Session 2 : Positionnement, productivité et gestion des forages: Côte d’Ivoire
208
Les relations possibles entre les quatre principales directions d’accidents relevées sur les
images (NS, NE-SO, E-O et NO-SE) et l’emplacement de 103 forages en exploitation
d’une part et la détermination des profils de productivité associés aux directions
préférentielles des linéaments de l’autre, montrent que, dans le bassin, 68 % des forages
sont implantés à l’intérieur d’un rayon de 200 mètres des linéaments, avec 27 % des
forages creusés directement sur le linéament. Tous les forages en exploitation dans le
bassin présentent une position plus ou moins idéale par rapport à au moins une direction
majeure de linéament, la distance d’éloignement étant de 1,1 km pour les quatre
directions. Les accidents NO-SE sont les plus productifs, avec 63 % des débits élevés,
compris entre 6 et 14 m
3 /h, contre 25 % pour ceux orientés N-S. Ces deux directions ont
guidé l’implantation de 62 % des forages de la région.
ABSTRACT
Digital processing of satellite images of the catchment of the Haute Marahoué was mainly
based on resolution enhancement techniques, principal components, the ratio TM7TM4/TM7+TM4 and the combination TM6+TM7. Data obtained were integrated and
interpreted within a Spatially Referenced Hydrologic Information System (SRHIS), which
made it possible to identify the volcano-sedimentary complex of Boundiali and the
localised granite mass of Seguela, whose shape had not yet been accurately described with
traditional methods. The lower limits of open fractures and the alteration profile of the
crystalline basement, which feed several wells in the region, were better defined with the
creation of a block diagramme in the SRHIS. A map of fractures, covering an area of 12
500 km
2 , illustrates very well the megafractures of regional importance in the fault plain.
They are oriented N170-190, N30-60, N80-100 and N120-160 and characterise fracturing
tectonics in West Africa. A sector study on the relationship between the accidents and the
hydrographic network revealed that the main rivers essentially flow along the tectonic
axes.
Runoff and the possibilities of creating water reservoirs in fractured environments can be
evaluated through the study of permeability caused by fractures. Therefore, until today, the
results obtained by using this technique remained a function of the density of fractures that
could be visualised on traditional aerial photographs. Landsat TM images of the Haute
Marahoué watershed, digitally processed and interpreted within a SRHIS, have
considerably improved this approach thanks to the high density of fractures (an average of
62 fractures per 1.44 km
2 ) visible on the enhanced satellite images. In fact, the values of
induced permeability, obtained with this procedure vary from 0.11 to 0.66 10
-4 m/s,
against values of 10
-8 provided by traditional methods. As a consequence, they are close to
the actual permeability of the fractured soil interested by the alteration.
The possible relationships between the four main directions of events revealed on the
images (NS, NE-SW, E-W and NW-SE) and the positioning of 103 drilling sites on one
hand, and the determination of productivity profiles associated with the preferential
directions of linear features on the other, show that in the catchment, 68% of the sites are
located within 200 meters of the fractures, with 27% of the sites placed directly above the
fracture. All the drilling sites in the catchment have a more or less ideal position in
relation to at least one major fracture, the distance being of 1.1 km for the four directions.
The NW-SE fractures are the most productive, with 63% of the yields being between 6 and
14 m
3 /h, against 25% for those oriented N-S. These two directions account for the
positioning of 62% of the wells in the region.
208
Les relations possibles entre les quatre principales directions d’accidents relevées sur les
images (NS, NE-SO, E-O et NO-SE) et l’emplacement de 103 forages en exploitation
d’une part et la détermination des profils de productivité associés aux directions
préférentielles des linéaments de l’autre, montrent que, dans le bassin, 68 % des forages
sont implantés à l’intérieur d’un rayon de 200 mètres des linéaments, avec 27 % des
forages creusés directement sur le linéament. Tous les forages en exploitation dans le
bassin présentent une position plus ou moins idéale par rapport à au moins une direction
majeure de linéament, la distance d’éloignement étant de 1,1 km pour les quatre
directions. Les accidents NO-SE sont les plus productifs, avec 63 % des débits élevés,
compris entre 6 et 14 m
3 /h, contre 25 % pour ceux orientés N-S. Ces deux directions ont
guidé l’implantation de 62 % des forages de la région.
ABSTRACT
Digital processing of satellite images of the catchment of the Haute Marahoué was mainly
based on resolution enhancement techniques, principal components, the ratio TM7TM4/TM7+TM4 and the combination TM6+TM7. Data obtained were integrated and
interpreted within a Spatially Referenced Hydrologic Information System (SRHIS), which
made it possible to identify the volcano-sedimentary complex of Boundiali and the
localised granite mass of Seguela, whose shape had not yet been accurately described with
traditional methods. The lower limits of open fractures and the alteration profile of the
crystalline basement, which feed several wells in the region, were better defined with the
creation of a block diagramme in the SRHIS. A map of fractures, covering an area of 12
500 km
2 , illustrates very well the megafractures of regional importance in the fault plain.
They are oriented N170-190, N30-60, N80-100 and N120-160 and characterise fracturing
tectonics in West Africa. A sector study on the relationship between the accidents and the
hydrographic network revealed that the main rivers essentially flow along the tectonic
axes.
Runoff and the possibilities of creating water reservoirs in fractured environments can be
evaluated through the study of permeability caused by fractures. Therefore, until today, the
results obtained by using this technique remained a function of the density of fractures that
could be visualised on traditional aerial photographs. Landsat TM images of the Haute
Marahoué watershed, digitally processed and interpreted within a SRHIS, have
considerably improved this approach thanks to the high density of fractures (an average of
62 fractures per 1.44 km
2 ) visible on the enhanced satellite images. In fact, the values of
induced permeability, obtained with this procedure vary from 0.11 to 0.66 10
-4 m/s,
against values of 10
-8 provided by traditional methods. As a consequence, they are close to
the actual permeability of the fractured soil interested by the alteration.
The possible relationships between the four main directions of events revealed on the
images (NS, NE-SW, E-W and NW-SE) and the positioning of 103 drilling sites on one
hand, and the determination of productivity profiles associated with the preferential
directions of linear features on the other, show that in the catchment, 68% of the sites are
located within 200 meters of the fractures, with 27% of the sites placed directly above the
fracture. All the drilling sites in the catchment have a more or less ideal position in
relation to at least one major fracture, the distance being of 1.1 km for the four directions.
The NW-SE fractures are the most productive, with 63% of the yields being between 6 and
14 m
3 /h, against 25% for those oriented N-S. These two directions account for the
positioning of 62% of the wells in the region.