Détection des cavités à l'aide de mesures électriques et du géoradar dans une galerie d'amenée d'eau

Old water-delivery tunnels can become leaky and produce water losses of some economic importance. The detection of solution cavities behind the concrete lining was the purpose of an investigation in a 2 km long part of a water-delivery tunnel in Switzerland. A gypsum zone of about 700 m length was considered to be especially critical. Two geophysical methods were used at the same time: resistivity profiling with three different array lengths, and ground-penetrating radar. The cavities were characterized by higher electrical resistivity, principally for the short array (AB = 3 m). Detailed measurements with a pole-dipole array were made for a better depth interpretation of anomalies in some critical zones. The GPR investigations were made with an antenna of 500 MHz. Some transverse profiles were carried out with GPR in order to better delineate the discovered cavernous or weathered zones. GPR located cavities up to a few meters in length, at depths between 0 and 4 m behind the lining. Anomalies found by one investigation method were often confirmed by the other one. Their complementarity thus allowed a more reliable interpretation of the gathered data. Verifying drillholes have mostly confirmed the good correlation between geophysical interpretation and drilling results. Endoscopic investigations in the drillholes enabled us to have a real vision of discovered cavities.Although GPR-profiling has the advantage of a continuous underground imaging and of greater efficiency during data acquisition (investigation speed) we consider the combination of both methods in the present study as a good example of an integrated geophysical survey, which enables a more reliable interpretation of the gathered data.

Résumé

Les galeries d'amenée d'eau d'un certain âge peuvent poser des problèmes à cause de pertes d'eau parfois importantes. L'objectif de cette étude a été la mise en évidence et le positionnement de cavités derrière le revêtement d'une galerie en Suisse, sur une longueur de 2 km. Une zone de gypse longue de 700 m était considérée comme particulièrement critique. Deux méthodes géophysiques ont été conjointement utilisées: le traîné électrique multiple avec trois longueurs de ligne d'envoi de courant et le géoradar. Les cavités, caractérisées par des résistivités élevées, ont surtout été mises en évidence par le dispositif le plus court (AB = 3 m). Des mesures de détail ont été réalisées dans les zones critiques avec un dispositif pôle-dipôle en vue d'une meilleure interprétation des anomalies en profondeur. Les mesures de géoradar ont été exécutées avec une antenne de 500 MHz. Quelques profils transversaux ont été effectués pour vérifier l'extension des zones d'altérations ou des cavités. Le géoradar a pu localiser des cavités mesurant jusqu'à plusieurs mètres en longueur à des profondeurs variant entre 0 et 4 m derrière le revêtement de la galerie. Les anomalies décelées par une méthode ont souvent pu être confirmées par l'autre et leur complémentarité a permis une interprétation plus sûre. Des forages carottés ont montré une bonne corrélation entre les anomalies géophysiques et la réalité. Des mesures de fibroscopie dans les forages nous ont permis de déterminer la géométrie des cavités.Le géoradar possède des avantages certains: imagerie continue du massif rocheux et grande rapidité dans l'acquisition des données. Néanmoins, nous considérons la combinaison des deux méthodes utilisées dans cette étude comme le bon exemple d'une investigation géophysique combinée permettant une interprétation optimale de tous les résultats.     

Predisposition to Induced Seismicity in Some Czech Coal Mines

—Mining-induced seismicity associated with longwall face operations in the Ostrava- Karviná coal mines, Czech Republic, has been investigated in order to establish the conditions leading to a focal zone generation. The study, based on macroseismic and instrumental observations, proved that seismicity is influenced by natural as well as mining conditions. The first group includes the influence of faults, washouts and red beds, while the second one is represented by shaft and/or crosscut safety pillars and various types of remnant pillars. All the cases discussed show that many focal zones are generated in overstressed strata as a consequence of interaction of natural conditions and/or old workings with the active coal face. Received June 18, 1996, accepted March 10, 1997     

Quantitative analysis of travel-time curves of seismic events in the Ostrava-Karviná coal mining district

Summary The paper is intended as a contribution to the quantitative analysis of travel-time curves of seismic events recorded in the Ostrava-Karviná District (OKD). The input data represent a set of 2621 seismic events, recorded by the local seismological network of 26 mine stations DSLA and a regional diagnostic polygon consisting of five surface Lennartz stations. All the events were processed automatically in the Operational Seismological Centre of the Czechoslovak Army Mine in Karviná and stored in the seismological data base. The results are presented in the form of graphs of arrival times versus distance for the whole OKD, for two mines and one tectonic block.Travel-time curves of direct P and S waves, as well as of reflected and refracted waves are given. The direct P and S waves propagate well practically throughout the whole region studied, but their apparent velocities of propagation are affected by the properties of the rock medium.As a result of the complicated geological conditions, the recorded wave image is quite complicated. Methods of mathematical modelling, using kinematic and dynamic parameters of seismic waves, will have to be applied to identify the separate wave groups uniquely.     

Earthquakes in the Czech Republic and Surrounding Regions in 1995–1999

In the time span from January 1995 to December 1999 the Czech National Seismological Network (CNSN), consisting of ten permanent digital broadband stations, several local networks and two data centers, detected and recorded 9530 regional natural seismic events, 27 greater than magnitude 2. Most of these events were located by the Czech Seismological Service (CSS), and the most prominent of them were analyzed in detail. A large number of quarry blasts were recorded as well but were not included in the analysis. We provide basic information on the configuration of the CNSN and on the way of routine data processing employed by the CSS in this paper. The over-all regional seismicity monitored by the CNSN in 1995–1999 is briefly reviewed. The main results of observations and evaluation of the local (NW-Bohemia/Vogtland, South Bohemia, Sudeten) and induced (Kladno, Píbram, Upper Silesia, Lubin/Poland) seismic activity within this period are presented in a condensed form. Finally, a summary on macroseismic observations on the territory of the Czech Republic in 1995–1999 is also presented.     

Seismic noise at the international gravity point in Prague-Ruzyně (Czechoslovakia)

Summary The vertical component of ground displacement was measured at the Prague - Ruzyn International Gravity Point in the frequency range of 1–300 Hz. The permanent noise, the vibrations caused by the observers during gravimetric observations and by the wind, as well as those due to normal operations at the airport, display maximum peak-to-peak amplitudes of 0.06 µm in the frequency range of 1–50 Hz; with a CG-2 gravimeter this is not detrimental to the accuracy of the observations. The taxiing of turbo-jet and jet aircraft and engine tests of aircraft generate vibrations in frequency ranges of 75–90 and 190–270 Hz. Their amplitudes, according to the results of laboratory tests published for various types of gravity meters (CG-2, GAK-PT, GVP-3, KVG), are of magnitudes which generate errors in tenths of mgl.     

Results of the earth-tide measurements at the Pecný station (CSSR)

¶rt;mam mam a¶rt;u, m u a nuu mauu nu¶rt; 1970–1977. nauaumGs 11 No 131, 201 uGs 15 No 228. a¶rt; ¶rt;uam mauau ¶rt;u um. ¶rt;uu au naam nuu nu¶rt; ma. 2,uuu au ma. 3.

---

---

Paper presented at the IUGG XVII Plenary Meeting, Canberra, Dec. 1979.     

A note on the microseismic noise in the western carpathians

Summary The level of microseismic noise is studied at locations in the Western Carpathians where, apart from the vertical component of the noise, also its horizontal component was recorded by a broad-band seismograph. The amplitudes of the permanent noise in the horizontal component ranged from 10 to 83 nm and, at most locations, the predominant periods T>0.45 s, although the predominant periods of the vertical component in the Carpathian region are mostly within the interval T 0.24–0.37 s.     

Geochemistry and petrogenesis of the tertiary alkaline volcanic suite of the Labe tectonovolcanic zone,Czech Republic

Summary The Tertiary alkaline, nepheline-normative olivine melanephelinite-basanite-phonolite suite in the Labe tectono-volcanic zone, Czech Republic, displays mineralogical and geochemical features indicative of the presence of products of more than one fractionation series and of more than one parental magma. The Mg- and Ni-rich olivine melanephelinites (ankaratrites), which contain disintegrated lherzolite xenoliths, represent low degree (c. 4%) partial melts of a highly enriched mande source. Many of the abundant basanitic rocks also represent primary or near-primary basaltic melts subject to a small but variable degree Fof olivine fractionation. Other basanites with lower mg-numbers and Mg and Ni contents had a crystallization history involving hornblende and phlogopite.Melaphonolites (tephriphonolites) provide a link between basaltic rocks and phonolites with plagioclase fractionation progressively playing a more significant role. The phonolites represent products of crystallization of highly evolved melts but poor correlation between SiO₂, MgO, CaO, Na₂O and degree of undersaturation as well as between major and trace element contents indicate that a single line of descent cannot have been in operation: the products of different parental magmas showing a wide compositional range that evolved under different conditions of fractionation are represented. A very advanced stage of magma evolution, characterized by the dominance of alkali feldspar fractionation is represented by phonolites that contain the highest Hf, Nb, Rb, Ta, Th, Y, Zr and REE in the whole Tertiary volcanic suite.

---

Geochemie und Petrogenese der tertiären alkalischen Vulkanit-Serie der Labe tektonovulkanischen Zone, Tschechische Republik

Zusammenfassung Die tertiäre alkalische Nephelin-normative Olivin Melanephelinit-Basanit-Phonolit Serie in der Labe tektono-vulkanischen Zone, Tschechische Republik, zeigt mineralogische und geochemische Eigenschaften, die auf die Anwesenheit von Produkten von mehr als einer Fraktionierungsserie und von mehr als einem Ausgangsmagma hinweisen. Die Mg- und Ni-reichen Olivin Melanephelinite (Ankaratrite), die umgewandelte LherzolitXenolite enthalten, stellen einen niedrigen Grad (ca. 4%) teilweiser Aufschmelzung einer stark angereicherten Mantelquelle dar. Viele der verbreiteten basanitischen Gesteine stellen auch primäre oder fast-primäre basaltische Schmelzen dar, die in kleinem, aber schwankendem Ausmaß von Olivinfraktionierung betroffen wurden. Andere Basanite mit niedrigen Mg-Zahlen und Mg- und Ni-Gehalten hatten eine Kristallisationsgeschichte in der Hornblende und Phlogopit eine Rolle spielte.Melaphonolite (Tephriphonolite) stellen ein Verbindungsglied zwischen basaltischen Gesteinen und Phonoliten dar, wobei Plagioklas-Fraktionierung eine zunehmend größere Rolle spielt. Phonolite sind Produkte der Kristallisation von Schmelzen, aber die schlechte Korrelation zwischen SiO₂, MgO, CaO, Na₂O und dem Grad der Untersättigung, wie auch zwischen Haupt- und Spurenelementen weist darauf hin, daß eine einzelne Entwicklungslinie nicht in Frage kommt. Die Produkte verschiedener Ausgangsmagmen zeigen ein weites Spektrum verschiedener Zusammensetzung, das sich unter verschiedenen Bedingungen der Fraktionierung entwickelt hat. Ein sehr fortgeschrittenes Stadium der Magmaentwicklung, charakterisiert durch die Vorherrschaft von Alkali-Feldspat-Fraktionierung wird durch Phonolite dargestellt, die die höchsten Gehalte an Hf, Nb, Rb, Ta, Th, Y, Zr und REE in der gesamten tertiären vulkanischen Abfolge führen.

---

---

With 7 Figures