Fractured Aquifers - Hydrogeology: Objectives, Methods, Applications - page 45

C HAPTER A7
Fractured Aquifers
Basement rocks or their ancient sediment covers, previously considered
impermeable bedrock due to their very poor matrix porosity, have, since the
60s and 70s, been drilled into and exploited, due to the following factors:
• hydrologic: the absence of alternative local resources (intertropical
zones);
• climatic: droughts (starting in 1970 in the Sahel, and in 1976 in western
France);
• technological: the development of the down-the-hole hammer.
Previously, cable-drilling, a slow drilling mechanism, was used, and
drilling was therefore costly.
Hard basement rocks have a variable primary porosity: the interlayer
porosity of metamorphic rocks can reach 7.5%, while in granites and gneisses
it does not generally exceed 1.8%. These rocks underwent fracturing due
to contraction as they cooled, then tectonic fracturation over the course of
their long geologic history. They include fractures tens of meters long as
well as fractures that can be mapped over hundreds of kilometers. Water is
therefore present within them in a discontinuous manner, and prospecting
for it appears chancy: the productivity of exploration wells varies greatly,
from poor to excellent.
1 GLOBAL DISTRIBUTION AND ECONOMIC
IMPORTANCE
On Earth, crystalline rocks represent 30% of the continents. They cover a
wide extent in western, central, and southern Africa, in Asia (the Middle
East, India, South Korea), in South America (Brazil), in Scandinavia, in
Australia, in a wide variety of physiographic and climactic contexts.
Historically, springs were tapped in the fractured rocks of temperate
zones, but they often stopped fl owing during the dry season. In Africa,