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54th EAEG Meeting
- Conference date: 01 Jun 1992 - 05 Jun 1992
- Location: Paris, France
- ISBN: 978-90-73781-04-7
- Published: 01 June 1992
21 - 40 of 405 results
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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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