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8th EEGS-ES Meeting
- Conference date: 08 Sep 2002 - 12 Sep 2002
- Location: Aveiro, Portugal
- ISBN: 972-789-071-7
- Published: 08 September 2002
61 - 80 of 131 results
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Tomographic formation mechanical stability estimation using borehole radial gradient of bulk modulus distribution
Authors C. Pretzschner and H. LindnerKnowledge of mechanical stability of the near borehole formation is fundamental in: geotechnical investigation • • • defining optimal locations for deviation of horizontal drilling paths reducing risks of borehole collaps through scattering of formation particles in the the borehole while production (sanding) The respective information may be obtained by laboratory measurements directly on cores or in situ as a result of borehole-caliper evaluation or sonic-log interpretation. Normally the borehole-surface is destroyed by the drilling process (microfractures, breakouts) and the measured travel times will be influenced by a medium of lower-velocity. Because of increasing velocity in higher penetration r the seismic energy is traveling as diving waves through the formation in this case. Using a multi-array sonic-probe and low shot-point distances, the high density of travelpaths through the formation allows imaging the formation velocity distribution by an In-Borehole Tomographic Reconstruction (IBTR). These 2D-images especially reflect the sensitive mechanical behaviour in the transition from mud to formation besides an accessible higher vertical resolution and exclusion of caliper effects (Fig. 1).
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Contribution of well logging to hydrogeology investigation
More LessA study area – “The Bohemian Cretaceous Basin” forms the north-eastern part of Bohemia (Fig.1). The main reason for selecting the central part of this structure as an object of the study was its significance for water management, in terms of important natural groundwater resources of mineral water in the vicinity of the Poděbrady spa. The aim of the study is to synthesise available data, to assess natural groundwater resources and to keep the groundwater safe. Furthermore, the goal is to propose ways of rational groundwater development, to consider the impact of groundwater overexploitation in the Poděbrady spa, as well as that of industry, agriculture and landfills on the Poděbrady vicinity environment. This study tends to summarize the results of different earlier logging measurements (62 boreholes) together with surface geophysical interpretation and by this way to yield important information for hydrogeological conclusions.
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Designing surface-to-borehole electrical resisitivity tomography surveys using the frechet derivative
Authors L. Marescot, S. Palma Lopes, R. Lagabrielle and D. ChapellierCrosshole resistivity imaging is increasingly used in civil engineering, groundwater and environment investigations: the cost of equipment is low and the field measurements are easy to perform. Such arrays are able to yield valuable information on the variation of resistivity between the boreholes (e.g. Daily and Owen, 1991). Resistivity imaging using buried electrodes permits a greater accuracy and resolution than what can be obtained with surface arrays. The strong imprints of near surface inhomogeneities are reduced whereas the resolution at depth is increased since the sensors are closer to the structures of interest. Nevertheless, crosshole resistivity imaging surveys are frequently prohibited on the working site. There is either one single borehole available or the distance between two boreholes is too large for crosshole resistivity investigations (the distance between the boreholes should be comparable to the length of the boreholes). In this case, surface-to-borehole (or borehole-to-surface) resistivity measurements are performed by placing a current source on the surface (or downhole) and measuring the potential field in a borehole (or on the surface). Geophysicists are probably more familiar with surface arrays since they have been using Wenner, Schlumberger or dipole-dipole configurations for many years. Resistivity surveys with non-conventional arrays (e.g. surface-to-borehole arrays) are more rarely carried out. This prompts us to find a tool for designing such surveys.
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Multifrequency microwave diffraction tomography: Application to ground imagery by transmission
Authors R. Tarnus and X. DérobertAmong the important number of geophysical methods dedicated to ground analysis, those based on the propagation of electromagnetic waves at radar frequencies have gained a lot of interest these last ten years. Their non-destructive capabilities in addition to the technological advances in antenna design and signal processing played an important role in their success, especially for the Ground Penetrating Radar, which is commonly used from Field Geophysics field to Civil Engineering, and also for tomographic applications [1]. The multifrequency microwave diffraction tomography, presented in this paper, is based on the measurement of the scattered electric field for several frequencies after it has diffracted in the ground. There are several applications for this technique, such as human body imaging [2]. Applied to ground analysis, its goal is to provide an image of the ground that is relevant of its electromagnetic properties. The technique is applicable, in theory, in reflection from the surface as well as in transmission around walls, or from boreholes [3]. In the latter case, we would obtain an image of the vertical section of ground between the two boreholes, enabling to reduce the number and therefore the cost of the investigations. This technique has already been studied trough several PhD theses in the LPC network (Laboratoires des Ponts et Chaussées) [4,5]. In the present work, the interest will be mainly focused on the transmission mode from boreholes, with an approach of numerical modeling using laboratory experimental results. The goal is to develop a cross-hole microwave imagery technique that could be applied to the localization of objects like cavities or utilities.
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Use of geophysical borehole logging techniques and depth sampling methods to characterize the hydrogeochemical heterogeneity of the aveiro cretaceous multilayer aquifer
More LessThe Aveiro Cretaceous aquifer is a multilayered coastal aquifer formed by several hydrostratigraphic units with distinct grain sizes and mineralogical compositions, and with different hydrogeological properties. As is common in the region, most boreholes exploiting the aquifer have increasing depths towards the coast following the aquifer thickening and are multi-screened, pumping water from the different aquifer layers with known distinct hydrochemistry and residence times. The use of conventional methods of groundwater sampling from pumping multiscreened boreholes in the Aveiro Cretaceous multilayer aquifer inevitably leads to water mixing between the different aquifer layers, making the understanding of the water quality patterns more difficult, and eventually, leading to misinterpretations. To better address this problem or this uncertainty, it was decided to select seven non-equipped boreholes within the study area for detailed geophysical logging studies and depth sampling. The integration of lithologic logs, geophysical logs and depth sampling hydrogeochemical results allowed to confirm the borehole construction and to observe the vertical variation in the formation, aquifer and fluid properties (temperature and salinity). Flow logs showed that the Furadouro sandstone formation is contributing with most of the water supplied by the aquifer, with a ratio for the transmitted flow that could be as high as 9:1 when compared to the other aquifer layers. The depth sampling allowed the hydrogeochemical characterisation of each aquifer layer confirming the most permeable and productive layers as the ones with youngest and lowest salinity waters.
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Electromagnetic fields generated by vertical current in volcano Etna
Authors E. Fainberg and B. Singere based on electrical methods, magnetotellurics, transient electromagnetics, and other electromagnetic methods. The source of the electromagnetic field in these methods is located either immediately at the Earth surface or above it. Methods using external sources to monitor tectonic processes are classified as active methods (Svetov 1992). Alternatively, passive methods of monitoring rely on measurement of electric and magnetic fields caused by volcanic or tectonic processes inside the Earth. A number of electrokinetic, piezoelectric, thermoelectric, and other physical and chemical phenomena can be responsible for generating electrical currents in the interior of a volcano. Electric and magnetic fields induced by these currents are known to be measurable. Electrical structure of the Earth near a volcano is always laterally heterogeneous. Interpretation of measurements made in such conditions is a challenging task. On the other hand, as shown by Fitterman (1979) and will be further discussed in this publication, the magnetic field of the internal origin is often observed at the Earth surface only because the Earth is electrically heterogeneous. The magnetotelluric field caused by ionospheric and magnetospheric currents obscures the internally generated field. A significant suppression of the magnetotelluric noise can be achieved by using linear relationships between components of the electromagnetic field at different locations on the Earth surface. As shown by Svetov (1992), a reduction of the magnetotelluric noise by the orders of magnitude can be accomplished by finding the transfer functions that relate the electromagnetic field at an observation site to the magnetic field at the reference site (base), and using these transfer functions to forecast the magnetotelluric noise at the observation site from the magnetic field measured at the base during the monitoring periods. In this approach, the transfer functions are determined during "seismically quiet" periods. During the “monitoring” periods, the transfer functions are used to eliminate most of the magnetotelluric noise from the signal recorded at the monitoring site. The sites must satisfy rather different conditions. The monitoring site must be located in the area where the signal of the anticipated internal source is sufficiently strong to be registered. The reference site should satisfy the exactly opposite requirement, i.e. the field of the internal source should remain at a negligible level even during periods of seismic activity. On the other hand, the monitoring and reference sites must remain within the coherency distance of the magnetotelluric field. Both the internal and magnetotelluric fields are scattered by geoelectric heterogeneities. It can be expected that features of the subsurface conductivity distribution, relief, proximity to the seashore, as well as presence of active faults in the Earth crust may have a direct effect on the level and structure of the internal and magnetotelluric fields. The main purpose of this publication is to demonstrate on the example of volcano Etna that modern approaches to 3D modeling, including the so-called generalized thin sheet modeling, allow for a reliable simulation of effects of lateral heterogeneities on the field of the internal source, thus allowing for developing recommendations for the optimal allocations of the observational and reference sites.
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Geophysical surveys for the investigation and monitoring of landslide areas
More LessDuring recent years several large landslide hazards took place in Austria. One of the largest happened at Sibratsgfäll, where about 70 million m3 and an area of 1,4 km2 was involved in a hazardous mass movement, that disturbed 13 buildings completely. Starting on the 19.May 1999 after three days of heavy rainfall, the whole area of Rindberg started to move with a maximum speed of 10m/day. Immediately after this event, a multi-disciplinary research program was started to investigate the structure and triggering cause of this landslide and to develop a geoelectrical monitoring system to register pre-alarm critical values. The research program included: • GPS measurements to monitor actual movements • Airborne geophysical measurements involving measurement of electromagnetic field, gamma ray, magnetics, infra red temperature and soil humidity • Detailed geological and hydrological mapping • Ground geoelectrical surveys on selected lines • Development of a geoelectrical monitoring system, with additional hydrological and meteorological monitoring parameters for calibration purposes Within this talk result of the structural investigations, focusing on the geophysical work, as well as the first results from a new geoelectrical monitoring network will be presented.
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Slow active fault detection and imaging using multiple geophysical methods
Authors F. Nguyen, S. Garambois, D. Chardon, D. Jongmans, O. Bellier and D. l’HermitteDespite its low to moderate seismic activity, most of Western Europe is characterized by a relatively high seismic risk due to the concentration of human and material properties presenting high-vulnerability. These regions have been affected by destructive earthquakes like the Ms=6 Lambesc earthquake, France, 11th June 1909, generating VIII-IX intensity values. Consequently, detecting tectonic structures that may lead to destructive earthquakes in such areas characterized by slow deformation rates, weak seismicity and dense vegetation coverage requires a multi-disciplinary approach which includes applied geophysics. The present geophysical study has been conducted in Provence (France), in the area touched by the 1909 Lambesc earthquake (Champion et al., 2000). The Provence domain is located at the transition between the Pyrenean and the Alpine mountain belts. It is bounded by the Nîmes fault to the west and by the external alpine thrusts to the east (Figure 1). Provence is affected by numerous east-west trending thrusts that have either northward (Sainte-Baume, Etoile, Ventoux-Lure) or southward (Alpilles, Lubéron, Costes-Trévaresse) vergences, and by regional NE-SW to NS trending strike-slip faults (Nîmes, Durance, Salon-Cavaillon faults).
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A combined geophysical survey on an active debris flow
Authors F. Bruno and F. MarillierThe Peillettes landslide in the western Swiss Alps is a complex landslide threatening villages of the main Rhone Valley. The slope is between 25 and 30 degrees. Trees cover a large part of the unstable zone which extends between elevations of 1000 to 2100 m; its width is 1000 m about. This slope is a known unstable zone and it was the subject of many studies, considering the potential impact on the neighbouring communities. In spite of many works to increase drainage of the run-off water, the zone remains unstable and it is reactivated after each particularly rainy period. The lower part of the landslide is the most active of the instability. Here a debris flow called le Grand-Tsâble is located few meters away from the closest country cottages. The goal of this work is to evaluate the contribution of several geophysical methods and in particular we will discuss the results of combined geophysical surveys: high-resolution P and S wave seismic reflection (PHR and SHR) and georadar (GPR). We focus on imaging the subsurface of the active debris flow and comparing the results of each method, which are often complementary.
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An underground railway project in Lausanne, Switzerland, as urban micro-gravity test site
Authors P. V. Radogna, R. Olivier and P. LogeanA new underground railway, named M2, is planned to be constructed within the next decade in Lausanne, the fifth densest populated city of Switzerland. In 1994, a preliminary geological report of the quarters chosen by the administration had shown the general conditions and features of the lithology of interest. The involved geology constitutes alpine molassic bedrock, an overlaying quaternary glacial fill and an overburden of loose soil. The choice of the corridor had to consider the exact thickness of these geological units. It was necessary to find out whether the best effective cost ratio would be an easier, but deeper, tunnel in a favourable lithology (Bedrock) or the technically more difficult tunnel in a superficial lithology (Glacial fill). Nearly 750 existing well loggings over the city, conducted for geological superficial surveys, test pits and geological maps were used to build a map of the depth to bedrock. Its surface was positioned with a precision of 3 to 5 m. The depth is varying between 2 and 15 meters. Once the corridor was chosen, the needed precision was increased. The requested precision in depth determination was about 1 meter. For this reason, more drillings were executed in 1999 and 2001 along the chosen corridor. The free availability of all data by the Road Service of the canton Vaud was an opportunity for testing a micro-gravity survey, integrated by other geophysical techniques, along the 6 km metro corridor.
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Geoelectric measurements in claystone for exploration of anomolous zones
Authors S. Kruschwitz and U. YaramanciUnderground excavations provoke micro- and macrofractures in the vicinity of drifts and galleries. A detailed knowledge of this excavation damaged rock zone (EDZ) is indispensable for the planning and dimensioning of geotechnical barriers for underground ultimate waste disposals in particular that for chemical, toxic and radioactive wastes. The host rock as well as the artificial, geotechnical barrier should avert any propagation or trespassing of virulent and radioactive substances. Still different geophysical methods are tested and adapted to find out the best assessment strategy for each kind of host rock. Seismics [1], microacoustics [7] and geoelectrics [8, 9, 10] turned out to be the most promising methods in claystone. Since the beginning of 1999 also complex valued geoelectric measurements underground in tunnels the have been successfully conducted
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Coastel erosion monitoring by means of geophysical methods
Authors G. Ranieri, R. Deiana, G. P. Deidda, S. Erriu and M. NuvoliOne of the most frequent problem in the islands is given by the erosive coastal process, due to the atmospheric agents and to the wavy motion. Moreover human activities favor and accelerate the modifications of the sea tides and the superficial erosive processes. Often, in fact, the losses from water-pipes or the construction of buildings and roads, modify soil structure, limiting and varying the natural groundwater flow and exposing the most alterable rocks to the action of erosion by waters and winds. In the present work the “physical condition” of the Promontory of Balai (Sardinia-Italy) is examined (fig.1). This zone is exposed to predominant winds in the island and wavy motion. Recently the construction of docks, near the harbour of Porto Torres, has produced acceleration of the erosion phenomena. The area is of particular interest for the presence of an old church (S. Gavino a Mare, XI a.C.) and for a built-up village. The final aim was to establish the sinking and settling risk. In order to know the evolution of the erosion phenomena in the inner part of the promontory, boreholes, electrical tomographies, seismic tomographies and a microgravimetric survey, have been executed.
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Nuclear magnetic resonance (NMR) properties of unconsolidated rocks and synthetic samples
Authors M. Müller, U. Krüger and U. YaramanciThe nuclear magnetic resonance (NMR) technique is used in geophysics mainly for well logging and laboratory applications. Consequently, the available investigations focus on the NMR properties of solid rocks, because of their significance as hydrocarbon reservoir rocks (Kenyon, 1992). In recent years Surface NMR (SNMR) has become available for hydrogeological applications (e.g. Shirov et al., 1991, Yaramanci et al., 1999). The need for well founded interpretation of SNMR field data has steeply increased the interest in NMR properties of unconsolidated rocks. In our work we therefore analyzed the NMR properties of synthetic and natural unconsolidated samples to enable an enhanced understanding and interpretation of SNMR data.
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Some features of the subsurface geoelectric structure in the sinkhole occurance sites along the dead sea shore line.
By M. EzerskyHazardous sinkholes started to appear in alluvial fans and unconsolidated sediments along the Dead Sea coast in 1990, when a small group of sinkholes 2 to 15m in diameter and up to 7m deep were formed in alluvial sediments in and around the middle of the main asphalt road close to Neve Zohar. Since then hundreds of sinkholes have appeared along the western coast from the Attraction Beach in the north to Nahal Hamar to the south of the Dead Sea. There are currently three theories explaining the sinkhole forming mechanism in the Dead Sea region. This could be a pseudo-sinkhole phenomenon (piping) for alluvial fan gravel holes, or mass mobilization and suspended clay transfer hypothesis for mud holes or salt layer dissolution mechanism. However, while none of these explains the sinkhole phenomenon completely, all the investigators agree that sinkhole formation is a dynamic process. It follows in time and could be accompanied by variation of the medium properties (density), salt layer or saline clays dissolution and/or fines washing out and transfer. The presence of fresh or salty groundwater plays an important role in sinkhole formation. Numerous geophysical investigations carried out in recent years were mainly aimed at detecting the sinkholes formation mechanism at depths of tens meters (salt layers, fresh water and large voids detection). In the present study we have used geoelectric methods to achieve a better understanding of the subsurface geoelectric structure at the sinkhole development sites, taking into account that electric parameters (such as resistivity or conductivity) are very sensitive to medium properties and their variations in time, for example, high porosity or voids including medium manifests the same as a high resistivity anomaly zone. In contrast, the water (different degree of salinity) as well as zones including clay appear as low resistivity anomalies. First, we reinterpreted previously obtained and published results of geoelectric surveys at the Neve Zohar, Nahal Hever southern and Ein Gedi Holiday Village sinkhole development sites. We then performed geoelectric mapping of the Ein Gedi Holiday village and palm tree plantation areas using the Continuous Vertical Electric Sounding (CVES) method (Ezersky, 2002) .
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Preliminary geological and geophysical investigations on a complex karstic area
Authors M. T. Carrozzo, G. Leucci, S. Margiotta, S. Negri and L. NuzzoThe growing urbanization of karstic areas and the diffuse utilization of chemical products in agricultural increases the probability of soil and groundwater pollution. The relevance of these problems in karstic areas forced decision-makers to produce a number of laws and regulation on this subject and to demand an increasing number of environmental assessment studies in those areas. The research we are at present carrying on is to define the impact of agricultural practices in a complex karstic area of Salento (southern Apulia, Italy), placed among Carmiano, Copertino, Leverano, Salice Salentino and Veglie villages (Fig. 1).
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Application of the seismic reflection method to abandoned mine detection in Óbidos, Portugal
Authors J. Carvalho, L. Torres, C. Galhano and L. Mendes-VictorThe municipality of Óbidos wanted to investigate the suitability of an area in Casal Caxinas, for construction of one or two story building. The surrounding area was an active gypsum mine, now closed for a few decades. An old mine map exists but serious doubts are cast over its validity since, for example, no gallery connects the shaft well to the extensive net of galleries, and some references seem misplaced. Several areas a few tens of meters from the planned construction site collapsed but the map shows no galleries at the site. The spot has been covered by several meters of mine wastes, that show some irregularities and fissures that can be caused by natural subsidence due to gypsum withdrawal or by galleries collapse. To verify the landfill competence and thickness and detect the eventual presence of old mine galleries, a reflection seismic survey was carried out. The water table was expected to be a few meters deep and the fact that the landfill materials were clayish prevented the use of georadar. The depth of the galleries (over 20 meters deep) and the short space available in some areas of the construction site (less than 60 meters) also did not allow the use of geolectric methods. A grid of seismic reflection lines was realised and the results interpreted with a few geothecnical mechanical soundings. Several meters of alluvium were detected beneath 5.5m of waste material, while the water table showed at about 7 meters depth. None of the soundings exceeded 20 meters and so no cavity was detected but also no signs of decompression in the alluvium were detected, indicating that no collapse occurred, even if galleries beneath the site of drilling exist. To the South of the study area, the presence of galleries is strongly suggested by the seismic data and a new map of the mine is created showing the limit of the galleries several meters further North. However, space is still available for the building construction and though the landfill material competence is weak, construction is possible provided some measures are taken.
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Detection of subsurface cavities by gravity methods
More LessThe detection of subsurface cavities by gravity methods has been investigated in recent years. We use some quantitative interpretation methods to determine the depth and location of the subsurface caves in a mountainous area located in the west of Iran. Key words: Euler deconvolution, analytical signal, Filon frequency, maximum depth, minimum depth.
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Field measurements of spectral induced polarisation on permo-triassic sandstone in the UK
More LessThe Triassic Sandstone is a major aquifer for western England and it has also historically been the location of much heavy industry. As industrial use of local groundwater recedes, water tables are rising and may remobilise contaminants held near the surface in the vadose zone. A robust method to survey for contaminants, and to be able to discriminate between different types, is desired. Measurements of Spectral Induced Polarization (SIP) is a potentially powerful discriminatory technique; however, before investigating the SIP properties of contaminated sandstone, it is important to understand the response of natural uncontaminated sandstones. This paper describes the use of SIP in field measurements on uncontaminated sandstones in various sites in the west of England.
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The spectral induced polarisation response of triassic sandstone from the UK
More LessThe Permo-Triassic sandstones form the second most important aquifer in the United Kingdom (U.K.) supplying about 25% of licensed groundwater abstractions in England and Wales (Allen et al., 1997). Geophysical methods have shown to have important applications in locating and characterising contaminants and determining the hydrological properties of the sandstone. In particular, SIP spectra of Triassic sandstone from the U.K. have already been shown to change dramatically with contamination (Scott and Barker, 2001). Low frequency Spectral Induced Polarisation (SIP) has been used to measure the electrical properties of samples of Triassic sandstone from around the U.K. in order to characterise the different SIP responses of this sandstone. Here we present the work carried out so far.
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Hidrogeological implications of Beja fault location using geoelectrical methods
Authors R. Rocha, A. Andrade Afonso and E. RamalhoDue to widely spread nitrate pollution that reaches the groundwater supply in the Beja area, deeper aquifer zones that may be considered as an alternative to the present situation must be identified. A deep unpolluted aquifer below the unaltered rock was detected using several surface geoelectrical methods: a dipole-dipole survey was therefore conducted, which made it possible to detect the main fracturing zones to a depth of about 600 metres, as well as deep alteration zones corresponding to possible groundwater circulation. More precise studies were then carried out in areas of interest detected during the above-mentioned survey, namely a Rectangular Survey and three Vertical Electrical Soundings (VES). Data interpretation has revealed a good adjustment with local geology and has allowed the determination of the approximate geometry and type of geological formations at these depths. It has also indicated the existence of two zones with potential groundwater circulation: one shallower and more vulnerable to the pollution; and a deeper aquifer, associated with the Beja Fault, less vulnerable to pollution.
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