Aide apportée par la prospection sismique à l’alimentation en eau de la ville de Conakry (Guinée)

Résumé En Fr

The usefulness of seismic methods in water supply prospection work for the city of Conakry (Guinea). The prospection was carried out as part of a plan to supply Conakry with water from the laterite complex overlying the local dunite. The permeability of this complex is determined by its many fissures. Since these fissures are not distributed according to any particular law, neither their size nor their position could be forecast. Probability considerations therefore seemed to imply that pervious passages would most likely be found in depressions in the dunite. The profile and arc shooting techniques — both seismic refraction methods — were used to locate these depressions. The seismic wave propagation rates ranged from 1,000 to 2,000 m/s in the laterite and exceeded 3,000 m/s in the dunite. Numerous basins and furrows were discovered in this way. The seismic prospection data have been confirmed by nine different boreholes since drilled. A systematic study of the wave propagation rates in the laterite covering showed furthermore that an idea of the permeability could be obtained from the seismic prospection data, and eight of the nine, boreholes are now productive. Conakry has been using up water at a very rapidly increasing rate since 1950. According to the initial geological and hydrological surveys, it had been considered that the city could be supplied with water from the Kaloun peninsula without having to draw supplies from the Grandes Chutes falls some 100 kms away. In a report entitled “A study of water supply conditions for the city of Conakry — State of progress in May 1956”, Mr. Jean Archambault presents the main results of the successive borehole observations and surveys carried out since 1950, as well as showing the general underground flow conditions in the peninsula and suggesting a preliminary general research programme including, among others, the use of seismic prospection methods. These were subsequently carried out in March 1957. The present paper begins by reviewing geological and hydrological conditions in the Kaloun peninsula (from the above report), the principles of the seismic refraction method, and the techniques applied for the considered problem, followed by an exposé of the results of the September 1956 — March 1957 seismic prospection.

Pour son alimentation en eau, la ville de Conakry ce fait appel à la nappe du complexe latéritique qui surmonte la dunite. Ce complexe présente une perméabilité de fissures ; leur répartition n'obéissant à aucune loi, toute prévision concernant leur existence et leur importance était impossible. Par le jeu des probabilités c'est donc au droit des dépressions de la dunite qu'on a le plus de chance de rencontrer des passées perméables. Ces dépressions furent recherchées par prospection sismique réfraction. Les techniques utilisées furent le profil sismique et le tir en arc. Les vitesses de propagation des ondes sismiques variaient de 1 000 à 2 000 m/ s dans les latérites et étaient de plus de 3 000 m/ s dans la dunite. De nombreuses cuvettes et sillons furent ainsi mis en évidence et les neuf sondages exécutés depuis ont bien vérifié les prévisions sismiques. L'étude systématique des vitesses de propagation des ondes dans le recouvrement latéritique a de plus montré que la méthode sismique permettait de se faire une idée de la perméabilité et huit sondages sur neuf sont productifs.