methodology for the geological calibration of a 2D seismic data set
Authors F. Fournier and J. -F. DerainMore and more sophisticated techniques such as stochastic simulations are used to build the reservoir model, which is mainly based on well data. These data are often not sufficient for a reliable reservoir description because of the well scarcity or poor spatial distribution. Indeed, reservoir description would greatly be improved by accounting for some extra geological information between the Wells. Such geological knowledge may be derived Erom the seismic data which are welt spatially distributed over the field. In particular, multivariate statistical techniques could be applied to achieve this goal (Thadani et al ., 1987, Fournier, 1989) .
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Wave equation based seismic processing: in which domain?
More LessThe forward model of prestack data that we use in our seismic inversion project DELPHI (Berkhout and Wapenaar, 1990) is based on a number of matrix multiplications. In its simplest form, (i.e. after pre-processing), it is given by P = W RW+S+ Where S+ contains the downgoing (source) wave fields at the surface, W+ describes downward propagation into the subsurface. W describes upward propagation to the surface and, finally, P contains the upgoing wave fields at the surface
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Hume, popper, bayes and geophysical inverse problems
Authors G. G. Drijkoningen and A. M. ZiolkowskiWe distinguish two approaches to geophysical inverse problems. One is the opposite, or inverse, of the forward problem. Signature deconvolution and migration are examples of this kind of inversion. The data are inverted to arrive at the Barth model. The other approach is iterative forward modeping, in which we try to match the measured data with synthetic data created with a model and the given theory. In recent years Bayes's rule has been a popular way to measure the quality of the result obtained by iterative forward modelling. We focus on the philosophical arguments for the two approaches and on the difficulties in applying them to geophysical inverse problems.
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Constrained AVO inversion
Authors G. Roberts and P. N. ArmstrongMuch AVO (Amplitude versus Offset) analysis involves linearised inversion of simplified Zoeppritz equations (Smith and Gidlow, 1987) . The approximations are only valid for small changes of physical properties, and for angles of incidence significantly leas than critical. Therefore, weighted stack AVO analysis is only applicable over a restricted range of conditions .
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Bright spot anomalies studies: an integrated approach
Authors A. Bruneton, D. T. van Nhieu and D. WloszczowskiAn integrated lithological and seismic interpretation method has been used in a real case study of seismic amplitude anomalies ("bright spots") related to shallow gas Bands encased in marine shales. Using a statistical analysis of geological and petrophysical data from core and wire-line logs data, the distribution of parameters to which seismic reflection is sensitive such as acoustic and elastic properties and bed thicknesses, is determined for the encountered lithologies . Qualitative interpretation of reprocessed seismic data witti we amplitude recovery before stack, quantitative seismic modelling and inversion are carried out . The results of this study calibrated on wells serve as guide lines in prospect evaluation whose expressions on conventional seismic sections are "bright spots".
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Porosity and lithology mapping from seismic data
Authors H. Özdemir, L. Jensen and A. StrudleyMapping of the reservoir zones and reservoir parameters between the Wells is needed at the field development stage, and later for reservoir characterization as inputs to reservoir simulations. Increasing use of horizontal Wells has also emphasized the need for detailed delineations of reservoir zones using surface seismic data. Improvements in the resolution obtained Erom seismic measurements can be achieved through post stack inversion scheures to assist in drilling prognosis. Key issuses in such inversion procedures are a relative amplitude proceseed surface seismic data set and a reliable method of wavelet extraction Erom surface seismic data over the zones of interest. Well seismic calibration is required at key well5 for calibration of inverted acoustic impedance and for the most rubast wavelet extraction scheures which make use of well acoustic impedance.
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Direct and inverse problem estimating of reservoir structure
Authors E. M. Chesnokov and V. V. TertychnyiThis report is a review of recent results obtained through the study of physical parameters carbonate reservoirs, which is presented by pore-cracked model under stress . As a basic model of cracked solid, we assume a linear elastic matrix with some system of distributed penny-shaped of ellipsoidal inclusions. The distribution function of inclusions with respect to orientation angle and aspect ratio is considered to be a function of applied stress. The latter predicts non-linear elastic behaviour of cracked solid and theoretically determines the dependence of effective elastic modules, heat and electrical conductivity on applied stress and fluid pressure .
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Seismic physical modelling facility at Delft university
Authors E. A. Koek and G. FaberThe use of physical modelling as a research tool in 3D-seismology has been known and used for several years. The choice of model scaling factors was mainly limited by the availability of high frequency transducers. Common transducers used for investigations were usable up to a maximum frequency of 300 kHz and showed rather poor directivity. As a result a maximum reduction in model size was limited to a factor of 1 :12,000 requiring also velocity scaling. The models used in these experiments are still rather large and thus expensive.
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Borehole coupling in cross-well wave propagation
More LessWe present the results of large-scale simulations aimed at studying the propagation and scattering of elastic waves in cross-well seismic experiments and their influence on the migrationinversion algorithms used in processing the data. The objective of this study is twofold: • To understand coupling phenomena in order to better design the experiment and interpret the observed data; • To providé realistic 3D elastic synthetics to be used in algorithmic validation of inversion techniques .
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Modelling mach wave propagation between boreholes in layered media
More LessCrosshole seismic experiments often yield data containing arrivals which are difficult to understand and explain, due in part to the location of the sources and receivers in boreholes. In order to use the data from these experiments to infer properties of a petroleum reservoir, an improved knowledge of the effects of source and receiver boreholes on the waves propagating in these experiments is therefore very useful. An example of this type of problem is the interpretation of the energy radiated from tube waves propagating in the source well, since in slow formations where the tube wave velocity is larger than the formation shear wave velocity, this energy can result in the recording of large amplitude Mach waves, or shock waves, in the receiver well (Meredith 1990 ; de Bruin and Huizer 1989). This phenomenon typically will occur in shallow, low velocity sedimentary layers . The conical Mach waves will not only have an amplitude much larger than that predicted for radiation directly by the source, but will also have a linear moveout velocity which cannot be explained by radiation directly Erom the source.
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3D Seismic modelling of general material anisotropy in the presence of the free surface by a Chebyshev spectral method
Authors E. Tessmer and A. BehleA new 3-dimensional spectral method for séismic modelling in general anisotropic media is presented. An important feature of the method is its ability to handle the free surface boundary conditions very accurately . Moreover the algorithm is able to account for complete material variability, since it is a grid method, where every grid node can represent different material properties . The modelling scheme is based on the velocity-stress formulation of the equations of dynamic elasticity, but does not make use of staggered grids. For the horizontal directions spatial derivatives are carried out by the Fourier method which has periodic boundary conditions, whereas differencing with respect to the vertical direction is performed by a Chebyshev derivative operator, which is non-periodic and therefore can account for specified boundary conditions. Time integration is achieved by a fourth order Taylor expansion of the formal solution .
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3D Surface consistent decomposition for refraction modelling: a workstation implementation
Authors W. R. Stresau, R. C. Farrell and K. P. FordRefraction analysis, modeling, and statics, are often standard components of the production processing sequence for 3-D data volumes. Many algorithms exist to define a near surface model from the refraction analysis of 2-D and 3-D production field records. A new surface consistent decomposition algorithm has been developed and implemented on the workstation that will efficiently model the near surface based upon travel times derived from 3-D production field records. The algorithm is implemented using a conjugate gradient technique. The least squares error minimization is statistically analyzed to gain confidence in the solution. This error analysis facilitates identification of picking problems, inadequately identified refractor segments, or geometry problems. The interactive implementation allows problem areas to be identified, while iterating to the final model . Capabilities exist to interactively correct the problems before proceeding further with the modelling. This method ensures an optimal near surface model is derived in an efficient manher for 3-D data volumes, crooked lines, or 2-D data. Results of the 3-D implementation of the surface consistent decomposition algorithm on a 3- D data example will be provided .
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Angle dependent attenuation of seismic waves in a random stack of layers: examples of exponential and fractal media
Authors S. A. Shapiro and H. ZienThe effect of waves attenuation because of scattering in layered media can be observed and analyzed in data of seismic measurements like VSP and cross-hole sounding. Theoretically and numerically we consider the transmission of a pressure plane wave through an extensive fragment of a stratified disordered medium with constant density and without intrinsic attenuation. We study effects of multiple scattering.
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Structural model uncertainty of a prospect - a geostatistical approach
More LessThe structural geometry of a Prospect is usually defined using some Time to Depth migration technique, as a function of the time map and of the corresponding seismic velocity map . For time migrated data, a simpte Time to Depth conversion is needed, with an adjustment of the depth surtace to Well data, if necessary . Depth reconstructions are always affected by quality and spatial frequency of vetocity information; if, as in the study presented, the structures are very gentle, a special precision would be necessary for a good prospect reconstruction and evaluation. For the study presented, two seismic markers were interpreted and mapped, corresponding to the top (level B ) and the bottom (level C ) of a sequence of finely stratified sand and clay layers, structured in a gentle anticline, which bears gas in correspondence of Well A1. The targets of the study were the reconstruction of their depth maps, the evaluation of the trap potentiality and the estimation of the result reliability . To reach these targets we put our main effort in improving the estimation of the velocity maps and to establish a method for providing results in term of probability .
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Seismic wavefield fluctuations and attenuation in random media: numerical modelling and theory
Authors S. A. Shapiro and G. KneibSeismic wave propagation in random media yields complex wavefield fluctuations and apparent attenuation by scattering. Both can be observed and analyzed in VSPs and cross-hole data .
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Coupled tube waves between two boreholes: a theoretical approach
Authors J. -L. Boelle, M. Dietrich and B. PaternosterCross-hole seismic data are usually processed and interpreted by ignoring the presence of the borehole i .e., by assuming that the sources and receivers are simply buried in the subsurface . This assumption is generally acceptable for ray tomography methods using first arrivals only, but must be revised in diffraction or full waveform tomography techniques.
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Multichannel estimation of frequency-dependent Q from VSP data
Authors P. E. Harris, C. Kerner and R. E. WhiteMeasurements of attenuation (or Q) are frequently made from VSP data. Unfortunately, whatever method of estimation is chosen, the statistics of the Q estimates are poor. To achieve low variance, a large depth interval must be used, leading to poor resolution. This in turn produces a high bias in the estimates. The optimal depth range for estimation is thus a trade-off between variance and bias .
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Seismic attenuation measurements from crosshole data
Authors I. R. Portsmouth and M. H. WorthingtonOne compelling reason for performing crosshole seismic experiments is that relatively high frequency data are usually obtained compared to surface or VSP surveys. It may seem logical to conclude from this general observation of many published results that the seismic Q of the earth beneath the near surface weathered layer is high . However, in this experiment crosshole seismic data with a, frequency range of 400 to 2500 Hz have been analysed and estimates of the intrinsic Q of sedimentary rocks of approximately 18 have been obtained.
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P and S well seismic attenuation studies in a fractured reservoir
Authors A. Herrenschmidt and G. OmnesThe attenuation studies in this paper are based on borehole seismic data acquired in February 1989 in a vertical producing well in the Romaskhino field in Tatary (Republic of Russia). The well produces about 2. 5m3 (21 barrels) per day from a lower carboniferous (Bashkirian) carbenate reservoir within the 788-860 m depth interval. Average total porosity is 8 .4%, . Production is controlled by vertical fractures or joints .
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More reliable shear-wave data from VSP using CIPHER technique
Authors T. Gut, W. Sollner, E. Luschen and H. A. K. EdelmannIt has long been recognized that fracture systems in rocks can significantly influence the flow of fluids in the subsurface. Geophysical fracture characterization techniques based on seismic anisotropy and related shear-wave splitting have been under investigation for many years. It is now established as an observable effect. To keep rig time costs low, a 3-C multilevel downhole sonde has been introduced many years ago. Additional horizontal vibrators and additional survey time and processing costs for shear-wave data, however, are still important constraints. The CIPHER method (Edelmann, 1992) described below aims both at the reduction of costs and at an improvement of VSP shear-wave data.
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Present limitations in cross-well reflection seismic
Authors B. Paternoster and V. DesouchesHigh frequency seismic propagation can improve spatial resolution in Reservoir Descripton. Cross-well seismic has great potential for this. Based on three actual cross-welt seismic surveys conducted in 1990, the objective of this paper is to discuss some of the limitations that these surveys are presentIy facing in terms of acquisition, processing and interpretation .
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Drill bit wavefields
By J. W. RectorThe seismic body wave radiation pattern of a working rock bit can be characterized by theoretical modeling and field data examples. My model of drill-bit signal generation is a pseudo-random series of bit tooth impacts that create both axial forces and tangential torques about the borehole axis. Each drill tooth impact creates an extensional wave that travels up the drillstring and body waves that radiate into the Barth. The model predicts that P waves radiale primarily along the axis of the borehole, and shear waves radiate primarily perpendicular to the borehole axis. In a vertical hole the largest P waves will be recorded directly above and below the drill bit; whereas, the largest shear waves will be recorded in a horizontal plane containing the drill bit. In a deviated borehole, the radiation patterns should be rotated by the inclination angle of the bit. The proposed seismic body wave radiation is investigated with field data examples using multichannel arrays of vertical geophones. The modeled radiation pattern is also investigated with a three-component inverse VSP data example shown in Figure 1 . The variation in the signal-to-noise ratio witti drill bit depth and the partitioning of S-wave energy between radial and transverse components is consistent with the proposed radiation pattern. A quantitative measure of the drill bit radiation pattern, taken from calculating the rms ratio between the shear wave direct arrival on the radial component and the P-wave direct arrival on the vertical component, is also consistent with the proposed radiation pattern .
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Acoustics on real scale models
Authors P. Gaudiani and J. L. MariA full wave form acoustics tool is a very high frequency geophysical tool, for imaging near borehole structure with an investigation of several meters. We have built a 3-dimensional real scale model, with a substratum made of concrete (the model is composed of 3 zones, figure 1). The borehole profile can be modified. For an horizontal borehole profile, the substratum'is parallel to the borehole in zone 1. It has a 1 .2° dip in zone 2, and a 4 .9° dip at the beginring of zone 3. At the end of this zone, it is parallel to the borehole. The reservoir layer is made of concrete in zone 1. Zones 2 and 3 can be filled with different types of material (i .e . sand) to simulate different types of reservoir layers .
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Steep dip wave equation migration by the finite difference plus phase shift hybrid method
By C. M. HaddowTwo vital requirements of a practical migration scheme are that it should be able to accommodate velocity variations and that it should have no dip limitation. The finite differente method for seismic migration has been known for many years, and remains popular since it caters well for the first of these requirements. However, the low order approximations, usually known as the 15 degree and 45 degree equations, are dip limited and do not preserve steeply dieping events. The dip response can be improved by using higher order approximations to the wave equation; for example, Ma (1982) showed how a factorization of the continued fraction expansion could be combined with a splitting techniquè to obtain steep dip solutions. These higher order methods may be computationally inefficient, however, due to the number of iterative stages that make up a single step of wavefield extrapolation . Other techniques exist which aim for better dip handling capability. These include the explicit formulations developed by Harris (1979), Koehler (Sengbush, 1983), Beaumont et al. (1987) and Holberg (1988). They take the foren of convolutional operators and are usually table driven since the operator is velocity dependent. The accuracy of the result will depend on the length of the operator, but of course longer operators will be more expensive to use.
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True amplitude aspects of pre-stack depth migration
More LessIn the past decade, pre-stack depth migration algorithims have been developed to image complex geologic structures in a proper way. however, in recent years it has been shown that, if seismic inversion is formulated as a sequence of processing steps, pre-stack depth migration can also be sean as one of the principal steps in seismic inversion. This means a.o. that the interest in the amplitude behaviour of seismic migration has been significantly increased. Bleistein (1987) uses the method of stationary phase to compute true amplitude reflection coefficients. In a recent paper Hubral et al (1991) discus the problem of true amplitude zero offset time migration, using raylracing arguments . In the past we have shown by wave theoretical argument show to apply true amplitude pre-stack depth migration to shot records (see e.g . Berkhout, 1984, chapter VII and Wapenaar and Berkhout, 1989, chapter XI). In this paper we will show, also by wave theoretical arguments, how to apply true-amplitude pre-stack depth migration to any subset of pre-stack data such as CMP gathers and common offset sections. Of course, the proposed process is suitable to handle irregular offsets as well.
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Steep DIP migration by replacing vertical with horizontal propagation
By L. E. BergThe algorithm presented in this paper uses the implicit finite difference scheme (45 degree approximation), for the propagation. The advantages by choosing this alternative for a migration implementation are well known: Since the extrapolation operator is only 3 points long, it is robust in cases with considerable velocity variations. In addition, run time on computer is very short, compared to some other algorithms. The disadvantage is of course the low performance for steep dips
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FK DMO for depth variable velocity
Authors M. A. Brzostowski, H. A. Meinardus and K. SchleicherWe have implemented a practical DMO algorithm by incorporating Hubral 's stacking velocity theory into the Jakubowicz approach. This methodology allows for arbitrary velocity variations in depth and producer DMO impulse responses with triplications. We have analyzed the operator response for a constant velocity gradient. The resulting DMO impulse response adequately matches the ray-theoretical response. The method may also be extended to 3-D.
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Equalization of DMO for irregular spatial sampling
Authors C. J. Beasley and R. KlotzSince the introduction of dip moveout (DMO), various authors have extensively explored its amplitude and phase accuracy, the effects of variable velocity, operator aliasing, discretization, and computational efficiency. A lens familiar aspect of DMO that is often of crucial importance in everyday seismic data processing is the effect of sparse or irregular spatial sampling on DMO-processed data. While different facets of the problem have been addressed (see e.g., Black and Schleicher, 1989; Ronen, 1987; and Williams and Marcoux, 1989), a comprehensive, accurate, and efficient solution to the problem is still needed. In this paper, we propose a method based on the decomposition of DMO into its constituent dip components that accounts for the effects of irregular spatial sampling for both flat and dipping events, can be applied to any 2-D or 3-D DMO algorithm, and is computationally efficient.
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An implementation of anisotropic migration: some issues and examples
More LessA phase-shift based migration algorithm for transverse isotropy (TI) is described where the downward continuation operators are derived from the corresponding phase-shift propagators. The operators are applied in the space-frequency domgin either explicitly or implicitly. Results demonstrate that an extension to the standard implicit stencil is required when migrating with nonelliptical velocity models. The number of coefficients required depends very much on the degree of anisotropy and the mode in question. Some conclusions are drawn as to the number of coefficients required for a given maximum phase angle . The modification of explicit operators for anisotropy is relatively straightforward and they are used for TI media with tilted symmetry axes. For this implementation (one-way downward continuation) both the ray velocity and phase velocity have to be downwand going. For media with tilted symmetry axes, where some of the upward going energy corresponds to downward going plane waves and vice versa, the range of phase angles used in the operator design has to be limited accordingly. The method has been implemented for both prestack and poststack migration. The issue of deriving the velocity model using modifications to the standard surface seismic processing sequence is also addressed .
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