Characterizing Aquifers - Hydrogeology: Objectives, Methods, Applications - page 131

obtained by subjecting samples to laboratory testing, through selective tests
in boreholes, or through more or less prolonged pumping tests.
5.1 Laboratory measurements
These are measurements of the permeability, K, of the terrain, using samples
taken during drilling. They are only cited, as the reworking of the samples
causes a more or less marked modifi cation of the terrain's initial structure,
and can affect the validity of the results.
Such methods are generally more used in geotechnics (soil mechanics),
with the help of permeameters and oedometers (during compressibility
tests).
5.2 Selective testing in wells or boreholes
These allow for an understanding of the vertical variations in permeability
of the terrain, by taking measurements over successive slices of a well. When
the tests are performed in a group of wells, the horizontal permeability
variations can also provide a good illustration of the aquifer's general
anisotropy.
Different types of tests can be considered, depending on the terrain
under consideration and the goals of the study. Their comparison, however,
requires total control in order to ensure that the methods used and the well
diameters remain constant.
5.2.1 Lefranc tests
The Lefranc method is used in unconsolidated terrain (scree slopes,
alluvium, moraines), during the drilling process, over successive slices of
the terrain. Each slice is tested, over a height of 0.5 to 2 m, named lantern,
where the terrain, exposed at bottom and on the sides of the well, is isolated
above the relevant section by tubing or a valve.
The terrain's absorption capacity is determined by pumping or
injecting a constant fl ow of water into the lantern, until the level in the
well stabilizes. This is a constant-level test, well-adapted to permeable soils
(K > 10 -4 m·s -1 ).
One can also extract or inject a given volume of water and then follow
the variations in water level over time. This is then a variable-level test,
well-adapted to poorly permeable soils (K < 10 -4 m·s -1 ).