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5th EEGS-ES Meeting
- Conference date: 06 Sep 1999 - 09 Sep 1999
- Location: Budapest, Hungary
- ISBN: 978-94-6282-119-4
- Published: 06 September 1999
121 - 140 of 196 results
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Natural hazard and technogenic changes in the Zlatitza-pirdop graben system, Bulgaria
By D. AngelovaThe Zlatitza-Pirdop graben system is situated between the Balkan, the Sredna Gora, Galabets and Koznitsa mountain ridges. It includes three grabens formed during the Neogene and the Quaternary. The origin of the oldest of them was during the Middle Pontain. The contemporary outlook of the system was formed during the Plio-Pleistocene. The total area is 220 km2. In a regional plan the system represents a part of the Sub-Balkan graben system. It was formed as a result of a regional extension at the background of the Sub-Balkan listric fault zone (Tzankov et al., 1996). Rocks of diverse and rapid facial changeability (granites, diorites, Quaternary sediments, etc.) are observed in the grabens and along their periphery.
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Characteristics of the natural hazard and degree of disturbance of the ecelogical comfort in the Varna Bay, Bulgaria
By D. AngelovaThe Varna Bay is one of the greatest limans along the Black Sea coast with its area of 41.8 km2. It is situated in the Moezian microplate with craton structure, in the North Bulgarian Arch deformed morphostructure. The development of the Varna Bay is closely connected with the development of Euxino-Caspian and Black Sea basins during Neogene and the Quaternary. The bay’s territory was a part of the Varna depression during the Neogene. As a result of the listric extensional tectonics during the whole period of the Neogene and the Quaternary, it had been differentiated in blocks and gravitation benches were formed. In a contemporary structural plan they include land blocks and transient blocks from the land to the sea water area. This part of the Black Sea continental borderland preserved its negative deformations and was loaded by sediments during the whole period of the Neogene and the Quaternary.
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Seismic field measurements applied to static geotechnical problems
More LessThe paper tries to illustrate with practical examples that the loading rate during seismic field and laboratory tests is lower than that of many conventional geotechnical field and laboratory tests. Geotechnical parameters are needed for different design situations, such as: 1) construction activity, e. g. pile installation, soil excavation or soil placement, soil compaction, construction of structures or buildings etc. 2) short-term loading due to cyclic or dynamic loading, e. g. earthquakes, wind or water wave action, filling or emptying of silos, building live loads etc., and 3) long-term loading of structure, e. g. settlements of structures and buildings. Figure 1 shows typical loading rates for construction activities and building loads, which extend over many orders of magnitude.
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Comparison between different S waves velocity determination methods in soft formation
Authors B. Ludovic, C. Dominique-Marie and H. FrançoisIn situ determination of elsatic moduli is a helpful tool for geotechnical engineers in evaluation of soil stress-strain response. The link between static elastic moduli obtained in laboratory and dynamic ones have been shown using the frequency dependant Biot’s theory. Soil samples have been collected in a 28 m borehole (See Fig. 6). Density, matrix density and porosity have been determined in laboratory as well as P and S wave velocities in a triaxial cell in order to confined the sample as it was in situ. In addition, static elastic moduli in very small deformation area have been performed.
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Seismic investigations for a metro line crossing a river
Authors Zs. Prónay, E. Törös, L. Hermann, B. Neducza and B. ElekIn Budapest, Hungary a new metro line is plant which crosses the river Danube about 20 m below the bottom. The geology is rather complex. On the Buda side there is the dolomite structure of the Gellért hill with a height of 150 m above the Danube level. On the Pest side the dolomite can be found in the depth of about 200 m. Between them there are classical horst and graben structures under the river bottom covered by eocene and oligocene clay and marl and holocene sandy gravel. The fractures in the dolomite are in connection with the famous hot springs of Buda. Beating those fractures with the tunnel can cause the dry out of the springs.
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Three dimensional characterisation of rocks using reflection seismic tomography
Authors B. Celse, Ph. Côte, O. Abraham, B. Piwakowski, B. Vayssade and J. P. HenryIn mining works, seismic methods can be used to characterise rocks and to detect and localise accurately ore. In this paper, we present results of an experimental study in a mine. The method proposed is similar to Vertical Seismic Profiling (VSP) which has been widely used in the field of borehole seismic. The major difference between these two methods is in the configuration of the survey line. In contrast with VSP, the borehole is horizontal and shotpoints are located in the tunnel face. A reflection tomography algorithm ([1]) is then used because it can provide both a map of velocity and reflector location.
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Ultra-high resolution shallow water-borne seismics
Authors R. Vida, T. Tóth, F. Horváth and P. SimpkinDuring the past five years several single-channel water-borne shallow seismic surveys have been carried out using a special instrument called IKB SeistecTM. One of the main advantages of the SeistecTM profiler is that the source (a boomer) and the receiver (a line-in-cone group of piesoelectric hydrophones) are mounted on the same catamaran body. This way a fix offset is ensured during the whole time of operation. The 90° aperture focusing cone around the hydrophones has a very good vertical selectivity and almost completely eliminates direct waves and side reflections. In addition, as the water is almost still inside the cone no noise is created by the friction between the hydrophones and the water. Therefore signal-to-noise ratio of the SeistecTM records is usually higher than for conventional streamer records.
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Shear wave energy from seismic sources subjected to varying soil-coupling stresses
Authors L. Areias, W. F. Van Impe and W. HaegemanABSTRACT: Modern downhole seismic cone penetration tests (SCPTs) are used in geotechnical studies to determine shear wave velocity and dynamic modulus profiles in soils. Although recommended test procedures exist for the SCPT, no standard specification is available for the source's size, type, impact energy and coupling stresses. Results of tests performed on three steel beams subjected to varying coupling stresses, as part of an ongoing research program on seismic response of SCPT sources, are presented. The test results confirm an increase in signal energy with increasing coupling stress as previously known. However, the results show that this increase occurs relatively fast and reaches a maximum at low coupling stresses between 15 kN/m2 and 28 kN/m2, irrespective of size and type of the source, for similar impact energies. One of the beams with metal blades, which were added to improve coupling, performed better than the others with smooth coupling surfaces.
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Using surface waves for estimating shear wave velocities in the shallow subsurface onshore and offshore Israel
More LessAnalysis of data from conventional (P-wave) seismic refraction and shallow reflection surveys conducted in various areas of Israel shows that in many cases these data can be successfully used to retrieve information on shear wave velocity (Vs) in the shallow subsurface. The Vs estimation procedure is based on dispersion analysis of different kinds of surface waves contained in seismic records.
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Anisotropic seismic and electric cross borehole tomography at a hydrogeological test-site
Authors J. Herwanger, C. Pain, A. Binley and M. WorthingtonUnderstanding fluid flow in fractured rock is of importance in many different fields as diverse as the development of oilfields, contaminant transport along fractures and the supply of drinking water. Geophysical tomographic techniques yield structural information on the examined rock mass, e.g. the distribution of seismic velocities in seismic tomography or the distribution of electrical resistivities in electric tomography. The advantages of joint inversion of seismic and electrical data are frequently discussed although there are relatively few examples of field experiments. The main emphasis of this study is the relationship between seismic and electrical anisotropy within fractured rock. It has long been recognised, that the presence of fractures will result in anisotropy of the physical parameters examined. For example electrical current will flow preferentially along fractures, resulting in a high conductivity along the fracture plane and low conductivity perpendicular to the fracture plane. If this anisotropy of physical parameters is not accounted for in the tomographic reconstruction algorithm, the resulting image of the subsurface can be severely distorted. The Reskajeage Quarry Test Site offers the unique opportunity to carry out geophysical experiments and ground truth the results in a well studied hydro-geological locality.
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Ultra-high resolution seismics for monitoring the distribution of fluvial deposits below the river Danube
Authors T. Tóth, R. Vida and P. SzafiánSeveral ultra-high resolution seismic surveys have been carried out on the river Danube during the past few years. Main objective of the surveys nearby Budapest was to map the distribution of fluvial deposits, mainly gravel and sand, under the river bottom. Mapping and monitoring the distribution of gravel beds is crucial for water management, as water supply for the whole city (more than 2 million people) is provided from bank-filtered water resources. Ultra-high resolution river seismic surveys can provide a very detailed (0.1 m resolution) image of the river deposits and the underlying older strata. The method has been demonstrated to be a powerful tool for mapping the 3D distribution of the fluvial sediments.
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Seismic hazard estimates using site intensity data and correlation with In-Situ measures of amplification effectsin the Val D’Agri area (Southern Italy).
Authors M. R. Gallipoli, D. Albarello, G. Calvano, V. Lapenna and M. MucciarelliIn this work we approach the problem of the estimates of site seismic hazard for many towns and villages located in the Val d’Agri area (Southern Italy) that in the past were interested by many seismic events (Fig.1). The estimates have been carried out using a method that is based on the analysis of site seismic history extracted from macroseismic catalogs (Magri et al., 1994). To point out the influence of site effects two different procedures have been performed: in the first one seismic hazard estimates have been deduced from epicentral data only, in the second, intensity data observed at the site have been also considered. In tab.1 the results are shown, in the first column seismic hazard estimates deduced from epicentral data are reported, while in the second one seismic hazard estimates obtained not only from epicentral data, but also intensity dates observed at the site are indicated. The differences between the two estimates can be correlated with local variations of seismic response due to local geological features which are responsible for possible cases of amplification. In order to validate the presence of such correlation, seismic hazard estimates have been integrated by the use of site amplification measurements obtained using the Nakamura’s technique (Mucciarelli, 1998). This method is based on spectral analysis of the ratio between horizontal and vertical component of microtremors (Fig.2). Our findings reveal a good correlation between seismic hazard variations and the presence of site amplification effects. With a non-parametric test it is deduced a good correlation between hazard estimate augmentations and site amplification in the frequency band of building (Fig.3). The application of this kind of analysis to Val d’Agri area has pointed out that the estimates of site seismic hazard and site amplification measurements could be a powerful tool to solve many problem related to seismic microzonation.
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Attenuation versus fracture using different methods
Authors M. Ali Riahi and M. PirasstehAccording to the present study about seismic field data of the North east part of IRAN, relationship between velocity, fractures and attenuation of rocks was investigated. The seismic refraction data was produced by blow and explosion sources. In this paper, an algorithm was developed that performs many stages. The stage's sequences are consist of separation of refraction and reflection signals, correction due to geometrical spreading, resampling and transformation of data to frequency domain, filtering (low pass or band pass filtering), spectrum of data, and then pass from pan filter. Consequently, determination of absorption spectrum of rocks and quality factor (Q) was attained.Processing of these data showed that due to increase of fractures, the values of VP and VS decrease considerably. Then, amplitude of observed data and determination of quantities such as absorption coefficient α(f) and quality factor (Q) for P- and S-waves in rocks were performed. For a partially dry rock the results obtained in fractured area suggest that, while fracture increases in rocks the absorption increase and factor decrease.
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Impact shear wave generator for dynamic shallow soil characterisation
Authors F. E. Almeida and R. Marques MouraP and S waves are used to determine Poisson ratios that are useful for micro zoning in seismic engineering studies. P waves are easily generated by many vertical impact sources. On the other hand S waves are commonly obtained by a polarised sideways impact on a heavy beam perpendicular to the line of geophones (MOONEY, 1973). Any S wave source always produces some P wave component that can be extracted from the paired field data. Each pair consists of two opposite polarity records generated by inverted directional impacts. Recently developed sources are based on a vibration sweep pulse (GHOSE et al, 1996) requiring dedicated hardware and processing software and thus making it impossible to use with standard engineering seismographs.
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P- and SV-wave velocity-depth profiles from 2-component recordings
Authors B. Novotny and E. BrücklFor a great variety of engineering geophysical projects information about P- and Swave velocities is important. One way to get this information is the recording of standard Pwave lines and an extra excitation and recording of SH-waves. The value of this technique is proven; however, the separate excitation of SH-waves by inducing horizontal forces in the ground is sometimes difficult and time consuming. An alternative way of obtaining the equivalent information is to excite both P- and SV-waves by a single vertical force applied to the ground surface, and to record simultaneously the X- and Z-component of the ground motion. Applying this technique both P- and SV-waves are present in the records and they have to be separated by proper processing. The investigations presented here are an application of this alternative method, concentrating on direct and refracted waves. They are based on 2- and 3-layer synthetic data sets generated by elastic finite element calculations and on real data sets acquired in the course of investigations into large landslides in Austria within the scope of IDNDR. The topographical and ground conditions here would have made it very difficult to induce the horizontal forces in the ground required for the SH-technique.
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Shallow seismic reflection results in the Vouga basin region (Northern Portugal)
Authors R. Marques Moura and M. Senos MatiasThe shallow seismic reflection method has two basic modes of employment: CMP gather acquisition (multichannel) and Optimum Offset acquisition (single channel). These two modes of acquisition were used in a coastal area near Ovar (Northern Portugal). In this setting, a series of profiles were conducted with varying modes of acquisition. The main variations were placed in terms of geophone frequency (damped 14 and 100 Hz), source (hammer and buffalo gun) and single or multichannel acquisition. Here in, a comparative study of the results, based on acquisition, processing and interpretation will be shown.
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Multiple suppression for shallow water riverine seismic sirveys: processing tricks versus a new data acquisition method
Authors Dr. L. Oszkó and Z. HámoriWater-bottom multiples represent a principal limitation of the application of single channel riverine seismic surveys. The high impedance contrast at the bottom and surface of the water column causes strong reverberations, which often mask the deeper reflections and make the interpretation difficult or impossible. This paper shortly addresses the advantages and problems of the different means of multiple suppression and suggests an alternative new solution applicable for the special case of single channel shallow water seismic profiling. The suggested method has been tested by finite difference modeling and a small-scale basin model.
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“SwyphoneTM”: A New seismic sensor with increased response to SH-waves
Authors L. Sambuelli and G. Piero DeiddaIt is well known that reflection seismic is an extremely powerful technique of investigation but, at the same time it is also quite expensive. The related costs, mainly as far as the shallow high resolution surveys are concerned, limit an extensive application of it. This is the reason why today research is mainly focussed to a technological improvement so that data could be acquired more and more quickly and with a signal to noise ratio more and more favourable. Although several authors have demonstrated that certain situations could be addressed more effectively through the use of SH-waves rather than the more commonly used P-waves, SH-waves are far from being a widely used exploration tool since they involve higher costs than P-waves. When suitable circumstances occur, and the appropriateness of the method is verified, the key to the successful use of shallow seismic reflection is its cost-effectiveness, that is, a quick data-acquisition and minimal data processing. Usually, SH-wave data acquisition involves the use of polarized sources. SH-wave records require two shots, striking the source in opposite directions perpendicular to the seismic line, which must be subtracted to eliminate P-wave contamination. This means an increase of survey costs with respect to similar P-wave survey. Also, it is common experience that effective P-wave removal requires the equalization and the removal of time break variations of shot records prior to subtraction. This means additional processing time and, as a consequence, additional costs. In order to avoid the above drawbacks we have designed and realized (Sambuelli and Deidda, 1998) a new receiver that enables to acquire SH-wave data without the abovementioned additional costs. Using this receiver, SH-wave records, without or with minimal P-wave contamination, can be achieved performing single shots and avoiding extra processing. Consequently, SH-wave survey can be performed as well as P-wave survey, without involving traditional extra costs. In an attempt to quantify the significant characteristics and field performances of the new receiver, some laboratory (electrical and mechanical) and field (with different acquisition parameter settings) tests with consistent testing procedures and equipment were carried out. Besides, in order to have a comparison between this receiver and the traditional one, all tests were carried out on two sets of receivers: 24 SwyphoneTM type receivers (prototypes) and 24 Mark Product (L-40 A2) 100 Hz horizontal geophones. All results prove that the SwyphoneTM has better performances and so, it is an improvement in SH-wave reflection seismology that could be of great assistance to engineering seismologists. In particular, with respect to traditional geophones, it has a higher sensitivity to SH-waves, higher energy efficiency and allows cost-effectiveness data acquisition and data processing. The higher sensitivity to SH-wave and higher energy efficiency of the SwyphoneTM type receiver, if it is mounted together with a vertical geophone, allow performing P- and SH-wave simultaneous data acquisition. Figure 1 shows a P- plus SH-wave record acquired with a 48- channel ABEM Mark VI seismograph. Odd channels recorded P-wave signals while even ones recorded SH-wave signals. The P- and SH-wave records can be obtained by traces sorting.
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Some evidence of the recent tectonism in the central part of the Pannonian basin
Authors G. Detzky, K. D. Lőrincz, K. Tevan and T. MarkosFormer presumptions about the presence of the recent tectonic processes in the central part of the Pannonian basin have been emphasised by presentations and some papers in the late years (References). Two shallow seismic sections have been measured by ELGI on the frozen lake surfaces in promising locations relaying upon known exploration seismic sections (Fig.1.). The result of the finite difference seismic modelling carried out for the preliminary testing of special conditions and certain aspects on the field execution of the measurement will be provided. The technical information of the equipment and measurement technology also will be presented. The distinct quality increasing operations in the processing of the actual shallow seismic data will be shown which had been made possible by the presence of the ice surface on the site. On the basis of the integrated interpretation of these shallow seismic sections (Fig.3.), former hydrocarbon exploration seismic profiles measured by ELGI and well data (Fig.2.) of the studied area, a conclusion can be drawn about the presence of such recent compressive tectonic processes related to the tectonic evolution of the Szolnok-Flysch, which are already proven on the other parts of the Pannonian Basin.
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High resolution multi-channel seismic reflection imaging of the Rhone delta, Lake Geneva (Switzerland)
Authors G. Tacchini, O. Zingg, M. Beres, F. Mariller and M. ScheidhauerLake Geneva, located in western Switzerland along the French border, is one of the biggest European peri-alpine lakes. The Rhone River contributes 80% of its sediment influx from the alpine valleys, and its sublacustrine delta is 10 km long. This lake formed in a glacial trough that was carved in an Oligocene-Miocene molasse basement. The deposits left by the retreating glacier and the fluvio-lacustrine sediments are up to about 500 m thick. Past seismic studies in this area, mostly single-channel surveys, were unable to provide sufficient information on the deep internal structure of the delta. The aim of the present work was to test more efficient and powerful methods of water-borne high-resolution seismic reflection that would allow a deeper penetration. During our 1998 acquisition campaign, we recorded 24 km of data that were divided into 4 lines (Fig. 1). The following instruments were used: a double-chamber 30 cu. in. airgun (bubble-suppressor Mini GI gun), a 48-channel streamer array, a Bison seismograph and a differential GPS positioning system. The air gun covered a frequency spectrum of up to 400 Hz, and the survey design allowed a 9-fold data coverage. GeovecteurPlus software from the Companie Générale de Géophysique was used to process the data. Conventional processing was applied: trace editing, geometry assignment, spherical divergence correction, spiking deconvolution, bandpass filter (40-56-350-494 Hz), AGC gain (30 ms window) and coherency filter. The 9-fold coverage provides a better signal/noise ratio than single-fold coverage and allows different seismofacies to be more clearly distinguished (compare Figs. 2a and 2b with 2c). Migration corrected reflection geometry and collapsed numerous diffractions associated with complex structures (compare Figs. 2a and 2b). Currently, attempts are being made to attenuate an intermittent but strong water-bottom multiple that may obliterate some deep reflections (Figs. 2a and 3). In many areas, the profiling penetration reached more than 100 ms beneath the water bottom which is equivalent to a sediment thickness of about 100 m. The high resolution of the data allows us to identify several typical sedimentary features within the delta. Figure 2 shows a prograding facies (facies A) with foreset (dipping NW), bottomset beds and the underwater Rhone channel with its natural levees. This facies is interpreted as prograding sediments originating from the actual channel and downlapping onto horizontaly bedded deposits (facies B). These deeper layers may have formed when the mouth of the Rhone River was further east of its present location. The top of this facies can be identified on both lines (event D). A third, relatively hummocky and faulted facies (facies C) is identified underlying facies B. Some of the folds are indicated in Figs. 2a and 2b. All three facies belong to the Quaternary fluvio-lacustrine sequence of the Rhone delta. In places, the absence of seismic reflection may be caused by the presence of gas within the upper sedimentary layers (Fig. 3). Gas occurrence has already been observed elsewhere in Lake Geneva.
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