This paper describes a tomographic method for estimating the spatial distribution of seismic velocity in rock mass, and presents the results of its application to cross-adit seismic data. With this method, a two- or three-dimensional velocity image can be reconstructed from the traveltime data of transmitted waves through the surveyed area. The performance of the reconstruction algorithm was examined using artificial model data. It was then applied to the seismic field data observed in adits at the dam site under survey, and compared with the inferred geotechnical data. These comparisons suggest that seismic tomography is useful in mapping the in situ mechanical properties of rock mass and in locating fractured zones and cracks.
Ce document decrit une methode tomographique d'estimation de la repartition de la vitesse seismique dans une masse rocheuse et presente les resultats de son application à des donnees seismiques simultanees de galeries à flanc de coteau. Avec cette methode, une image à deux ou trois dimensions de la vitesse peut être reconstituee, à partir des donnees relatives au temps de deplacement des ondes transmises à travers la zone etudiee. Les performances de l'algorithme de reconstitution ont ete examinees à l'aide des donnees d'un modèle artificiel. On l'a ensuite applique aux donnees seismiques reeles observees dans des galeries à flanc de coteux, sur le site du barrage faisant l'objet de l'etude et compare aux donnees geotechniques deduites. Ces comparaisons suggèrent que la tomographie seismique est utile pour fixer en plan sur le terrain des proprietes mecaniques d'une masse rocheuse et pour situer les zones fracturees et les fissures.
In diesem papier wird ein tomographisches Verfahren zur Schatzung der raumlichen Verteilung seismischer Wellengeschwindigkeit in einer Felsmasse beschrieben und die Ergebnisse seiner Anwendung auf seismische Daten von Querstollen dargestellt. Mit diesem Verfahren kann ein zwei- oder dreidimensionales Geschwindigkeitsbild aufgrund der Wegzeitdaten der uebertragenen Wellen durch das untersuchte Gebiet rekonstruiert werden. Die Leistung des Rekonstruktionsalgorithmus wurde unter Verwendung von Daten kuenstlicher Modelle untersucht. Es wurde dann auf seismische Felddaten, die wahrend der Untersuchung in Tunneln an der Dammseite ermittelt worden waren, angewendet und mit den abgeleiteten geotechnischen Daten Verglichen. Diese Vergleiche zeigen, daβ seismische Tomographie fuer die Erfassung der mechanischen Eigenschaften von Felsmassen und der Anordnung von Bruchzonen und Rissen vor Ort nuetzlich ist.
The mapping of weak zones, such as crack, fracture and fault, is one of the most important problem in many geotechnical projects. Although, it may be possible to detect such weak zones by drilling or tunneling, it is difficult to estimate the spatial distribution of weak zones through such means alone, because of its one-dimensional character. Conventional geophysical techniques, which have been widely applied to geotechnical investigation, are also inadequate to determine the pattern of weak zones because they give information on the limited region close to the surface. New methods to detect remotely weak zones covering effective depth and of sufficient resolution have been expected for long times. Nowadays, several methods have been developed as remote sensing techniques, and among them, tomographic method has been successfully applied to many geophysical applications: for example, Bois et al. (1972), Cosma (1983) and Gustavsson et al. (1986) used cross-hole tomography to reconstruct velocity structure between boreholes. Some field experiments of fracture mapping were presented by Mason (1981) and Ramirez (1986). Laine et al. (1980) applied it to geotechnical determination of grouted zone. Tomography is useful for fracture-mapping and also for evaluation of rock mass quality, because there is a close relation between seismic parameters and mechanical properties. In this paper, we describe the concept of seismic tomography used in our experiments, and present the result of model simulation and its application to corss-adit seismic data.
Seismic tomography is a method for reconstructing unknown velocity distribution of the region of interest from travel time data. The techniques of seismic tomography are similar to those used in medical X-ray CT (Computed Tomography), but differ in restricted pattern of measuring geometry and ray bending. Namely, in medical X-ray CT, the object can be scanned from all direction by rotating the transmitter and detectors. On the contrary, the geometry of the sources and receivers in seismic tomography is fixed in, space, restricted by the position of boreholes or adits and provides the limited projection data. This makes the reconstruction techniques of seismic tomography sensitive to data noise. The second and much more important difference is in propagation property of wave. While the assumption of ray in simple straight line is adequate for X-ray CT, it is not appropriate for seismic tomography because spatial variations in the velocity cause ray to curve. Consequently, the algorithm of reconstruction for X-ray CT may not be suitable for seismic tomography, and new algorithm taking account of ray bending must be employed.
