Curved ray seismic tomography: application to the Grand Etang Dam (Reunion Island)

Curved ray seismic tomography is used regularly by Compagnie de Prospection Géophysique Française (CPGF) for dam and nuclear site investigation. Measurements are made between tunnels, from hole to surface or, as here from hole to hole. The hydroelectric power development proposed for the Marsouins river (Réunion Island) involves a large capacity dam. Thc only suitable site is Grand Etang, a temporary lake situated in a deep valley, fillcd with heterogeneous materials, several hundred metres thick. During field investigations, a recent basaltic flow 5-3O m thick has been detected at a dcpth of 20-40 m, interbedded with silt and scoria. The following are successively examined in this article: -the data processing programme which takes into account curved ray paths and, after several iterations ends up with an optimum solution; - the hole-to-hole tornography surveys made between 13 drill holes on a 600 m profile; - the overall results, with thc limits and the discontinuities of the basaltic flow, showing that there is no link between the recent flow and older basalts of the valley borders. The hydroelectric development project on the Marsouins river, Reunion Island in the Indian Ocean is situated in a volcanic environment. It includes thc existing Takamaka dam on the river itself; it also involves a large capacity dam for flow regulation, for which the only suitable site is in an adjacent valley at Grand Etang (Fig. 1). This small temporary lake is situated in a deep valley, filled with several hundred metres of heterogeneous material including a recent basaltic flow. The Grand Etang water level varies notably according to rainfall; it does not have any free outlet and empties only by leakage through various pervious levels. Before undertaking the design of the dam and if necessary its impervious cut-offs, it seemed neccssary to secure better knowledge of lithology, structure and permeability of different horizons. In 1981-1982, preliminary surveys included surface resistivity soundings, seismic refraction and drilling. At the projected dam site, a recent basalt flow, 5 to 30 m thick and lying on silt, was discovered under 20 to 40 m of silt and scoriaceous overburden. Figure 2 shows a geological section along the valley. In order to complete information on this flow, a line of 13 drill holes, 40 to 60 m deep, were drilled in 1982-83 along the dam axis at an average spacing of 50 m. Gamma-ray logs were run in these holes. In order to determine the continuity of the flow between the holes and to appreciate the extent of weathering, a hole-to-hole seismic tomography survey was carried out. It should be noted that a previous surface seismic refraction survey had been unable to 'see' through the high velocity basalt flow.