Seismic tomographic surveys were carried out from within the pilot heading of a large storage cavern in chalk. The tomograms constitute Class' A' (a priori) predictions of the faulting patterns that were subsequently revealed and logged during the later excavation of the full cavern. These surveys offer a unique means of assessing the effectiveness of seismic tomography as an in-situ investigation and characterisation tool.
La methode tomographique avec les ondes sismiques a ete utilisee dans le tunnel d'avance d'un grande caverne d'entreposage en craie. Les tomogrammes presentent les predictions a priori des zones de fracture revelees ulterieurement lorsque la caverne entière a ete creusee plus tard. Ces arpentages offrent une occasion unique pour evaluer l'efficacite de la methode tomographique comme moyen in-situ d'investigation.
Das Vorhandensein eventueller Gesteindiskontinuitaten in einer großen Speicherkaverne aus Kalkstein wurde mittels seismischer Tomographie in Pilotbohrungen untersucht. Die Meßergebnisse wurden mit den bei den anschließenden Ausschachtungen gefunden Gesteinformationen verglichen. Diese Untersuchungen ergaben die einzigartige Möglichkeiten, die Verwendbarkeit von seismischer Tomographie fuer "von Ort" Untersuchungen und zur Charakterisierung von Gesteinformationen zu analysieren.
The process of ground imaging using seismic tomography is becoming an increasingly widely applied technique in rock mechanics and civil engineering. In most ground investigations, only a very small proportion of the ground can be investigated directly, for example by means of coring and sampling. Techniques such as tomography, by which cross-sections of the ground may be produced, are therefore potentially very attractive. As with any complex and relatively untried technique, there is a need for objective appraisal of geotomography, preferably under real site conditions. This paper presents the results of a number of Class' A' (that is, a priori) predictions of the physical locations of major fractures located in and around the site of a large storage cavern, constructed in chalk. The data for the compressional wave tomographic surveys were obtained by a field geophysicist, and were then processed independently by the first author without detailed knowledge of the ground conditions obtaining at the site. Based on the resulting seismic velocity tomograms, predictions were made of the locations of the major discontinuities. The sides of the cavern were subsequently logged as the benched excavation of the cavern proceeded, and these results were made available to the authors. These records are used to compare objectively the predicted and actual locations of the discontinuities. The tomographic surveys were carried out in the pilot tunnel of the cavern, and utilised boreholes drilled upwards from the crown and downwards at invert level. Rather than adopting the conventional cross-hole tomographic acquisition arrangement, the seismic measurements were taken between receivers in a borehole and a seismic source located radially from the borehole on the inner surface of the tunnel. This approach was well-suited to the geometry of this particular imaging problem, and also offered advantages of economy and speed over two-borehole methods. The paper discusses the importance of selecting a suitable acquisition geometry for a survey, and how this choice can affect the ability to resolve specific structural forms.